Evolution

I

At the edge of the clearing, Purga crept out of a dense patch of ferns. It was night, but there was plenty of light — not from the Moon, but from the comet whose spectacular tail spread across the cloudless sky, washing out all but the brightest stars.

This scrap of forest lay in a broad, shallow lowland between new volcanic mountains to the west — the mountains that would become the Rockies — and the Appalachian plains to the east. Tonight the damp air was clear; but often mists and fogs blew in from the south, born over the great inland sea that still pushed deep into the heart of North America. The forest was dominated by plants that could extract moisture from the air: Lichen coated the gnarled bark of the araucaria trees, and even the low magnolia shrubs dripped with moss. It was as if the forest had been coated with a layer of thick green paint.

But everywhere the leaves were soured, the moss and ground cover ferns browned. The rains, poisoned by gases from the great volcanic convulsion to the west, had been hard on plants and animals alike. It wasn’t a healthy time.

Still, in the clearing, dinosaurs dreamed.

The thick night dew glistening from their yellow-black armor, ankylosaurs had gathered in a defensive circle, their young at the center. In the gentle Cretaceous air, these cold-blooded giants stood like parked tanks.

In the milky light Purga’s large black eyes had fixed on a moth. The insect sat on a leaf, brown wings folded, fat and complacent. With an efficient lunge Purga caught her prey in her paws. She severed off the wings with a couple of nips of her tiny incisors. Then, with a noise like the crunch of a tiny apple, she began to munch with relish at the moth’s abdomen. For this brief moment, with food in her mouth, Purga found a scrap of contentment in her crowded, difficult life.

The moth quickly died, its sparklike awareness incapable of recording much pain.

The moth consumed, Purga moved on. There was no grass cover here — the grasses had yet to dominate the land — but there was a green covering of low ferns, mosses, ground pine, horsetails, and conifer seedlings, even a few gaudy purple flowers. Through this tangle, scuttling between scraps of cover, she was able to progress almost silently. In the dark, solitary foraging was the best strategy. Predators worked by ambush, exploiting the shadows of the night; no group could have been as invisible as a lone prowler. And so Purga worked alone.

To Purga the world was a plain picked out in black, white, and blue, lit up by the uneasy light of the comet, which shone behind high scattered clouds. Her huge eyes were not as sensitive to color as the best dinosaur designs — some raptors could make out colors beyond anything that would be visible to humans, somber infrareds and sparkling ultraviolets — but Purga’s vision worked well in the low light of night. And besides she had her whiskers, which fanned out before her like a tactile radar sweep.

Purga looked more rodentlike than primate, with whiskers, a pointed snout, and small folded-back ears. She was about the size of a small bush baby. On the ground she walked on all fours, and she carried her long bushy tail behind her, like a squirrel. To human eyes she would have seemed strange, almost reptilian in her stillness and watchfulness, perhaps incomplete.

But, as Joan Useb would one day learn, she was indeed a primate, a progenitor of that great class of animals. Through her brief life flowed a molecular river with its source in the deepest past, its destination the sea of the furthest future. And from that river of genes, widening and modifying as thousands of millennia passed, would one day emerge all of humanity: Every human ever born would be descended from the children of Purga.

She knew none of this. She didn’t give herself a name. She was not conscious like a human — or even like a chimp or monkey; her mind was more like a rat’s or a pigeon’s. Her behavior was made up of fixed patterns, controlled by innate drives that constantly shifted in balance and priority, reaching a new sum each moment. She was like a tiny robot. She had no sense of self.

And yet she was aware. She knew pleasure — the pleasure of a full belly, the safety of her burrow, the snouts of her pups as they nuzzled her belly for milk — and, in this dangerous world, she knew fear very well.

She crept among the feet of dreaming ankylosaurs. As she moved beneath the immense bellies Purga could hear the huge rumble of the dinosaurs’ endless digestion, and the air was thick with their noxious farts. With their crude teeth, all the work of processing and digesting their coarse food had to be done in the dinosaurs’ vast guts, which labored even as the ankylosaurs slept.

The ankylosaurs were herbivorous dinosaurs. But this was a time of huge, ferocious predators. So these animals, larger than African elephants, were covered with armor, a fusion of bones, ribs, and vertebrae. Great yellow-black spines were embedded in their backs. Their skulls were so heavily reinforced there was little room left for brain. Their tails ended in heavy clubs that could smash legs or skulls.

The dinosaurs were too huge for Purga to comprehend. Hers was a small world, where a fallen log or a puddle was a major obstacle, where a scorpion could be a significant predator, where a fat millipede was a rare treat. To her, the dozing ankylosaur herd was a forest of immense stumpy legs and drooping tails that had no connection to each other.

But for Purga there was a rich prize here: dinosaur dung, immense heaps of it scattered in the muddy, trampled ground. Here, in fibrous mountains of roughly digested vegetation, she might find insects, even dung beetles, laboring to destroy the tremendous turds. She burrowed into the steaming stuff eagerly.

Thus had been the role of the ancestors of humanity, all through the long dinosaur summer: relegated to the fringe of the reptiles’ great society, emerging from their burrows only at night, foraging for a living from dung, insects, and the small pickings of the forest.

But tonight the rewards were meager, the droppings watery and foul-smelling. The volcano-damaged vegetation had provided poor fodder for the ankylosaurs, and what came out the other end was of little value to Purga.

She moved across the clearing and into the forest. Here conifers towered grandly, rising to spreading mats of leaves far overhead. Among them were smaller trees a little like palms, and a few low bushes bearing pale yellow flowers.

Purga scrambled briskly into the angular branches of a ginkgo tree. As she climbed she used the scent glands in her crotch to mark the tree. In her world of night, scent and sound were more important than sight, and if others of her kind found this mark, any time within the next week, it would be a sign like a neon light, telling them she had been here, even how long ago she had passed.

It was pleasing to climb, to feel her muscles work smoothly as they hauled her high above the dangerous ground, to use the delicate balance afforded by her long tail — and, most of all, to jump, to fly briefly from one branch to another, using all her body’s equipment, her balance, her agility, her grasping hands, her fine eyes. She was forced to shelter in burrows on the ground. But everything about her had been shaped by an existence in the complex three-dimensional environment of the trees, where almost all primate species, throughout the family’s long history, would find refuge.

But the acid rain of recent months had withered the trees and undergrowth; the bark was sour, and there were few insects to be found.

Purga was perpetually hungry. She needed to consume her body weight every day: It was the price of her warm blood, and the milk she must produce for her two pups, safe in their burrow deeper in the forest. She clambered reluctantly back down the trunk of the ginkgo. Fear and hunger warring in her mind, she tried one or two more trees, but with no better luck.

But now she lifted her head, whiskers twitching, bright eyes wide, to peer into the green dark of the forest. She could smell meat: the alluring stench of broken flesh. And she heard a forlorn, helpless piping, like that of baby birds.

She scuttled away, following the scent.

In a small clearing at the base of a huge, gnarly araucaria there was a heap of roughly piled moss. At its edge, a small patch of debris-littered silt began to move. Soon the patch rose like a lid, and a small, scrawny neck poked out of the ground and through the layer of mud and debris. A beaklike mouth opened wide.

Its little head quivering, tiny scales and feathers still moist with yolk, the infant dinosaur took its first breath. It looked like an oversized baby bird.

It was the moment the didelphodon had been waiting for. This mammal, the size of a domestic cat, was one of the largest of its day. It was low slung, with a black-and-silver coat. Now it lunged forward and grabbed the chick by its thin neck, hauling it from its shell and flinging it into the air.

The chick’s life was a handful of brief, vivid impressions: the cold air beyond its cracked shell, the blurred glow of the comet, a sense of flying. But now a hot cavern opened beneath it. Its skin still smeared with yolk, the chick died instantly.

Meanwhile more chicks were pushing out of the ground, hatching all at the same time. It was as if the ground were suddenly swarming with baby dinosaurs. The didelphodon, and more predatory mammals, closed in to feed.

An ancient survival strategy had been operating. Dinosaurs were reptiles who laid their eggs on the ground. Though some parents stayed with their brood, there was no way all the vulnerable eggs and chicks could be protected. So dinosaurs laid many eggs, and their hatchings were synchronized. There should have been dozens of broods hatching right now, scattered through this area of forest: hundreds of chicks. The idea was that suddenly the forest floor would be overwhelmed with baby dinosaurs, far too many for even the hungriest of predators. Most of the chicks would die, but that didn’t matter. It was enough that some would survive.

But here, tonight, the strategy had gone wrong — horribly so for the dinosaur chicks. The mother of these chicks was a hunter isolated from her pack. Confused, hungry, scared of predation herself, she had laid her eggs in the old, familiar place — this rookery was millennia old — and covered them with rotting vegetation for warmth. She had done just what she should have done, save that it was the wrong time, and the eggs had been forced to hatch without the cover of hundreds of others.

The air was filled with the stink of blood, the low growls of the predators, and the sad peeping of the doomed chicks. There were many species of mammals represented here at this grisly banquet. The largest was the big didelphodon. There was a pair of deltatheridiums, ratlike omnivores, neither marsupial nor placental, a unique line that would not outlive the dinosaurs. Many of the creatures here had potential far beyond their present standing; one unprepossessing little creature was an ancestor of the line that would lead to the elephants.

But for now, all that concerned them was their empty bellies. Dissatisfied with the slow emergence of the struggling hatchlings, the mammals had already started to dig into the loose silt, seeking unbroken eggs, scattering the cover of moss laid over the nest by the mother dinosaur.

By the time Purga arrived the rookery had become a killing pit, a squirming mass of feeding mammalian bodies. Purga, late to the fray, burrowed eagerly into the dirt. Soon tiny bones crunched in her mouth. And, so deeply did she immerse her head in search of the deep-buried goodies, she was the last to sense the return of the mother dinosaur.

She heard an angry bellow, felt the ground shudder.

Her snout sticky with yolk, Purga pulled her head out of the dirt. The other mammals were already vanishing into the forest’s welcoming green black. For one instant Purga saw the whole creature, an unlikely feathered monster suspended in the air, limbs splayed, mouth gaping. Then a vast clawed hand flashed out of the sky.

Purga hissed and rolled. Too late she learned that this was the nest of a troodon: an agile, fast-moving killer — and a specialist hunter of mammals.

The troodon’s name meant “Wounding Tooth.”

Wounding Tooth, the size of a dog, was not the largest of dinosaurs, but she was intelligent and agile. Her brain compared in size to that of the flightless birds of later eras she somewhat resembled. Her eyes were as large and as well night-adapted as Purga’s, and they could see forward, giving her binocular vision, the better to triangulate on her small, fast-moving targets. She had legs that enabled her to spring like a kangaroo, a long sicklelike claw on the second toe of each foot, and hands like spades evolved specifically to dig out and crush scuttling mammals.

She was coated in small sleek feathers, an elaborate development of scales. The feathers weren’t meant for flying, but for warmth during the night’s chill. In the equable climate that swathed the Earth in these times, you didn’t need a hot-blooded metabolic engine to keep warm: If you were big enough, your cold-blooded body would retain its heat right through the night, even if you lived at Earth’s extremes, at the poles. But smaller dinosaurs, like the troodon, needed a little extra insulation.

Small or not, she had one of the largest brains of all dinosaurs. All in all, she was a well-equipped hunter. But Wounding Tooth had problems of her own.

She could not know it, but they had been caused by the widening of the Atlantic, the huge geological event that had dominated the whole of this Cretaceous period. As the Americas were pushed west, North America’s huge inland seaway had shallowed and drained, and close to the western coast — just a few hundred kilometers from the troodon’s hatching site — that line of new volcanoes had erupted like an angry wound. The volcanism had disturbed the complex web of life in many ways. The young volcanoes were almost continually active, belching out smoke and ash ladened with sulfur that, mixing with the rain, turned to acid. Many species of plants had vanished, and trees on the higher ground had been reduced to bare trunks. Elsewhere the destruction had been more direct, with vast fingers of cold lava reaching deep into the forest.

The troodon’s mammalian food, relatively close to the base of the food chain, had been less disturbed than most of the larger species of predatory dinosaurs. In fact, with their tiny bodies, deep burrows, and fast reproductive rate, the mammals were better equipped to live through such times of stress than the land’s grander overlords.

But troodons were pack hunters. And this female had, some days ago, become isolated from her pack by a spectacular venting of hot steam from a fissure. Even though she was alone, Wounding Tooth was carrying eggs from her last fertilization. So she had come to the herd’s ancient roosting site. A deep part of her had hoped to find others of her kind here. But there was no other here, only herself.

Wounding Tooth was growing older. At fifty, she found many of her much-stressed joints were racked with the pain of arthritis. And, because of her age and loss of strength and flexibility, she herself was under threat: This was, after all, a time of predators powerful enough to justify armor plating on creatures bigger than elephants. She had to reproduce; every instinct demanded it.

She had laid her eggs, as she had before. What else could she do?

The nest itself was a circular pit scraped out of the dirt, and she had arranged the eggs with an odd, almost surgical precision. She made sure the twenty eggs were not too close together, and that the top of each elongated egg pointed to the center so the emerging chicks would have a good chance of digging their way out. Then she had covered over the eggs with dirt and moss. She had returned several times to the nest, probing with her claws to tap the shells. The eggs had developed well; she could see it. But now the eggs had hatched — her young had emerged — but nothing was left of them except scattered bits of red flesh and gnawed bone. And here, in the center of the smashed nest, was a mammal, her face stained by blood and yolk and dirt.

Which was why Wounding Tooth leapt.

Purga helplessly squirted urine and musk, leaving a scent warning: Beware! Mammal hunter about! Then she ran out of the forest, back to the clearing of the ankylosaurs.

But on the edge of the clearing Purga hesitated. She had a choice to make, a choice of dangers. She had to get away from the pursuing troodon. She was heading back toward her burrow, where her pups waited. But by crossing the clearing again she was leaving the safety of the trees. The unconscious calculus rapidly produced a result. She took the gamble; she raced across the clearing.

A sleepy infant giant raised one bony eyelid.

The light seemed brighter than ever now, exposing Purga clearly. But there was no dawn; it was only the comet, its nucleus huge, blurred, and bright, the gas jets that erupted from it clearly visible, even through the haze of air. It was an eerie, extraordinary sight that sparked a dim curiosity in her agile mind, even as she ran.

A shadow fled at the corner of her vision.

Instinctively she darted sideways — just as a dinosaur hand slammed into the ground where she had stood. She ran back into the ankylosaur herd, darting this way and that, seeking cover in the shadow of the lethargic dinosaurs.

The troodon chased her around immense legs. But even the enraged mammal hunter was reluctant to disturb these huge armored beasts. Their clubbed tails could have crushed her in a second. Purga even slithered perilously under one ankylosaur’s huge raised foot, which hovered like a falling moon above her, while Wounding Tooth, frustrated, hissed and scraped at the ground.

At last Purga reached the far side of the clearing. Her scent and instinct guiding her unerringly, she raced into the undergrowth.

Her burrow was pitch dark, too dark even for her huge eyes to make out anything. It was like entering a mouth set in the warm earth. But the burrow was full of the consanguineous smell of her family, and she could hear the snuffle of her two young as they squirmed blindly out of the dark. Soon their warm, tiny mouths were nipping at her belly, seeking her nipples. Her mate was not here; he was out foraging himself in this clear Cretaceous night.

But Wounding Tooth must be close; the scent of warm flesh, fur, and milk that had helped bring Purga home would draw the hunter here too.

The imperatives in her head shifted again. She tucked her infants behind her and pushed her way to the back of the burrow, away from its entrance. Purga, unlike the troodon, was young — just a few months old, in fact — and this was her first brood. And unlike the prolific dinosaurs, Purga’s kind bore few young. She could not afford to lose her brood. And now she prepared to fight for them.

There was a crash behind her.

The roof of packed earth imploded, showering Purga and the pups with dirt. Comet light flooded in, startlingly bright after the few seconds of darkness. It was as if a bomb had fallen. A huge grasping hand reached out of the sky into the burrow. The pups squirmed and squealed, but one of them was impaled on a bloody claw. In an instant its life was over. It was lifted up and out of the burrow, a naked, lifeless scrap, and out of Purga’s life.

Purga hissed her distress. She ran toward the burrow entrance, away from the claw. She could sense the remaining infant, naked and stumbling, hurrying after her. But the wily troodon had foreseen this. That claw now pushed into the entrance, breaking open its earthen walls. Reptile fingers closed, and the second pup’s life was squeezed out, its skull and tiny bones crushed, its organs pulped.

Purga, her world broken apart in a few heartbeats, scrabbled away from the debris of the entrance, away from the broken roof, back to the deepest recesses of the burrow. But again and again that machinelike clawed paw slammed through the roof, breaking it down and admitting more milky comet light.

Purga’s body urged her to flee, to find darkness, a new burrow, shelter — to be anywhere but here. She was even hungry; for such a fast-metabolizing creature as Purga, it had been a long time since she had supped the yolk of Wounding Tooth’s eggs.

But suddenly the strength drained out of her.

She huddled at the rear of her ruined burrow, shivering, folding her paws over her face as if to clean her fur of mites. From the moment of her birth into this world of huge teeth and claws that could flash from the sky without warning, she had struggled to survive by instinct and agility. But now her young were gone. The innate imperatives dissolved, and something like despair settled on her.

And while Purga trembled in the remains of her burrow, a world trembled with her.

If she submitted now, she would leave no living descendants: The molecular river of inheritance would be blocked, here, forever. Others of her kind would breed, of course; other lines would go on into far distant futurity, to grow, to evolve, but not Purga’s line, not her genes.

And not Joan Useb.

Life always had been chancy.

The great clawed hand slammed down one more time, centimeters from Purga. And now Wounding Tooth, impatient, rammed her great head into the burrow. Purga quailed before a wall of snapping teeth.

But as the dinosaur pressed closer, screeching, Purga smelled meat, and crushed bones, and a lingering sweetness of milk. The monster’s hot breath smelled of Purga’s babies.

With a spasm of rage Purga threw herself forward.

The great teeth snapped, scything through the air around Purga like some vast piece of machinery. But Purga squirmed to avoid their flashing arcs, and sank her own teeth into the corner of the dinosaur’s lips. The scaly skin was tough, but she felt her lower incisors sink into the warm, softer flesh inside the creature’s mouth.

Wounding Tooth bellowed and pulled back. Purga, hooked by her own teeth, was dragged out of her burrow and hauled up into the air, up through many times her own body height, up past the scaly belly of Wounding Tooth and into the cold night.

Her mist of rage faded. She twisted her head, ripping away a scrap of dinosaur flesh, and she tumbled backward through the misty air. Even as she fell a great clawed hand swept sideways at her, seeking to grab her. But Purga was a creature of the trees, and she twisted as she fell. Again luck favored her — though the grasping claw came close enough to make a breeze that ruffled the downy hairs of her belly.

She fell onto a patch of trampled dirt. She was momentarily winded. But already teeth and claws were descending again, painted silver by the eerie comet light. With a lithe wriggle Purga rolled over, got her feet under her, and ran into the roots of the nearest tree. Alone, eyes wide, mouth gaping, she huddled there, panting, twitching at every leaf that stirred.

There was a scrap of meat in Purga’s mouth. She had forgotten that it had come from the dinosaur. She chewed it quickly and swallowed, for a moment assuaging the hunger that clamored at her even now. Then she peered around, seeking a safer refuge.

Wounding Tooth paced and bellowed out her frustration.

Purga had chosen life. But she had found an enemy.

II

The Devil’s Tail was as old as the sun.

The solar system had been born out of a rich, spinning cloud of rock and volatiles. Battered by a supernova shock, the cloud quickly coalesced into planetesimals: loosely aggregated lumps of rock and ice that swam chaotically through the dark, like blind fish.

The planetesimals collided. Often they were destroyed, their substance returning to the cloud. But some of them merged. Out of this clattering violence, the planets grew.

Close to the center, the new planets were rocky balls like Earth, baked by the sun’s fire. Farther out, huge misty worlds were born, globes stuffed with gases — even the lightest gases of all, hydrogen and helium, gases manufactured in the first few moments of the universe itself.

And around these growing gas giants, the comets — the last of the icy planetesimals — swarmed like flies.

For the comets it was a dangerous time. Many of them were dragged into the gravity wells of Jupiter and the other giants, their masses feeding those growing monsters. Others were hurled inward by the giants’ gravitational slingshots to the warm, crowded center, there to batter the inner planets.

But a few lucky survivors were hurled the other way, away from the sun and into the huge, cold spaces of the outer dark. Soon a loose cloud of comets formed out here, all of them following vast, slow orbits that could reach halfway to the sun’s nearest stellar neighbor.

One such was the Devil’s Tail.

Out here the comet was safe. For most of its long lifetime its nearest neighbor was as remote as Jupiter was from Earth. And at the farthest point of its orbit, the Devil’s Tail sailed all of a third of the way to the nearest star, reaching at last a place where the sun itself was lost against the star fields, its huddled planets invisible. Away from the heat, the comet quickly cooled and froze hard. Its surface was made black by silicaceous dust, and an epochal frost carved exotic, fragile ice sculptures on its low-gravity surface, a meaningless wonderland that no eye would ever see.

Here the comet swam for four and a half billion years, while on Earth continents danced and species rose and fell.

But the sun’s gentle gravity tugged. Slowly, slower than the rise of empires, the comet responded.

And it began to fall back toward the light.

Red dawn light seeped into the eastern sky. The clouds had a bubbly texture, and the sky was tinged a peculiar bruise purple. In this remote time the very air was different — thick, moist, laden with oxygen. Even the sky would have looked alien to human eyes.

Purga was still traveling, exhausted, already dazzled by the gathering light. She had wandered far from any forest. There were only scattered trees here, spaced out over a ground made green by a dense mat of low-lying ferns. The trees were cycads, tall trees with rough bark that resembled palms, squat cycadeoids looking oddly like giant pineapples, and ginkgoes with their odd, fan-shaped leaves, an already ancient lineage that would survive into the human era and beyond.

In the stillness of the predawn, nothing moved. The dinosaur herds had yet to stir, and the hunters of the night had retired to their burrows and nests — all but Purga, who was stranded in the open, all her worn nerves sparking with an apprehension of danger.

Something moved across the sky. She flattened herself against the ground and peered up.

A winged form glided high over the roof of the sky, its profile picked out cleanly by the red-gray light of the dawn. It looked like a high-flying aircraft. It was not; it was alive.

Purga’s instinctive computation relegated the pterosaur to a matter of no concern. To her the most ferocious flying creature was of much less immediate peril than the predators who might lurk under these cycads, the scorpions and spiders and ever-ravenous carnivorous reptiles, including the many, many small and savage dinosaur species.

She stumbled on, toward the gathering dawn. Soon the greenery started to thin out, and she scrambled over hard-packed dunes of reddish sand. She topped a short rise — and found herself facing a body of water, which lapped languidly to the horizon. The air smelled strange: full of salt, and oddly electric.

She had come to the northern shore of the great slice of ocean that pushed into the heart of North America. She could see vast, languid forms break the water’s surface.

And to the southeast, where the dawn light was gathering, the comet was suspended in the sky. Its head was a milky mass from which immense fountains of pearl-white gas gushed, visibly evolving as she watched. Its twin tails, streaming away from the sun, flailed around the Earth, making a confusing, billowing mass. It was like looking down a shotgun blast. The whole immense, brilliant show was reflected in the shallow sea.

Listlessly she stumbled forward, descending to a shallow, sloping beach. The shore was littered with clam shells and half-dried seaweed. She prodded at this detritus, but the seaweed was stringy, salty stuff. And she could smell the salt in the water; there was nothing to drink here.

On her little rise, Purga was increasingly exposed, as if picked out by a spotlight.

She spotted a tree fern, no more than a meter tall. She stumbled to it and began to dig at its roots, hoping to make a rudimentary burrow. But the soft sand fell back into her trenches. At last, as the ruddy sun rose above the horizon, Purga managed to dig out a hole big enough to shelter her body. She tucked her tail in behind her, put her paws over her face, and closed her eyes.

The warmth and darkness of the burrow reminded her of the home she had lost. But the smell was wrong: nothing but salt and sand and ozone and decaying seaweed, the sharp stinks of this place where land met sea. Her home burrow had smelled of herself, of that other who was her mate, of the pups who had smelled like a mixture of herself and her mate — a wonderful mélange of selves. All gone now, all lost. She felt a deep pang of regret, though her mind was not rich enough to understand why.

As she slept out the long day her legs scraped and scratched at the gritty young sand.

Cretaceous Earth was a world of ocean, of shallow seas and shore.

A giant ocean called the Tethys — like an extension of the Mediterranean — cut off Asia from Africa. Europe was little more than a scattered collection of islands. In Africa, even the mid-Sahara was an ocean floor. The world was warm, so warm there were no polar ice caps. And for eighty million years the sea levels had been rising. The post-Pangaea spreading of the continents, and the formation of huge reefs and shelves of chalk around their coasts, had pushed huge volumes of solid matter into the oceans: It had been like putting bricks into an already full bucket of water, and the brimming oceans had flooded the continents. But the vast shallow oceans were almost tideless, and their waves were gentle.

Life in the sea was richer and more varied than at any other time in Earth’s long history. Tremendous blooms of plankton filled the waters, drinking the sunlight. The plankton were the base of the ocean’s vast pyramid of eaters. And in the plankton were microscopic algae called haptophytes. After a brief free-swimming phase, the haptophytes constructed for themselves tiny, intricate suits of armor from calcium carbonate. As they died, billions of tiny corpses sank to the warm seabeds, where they settled and hardened into a complex white rock, chalk.

Eventually tremendous chalk beds, kilometers thick, would smother Kansas and the gulf coast of North America, and stretch along the southern half of England and into northern Germany and Denmark. Human scientists would call this era the Cretaceous — after creta, meaning “chalk” — for its most enduring monuments, constructed by the toiling plankton.

When the light began to seep out of the sky, Purga emerged from her shelter.

She scampered with difficulty through dry sand that yielded with every step, sometimes billowing around her belly. She was rested. But she was hungry, and confused, and pulsed with loneliness.

She came to the top of the rise she had crossed yesterday. She found herself facing a broad, gently rolling plain extending to the rising smoke-wreathed mountains to the west. Once the great American inland sea had flooded this place. But now the sea had receded, leaving a plain littered with broad, placid lakes and marshes. Everywhere there was life. Giant crocodiles cruised like gnarled submarines through the shallow waters, some of them with birds riding on their backs. There were flocks of birds, and birdlike, furry pterosaurs, some of whom built huge rafts to support their nests at the center of the lakes, far from the land-based predators.

And everywhere she looked there were dinosaurs.

Herds of duckbills, ankylosaurs, and a few gatherings of slow, clumsy triceratops clustered around the open water, jostling and fighting. Around their feet ran and hopped frogs and salamanders, lizards like iguanas and geckos, and many small, snapping dinosaurs. In the air pterosaurs and birds flapped and called. On the fringe of the forest, raptors could be seen stalking, evaluating the jostling herds.

The hadrosaurs, the duck-billed dinosaurs, were this era’s most common herbivores. Though they were larger than later mammalian equivalents like wildebeest or antelope, they walked on two legs like outsized ostriches, their strides long, their heads bobbing. Males led the way, elaborately ornamented by huge crests over their noses and foreheads. The crests acted as natural trumpets, capable of producing notes as low as a piano’s bottom register. Thus the voices of the duckbills hooted mournfully across the misty plain.

In the foreground a herd of vast anatotitans was crossing the floodplain. It was a convoy of flesh. These immense creatures looked oddly unbalanced, with powerful hind legs — each of them taller than an adult human — but comparatively spindly forelegs, and they trailed long, fat conical tails. The air was filled with their rumbles: the churning of the herbivores’ huge stomachs and the deeper growl of their voices, reaching deep into the infrasonic, deeper than any human ear could have detected, as they called reassurance to each other.

The anatotitans converged on a grove of cycads. The cycads’ mature leaves were thick and spiny, but their fresh growth, protected by a crown of older leaves, was green and luscious. So the anatotitans rose up on their heavy hind legs and cropped the new growth. As their great feet fell back on the undergrowth of ferns, clouds of insects rose up. The phalanx of titans would leave the cycads smashed and broken. Though the anatotitans would scatter seeds for future growth far from here, the vegetation would take a long time to recover from the devastation they caused.

There was noise everywhere: the mighty foghorn honks of the duckbills, the bellows of the armored dinosaurs, the screeching of birds, the leathery flapping of the huge flocks of pterosaurs. And, under it all, there was the ugly, unstructured roar of a female tyrannosaur, the area’s top predator: All of these animals were within her domain, and she was letting them, and any competitor tyrannosaur, know about it.

The scene might have reminded a human of Africa. But though there were great herbivores to fill the roles of antelopes, elephants, hippos, and wildebeests, and predators who hunted like lions, cheetahs, and hyenas, these animals were more closely related to birds than to any mammal. They preened, displayed, fought, and nested with oddly rapid motions fueled by the rich oxygen of the thick air. The smaller, more lithe dinosaurs that ran or stalked through the undergrowth would have seemed surreal: There was nothing like these bipedal runners in human times. And there was no sight in twenty-first century Africa like the two ankylosaurs who now began to mate, backing their rear ends together with the most exquisite care.

It was a landscape of giants, in which Purga was a lost, helpless figure, utterly irrelevant. But to the west, Purga made out a storey of denser forest, layer on layer of it rising up toward the distant volcanoes.

Purga had run the wrong way, coming to this place of the sea. She was a creature of the forest and the dirt; that was where she must go. But to get there she had to cross the open plain — and evade all those mountainous feet. With trepidation she slid down the sand bank.

But now she glimpsed stealthy movement through low ferns. She hurried beneath an immature araucaria and flattened herself against the ground.

A raptor. Standing as still as a rock, it was studying the jostling anatotitans. It was a deinonychus, something like a featherless, flightless bird. But it was as still as a crocodile. The raptor had only a faint scent — its skin was not as glandular as mammals’ — but there was a dry pungency in the air, a spiciness that filled Purga with a sense of peril.

It was very close to Purga. If it caught Purga the raptor would, of course, kill her in a second.

A bird was climbing into the tree above her. Its feathers were bright blue and it had claws on its wing bones and teeth in its beak. This creature was a relic of ancient times, of archaic linkages between birds, crocodiles, dinosaurs. The bird was climbing to feed its brood of fat, squawking chicks. Apparently it had not seen the raptor.

But for now the raptor was stalking larger prey.

The raptor watched the anatotitan herd with blank, hawklike eyes, its only calculation was which of the titanic herbivores might serve it as Prey. If necessary, it would harass the herd, seeking to make one of them peel away and thereby become vulnerable.

But that proved unnecessary.

One of the adult titans fell behind the rest. This female, walking tiredly, was more than seventy years old. Her growth had continued all her life, and now she was the largest in the herd — one of the largest of her kind anywhere, in fact. Now she dipped a heavy head into the scummy water of a shallow pond.

The raptor began to stalk steadily, silently, toward the old titan. Purga cowered in the shelter of her araucaria.

The raptor was three meters tall — compact, agile, with slim legs capable of high-speed running and a long stiff tail for balance. It had a huge claw on each hind limb; while the raptor walked, its toes lifted the claws up and clear of the ground.

The raptor wasn’t so smart. Its brain was small — no larger than a chicken’s or a guinea fowl’s. And it was a solitary hunter; it wasn’t smart enough to hunt in a pack. But it didn’t need to be.

The great anatotitan still had no idea of the danger it was in.

The raptor erupted from cover. It spun in the air, and its grime-crusted hind claws flashed cruelly. The strikes were made well.

Blood gushed. Bellowing, the anatotitan tried to back away from the water. But the titan’s black entrails slid out of immense, deep belly wounds, steaming. At last she caught her forefeet in the slippery mess. With a sound like thunder she slid forward on her chest. And then, with a spasm, the great hind legs collapsed, rolling the great bulk of her body onto its side.

One of the other anatotitans looked back and lowed mournfully, a deep noise that made the ground under Purga tremble. But the herd was already moving on.

The raptor, panting rapidly, waited for the titan to weaken.

The dinosaurs had first emerged more than a hundred and fifty million years ago, in a time of hot dry climates more welcoming to reptiles than mammals. In those days the continents were fused into the single vast Pangaean landmass, and the dinosaurs had been able to spread across the planet. Since then, continents had fissioned, danced, and whirled, and bands of climate had shifted across the planet. And the dinosaurs had evolved in response.

Dinosaurs were different.

They did not hunt like the mammalian killers of later times. Their cold blood meant they were poor at sustaining speed for long distances; they could never be endurance hunters, running down their prey like wolves. But they had versatile, high-pressure hearts. And the design of their bodies had much in common with birds’: This raptor’s neck bones and torso contained a duct system that drew the air through its lungs, and oxygen could be supplied to its tissues at a tremendous rate. It was capable of short sprints, and could pour a great deal of energy into its attacks.

Dinosaur hunts were events of stillness, of ambush and silence and motionlessness, broken by brief bursts of savage violence.

Mammals were not poorly evolved compared to the dinosaurs. The product of her own track of tens of millions of years of evolution, Purga was exquisitely adapted for the niche in which she made her living. But the brutal facts of energy economics kept mammals caged in the neglected corners of a dinosaur world. Overall, a dinosaur killer made better use of energy than mammals: This raptor could run like a gazelle but it rested like a lizard. It was that combination of energy efficiency and lethal effectiveness that had kept the dinosaurs supreme for so long.

The raptor was something like a huge, ferocious bird, perhaps. Or something like a souped-up crocodile. But it was not truly like those animals. It was like nothing seen on Earth in human times, something no human eye would ever witness.

It was a dinosaur.

This raptor’s preferred way of killing was to burst out of cover and slash at its prey, inflicting wounds that were savage but often nonlethal. The prey might flee, but it would be weakened by raking wounds to its legs and flanks — or hamstringing — blood loss and shock would result. The raptor had poor dental hygiene — its breath stank ferociously — and its bite passed on a mouthful of bacteria. The raptor would follow, perhaps attacking again, perhaps just following the scent of the stinking, infected wounds, until weakness disabled the prey.

Today this raptor had been lucky; it had disabled its victim with a single blow. All it had to do now was wait until the titan was too weak to do the raptor any harm. It could even take its food while its prey was still alive.

The raptor would not trouble with such small fry as Purga while such a giant meal awaited it. Moving cautiously, watchfully, Purga left the shelter of the fern, and scurried across the scrubby floodplain, through the devastated track left by the anatotitan herd, until she reached the security of the trees.

For the first time in four billion years, heat had touched the Devil’s Tail. Fragile ice sculptures older than Earth were quickly lost.

Gases boiled through fissures in the crust. Soon a shining cloud of dust and gas the size of the Moon had gathered around the comet. The wind from the sun, of light and sleeting particles, made the gas and dust stream behind the falling comet nucleus in tails millions of kilometers long. The twin tails were extremely tenuous, but they caught the light and began to shine.

For the first time, uncomprehending eyes on Earth made out the approaching comet.

Spitting, rotating, its dark nucleus founting gases with ever greater vigor, the Devil’s Tail swam on.

III

Another long, hot Cretaceous day wore away.

Purga slept through the day, her new family curled around her. She slept even when her pups suckled. The snug burrow floor was littered with the primates’ soft fur — and it smelled, indubitably, of Purga, of her new mate, and of the three pups who were half of herself.

Purga’s mate gave himself no name, and nor did Purga name him, any more than she named herself. But if she had — in recognition that he could never be the first in her life — she might have called him Second.

As Purga slept, she dreamed. Primates already had brains large and complex enough to require self-referential cleansing. So she dreamed of warmth and darkness, of flashing claws and teeth, and of her own mother, huge in her memory.

Purga, like all mammals, was hot-blooded.

All animal metabolisms were based on the slow cellular burning of food in oxygen. The first animals to colonize the land — gasping fish, driven from drying rivulets, using swim bladders as crude lungs — had had to rely on metabolic engines designed for swimming. In those first land-walkers the metabolic fires had glowed dimly. Still, their decisive move onto the land had been successful; and now and into the future every animal — mammals, dinosaurs, crocodiles, and birds, even snakes and whales — would use a variant of the same ancient tetrapod body plan of four legs, a backbone, ribs, fingers, and toes.

But some two hundred million years before Purga’s birth, certain animals had begun to develop a new kind of metabolism. They had been predators, driven by selection to burn food more briskly in order to improve their luck in the chase.

It had meant a complete redesign. These ambitious predators needed more food, a higher rate of digestion, a more efficient system of waste elimination. All this had raised their metabolic rate, even when resting, and they had had to increase the size of heat-producing organs like the heart, kidneys, liver, and brain. Even the working of their cells had speeded up. In the end a new and stable high body temperature had been set.

The new hot-blooded bodies had had an unplanned advantage. Cold-bloods relied on drawing heat from the environment. But the hot-bloods did not. They could operate at peak efficiency in the cool of night, when the cold-bloods had to rest, or in extreme heat, when cold-bloods would have to hide. They could even prey on cold-bloods — frogs, small reptiles, insects — at times like dawn and dusk, when those slow movers were vulnerable.

But they could not topple the dinosaurs from their thrones; the dinosaurs’ supreme energy efficiency saw to that.

Purga’s dreams were disturbed by the immense stomping of the dinosaurs as they went about their incomprehensible activities in the world of day above. The ground would shake as if in an earthquake, and bits of the burrow walls crumbled and fell around the dozing family. It was as if the world was full of walking skyscrapers.

But there was nothing to be done about any of that. To Purga the dinosaurs were a force of nature, as beyond her control as the weather. In this huge, dangerous world, the burrow was home. The thick earth protected the primates from the heat of the day, and sheltered the still-naked pups from the night’s chill: The earth itself was Purga’s shelter against dinosaur weather.

And yet, at the back of her small mind, there was a tiny chapel of memory, a reminder that this was not her first home, not her first family — a lingering warning that she could lose all this, too, in another instant of light and flashing claws and teeth.

When the Earth turned and the air cooled and the dinosaurs settled into their nightly torpor, at their feet the dirt stirred. The creatures of the night emerged: insects, amphibians — and many, many burrowing mammals, rising like a tide of miniature life around the dinosaurs’ pillarlike legs.

This night Purga and her new mate traveled together. Purga, a little older and more experienced, led the way. A few centimeters apart, proceeding in cautious fits and starts, they made their way down the shallow slope toward the lake.

They did not usually forage together. But the weather had been dry, and the priority for both of them was to get a drink.

This part of America had endured a long, epochal drying. Here the relic of the ancient inland sea was a great stretch of swampy land, drowned by new sediment from the Rockies to the west, young mountains eroding almost as quickly as they were born. And in this time of relative drought, any standing water was a focus for animals large and small.

And so the lakeshore was crowded with dinosaurs.

Here was a herd of triceratops, three-horned giants with huge bony frills that covered their shoulder regions. They were like heavily armed rhinos, dozing in their loose circles, the adults’ ferocious horns pointing outward to deter any hungry, night-prowling aggressor.

There were many duck-billed hadrosaurs. Herds had gathered around this shallow lake, a bewildering, brightly colored array of them, and Purga and Second had to creep past forests of their great, immobile legs, like refugees in an immense sculpture park. Even now, as the duckbills slumbered, their unconscious snoring was a cacophony of deep and mournful hoots, honks, and cries; they sounded like fogbound ships.

At last Purga and Second reached the edge of the lake. The water had receded, and they had to cross a stretch of stony, half-dried pond-bottom mud, slick with mucus and sheets of green vegetation. In the eerie, still light, Purga drank quickly, her eyes wide and whiskers twitching.

Their thirst sated, the primates split up. Second began to track over the shallow beach, looking for the little piles of coiled sand that marked the presence of a worm.

Purga moved up the beach to the fringe of the scrub, following a more intriguing smell.

She soon found the source of the stink: a fish. It was lying in a heap of rusty fern fronds, its carcass shriveled within its silvery skin. Somehow stranded far from the water, it had been dead many hours. When Purga poked the fish’s skin it burst, releasing a voluminous, noxious stench — and a squirming mass of ghost-pale maggots. Purga plunged her paws into the carcass. Soon she was pushing maggots into her mouth; the salty goodies burst between her teeth, releasing delicious body juices.

But now another fish came flying over her head, to land deeper into the scrub. Startled, she flattened herself, whiskers twitching.

A dinosaur was standing stock-still in the shallow water. She was tall and upright, some nine meters tall, with a jaw like a crocodile’s and a great purple-red sail on her back. Her teeth were hooked, and her hands were equipped with claws like great blades all of thirty centimeters long. Suddenly she plunged her claws into the water, breaking the glimmering surface into shards. A handful of silvery fish flew up, wriggling and squirming, and the dinosaur skillfully snapped most of them out of the air with her long mouth.

This was a suchomimus, a specialist hunter of fish. Her kind was a relatively recent immigrant from Africa, having traveled the land bridges that sporadically connected the continents. She dragged for fish like a bear. She could take her prey with her claws or by scooping her crocodilian jaw through the water, relying on her hooked teeth. She was hunting in the night — when most creatures of her size had become dormant — because now was the time when the fish, lulled by the dimming of the daylight, came to the surface and the shore to feed.

Some meters behind her, a second suchomimus followed. This was a male; like many hunting dinosaurs, the suchomimus traveled in mating pairs.

The female suchomimus swiped again, and fish rained onto the dry shore, where they flopped briefly, suffocation quickly extinguishing their pinpoint sparks of consciousness. But the female suchomimus ignored such easy takings, apparently preferring the game of the hunt.

As did the watching deinosuchus.

The deinosuchus was a giant crocodile. She glided through the water of the lake, almost silent, hidden from view by a thin surface layer of aquatic fern. Her transparent eyelids slid over yellow eyes, keeping the tiny green leaves away.

This deinosuchus was a female: already sixty years old and twelve meters long, many of her offspring had grown to hunters themselves. A time like this — a time of drought, a time when animals came clustering to the water, in their thirst losing some of their native caution — was a bonus time for the crocodiles, a time of easy pickings. But the deinosuchus was a creature capable of taking on a tyrannosaur; she seldom went hungry, whatever the weather.

The crocodiles were already ancient, descended from bipedal hunters some hundred and fifty million years before. They were supremely successful, dominating the shallow waterways and lakes all over North America and beyond: They were among the few animals of the Cretaceous to die of old age. And they would survive to the time of humans and far beyond.

The exquisitely adapted nostrils of the deinosuchus could sense the motions of the suchomimus pair at the edge of the lake. It was time. Her mighty tail flexed once.

Purga saw a kind of eruption at the edge of the lake. Pterosaurs and birds rose from floating nests, cawing throatily in protest. The male suchomimus barely had time to turn his expressionless head before the crocodile’s jaws locked around one of his great hind legs. The crocodile hauled backward, bringing the suchomimus crashing to the mud, crushing his beautiful crest. The suchomimus hooted and fought, trying to bring its long, bloody claws into play. But the crocodile slithered back toward the water, taking the suchomimus with her.

Barely a minute after the deinosuchus had emerged, the turbulence of its passing soothed away from the surface of the water. The female suchomimus seemed baffled by her sudden loss. She patrolled the water’s edge, hooting mournfully.

The crocodile had been a messy killer. The mud of the shore was left soaked in blood, and littered with scraps of the suchomimus — lengths of glistening entrails, chunks of ripped flesh, even its staring, dismembered head. The first scavengers on the scene were a pack of small, agile raptors; they burst from the undergrowth, hopping, jumping, and swiveling, lashing out at each other like kickboxers as they fought over the juicy scraps of flesh.

They were soon joined by pterosaurs, flapping in noisily. They landed on the mud and walked clumsily, with legs and elbows splayed like a bat’s. Their heads were long, their beaks narrow and equipped with sharp teeth. The beaks dug deep into the remnants of the suchomimus. As more pterosaurs were attracted, the sky became darkened by their gaunt wings. One pterosaur in particular descended toward the two toiling primates.

Purga saw it coming. Second did not.

His only warning was a gush of leathery air, a glimpse of huge, hair-covered wings flapping across the sky above him. Then clawed feet fell out of the sky and enclosed him like a cage.

It was over before Second knew what had happened. From the comforting noises of the ground he was lifted into a silence broken only by the rustle of the pterosaur’s huge flapping wings, the silky straining of its wirelike muscles, and by the rush of the wind. He glimpsed the land, dark green and pocked by blue-glimmering ponds, falling away beneath him. And then the view opened up spectacularly to the southeast, the direction where the comet lay. The comet’s head was a vast unearthly lantern hanging over the tongue of sea that pushed into the land from the Gulf of Mexico.

Second longed only to get out of this cage of scaly flesh, back to the ground and his burrow. He thrashed at the talons that contained him, and tried to bite into the flesh; but the scales of the huge creature defied his small teeth.

And the pterosaur squeezed until small primate ribs cracked.

The pterosaur was an azhdarchid. She was the size of a hang glider. Her massive, toothless head, with a pointed triangular beak at the front and an elaborate crest at the rear, was sculpted to serve as an aerodynamic aid. Her hollow bones and porous skull made her remarkably light, and her body was tiny. She was nothing but wings and head. She looked like a sketch by Leonardo da Vinci.

The spar of each pterosaur wing was a single tremendous finger. Three remnant fingers created a small claw in the middle of the leading edge. The wing was held open by her hind legs. With all four limbs occupied in controlling the aerodynamic surfaces, the azhdarchid’s relatives could never diversify, like the birds, into running or aquatic forms. But the pterosaurs had been astonishingly successful. Along with birds and bats, they had been one of only three groups of backboned animals to have mastered flight — and they had been the first. By now pterosaurs had darkened Earth’s skies for more than a hundred and fifty million years.

The azhdarchid was capable of taking fish from shallow waters, but made most of her living as a scavenger. She rarely took live mammals. But Second — who had been engrossed in devouring a worm he was pulling from the sand — had not realized how visible the bright comet light had made him. He was not the only animal whose rhythms and instincts were disturbed by the new light in the sky. He had been an easy capture.

Second lay still, encased in pain, as cold air washed over him.

He could see the great outstretched wings above him, comet light shining blue through the translucent skin. Tiny creatures squirmed: a pterosaur’s wing was an enormous expanse of almost hairless skin packed with blood vessels, a powerful lure for parasitic insects. Every square centimeter of the pterosaur’s wing surface was controlled by an underlying mat of muscle fabric, enabling the azhdarchid to control her aerodynamics with exquisite precision; her body was a better engineered glider than any manufactured by human hands.

The azhdarchid banked to avoid a smudge of volcanic cloud that hung above the young mountains. It would be fatal for her delicate wings to be caught in such foul air. She was expert at spotting upwelling fountains of warm air — marked by cumulus clouds or over the sun-facing slopes of hills — that she could exploit for free lift. To her the world was a three-dimensional web of invisible conveyor belts, capable of carrying her anywhere she wanted to go.

The azhdarchid’s nest was in a foothill of the Rockies, above the tree line. A steep wall of young rock soared above a guano-stained ledge littered with eggshells and bones and beaks. Chicks stalked noisily around this confined area, scattering the bits of shells from which they had emerged a few weeks earlier. There were three of them; they had already devoured a weakling fourth sibling.

The parent worked a spur of bone in her wrist that changed the shape of the wings’ membrane: like air brakes, this enabled her to slow without stalling. She came to a halt a meter above the ledge, and dropped onto her hind legs. She stowed her delicate wing membranes, folded her flight fingers across her back, and walked forward, her knees bent outward and her elbows bent.

Second was dropped. He clattered against bare rock. He glimpsed the adult azhdarchid flap away. He scrabbled at the rock, but it was too hard to burrow into.

And little monsters closed in on him, blue-black in the comet light. Fed by their parents’ proteinaceous offerings of fish and meat, the chicks were growing quickly. But their wings were still undeveloped, and their bodies and heads were relatively large. They looked like miniature dinosaurs.

The first beak nipped at Second’s hind leg, almost playfully. The scent of his own blood evoked sudden memories of the burrow. He experienced a kind of regret. He bared his teeth. The ravenous chicks closed around him. It was over in a heartbeat, his warm body torn apart.

But now something moved, far above the mother azhdarchid. She twisted her sketch of a head to peer upwards. In these tall Cretaceous skies, fueled by the oxygen-rich air, a pyramid of predators had erected itself, with all the savagery of its landbound analogues. But when she saw the vast sprawled shadow, skimming over the comet-bright sky above the lowest of the clouds, she knew she was in no danger.

It was only an air whale.

The largest flying animal ever discovered by humans was a type of azhdarchid christened Quetzalcoatlus. Its wingspan of fifteen meters was four times that of the largest birds, condors; it had looked like a light airplane.

But the greatest pterosaur of all was an order of magnitude larger again.

The air whale’s tremendous, delicate wings were one hundred meters across. His bones were little more than sketches, strut-filled and hollow, astonishingly light. His mouth was vast, a translucent cavern. His main danger was overheating in the unfiltered sunlight of the high air, but his body had a number of mechanisms to compensate, including the capacity to vary the flow of blood in his tremendous wings, and air sacs placed in his body that enabled his internal organs to lose heat.

He lived his life in that thin, high layer of air called the stratosphere, higher than the mountains, above most of the clouds. But even this far from the ground there was life: a thin ethereal plankton of insects and spiders, windblown. Sometimes mating swarms of midges, or even locusts, could be blown up into this lofty realm. This was the whale’s thin bounty, which he scooped endlessly into his vast mouth.

Far below, if he had chosen to look, the air whale might have glimpsed the little drama of Second, the azhdarchid chicks, and the pterosaur. But from up here such remote events were of little interest. When he looked down over his airy domain, the whale could see the curve of the Earth: the fat blue band of thicker air that marked the horizon, and the glimmering of the sea in the light of the comet. The sky above faded to violet at the zenith. He was so high that there was too little air to scatter the light effectively; despite the brightness of the comet, he could see stars.

The air whale was capable of circumnavigating the globe, following the stratospheric winds and seeking updrafts, without once touching the ground. His kind made up a thin population — the aerial plankton could sustain no more — but they were scattered all over the planet. Three or four times in his life he had mated, summoned to the planet’s highest mountain peaks by innate timing mechanisms triggered by the motion of the sun. Mating was perfunctory and uninteresting; such huge, delicate creatures couldn’t afford the displays and courtship rituals of more terrestrial species. Nevertheless ancient instincts did sometimes come to the fore. There could be fights, often savage, almost always lethal, and when that happened, huge flimsy bodies would rain out of the skies, to baffle ground-based scavengers.

The whale was the end product of a brutal evolutionary competition, mostly aimed at removing weight; everything that had been surplus to requirements had, over the generations, been selected out or shriveled to insignificance. And, since nothing ever happened up here in the cool stratosphere, those diminished organs included the whale’s brain. The whale was at once the most spectacular but among the most stupid of his great family; his brain, though a fine control center for his elaborate flight systems, was little more than an organic adding machine. So the magnificent astronaut’s eye view before him meant nothing.

Only the warm oxygen-laden air of the late Cretaceous would allow such immense, delicate creatures to escape from gravity’s clutches, and never again would there be a gene bank like the pterosaur’s to supply raw material for similar evolutionary experiments. Never again would any creature fill this particular ecological niche, and in the future windblown insects would sail in peace.

And human paleontologists, piecing together this remote era from fragments of bone and fossilized plant, would learn little of the true giants. Most pterosaur bones found would be of marine and lakeland species, because that was where fossils were most easily preserved. By comparison the creatures that dominated the roof of the world, the upland areas and mountaintops, left few traces, for their habitats were subject to ferocious uplift and erosion. The highest mountains of the human era, the Himalayas, did not even exist in the Cretaceous.

The fossil record was patchy and selective. All through time there had been monsters and wonders that no human being would ever know had existed — like this immense flyer.

With the most delicate of touches from his immense extended forefingers the whale banked his wings and soared toward a particularly rich layer of aerial plankton.

The cruel night was not yet done with Purga.

Despite the loss of Second, she continued to forage. There was no choice. Death was common; life continued. There was no time to grieve.

But when she returned to her burrow a small, narrow face came pushing out of the dark toward her, a twitching, mobile snout, bright black eyes, quivering whiskers: one of her kind, another male.

She hissed and backed out of the burrow entrance. She could smell blood. The blood of her pups.

It had happened again. Without hesitation Purga launched herself at the male. But he was fat and strong — evidently a good forager — and he pushed her away easily.

In despair she ran out into the dangerous dawn, where mountainous dinosaurs were starting to stir, the air resonating to the first long-distance calls of the hadrosaurs. She made for an old fern she knew, around whose roots the ground was dry and crumbling. Quickly she dug herself in, ignoring the moist squirms of the worms and beetles. Once she was safe in her cocoon of soil she lay there trembling, trying to shut out of her head the dread stink of her pups’ blood.

The strange male, on discovering Purga’s scent marks — the scent of a fertile female — had followed them back to the burrow, carefully covering over her marks with his own in order to hide her from any other males.

When he had entered the burrow the pups had clustered around the stranger, his same-species smell overwhelming his warning aura of not-of-my-family. He could smell from the traces of fur and dung that a healthy, fertile female lived here. The female was of use to him, but not the pups. They did not smell of him; they were nothing to do with him. Without them, the female would have that much more incentive to raise the litter he would give her.

For the male, it was all utterly logical. The two larger pups had mouthed his belly, seeking milk, even as the male had consumed their younger sister.

The night after that the male found her again, having tracked her scent. He still stank of her dead babies, of the lost part of her. She fought him off savagely.

It took her two more nights before she accepted his courtship. Soon her body would begin to incubate his young.

It was hard.

It was life.

It would have been of no consolation to Purga to know that this brutal landscape, which had swallowed up two of her broods, would soon be overwhelmed by a wave of suffering and death to dwarf anything she had endured.

IV

Earth was now inside the comet’s swelling coma, the loose cloud of gases that swathed the nucleus itself.

All over the night side of Earth the tail could be seen stretching away from the sun. It was as if the planet had drifted into a sparkling tunnel. The sky glittered with meteors, tiny bits of comet falling harmlessly into the high atmosphere, creating a light show glimpsed by uncaring dinosaurs.

But the comet’s nucleus was bigger than any meteor. It moved at twenty kilometers per second, at interplanetary speeds. It had already crossed the orbit of the Moon.

From where it would take just five more hours to reach Earth.

All night long, the birds and pterosaurs sang their confusion; during the day, they slumped with exhaustion. There was no room in their neural programming for a new light in the sky, and they were disturbed at a deep cellular level. In the shallow seas, too, the unending light had disturbed the plankton and larger creatures like crab and shrimp; the cynical hunters of the reef fed well.

Only the great dinosaurs were unperturbed. The comet light made no difference to the air temperature, and when true night had fallen they slipped into their usual torpor. On the last night of a reign that had lasted nearly two hundred million years, the rulers of the Earth slept untroubled.

If not for the tyrannosaur eggs, the young giganotosaur would have spotted the disturbed troodon earlier. In the lee of the mountains, he stalked silently through the green shadows. His name meant Giant.

The forest here was sparse, spindly araucaria and tree ferns, scattered over a ground littered with sharp-edged volcanic rocks. Nothing moved. Anything that could hide had already hidden; anything else lay still, hoping for the shadow of death to pass by.

He came to a pile of moss and lichen. Superficially it looked like a heap of debris piled up at random by wind or the passage of animals. But Giant recognized the characteristic scrapings, the lingering smell of meat eater.

It was a nest.

With a rumble of anticipation, he fell on the nest and began to dismantle it with his stubby forearms. When he had exposed the eggs, Giant dug his clawed thumb, with a surgical precision, into the top of the largest. He pulled out the embryo head first. As the mucus and yolk drained from it, lurid colors bright, Giant saw the chick squirm feebly, even saw its tiny heart beat.

Just as the embryos of chimps, gorillas, humans would all be disturbingly similar, so dinosaur fetuses all looked alike. There was no way to know that this chick would have been a female tyrannosaur. Blind, deaf, immature, the embryo struggled to open her mouth, dimly imagining the hulking shape of a mother who would feed her. Giant flicked the embryo into his mouth and swallowed it without chewing. The chick’s life ended in crushing, acidic darkness.

It didn’t matter. Even if no predator had come this way, her egg would have been destroyed before she could have hatched by a monster even more terrible than a giganotosaur.

Giant was descended from South American stock that had crossed a temporary land bridge into this continent a thousand years earlier.

In a world of slowly separating island continents, the dinosaur fauna had become diverse. In Africa there were archaic-looking long-necked giant herbivores and creatures like hippos with fat, low-slung bodies and powerfully clawed thumbs. In Asia there were small, fast-running horned dinosaurs with noses like parrots’ beaks. And in South America large sauropods were hunted by giant pack-hunting predators; there it was like a throwback to earlier times, to Pangaea. The giganotosaurs had cut their evolutionary teeth hunting the great South American titanosaurs.

Giant was an immature male, and yet he already outmassed all but the very largest carnivores of the era. Giant’s head, in proportion to his body, was larger than a tyrannosaur’s — and yet his brain was smaller. The giganotosaurs were less agile, less fast, less bright; they had more in common with the ancient allosaurs, equipped to kill with teeth and hands, whereas the tyrannosaurs, all their evolutionary energy funneled into their huge heads, specialized in immense, sharklike bites. Where the tyrannosaurs were solitary ambush hunters, the giganotosaurs were pack animals. To bring down a sauropod fifty meters long and weighing a hundred tons, you didn’t need brains as much as raw strength, rudimentary teamwork — and a kind of reckless frenzy.

But, coming across that land bridge into a new country, the giganotosaurs had been forced to confront an established order of predators. The invaders had quickly learned that their takeover of a region could not succeed unless they first mounted a bloody coup against the ruling carnivore.

Which was why this young male giganotosaur was munching slippery tyrannosaur embryos. Resolutely Giant cracked one egg after another. The carefully constructed nest turned into a mess of shattered eggs, scattered moss, and chunks of dismembered chick. Giant was feeding well — and issuing a challenge.

It would be a transfer of power. The tyrannosaur had been the top predator, mistress of the land for a hundred kilometers around, as if the whole elaborate ecosystem was a vast farm run for her benefit. The prey species had come to terms with the formidable presence that lived amongst them: with their armor or weapons or evasive strategies, each of the hunted had reached a point where their losses to predators were not a threat to the endurance of the herd.

Given time, all that would have changed. The impact of the invaders’ hunger would have rippled down the food chain, disturbing creatures large and small, before a new equilibrium could be established. It would have taken longer still for the prey species to learn new behaviors, or even evolve new coping systems or armor to deal with the giganotosaurs.

But none of that would happen. There would not be time for the giganotosaur clan to exploit their triumph. Not in the few hours left.

The nest destroyed, Giant wandered away. He was still hungry, as always.

He could smell putrefaction in the still, misty air. Something huge had died: easy meat, perhaps. He pushed through a bank of tree ferns and emerged into another small clearing. Beyond, through a screen of greenery, he could dimly see the black flank of a young volcanic mountain.

And there, in the middle of the clearing, was a dinosaur — a troodon — standing quite still above a scraping of earth.

Giant froze. The troodon had not seen him. And she was alone; there were none of the watchful companions he associated with the packs of this particular agile little dinosaur.

There was something wrong with the way she was behaving. And that, so the grim predatory calculus of his mind prompted him, gave him an opportunity.

Wounding Tooth should have been able to overcome the loss of a clutch of eggs.

This was a savage time, after all. Infant mortality rates were high; and at any time of life, sudden death was the way of things. This was the world the troodon had been evolved to cope with.

But she could not cope, not anymore.

She had always been the weakest of her brood. She would not even have survived the first days after her hatching if not for the chance decimation of her siblings by a roaming marsupial predator. She had grown to overcome her physical weakness, and had become an effective hunter. But in a dark part of her mind she had always remained the weakest, robbed of food by her siblings, even eyed as a cannibalistic snack.

Add to that a slow poisoning by the fumes and dust of the volcanoes in the west. Add to that an awareness of her own aging. Add to that the hammer blow of her lost brood. She hadn’t been able to get Purga’s scent out of her head.

It had not been hard to pursue that scent out of her home range, across the floodplain to the ocean shore, and now to this new place where the scent of Purga was strong.

Wounding Tooth stood still and silent. The burrow, her nose told her, was right under her feet. She bent and pressed one side of her head against the ground. But she heard nothing. The primates were very still.

So she waited, through the long hours, as the sun rose higher on this last day, as the comet light grew subtly brighter. She did not even flinch when meteors flared overhead.

If she had known about the giganotosaur that watched her she would not have cared. Even if she could have understood the meaning of the comet light, she would not have cared. Let her have Purga; that was all.

It was a peculiar irony that her high intelligence had brought Wounding Tooth to this. She was one of the few dinosaur types smart enough to have gone insane.

It was not yet dark. Purga could tell that from the glint of light at the rough portal to the burrow. But what was day, what was night in these strange times?

After several nights bathed in comet light, she was exhausted, fractious, hungry — and so was her mate, Third, and her two surviving pups. The pups were just about large enough to hunt for themselves now, and therefore dangerous. If there was not enough food, the family, pent up in this burrow, might turn on one another.

The imperatives slid through her mind, and a new decision was reached. She would have to go out, even if the time felt wrong, even if the land was flooded with light. Hesitantly she moved toward the burrow entrance.

Once outside, she stopped to listen. She could hear no earth-shaking footsteps. She stepped forward, muzzle twitching, whiskers exploring.

The light was strong, strange. In the sky cometary matter continued to fall, streaking across the dome of the sky like silent fireworks. It was extraordinary, somehow compelling — too remote to be frightening.

An immense cage plunged out of the sky. She scrabbled back toward the burrow. But those great hands were faster, thick ropy muscles pulling the fingers closed around her.

And now she faced a picket fence of teeth, hundreds of them, a tremendous face, reptilian eyes as big as her head. A giant mouth opened, and Purga smelled meat.

The dinosaur’s face, with its great, thin-skinned snout, had none of the muscular mobility of Purga’s. Wounding Tooth’s head was rigid, expressionless, like a robot’s. But though she could not show it, all of Wounding Tooth’s being was focused on the tiny warm mammal in her grip.

Her limbs pinned against her belly, Purga stopped struggling.

Oddly, Purga, in this ultimate moment, knew a certain peace that Wounding Tooth would have envied. Purga was already in her middle age, already slowing in her movements and thought. And she had, after all, achieved as much as a creature like her could have hoped for. She had produced young. Even encased in the troodon’s cold reptilian grip, she could smell her young on her own fur. In her way she was content. She would die — here and now, in heartbeats — but the species would go on.

But something moved beyond the troodon’s bulky body, something even more massive, a gliding mountain, utterly silent.

The troodon was unbelievably careless. Giant didn’t care why. And he didn’t care about the warm scrap she held in her hands.

His attack was fast, silent, and utterly savage, a single bite to her neck. Wounding Tooth had time for a moment of shock, of unbelievable pain — and then, as whiteness enfolded her, a peculiar relief.

Her hands opened. A ball of fur tumbled through the air.

Before Wounding Tooth’s body fell Giant had renewed his attack. Briskly he slit open the belly cavity and began pulling out entrails. He expelled their contents by shaking them from side to side; bloody, half-digested food showered the area.

Soon his two brothers came racing across the clearing. Giganotosaurs hunted together, but their society was fragile at the best of times. Giant knew he couldn’t defend his kill, but he was determined not to lose it all. Even as he chewed on the liver of Wounding Tooth, he turned to kick and bite.

Purga found herself on the ground. Above her, mountains battled with ferocious savagery. A rain of blood and saliva fell all around her. She had no idea what had happened. She had been ready for death. Now here she was in the dirt, free again.

And the light in the sky grew stranger yet.

The comet nucleus could have passed through the volume of space occupied by Earth in just ten minutes.

In the great boiling it had endured the comet had lost a great deal of mass, but not a catastrophic amount. If it had been able to complete its skim around the sun, it would have soared back out to the cometary cloud, quickly cooling, the lovely coma and tail dispersing into the dark, to resume its aeonic dreaming.

If.

For days, weeks, the great comet had worked its way across the sky — but slowly, its hour-by-hour motion imperceptible to any creature who glanced up at it, uncomprehending. But now the bright-glowing head was sliding: sliding down the sky like a setting sun, sinking toward the southern horizon.

All across the daylit side of the planet, silence fell. Around the drying lakes the crowding duckbills looked up. Raptors ceased their stalking and pursuit, just for a moment, their clever brains struggling to interpret this unprecedented spectacle. Birds and pterosaurs flew from their nests and roosting places, already startled by a threat they could not understand, seeking the comfort of the air.

Even the warring giganotosaurs paused in their brutal feeding.

Purga bolted for the darkness of her burrow. The disembodied head of the troodon fell behind her, lodging in the burrow’s entrance, following Purga with a grotesque, empty stare as the light continued to shift.

I

Once interplanetary impacts had been constructive, a force for good.

Earth had formed close to the brightening sun. Water and other volatiles had quickly boiled away, leaving the young world an empty theater of rock. But the comets, falling in from the outer system, delivered substances that had coalesced in that cooler region: especially the water that would fill Earth’s oceans, and compounds of carbon, whose chain-based chemistry would lie at the heart of all life. Earth settled down to a long chemical age in which complex organic molecules were manufactured in the mindless churning of the new oceans. It was a long prelude to life. It would not have come about without the comets.

But now the time of the impacts was done, so it seemed. In the new solar system, the remaining planets and moons followed nearly circular orbits, like a vast piece of clockwork. Any objects following more disorderly paths had mostly been removed.

Mostly.

The thing that now came out of the dark, its surface of dirty slush sputtering in the sun’s heat, was like a memory of Earth’s traumatic formation.

Or a bad dream.

In human times, the Yucatan Peninsula was a tongue of land that pushed north out of Mexico into the gulf. On the peninsula’s northern coast there was a small fishing harbor called Puerto Chicxulub (Chic-shoe-lube). It was an unprepossessing place, a limestone plain littered with sinkholes and freshwater springs, agave plantations, and brush.

Sixty-five million years before that, in the moist age of the dinosaurs, this place was ocean floor. The plains of the Gulf of Mexico were flooded up to the foothills of the Sierra Madre Oriental. The shallow Yucatan Peninsula itself lay under nearly a hundred meters of water. The sediments that would later form Cuba and Haiti were part of the deep seafloor, yet to be lifted by fault movements to the surface.

In an age dominated by warm shallow seas, drowned Chicxulub was an unremarkable place. But it was here that a world would end.

Chicxulub is a Mayan word, an ancient word coined by a lost people. Later, when the Mayans were gone, nobody would know for sure how it translated. Local legend said it meant the Devil’s Tail.

In its last moments the comet flew in from the southeast, passing over the Atlantic and South America.

II

In bright, shallow waters the huge ammonite cruised.

This sea-bottom hunter, the size of a tractor tire, looked something like a giant snail, with an elaborately curved spiral shell from which arms and a head protruded cautiously. As it had grown, it had extended its shell’s spiral structure, gradually moving from one chamber outwards to the next; now the linked, abandoned chambers were used for buoyancy and control.

The ammonite moved with surprising grace, its upright spiral cutting through the waters. And it scanned its surroundings with wide intelligent eyes.

The sunlit sea was crowded, translucent, full of rich plankton. Some of the creatures here — oysters, clams, many species of fish — would have been familiar to humans. But others would not: there were many ancient species of squid, the ammonite itself — and, dimly visible as shadows passing through the blue reaches of the deeper ocean — giant marine reptiles, mosasaurs and plesiosaurs, the dolphins and whales of the age.

As the daylight gathered, more of the ammonite’s kind were rising, to hang like bells in the translucent water.

But the ammonite spotted movement on the seabed. It descended quickly, sensory tentacles pushing out of its shell. By sight and feel it quickly determined that the scuttling, burrowing thing in the gritty sand was a crab. More arms slid out of the shell and wrapped around the crustacean, tiny hooks on each arm helping to secure their grip. The crab was pulled easily away from the soft seabed. A heavy birdlike beak protruded, and the ammonite bit through the crab’s shell, between its eyes. It injected digestive juices into the shell, and began to suck out the resulting soup.

As particles of meat diffused in the water, more ammonites came sliding in.

But the ammonite with the crab saw a shadow moving above, a shadow with a snout and fins, silently sliding, rapidly resolving. It was an elasmosaur; a marine reptile, a kind of plesiosaur with an immensely long neck. Abandoning its kill, the ammonite ducked into its shell. The opening in the shell was immediately sealed off with a heavy cap of hardened tissue.

The elasmosaur fell on the ammonite, pushed its shell over, and clamped its powerful jaws around the narrowest part of the spiral. But it could not break through. After breaking a cluster of teeth, the elasmosaur dropped the shell, letting it drift back to the ocean floor. Frustration and pain seethed in its one-dimensional awareness.

The ammonite had endured violent shaking, but it was safe in its armored home.

But one immature ammonite had not been so wary. It tried to flee, its jets pushing it this way and that.

The elasmosaur took its consolation kill well. Its teeth sliced expertly across the spiral shell at the place where the body was attached to the inner surface. Then it shook the shell hard until the ammonite, still alive, tumbled out into the water, naked for the first time in its life. The fish-lizard took its prize in a single gulp.

Now the elasmosaur spotted a cloud in the water. It plunged in without hesitating.

The cloud was a shoal of belemnites, thousands strong. The little squid had gathered for protection, and their defensive systems, of sentries and ink and shimmying, deceptive movements, were usually effective even against predators as fast as this elasmosaur. But they had been caught out by this creature’s angry lunge. They darted away, venting ink furiously at the immense invader, or even leaping out of the ocean altogether and into the comet-bright air. Still, hundreds of them died: each a pinpoint of awareness, each of them in its way unrepeatable and unique.

Meanwhile, cautiously, the crab-killer ammonite had opened its shell once more. A tube of muscle protruded from the opening, and a high-pressure stream of water pulsed out, jetting the ammonite up and into the blue waters. It had lost the crab. But no matter. There was always another kill to make.

So it went. It was a time of savage predation, in the sea as on land. Mollusks hunted ammonites, boring through shells, poisoning prey animals, and firing deadly darts. In response, bivalves had learned to bury themselves deep in sediment, or had evolved spines and massive shells to deter attackers. Limpets and barnacles had forsaken the deep sea, colonizing shallow environments on the shore where only the most determined of hunters could reach them.

Meanwhile, the seas teemed with predatory reptiles. Carnivorous turtles and long-necked plesiosaurs fed on fishes and ammonites — as did pterosaurs, flying reptiles who had learned to dive for the riches of the ocean. And huge, heavy-jawed pliosaurs preyed on the predators. Measuring some twenty-five meters long, with jaws alone some three meters long, their sole stratagem to rip and shake their prey apart, the pliosaurs were the largest carnivores in the history of the planet.

The rich Cretaceous oceans teemed, enacting a three-dimensional ballet of hunter and hunted, of life and death. It had been so for tens of millions of years. But now a bright light was building above the glimmering surface of the ocean, as if the sun were falling from the sky.

The ammonite’s eye was drawn upwards. The ammonite was smart enough to feel something like curiosity. This was new. What could it be? Caution prevailed: Novelty usually equated to danger. Once more the ammonite began to withdraw into its shell.

But this time even its mobile fortress could not protect it.

The comet punched through Earth’s atmosphere in fractions of a second. It blasted away the air around it, blowing it into space, leaving a tunnel of vacuum where it had passed.

The ammonite was trapped right under the comet’s fall. It was as if a great glowing lid closed across the sky. Its substance immediately vaporized, the ammonite died. So did the belemnites. So did the elasmosaur. So did the oysters and clams. So did the plankton.

The ammonites had stalked the oceans of the Earth, spawning thousands of species, for more than three hundred million years. Within a year, none of them would be left alive, none. Already, in these first fractions of a second, long biographies were being abruptly terminated.

The few dozen meters of water offered the comet nucleus no more resistance than the air. All the water flashed to steam in a hundredth of a second.

Then the comet nucleus hit the seabed. It massed a thousand billion tons, a flying mountain of ice and dust. It took two seconds to collapse into the seabed rocks, delivering in those seconds the heat energy released by all of the Earth’s volcanoes and earthquakes in a thousand years.

The nucleus was utterly destroyed. The seabed itself was vaporized: rock flashed to mist. A great wave pulsed outward through the bedrock. And a narrow cone of incandescent rock mist fired back along the comet’s incoming trajectory, back through the tunnel in the air dug out in the comet’s last moments. It looked like a vast searchlight beam. Around this central glowing shaft, a much broader spray of pulverized and shattered rock, amounting to hundreds of times the comet’s own mass, was blown out of the widening crater.

In the first few seconds thousands of billions of tons of solid, molten, and vaporized rock were hurled into the sky.

On the coastal plain of the North American inland sea, the duckbill herds gathered around the pools of standing water. They hooted mournfully as they clustered and nudged each other. Predators, from chicken-sized raptors upwards, watched stray duckbill young with cold calculation. In one place a crowd of ankylosaurs had gathered, their dusty armor glistening, like a Roman legion in formation.

An orange glow could be seen deep in the south, like a second dawn. Then a thin, brilliant bar of light arrowed into the sky, straight as a geometrical demonstration — straighter, in fact, than a laser beam, for the beam of incandescent rock suffered no refraction as it pushed out through the hole in the Earth’s superheated air. All of this unfolded in silence, unnoticed.

The crocodile-faced suchomimus stalked the edge of the ocean, her long claws extended. Just as she did every day, she was looking for fish. The death of her mate days before was a dull ache, slowly fading. But life went on; her diffuse grief gave her no respite from hunger.

Elsewhere a group of stegoceras was foraging, scattered. These pachycephalosaurs were about as tall as humans. The males had huge caps of bone on their skulls, there to protect their small brains during their earth-shuddering mating competitions, when they would crash their heads together like mountain sheep. Even now two great males were battling, ramming their reinforced heads together, the bony clatter of their collisions echoing across the plains. This species had sacrificed much evolutionary potential to these contests. The need to maintain such a vast protective cap of bone had limited the development of the pachycephalosaur brain for millions of years. Locked in biochemical logic, these males cared nothing for shifting lights in the sky, or the double shadows that slid across the ground.

On this beach it was just another day in the Cretaceous. Business as usual.

But something was coming from the south.

By now the crater was a glowing bowl of shining, boiling impact melt, wide enough to have engulfed the Los Angeles area from Santa Barbara to Long Beach. And its depth was four times the height of Everest, its lip farther above its floor than the tracks of supersonic planes above Earth’s surface. It was a crater ninety kilometers across and thirty deep formed in minutes. But this tremendous structure was transient. Already great arching faults had opened up, and immense landslides, tens of kilometers wide, began to collapse the steep walls.

And the seabed was flexing. The Earth’s deeper rocks had been pushed down into the mantle by the comet’s hammer blow. Now they rebounded, rising up through twenty kilometers, breaking through the melt pool to the surface. The basement rock itself, almost liquefied, quickly spread out into a vast circular structure, a mountain range forty kilometers across, erected in seconds. Meanwhile water strove to fill the pit that had been dug into the ocean floor. And already ejecta debris was falling back onto the crater’s shifting floor, a rain of burning rock. Temperatures reached thousands of degrees — enough to make the air itself burn, nitrogen combining with oxygen to form poisons that would linger for years to come. It was a chaotic battle of fire, steam, and falling rock.

From the impact site, superheated air fled at interplanetary speeds. A great circular wind gushed out from the Yucatan, down into South America, and across the Gulf of Mexico. The shock wave was still moving at supersonic speeds ten minutes later, when it reached the coast of Texas.

To the south of the beach, the thin pillar of light had fanned outward. It became more diffuse, and changed color, becoming a deeper orange white. Tiny flecks of orange could be seen flying up around its base. And now a band of darkness spread over the southern horizon. Still, all this unfolded in silence. What was coming was still moving much more rapidly than sound. The dinosaur herds were oblivious; still the young Pachycephalosaurs battled, locked into their Darwinian dance.

But the birds and the pterosaurs knew the sky. A group of pterosaurs had been working the ocean, skimming low over the surface seeking to scoop up fish in their hydrodynamically elegant beaks. Now they turned and headed inland, flapping to gain speed. A flock of small, gull-like birds followed, rising up on gray-white wings that seemed to pulse in the glowing rock light.

Of the thousands of dinosaurs, only the suchomimus reacted to the light show. She turned to the south, and her slit pupils narrowed at what she saw. Some instinct made her splash away from the water to run higher onto the shore. The warm sand was soft under her feet, slowing her down. But still the suchomimus ran.

Two young raptors, working playfully at the shell of a stranded sea turtle, lifted their heads with speculative interest as she passed. A corner of the suchomimus’s clever mind rippled with alarm signals. She was breaking many of her innate rules; she was making herself vulnerable. But a deeper instinct told her that the stain of darkness spreading over the horizon was more of a threat than any raptor.

She reached a bank of low dunes. A ball of fur squirmed indignantly out from beneath her feet and fled with blurring speed.

Over the coastal plain, the light began to fade.

At last the dinosaurs were disturbed. The great herbivore herds, the duckbills and ankylosaurs, lifted their heads from their browsing and turned to face the south.

The fan of ascending rock was invisible now, hidden by a wall of darkness that spanned the horizon. But it was a moving wall whose front bubbled and writhed. Lightning flickered over the moving surface, making it shine purple white.

Even now, in these last seconds, there was little sense of strangeness. It was like an eerie twilight. Some of the dinosaurs even felt drowsy, as their nervous systems reacted to the reduced level of light.

Then, from out of the south, the shock front exploded. From silence to bedlam in a heartbeat. The front smashed the animal herds. Duckbills were hurled into the air, huge adults writhing, their lowing lost in the sudden fury. The competition among the hard-skulled stegoceras was concluded without resolution, never to be resumed. Some of the great ankylosaurs stood their ground, turning into the wind, hunkering down like armored bunkers. But the very ground was torn up around them, the vegetation ripped out and scattered; even the lakes explosively emptied of their water. The shallow dune exploded over the suchomimus, instantly burying her in gritty darkness.

But as quickly as it had come, the shock wave passed.

When she felt the ground’s shuddering cease, the suchomimus began to scrabble at the earth. She sneezed the grit out of her nostrils, her great translucent eyelids working to clear her eyes, and clambered to her feet.

She stepped forward gingerly. The new ground was rubble strewn, uncertain, difficult to walk on.

The coastal plain was unrecognizable. The dune that had sheltered her was demolished, the wind’s patient, centuries-long work erased in seconds. The plain was littered with debris: bits of pulverized rock, seabottom mud, even a few strands of seaweed and smaller sea creatures. Above her, clouds boiled, streaming north.

Still the noise continued, great crackling shocks that rained out of the sky as sound waves folded over on themselves. But the suchomimus heard none of this. She had been deafened in the first instant of the shock’s passage, her delicate eardrums crushed.

Dinosaurs lay everywhere.

Even the largest duckbills had been smashed to the ground. They lay, broken and twisted, under scattered sand and mud. A group of raptors lay together, their lithe bodies tangled up. Everywhere the old lay with the young, parents alongside their children, predators with their prey, united in death. Most disasters, like floods and fires, selectively affected the weakest, the young and the old and the ill. Or else they targeted species — an epidemic, perhaps, carried by an unwitting host across a land bridge between the continents. But this time, none had been spared, none save the very fortunate, like the suchomimus.

The suchomimus saw a silver fish. It twitched, carried a dozen kilometers in seconds, still alive. The suchomimus’s gut rumbled gently. Even now, as the world ended, she was hungry.

But the wind’s work was not yet done. Already, over the ocean, the air was rushing back to fill the vacuum created at the impact site. It was like an immense inhalation.

The suchomimus, toying with her fish, saw the wall of darkness bear down once more. But this time it came from inland, and it was laden with debris, with dirt and rocks and uprooted trees and even a huge male tyrannosaur that writhed lifeless, high in the air.

Once more the suchomimus dived at the sand.

From the furies of the crater the shock front continued to spread out, like a ripple around a fallen stone. Further inland, where Giant had raided the tyrannosaur nest, the front had wrought devastation around a great circle big enough to have been wrapped around the Moon.

Tornadoes spun off the advancing front like willful, destructive children.

To Giant, the twister was a tube of darkness that connected sky to ground. At its feet, what looked like splinters rose up, whirled and fell back. The giganotosaurs’ ancestors had invaded a continent. Now Giant reared up and hissed, bobbing his head, eyes triangulating on the approaching menace.

But this was no saurian competitor. As the twister approached it grew ever larger, towering high above him.

At last something in Giant’s mind focused on those twigs scattered at the feet of this climatic monster. Those “twigs” were trees, redwoods and ginkgoes and tree ferns, scattered as easily as pine needles.

His brothers made the same calculation. The three of them turned and ran.

The base of the twister tore casually through the blanket forest, destroying trees, scattering rock. Animals weighing five tons or more were hurled into the air, great slow-moving herbivores suddenly flying. Many of them died of shock even before they hit the ground.

In her burrow, Purga was shaken awake by the rattling of the earth. She and her mate huddled closely around the two pups, and they listened to the howling of the wind, the clatter and crunch of trees being shattered, the scream of dying dinosaurs.

Purga closed her eyes, baffled, terrified, longing for the noise to stop.

And in the foothills of the Rockies, the mother azhdarchid sensed the approach of the mighty wind. Hastily she folded up her wings and waddled on wrists and knees toward her nest.

Her young clustered around, but she had no food to give them, and they pecked at her angrily. The chicks were still flightless, their wing membranes yet to develop. For now they had only loose, useless flaps of skin trailing between their flight fingers and hind legs. And yet they were already beautiful, in their way; the scales that clustered around their thin necks, a relic of their reptilian ancestry, caught the high sunlight, gleaming and glistening.

But now clouds raced across the sun. The twisters would not reach so high. But the shock front was still a broiling wall of turbulent air, still powerful even so far from the impact site.

A first gust buffeted the nest. The chicks screeched and stumbled.

Without thinking the mother flapped her wings, taking to the air. A primitive imperative had taken over. There would always be more broods, if she survived. The chicks, receding beneath her, squawked their anger and fear.

As the wall of wind approached, there was a moment of stillness.

The azhdarchid’s airspeed dropped. She turned and spread her wings, instinctive responses coming to play. She held out her long flight finger and her hind limb, and subtle twitches of thigh and knee adjusted the tension in her wings. She was an exquisite flying device, an apparatus of tendons, ligaments, muscle, skin and fur, shaped by tens of millions of years of evolution.

But the comet wind didn’t care about that, not at all.

The wind hit the nest first. The rock ledge was swept bare, the nest smashed to fragments. The bones of the pterosaurs’ victims — including those of Second — were sent whirling into the air with the rest of the debris. The chicks flew: if only briefly, if only once, if only to their deaths.

And then, for the mother azhdarchid, it was as if she had flown into a wall of dust and spray, and even bits of vegetation and wood and rock. She felt her fragile bones snap. She was tumbled over and over, helpless as a dead leaf.

Once more the suchomimus struggled to her feet. She ached in her legs, arms, back, tail, and head, where she had been struck by bits of flying debris, the wreckage of a world.

Again the beach had become an utterly unfamiliar place. The ground was now littered by debris from inland, bits of smashed trees and crushed animals, dead or dying pterosaurs and birds, even lake-bottom ooze. Nothing moved — nothing but dying creatures, and the suchomimus.

She remembered the fish she had been about to eat. The fish was gone.

Above her, dark banks of cloud whipped across the sky, like a curtain being drawn. The sun disappeared; it would not be seen again for a long time.

And to the south, the lid of sky began to glow an eerie orange. A breeze wafted a sharp, distinctive smell to her nose. Ozone. The smell of the sea. She thought of lapping water, the glittering fish of the shallows. She must get to the sea. She had always made her living from the sea; there she would be safe. With a mournful lowing even she couldn’t hear, she began to blunder in the direction of the scent, ignoring the grisly detritus under her feet.

The sea turtle had been fortunate. When the comet hit, she was cruising the sea bottom far from the impact zone.

Her kind was among the most primitive of the great reptile dynasties. But, primitive or not, this turtle was an effective hunter. Her body was undemanding, requiring only a twentieth as much food as a dinosaur of the same weight. Heavily protected by her powerfully reinforced shell, cautious even as a hunter, the only risks she ran in her life were the annual assaults she had to make on the beaches to lay her eggs, before hurrying back to the safety of the water.

Her brain was small, her consciousness dim. She lived alone, in a world of colorless monotony. She had no bonds with her parents or siblings, no real understanding that the eggs she laid would produce a new generation. But she was ancient, wary, enduring.

Now, though, something disturbed her blue, lonely world. A monstrous current began to drag the sea toward the south.

Grimly the turtle paddled at the water, heading downward. Her instincts, honed by millions of years of tropical storms, primed her with a simple instruction: dive deep, get to the bottom, find shelter.

But this was like no current she had ever experienced. Through the increasingly muddy and turbulent water she glimpsed much larger creatures, even giant pliosaurs, being dragged backward by this mighty tide. And as she descended she was battered by debris, helpless ammonites, clams, squid, even rocks torn from the floor.

At last she found soft mud. All her four fins working, she began to work her way into the dirt, ignoring the hail of objects that clattered off her shell. Eventually she would have to surface, for air and warmth; but she could last for a long time, perhaps until this monstrous storm had passed away.

But now the sea’s glimmering meniscus descended toward her — and the sea drained away — and she found herself in sunlight, with moist mud hissing all around her. Something like shock lit up her small mind. The world had turned upside down; this made no sense.

And now the sea bottom mud, exposed, began to shake.

By the shifting, strange light, at last the suchomimus saw the sea. With a hoarse cry of relief, she hurried forward.

But the sea ran away from her, exposing glistening mud. And as fast as she pursued it, the sea ran faster.

A fish flopped at her feet. She stopped and plucked it out of the dirty mud and popped it into her mouth. In the fish’s tiny awareness was a kind of relief; this death was quick compared to the grisly suffocation it had endured on the new beach.

The sea bottom, uncovered for the first time in millions of years, was a glistening floor of life. It was littered with clams, crustaceans, squid, fish, ammonites of all sizes, all of them drowning in the air.

Further south there were giant shapes. The suchomimus saw a giant plesiosaur, stranded like the rest. Eight meters long, it lay gasping on the mud with its four huge flippers splayed and broken around it. It struggled, tons of marine carnivore flipping this way and that, huge fins waving, savage teeth snapping in rage at the fate that had stranded it.

On any other day it would have been a remarkable sight. The suchomimus turned away, bewildered.

When she looked north to the land, she could see creatures creeping out of the devastated forests, the wind-scoured marshland. Many of them were ankylosaurs and other armored creatures, protected thus far by the heavy armor that had evolved to fend off the teeth and claws of tyrannosaurs. They crawled toward the exposed seabed, seeking sanctuary, to drink, to feed.

But now the ankylosaurs opened their mouths and began to retreat once more. The suchomimus watched them, baffled. They were bellowing, but she couldn’t hear them.

She turned back to face the sea. And then she saw what had frightened them.

As air, so water.

From the impact site, powered by the immense pulse of heat, a circular shock wave now marched outward through the body of the ocean. Its destructive power was limited because the impact had not occurred in deep ocean water. Still, as it neared the coastline of North America, the wave was already some thirty meters high. And as it reached the shallower water of the Texas coast, the tsunami gathered itself, rearing ten to twenty times its initial height.

Nothing in the suchomimus’s evolutionary heritage had prepared her for this. The returning sea was like a moving mountain range, hurtling out of the retreated ocean. She could not hear it, but she could feel how it made the exposed seabed shudder, smell the sharp stink of salt and pulverized rock. She stood upright and bobbed her head, baring her teeth defiantly at the approaching tsunami.

The water towered above her. There was an instant of pressure, of blackness, a huge force that compressed her. She died within a second.

The tsunami rolled landward, dwarfing the lumbering ankylosaurs before crushing them, armor and all. On it went, ramming its way into the ancient, long-dried sea way. When it receded, the water left behind debris, great banks of it dredged from the sea bottom. It had been an immense slosh, from the stone thrown into this Cretaceous pond.

On the land, in Texas, nothing survived.

In the sea, only a handful of creatures lived through the oceanic catastrophe.

One of them was the sea turtle. She had burrowed deep enough into the mud for the tsunami waters to spare her. When she could sense that something like calm was restored, she struggled out of the mud, and ascended up through water cloudy with debris and bits of dead animals and plants.

The turtles, ancient, had already passed the zenith of their diversity. But where more spectacular creatures had perished en masse, the turtle had survived. In a dangerous world, humility made for longevity.

The impact had sent an energy pulse through the body of the Earth. In North and South America, across thousands of kilometers, faults gaped and landslides crashed, as the shocked ground shuddered. The rocky waves weakened as they propagated, but the Earth’s internal layers acted like a giant lens to refocus the seismic energy at the impact’s antipode, the southwestern Pacific. Even there, the width of the planet away, the ocean floor heaved in swells ten times higher than the 1906 San Francisco earthquake.

The shock waves would continue to pass through the planet’s body, crossing, interfering, reinforcing. For days, the Earth would ring like a bell.

Seen from space, a glowing wound was spreading out over the Earth around the still-burning impact point. It was a great cloud of molten rock, hurled into space.

In the vacuum the scattered droplets were beginning to cool and condense into hard specks of dust. Some of this material would be lost to the planet forever, joining the thin drizzle of material that swam between the planets: In a few millennia fragments of Yucatan seafloor would fall as meteors on Mars and Venus and the Moon. And some of the space-borne material would, through chance configurations, enter orbit around the planet, making a temporary ring around the Earth — dark, unspectacular — that would soon disperse under the shifting gravitational tweaks of the sun and Moon.

But most of the ejecta would fall back to Earth.

Already the great hailing had begun. The first to fall was the coarser debris from the perimeter of the crater, much of it fragments of smashed-up ocean-bottom limestone. These chunks had not been melted by the heat pulse of the initial impact. But as they fell back into the Earth’s warm pond of air, they began to glow brightly. Streaks of light hundreds of kilometers long were drawn across the sky, like an insane geometrical exercise. Some of the debris chunks were large enough to crack open as they heated, and secondary tracks fanned out from sparking explosions.

Of all the creatures within a few thousand kilometers of the impact, the great aerial whale had been least affected so far.

He had watched the great light descend over the Yucatan Peninsula — had seen that stabbing laser beam of vaporized seabed and comet, had even glimpsed the formation of the crater, as great ripples of rock pulsed through the exposed seabed before congealing into place in a great chthonic clench. Had he been able to describe what he saw, the whale could have provided posterity with a compelling eyewitness account of the catastrophe, the most violent impact since the end of the formative bombardment four billion years earlier.

But the whale cared nothing for that. The whale had not even been troubled by the wind; he flew too high, and had been able to continue feeding as the great sheets of discolored air fled across the ground far beneath him. Distant lights in the sky, trouble on the ground — like the creamy-swirl weather systems that often crossed the land and oceans — meant nothing to a creature who flew at the fringe of space. So long as the wispy aerial plankton that fed him continued to drift up from the lands below, he prowled his thin niche untroubled.

But this storm was different.

The air whale was used to meteors. They were just streaks of light in the purple-blue sky above. Almost all of the billions of bits of cosmic debris that fell to Earth burned up far above the stratosphere, the whale’s realm.

But some of these tracks were reaching down into Earth’s thicker air, passing far below him. The whale had no hearing — he had no need of it in this thin, silent air, where no predators worked — but if he had he might have made out the thin howl of the meteors as they plunged back to the planet from which they had so recently been flung. He could even see where the first sea-bottom chunks fell: On the ground, far below, sparks of light bloomed like tiny flowers, one after the other. It was like the view from a high-altitude bomber.

For the first time since he was a chick the whale began to know fear. Suddenly this was no aerial light show but a rain of light and fire. It was a rain that was falling all around him — and it was getting thicker. Belatedly he turned. With a slow flap of his immense wings, he headed north.

Light pulsed.

The white-hot rock fragment was just a scrap. After the encounter with the whale it continued its descent toward the thick Cretaceous forests, only a fraction of its kinetic energy expended. But the whale’s complex nervous system brought his small brain messages of agonizing pain. When he turned his great head to the right, he saw that the surface of his wing was torn and scorched.

If the meteor had hit near the center of the wing, it might have made no more than a puncture, and the whale might have lived a little longer. But the whale had been unlucky. The meteor had punched through the joint of an immense, fragile flight finger. The wing began to fold up in great sections around the broken segment of bone.

The blue-gray Earth tipped over. Though he thrashed inelegantly with his good wing, the whale was already falling away from the horizontal — falling out of control, out of the sky. Still he remained conscious, slowly twisting, crumpling like a broken toy kite. But the meteor hail thickened. Bulletlike meteors tore tunnels through the fine caverns of his body, ripping open air sacs, smashing his delicate, light-as-air filigree skeleton, further puncturing his magnificent wings.

The pain became overwhelming. His mind filled with comforting, creamy memories of gliding high over an undisturbed Earth. He died long before the remnants of his torso reached the ground, his lungs crushed by the thick air.

Giant was struggling to get back to his feet.

Before him a stegoceras lumbered, bewildered, the scarlet-coated cap of bone and flesh on his head absurd. Thanks to a chance sheltering in a dense crop of araucaria this young male had survived the tornado, suffering no worse injury than a snapped rib. But his clan was gone, scattered by the wind. He lifted his head and howled, a great mournful lowing. It was like a chick’s call of distress, a lost call.

It wasn’t his mother who responded, but two huge carnivores, giganotosaurs, who came stalking slowly toward him, their heads bobbing, their eyes fixed on him. Even now, the game of predator and prey continued.

But through the adrenaline-induced fear that flooded his system, the stegoceras noticed something strange. A third giganotosaur, as big and powerful as the rest, was showing no interest in him. The third monster was head-bobbing, threatening, reacting to something that approached from out of the sky. Confused, fearful, the stegoceras turned to the south, where a lowering cancerous orange continued to spread through the racing black clouds.

The first meteor screamed overhead like a glowing hornet. It flew low over the smashed forest and slammed against a foothill beyond. Young volcanic stone exploded, and a secondary shower of steaming fragments hailed out, pattering against the debris-strewn ground. All the dinosaurs turned that way, shocked and startled, their innate animosity briefly forgotten.

And the second meteor passed through the stegoceras’s body, like a high-velocity bullet. A fraction of a second later, on meeting the impenetrable ground, the meteor dumped the last of its energy into the rock. The explosion burst apart the stegoceras’s body before it had time to fall. In the brief rain of blood, Giant cringed, uncomprehending.

Now the meteors began to land in the remains of the smashed forest. Fire splashed.

Giant and his brothers panicked and ran. But still the meteor rain thickened. The meteors pounded the ground around the giganotosaurs, digging shallow craters and starting fires even in the scattered undergrowth. It was as if the brothers were running through an artillery barrage.

Purga, too, could smell the smoke.

The primates could ride out fires in their burrows, buried deep in the cool earth, to emerge into the debris of a charred and ruined forest. But, Purga’s instincts warned her, this time was different. She pushed past her cowering mate and her pups, past the grisly severed head of the troodon. She emerged into daylight. She was immediately dazzled, her sensitive night-adapted eyes unable to cope with the unaccustomed flood of light. But she could nevertheless make out the main features of the terrible day: the spreading fires in the smashed blanket forest, the continual, incomprehensible rain of meteors.

She could not stay here. But where to go?

With much of the obstructing forest already demolished by the winds she could see the shoulders of the Rockies with their clouds of volcanic smoke lingering at their summits. And where the comet winds had pushed warm, moist air up the flanks of the rising ground, thick cumulus clouds clung to the mountain’s upper slopes.

Shade. Darkness. Perhaps there would even be rain.

She took a step further into the open, whiskers twitching. She moved in rapid jerks, pausing every few paces, flattening herself against the ground.

She looked back. Beyond the fallen head of the troodon, she could see her mate and pups, three sets of wide eyes peering after her. Instincts honed across a hundred million years urged her to return to the cool earth, or to clamber into the trees where she would find safety, for otherwise the terrible claws and teeth and feet of this giant world would surely claim her. But the trees were smashed and broken, her burrow no longer a sanctuary.

She scurried away, toward the cloud-draped mountains.

Her mate followed, more cautiously. One of the pups followed him. The second, terrified, bewildered, bolted back into the recesses of the burrow. There was nothing Purga could do for the second pup. She would never see him again.

So the three tiny, shrewlike creatures — carrying all the potential of mankind within them — made their way slowly across the battered, smoldering plain while meteors rained around them.

The fire fed on itself. The scattered pockets of fire were beginning to link up. As the temperature of the air rose even the damp undergrowth was starting to burn. A wind began to gather, the smoke to spiral overhead. Here, and all over North and South America, the fires began to exert a logic of their own, becoming self-feeding, self-perpetuating systems.

Thus the firestorms began. Everything that could burn did so: every scrap of vegetation, even lake plants still soaked from their immersion. Animals simply burst into flame: Raptors burned like saplings and great armored herbivores cooked in their own monstrous shells.

The three giganotosaurs burst at last from the forest. They had come to a clearing centered on a large lake. They were overheated, their great mouths gaping, their heads filled with the stink of the smoke.

The open sky was extraordinary. A lid of blackness was rushing up from the southeast, as if a great curtain were closing. That eerie orange glow was spreading too, growing brighter and ascending to yellow. And still the meteors hammered into the muddy ground.

Near the lake itself a desolate scene greeted the giganotosaurs.

Dinosaurs stampeded. Great herds of rival duckbill species mingled, armored beasts like ceratops and ankylosaurs jostled for room, herbivores ran alongside giant predators. There were even mammals, blinking in the light, running amidst giant feet. All the animals charged in panic, their feet burned by the smoldering ground, clattering into each other blindly. This would have been unimaginable just a couple of hours ago. The intricate ecological relationships of herbivores and carnivores, of predators and prey, built up over a hundred and fifty million years, had utterly collapsed.

Giant pushed forward, barging his way through the panicking mob, driven to the water by a deep instinct. He plunged into the lake, ignoring the smoldering debris that floated on the surface. The deeper layers were still blessedly cool. But even with his head submerged he could see more meteors hitting the lake, creating bubble trails in the water like bullets.

And now a missile shape rose before him, a great mouth gaped white, and through the murky water he could see rows of conical teeth. He flailed back.

The crocodile had lain at the bottom of her lake, silent, patient.

A distant cousin of the seagoing deinonychus, so far the events of this tumultuous day had meant little to her. She had felt the shuddering of the Earth and the responding ripple of the water, noticed the peculiar lights in the sky. But she expected to ride out this storm, as she had ridden out many before. She could stay underwater for an hour at a time, as her metabolism was capable of shutting down almost completely when necessary. Her thinking was slow, patient. She knew that all she had to do was lie here in the mud, and the storm would pass, and once more her food would come to her.

But now a dinosaur came blundering clumsily into the water — not just skimming the fringe to drink and browse, like the stupid duckbills, but immersing itself, actually swimming in her domain. She felt anger at this intrusion, mixed with anticipation at any easy meal. She pushed herself away from the mud and rose toward the surface, which glimmered with meteor light. But more massive bodies plunged helplessly into the turbid water, struggling in the clinging mud of the lake bottom.

She attacked, of course.

Giant thrashed, evading the crocodile’s reaching jaws, and in his blundering he managed to land a kick on the crocodile’s snout. The crocodile backed away briefly. But soon she was returning to the attack. Giant might have withdrawn. But a crowd of animals was pushing into the water behind him. The crocodile fought and snapped at the invaders; and the animals warred amongst themselves.

But now there was a mighty surge, as an aftershock of the comet’s seismic jolt shuddered through the basement rock. The ground was uplifted, cracked — and the water drained suddenly away, leaving Giant stranded amid drying vegetation and writhing animals.

The crocodile, suddenly exposed to hot, dry air, could not understand what had happened. She tried to burrow into the mud, instructed by instincts that had guided her as a baby from her shell to her first swim. But the mud was hardening, drying fast; she could not even dig into the ooze.

Still the meteors fell, lancing through the clouds of smoke like pillars of light.

The winds and the tsunami had already wiped out most of the living things, from insects to dinosaurs, in North and South America. Around the world, the gathering fires were now killing most of those who had survived.

But the worst was yet to come.

The coarser ejecta at the periphery of the comet impact had fallen back quickly, much of it pounding the disturbed ground within one or two diameters of the central crater, the rest falling as forest-igniting meteors. But the great central plume of rock vapor had continued to rise, propelled by its own heat energy. In the vacuum of space, solid particles condensed out of this glowing cloud, and, still white-hot, began to fall back to Earth. But where they had risen through a tunnel of vacuum, now they fell back into atmosphere, and they dumped their energy into the air. It was a lethal hail of fire, a planetwide blanket of uncounted billions of tiny, white-hot meteors.

All over the planet, the air began to glow.

Purga had reached a foothill. Her mate, Third, and her one surviving pup were at her side. They could go no further toward the true Rockies, for even here the land had been broken and jumbled by the ground waves, littered with boulders that were many times Purga’s height.

This would have to do. She began to dig into the loose dirt, seeking to build a burrow.

She glanced back the way she had come. Under banks of billowing smoke the whole of the land glowed bright orange; it was an extraordinary sight. Even here, on this rocky rise, she could feel the heat; even here she could smell the stink of smoke and burning flesh.

She could see the clouds that had drawn her here. They were ragged, but still clustered around the upper slopes of the mountains. Against a sky as black as night the clouds glowed orange white, reflecting the glow of the burning land. But now, beyond the clouds, that orange light from the south crept overhead. The sky itself began to glow, like a dawn erupting all over the sky, all at the same time. The color quickly escalated to orange, then yellow, then a dazzling white, sun-bright.

The heat’s first breath reached them.

The primates scrabbled desperately at the ground.

On the cracked pond floor Giant was somehow on his feet, surrounded by the dead. He couldn’t breathe; his chest strained at air that was dense with smoke and bits of glowing, charred vegetation. It was like being in a gray fog. He saw nothing but smoke, dust, swirling ash.

Heat pulsed, hot as an oven. There was a stink of burning meat.

He felt a sharp pain in his hand. He lifted it in dim curiosity. His fingers were burning, like candles.

His last thought was of his brothers.

His death came in a moment of fulminant shock. He knew nothing about it: His vital organs were destroyed too quickly for his brain to process a conscious reaction. Then his muscles cooked and coagulated. They contracted his arms and legs, but his spine was extended, so that in this moment of death he adopted a posture oddly like a boxer’s, head back, hands up, legs flexed. His flesh was seared away, and the enamel on his teeth began to shatter.

All this before Giant had time to fall to the ground.

And then the very rocks began to crack.

Jewel-like, its sudden brilliance reflecting from the ancient seas of its companion Moon, Earth was beautiful. But it was the beauty of a dying world.

Half of all the heat energy released by the burning air was injected into the deeper atmosphere and the ground. All over the planet, the sky was as hot and bright as the sun. Plants and animals burned where they stood. The trees of the mighty Cretaceous forests were consumed like pine needles. Any birds in the air disappeared in a puff of flame, and the pterosaurs vanished into the maw of extinction. The burrows of mammals and insects and amphibians turned into tiny coffins. Purga’s second pup, whimpering and alone, was quickly baked.

Purga was spared. The last clouds, shadowed black, became ragged, dispersing quickly, soon vaporized into steam — but for the crucial minutes of the great heat pulse they served to shield the ground beneath them from a sky as bright as the sun.

It was just an hour after the impact.

III

After the first few days the Earth’s shuddering died away, and the daily stamping of the great mountain-reptiles was gone.

Purga was used to darkness. But not to silence, this eerie stillness that went on and on.

For countless generations the dinosaurs had framed the lives of Purga’s kind. Even after this cataclysmic shock, she had vague visions of arrays of dinosaurs waiting in silent rows to trap any mammal unwise enough to poke her snout out of her burrow.

But she could not stay here, in this hasty burrow. For one thing there was nothing to eat; the family had quickly excavated and consumed any burrowing worms or beetles they could reach. They didn’t even know when it was day and when it was night. Their sleep cycles had been thrown off by their flight during the day of the impact, and they found themselves waking at different hours, their hunger conflicting with their fear of the strange, cold silence above. They bickered among themselves, snapping and biting.

And as time wore on the temperature plunged, from the intense heat of the hours of the burning sky to a bitter cold. The primates were sheltered by the thick layer of earth above them, but even that would not protect them forever.

Finally Third turned on the pup — Last, for she was Purga’s last surviving child. Purga couldn’t see Third. But with her whiskers and well-developed hearing she could sense her mate approaching the pup, step by step, mouth wide, as if stalking a centipede.

Third was angry, confused, frightened, and very, very hungry. But what he was doing made a certain sense. After all, there was nothing to eat here. If the flesh of the pup kept the adults alive a little longer, long enough for them to produce another litter, the genetic program would be fulfilled. The calculations were relentlessly logical.

Perhaps in other times Purga would have submitted to Third’s aggression, even helped him finish off the pup. But Purga’s life had already been long for her kind, and she had suffered a series of extraordinary events: the destruction of her first home, her dogged pursuit by Wounding Tooth — and now the nightmare of the comet impact and her stranding in this world of cold and silence.

The imperatives resolved themselves. She bit Third savagely on the thigh, and scrambled past him to stand alongside her daughter.

Last was just as confused as the others. But she figured out that her mother was defending her from some kind of attack by her father. And so she stood with Purga and bared her teeth at Third. For a full half-minute the burrow was filled with hissing and the sound of tiny paws scraping the ground aggressively; three sets of whiskers filled the space between the primates, each of them waiting for the other to strike.

In the end it was Third who backed off. He gave up quite suddenly, abandoning his aggressive posture and curling up alone in a corner of the burrow. Purga stood with her daughter until the anger and aggression had drained out of her system.

It was this final incident that changed the balance of the forces in Purga’s mind.

They couldn’t stay here, for they would starve, or freeze, if they didn’t kill each other first. They had to go out, regardless of what mysterious dangers lurked in the newly silent world above. Enough was enough. When her body clock next woke her, Purga pushed away the dirt that clogged the entrance to the burrow.

And emerged into the dark.

After two days, the fire in the sky had died. But now, from pole to pole, dust and ash covered the wounded Earth, a black shroud laced with wispy, yellow-white clouds of sulfuric acid. The Earth had been transformed from a starlike shining to a dismal, gloomy darkness, darker than the core of the comet which had wrought such devastation. Dust and ash: The dust was comet fragments, and sea-bottom dirt, and even volcanic debris spewed out after the immense seismic shocks that had rippled through the planet. And the ash was burned life, trees and mammals and divergent species of dinosaurs from America and China and Australia and Antarctica, burned to cinders by the global firestorms and then burned again in the pulse of superheat, now mingled together in the choked stratosphere. Meanwhile, sulfur, baked out of seabed rock in the first moments of the impact, had lingered in the air, forming sulfuric acid crystals. The high, bright acid clouds reflected away sunlight and drove the cold deeper still.

Followed by Third and Last, Purga crept cautiously away from her burrow’s mouth, whiskers twitching nervously. It was late afternoon, here in the chill heart of North America. If the sky had been clear, the sun would still have been well above the horizon. There was only the gloomiest of twilights, barely sufficient even for Purga’s huge, sensitive eyes.

She stumbled forward over bare, scorched rock. Everything was wrong. There was no scent of green growing things, nor the pungent, spicy stink of the dinosaurs, not even of their dung. Instead, she smelled only ash. The whole of the great thick green-brown layer of Cretaceous life had been burned off: even the dead leaves, even the dung, all destroyed. All that was left were minerals, lifeless dirt, and rock. It was as if Purga had been transported to the surface of the Moon.

And it was cold, a deep intense cold that quickly penetrated through diminished layers of fat to her bones.

She came to the ruins of what had been a small stand of tree ferns. She scraped at the ground with her claws, but the ground was strangely hard — and it was cold, deep cold, so cold it hurt the pads of her hands. But when she licked her hand, a slow trickle of water gathered in her mouth.

Just a few days earlier this had been a place of tropical forest and swamplands. No frost had formed here in millions of years. But now there was frost. Purga scrabbled at the ground, cramming the strange, chill stuff into her mouth. Slowly she got mouthfuls of water — and plenty of ash and dirt along with it.

She tried to dig deeper. She knew that even after the most ferocious fire there was food to be had: hardened nuts, deep-buried insects, worms. But the nuts and spores were trapped under a lid of frozen ground, too tough for Purga’s small paws.

She moved on, feeling her way through the dark with her whiskers.

She came to a shallow puddle. In fact it was the footprint of a vanished ankylosaur. Her snout hit a hard surface: brutally cold and hard as rock. The cold that stabbed through her fur was intense. She backed up hastily.

Like frost, she had never encountered solid ice before either.

More cautiously she poked at the ice with her snout and hands. She scraped and scratched — she could smell the water that was somehow hidden here, and it maddened her to be able to get no closer to it. Frustrated, she began to circle the little puddle, pushing and probing. At last she came to a place where the ankylosaur’s foot, pushing into what had been soft warm mud, had dug a somewhat deeper pit. The ice here was thin, and when she pushed at it, the surface cracked and lifted up. She jumped back, startled. The fragment of ice, upended, slid slowly into the black water. Cautiously she slid forward once more. And this time, when she tentatively dipped her snout, she found liquid water: chill, already frosting over with fresh ice, but liquid nonetheless. She sucked in great mouthfuls, ignoring the bitterness of the ash and dust that laced it.

Attracted by the sound of her drinking, Third and Last came hurrying to her side. They quickly extended the hole she had broken, jostling to slurp up the gritty water.

For the first time since the comet had struck, things had gotten better for Purga: not by much, but better.

But now something touched her shoulder: something light, cold. She yelped and turned. It was a wisp of white, already melting.

Now more flakes came drifting down out of the sky. They fell with a random, gentle movement. When a flake came close enough, she leapt up and took it in her mouth, like plucking a fly from the air. She got a mouthful of soft ice.

It was snowing.

Spooked at last beyond endurance, she turned and bolted for the security of the burrow.

The impact had hurled vaporized ocean water into the air. After weeks of suspension, it began to fall back.

There was a lot of vapor. An epochal rain fell, all over the planet.

But the rain itself brought further devastation. It was full of sulfuric acid from the ice clouds, and the impact had injected thin clouds of toxic metals into the atmosphere, metals that now rained out. Nickel alone reached twice the threshold of toxicity for plants. Runoff water washed substances like mercury, antimony, and arsenic out of the soils, concentrating them in lakes and rivers.

And so on. For years, every raindrop would be poisoned.

The great rain washed out the dust and ash. All over the world, a fine layer of blackened clay was laid down, a band of darkness that would forever show up as a punctuation in the sedimentary rocks of the future — a boundary clay, one day to be studied by Joan Useb and her mother, the last remnant of a biosphere.

After months of dark the sun showed, at last, through the planet-girdling layers of dust and ash. But it was only a pinprick, shedding barely any heat on the frozen land; there would be no more than a murky twilight for another year.

The returning sun illuminated a skeletal landscape.

Tropical plants, if not burned, had been killed by the sudden cold. Any surviving dinosaurs were succumbing to hunger and cold, their bones quickly stripped of flesh by the surviving predators. But here and there living things moved in the ash: insects like ants and cockroaches and beetles, snails, frogs, salamanders, turtles, lizards, snakes, crocodiles — creatures that had been able to hide in mud or in deep water — and many, many mammals. Their furry bodies and habits of burrowing into the shelter of the ground were protecting them from the worst of the cold. Their indiscriminate eating habits helped as well.

It was as if the world ran with rats.

And even now the survivors were breeding. Even now, despite the cold and the shortage of food, in the absence of their ancient predators, their numbers were increasing. Even now the blind scalpels of evolution took raw material adapted for a vanished world and cut and shaped it for the conditions of the new.

Alone, the female euoplocephalus stumbled through the endless cold, seeking the rough forage she needed.

She was of a species of ankylosaur. Her body was ten meters long and, before her slow starvation had begun, she had weighed as much as six tons. Her armor was bone: plates of it set in the skin of her back, neck, tail, flanks, and head. Even her eyelids were plates of bone. The plates were woven into a layer of tough ligaments, making the great carapace flexible, if heavy. Her long tail terminated in a fused mass of bone. Once she had used this club to lame a young male tyrannosaur, her greatest triumph — not that she was able to remember it; all that armor had left little room, and little need, for a large brain.

Though geologically sudden, the great death unfolding across the planet was not instant in the consciousness of those who endured it. For days, weeks, months, many of the doomed clung to life — even dinosaurs.

Relatively speaking, the euoplos had been well-equipped to survive the end of the world. Their massive bulk, great strength, and heavy armor — together with a fortunate placing beneath a thick layer of cloud close to the bank of a river — had enabled a few of her kind to endure the first few horrific hours. She had survived droughts before; she ought to withstand this unexpected calamity. All she had to do was keep moving and fend off the predators.

And so, wandering the freezing Earth, she sought food. And found hardly any.

One by one her companions had fallen away, until the euoplo was alone.

But, in a final irony, she had endured one last mating, from a male now dead; and she found herself heavy with eggs.

In this new world, a land of ice and blackness and a lid of gray-black sky, the euoplo had been unable to find the ancestral rookeries. So she had constructed a nest of her own as best she could, from the bare, cinder-strewn floor of what used to be a rich forest. She had laid her eggs, lowing, setting them carefully in a neat spiral on the ground. Euoplos were not attentive mothers; six-ton tanks were not well equipped to deliver tender loving care. But the euoplo had stayed close to her nest, defending it from the predators.

Perhaps, despite the cold, the eggs might have hatched. Perhaps some of the young could have survived the great chill; of all the dinosaurs, perhaps it was an ankylosaur who might have endured best in the new, harsher world to come.

But the stinging rain had leached away the nutrients the euoplo’s body needed to manufacture successful eggs. Some of them had been laid with shells so thick no infant could ever break out, others were so thin they were broken as soon as she produced them. And then the rain began to damage the eggs directly, the grimy downpour etching away their protective surfaces.

None of the eggs had hatched. The euoplo, mournful, baffled on a deep cellular level, had moved away. Immediately after she was gone, a furry cloud of mammalian predators had descended on the eggs, in their squabbling reducing the nest to a muddy battlefield.

The last of her kind, the euoplo wandered the land, driven on by a final imperative: to survive. But the poison and the rain worked on her too. Creatures like Purga sheltered from the worst of the rain in her burrow, or under rocks — or even, once, under the scooped-out shell of a dead turtle. The euoplo was too big: There was nowhere for her to hide, and she could not burrow into the ground. So her back was ferociously scalded, and great bony plates of armor were stripped of flesh, the connecting ligaments etched and burned.

Unthinking, she staggered toward the sea.

Three months after the impact, Purga and Last stumbled across ground frozen as hard as rock.

They saw few animals: Sometimes a cautious frog would watch them go by, or a bird would flee at their approach, chirping with eerie loudness, abandoning some frozen bit of offal on the ground. The relics of the lush Cretaceous vegetation, the stumps of trees and patches of undergrowth, were now frozen as hard as blackened sculptures, and any attempt to gnaw them was rewarded only with a mouthful of ice, and more often than not a chipped tooth.

There were only two of them. Third had gone, lost to hypothermia.

Purga longed for security, to clamber into a tree or dig into the soft earth. But there were no trees — nothing but ash and stumps and bits of roots — and the ground was too hard to burrow into. When they needed to rest they could only grub in the looser debris, making loose nests of ash and burned leaves and bits of wood, where they would lie shivering, huddled to share body warmth.

After days of wandering, Purga and Last made their way slowly along the fringe of America’s inland ocean.

Even here the gritty beach was frozen, and the sea itself, as charcoal gray as the sky above it, was littered with ice floes. But the gentle swell still breathed salty water over the sand. And here at the fringe of the ocean the primates found food — seaweed, small crustaceans, even stranded fish.

The oceans, too, had been devastated by the impact. The loss of sunlight and the acid rain had massacred the photosynthetic plankton that had populated the ocean’s upper layers. With this key foundation of the ocean’s food chain gone, extinctions were following like tumbling dominoes. On wounded Earth, death stalked every realm, and the ice-littered waters of the darkened ocean hid a holocaust as horrific as that which was unfolding on the land. It would take a million years for the seas to recover.

Purga came across a stranded starfish. Still new to ocean foraging, she had never seen such a beast before. She poked it with her snout, trying to determine which of her world categories it most closely fit: a threat, or good to eat.

Her movements were listless. In fact she could barely see the starfish.

Purga was weakening. She was constantly thirsty, with a nagging pain that clung to her mouth and throat and sank deep into her belly. Since the impact she had lost weight steadily, from a scrap of a body that had little excess to start with. And she was a tropical creature suddenly stranded in arctic conditions. Though her layer of fur helped trap heat, her body form was long and lanky, lacking the spherical, enclosing shape of creatures adapted to the cold. So she burned up even more energy and body mass in shivering.

She was bony, enfeebled, continually exhausted, her thinking increasingly fuddled, her instincts dulled.

And she was getting old. Living as vermin, the primates’ principal survival tactic had been fast breeding: there had always been simply too many of them to be eliminated by the dinosaurs’ ferocious hunting. For such creatures, there was no premium on longevity. Already Purga was coming to the end of her short, explosive life.

Last suffered, too, of course. But, younger, she had more strength to spend. Purga was aware of a growing distance between them. It was not a question of disloyalty. This was the logic of survival. Purga sensed, deep down, that the day would come when her daughter would see her not as a foraging companion, not even as a hindrance, but as a resource. After all she had survived, maybe Purga’s final memories would be of her own daughter’s teeth at her throat.

But now they smelled meat. And they saw more survivors, more ratlike mammals, scurrying across the beach. There was something to be had. Purga and Last struggled to follow.

At last, her awareness flickering like a failing lightbulb, the great euoplo stumbled to the shore of the ocean.

She looked down, uncomprehending. Water lapped at her feet, dappled by heavy raindrops. The sand was flecked with the black of soot and volcanic dust, and littered with the bones of tiny creatures. She made out the silvery bodies of fish, lifeless, their eyes pecked out by opportunistic birds. But the euoplo knew only her own weariness, hunger, thirst, loneliness, pain.

She raised her head. The sun, setting to the southwest, was a disk, bloodred, not far above a horizon that was charcoal against charcoal.

The euoplo stood motionless at the edge of the water. She was one of the last large dinosaurs left alive anywhere on Earth, and she stood now like a statue to her vanishing kind. Her head and tail felt very heavy, weighed down by all that armor. She let them droop. She was dying without ever having produced a single viable young. An abject misery clamored within the euoplo’s small consciousness.

There was a sharp nip at the pad on the base of her foot.

It was a therian mammal. It was no more prepossessing than Purga, and yet equipped with teeth that scissored — just as, one day, a lion’s would. It had run forward and bit her, with absurd boldness. The euoplo hooted her indignation. With a vast effort, she raised one immense foot. But when she slammed it down into the water she made only a splash; the scurrying mammal escaped.

But, all around her, more survivors gathered.

None of these animals were large. Purga and Last were here, and other mammals, little ratlike creatures that had kept themselves alive in their underground burrows, warmed through this long winter by their constant body heat. There were birds, protected by their hot blood and small size from an event which their more spectacular relatives could not endure. Here, too, were insects, snails, frogs, salamanders, snakes, creatures who had endured in burrows and riverbanks or deep holes. These small, scurrying creatures had been used to feeding off scraps and hiding in the corners anyhow; to them, the comet impact hardly made things worse.

Now they moved closer to this giant, the last of the monsters who had dominated their world for a hundred million years. In the long empty months since the impact, as they spread out through a world like a charnel house, many of them had learned to exploit a new food source: dinosaur flesh.

Times had changed.

Extinction was a terminus more drastic than death.

At least with death there was the consolation that your descendants would go on after you, that something of your kind would linger on. Extinction took away even that comfort. Extinction was the end of your life — and of your children, and all your potential grandchildren, or any of your kind, on to the end of time; life would go on, but it would not be your kind of life.

Dread though they were, extinctions had always been commonplace. Nature was packed thick with species, each connected to all the others through competition or cooperation, all endlessly struggling for survival. Though nobody could get permanently ahead, it was possible to fail — through bad luck, or disaster, or invasion by a better-equipped competitor — and the price of failure had always been extinction.

But the comet impact had now triggered a mass extinction, one of the worst in this battered planet’s long history. Dying was occurring in every biological realm, on land, in the sea, and in the air. Whole families of species, whole kingdoms, were falling into the darkness. It was a huge biotic crisis.

At such a time it didn’t matter how well adapted you were, how well you evaded the predators or competed with your neighbors, for the most basic ground rules were changing. During a mass extinction, it paid to be small, numerous, geographically widespread, to have somewhere to hide.

And, crucially, to be able to eat other survivors in the aftermath.

Even then survival depended as much on good fortune as good genes: not evolution, but luck. For all their smallness and ability to hide, more than half the mammals had gone extinct with the dinosaurs.

But the mammals owned the future.

The euoplo was not aware of her legs collapsing. But there was suddenly a damp cold under her belly, a gritty saltiness in her mouth where her head dangled into the water.

She closed her eyes. The heavy armor made the lids opaque. She rumbled deeply — a sound that another of her kind could have heard kilometers away, had there been any to hear — and tried to spit the brine out of her mouth. She retreated into her bony armor, like a turtle inside its shell. Soon it was as if she could no longer hear the hiss of rain on the sand and water, the scuffling of the ugly little creatures who surrounded her.

Even to the last she knew no peace, only a huge reptilian loss. But she felt little pain, when the small teeth went to work.

This last great dinosaur was a storehouse of meat and blood that fed the squabbling horde of animals for a week.

At the end of that time, as the acid rain began to leach the huge gnawed plates of the euoplo’s back gleaming white, Purga and Last encountered another group of primates. There were several of them, mostly about Last’s age or younger — so they had probably been born after the impact, and had known nothing, all their lives, but this straitened world. They looked lean, hungry. Determined. Two of them were male.

They smelled strange. They were not even distantly related to Purga’s family. But they were undoubtedly Purgatorius. The males had no interest in Purga; her subtle scent told them she was too old to bear any more litters.

Last gave her mother a final glance. And then she scampered over to the others, where the males, whiskers quivering, began to sniff her and to nuzzle her with bloodied snouts.

After that day Purga never saw her daughter again.

IV

A month later Purga, wandering alone, came upon the carpet of ferns.

Entranced, Purga hobbled forward as fast as she could. These were only lowly groundcover growths, but their fronds made a dim green shade. On the underside she could see little spore sacs, brown dots.

Green, in a world of soot and ash gray.

Ferns were robust survivors. Their spores were tough enough to withstand fire, small enough to be carried great distances on the wind. In some cases the new growths sprouted directly from surviving root systems, black, creeping roots that were far more indestructible than the roots of trees. In times like this, as the light slowly recovered and photosynthesis became possible, the ferns faced little competition. Amid the muddied ash and clay, the world was taking on a look it had not had since the Devonian age some four hundred million years before, when the first land plants of all — primitive ferns among them — had made their tentative colonies.

She climbed. The tallest of these ground huggers gave her a platform just a few centimeters off the ground, but she clambered onto the fronds gratefully. It was enough to release in her a flood of inchoate memories of how she had scurried along the branches of the great, vanished Cretaceous forests.

Later, she dug. The rain still fell, and the ground was boggy, but by digging close to the tough roots of the ferns she was able to construct a satisfactory burrow. She began to relax, for the first time since the impact — perhaps for the first time since the crazed troodon had begun to pursue her.

Life had nothing more to ask of Purga. One of her pups had survived, and would breed, and through her the great river of genes would pass on, on into an unknowable future. And it was an irony that in former times she would surely already have succumbed to predation by now: It was the great emptying of the world that had preserved her life — a few extra months won at the expense of uncounted billions of creatures.

As content as it was possible to be, she settled to sleep in a cocoon of earth that still smelled of the great burning that had ended a world.

The planet was filling up with fast-breeding, short-lived creatures. Already almost all of Earth’s population had been born into the new era, and had known nothing but ash, darkness, and carrion. But as Purga slept her hind legs convulsed and her front paws scrabbled at the ground around her. For Purga, one of the last creatures on the planet to remember the dinosaurs, the terrible lizards still stalked, at least in her dreams.

There came a morning when she did not wake, and the little burrow became her coffin.

Soon a blanket of sediment, deposited by the ocean, covered over the vast impact crater. The great geological deformation was eventually hidden under a layer of limestone a thousand meters thick.

Of the Devil’s Tail itself, nothing remained but traces. The nucleus had been destroyed in the first seconds of the impact event. Long before Earth’s skies cleared, the last remnants of the coma and the glorious tail — the tenuous body of the comet, now cut from its tiny head — blew away in the wind from the sun.

But still the comet had left a kind of memorial. In the boundary clay would be found tektites — bits of Earth that had been blasted into space and returned, melted into glassy dewdrop shapes like tiny space capsules by their re-entry into the air — as well as fragments of quartz and other minerals, shocked into strange glassy configurations by the impact energy. There were shards of crystalline carbon, normally formed only deep in Earth’s interior, but baked on the surface in those few ferocious seconds: tiny diamonds, littering an ash of burned Cretaceous forests and dinosaur flesh. There were even traces of amino acids, the complex organic compounds once delivered by long-vanished comets to rocky Earth, the compounds that had enabled life to emerge here: a wistful present from a visitor who had come too late.

And as the dust clouds finally cleared and the chill dispersed, the comet’s final gift to the Earth came into play. Vast amounts of carbon dioxide, baked out of the limestone of the shattered seabed, now lingered in the air. A savage greenhouse effect kicked in. The vegetation, striving to recover, struggled to cope. The first millennia were times of swamps, of marshes and rotting bogs, where dead vegetation choked lakes and rivers. All over the world coal was laid down in great seams.

At last, though, as spores and seeds blew around the world, new plant communities blossomed.

Slowly, Earth turned green.

Meanwhile, time worked on Purga’s tiny remains.

Within hours of her death, blowflies had laid eggs in her eyes and mouth. Soon flesh flies were dropping larvae on her skin. As maggots burrowed into the little corpse, so the gut bacteria that had served her all her life burrowed out. Intestines burst. The contents began to rot other organs, and the cadaver liquefied, with a powerful stink, like cheese. This attracted carnivorous beetles and flies.

In the days after her death, five hundred types of insects feasted on Purga’s corpse. Within a week, there was nothing left but her bones and teeth. Even the great DNA molecules could not survive long. Proteins broke down into their individual building blocks; amino acids in turn decayed into mirror-image forms.

Just a few days after that, a flood of acidic water swept away the little hollow. Purga’s bones were dumped in a shallow depression half a kilometer away, jumbled with the bones of raptors, tyrannosaurs, duckbills, and even troodons: enemies made equal in the democracy of death.

With time, more layers of mud were laid down by floods and bank-bursting rivers. Under pressure, the layers of silt turned to rock. And, in her rocky tomb, Purga’s bones were further transformed, as mineral-rich water was forced into their every pore, filling them with calcite, so they became things of rock themselves.

Buried deep, Purga began a spectacular journey lasting millions of years. As continents collided, the land was uplifted, bearing all its entombed passengers like some vast ocean liner riding a swell. Heat and compressing forces fractured and twisted the rocks. But erosion continued, a relentless, destructive force balancing Earth’s creative uplift. Eventually this land became an angular landscape of plateaus, mountains, and desert basins.

At last the erosion cut through the mass grave that had swallowed Purga’s bones. As the rock crumbled away bits of fossil bone emerged into the light, corpses bobbing to the surface, waking from a sixty-five-million-year slumber.

Almost all of Purga’s bones were lost, flashing to dust in geological instants, all that patient chthonic preservation wasted. But in 2010 a remote descendant of Purga’s would pick out a blackened shard in a wall of gray rock, just beneath a strange layer of dark clay, and recognize it for what it was, a tiny tooth.

But that moment lay far in the future.

I

Through the endless forest, Plesi climbed.

Squirrel-like, she scampered up a scaly trunk and along a fat branch. Though it was close to noon, the light was dappled, uncertain. The canopy was high above her, the floor lost in green layers far below. The forest was silent save for the rustling of leaves in the warm breeze and the call of the canopy birds, those colorful cousins of the vanished dinosaurs.

It was a world forest. And it belonged to the mammals — including the primates, like Plesi.

She glanced back along her branch. There were her two pups — both daughters — who she thought of as Strong and Weak. About half Plesi’s size, they clung to the angle of branch and tree. Even now, Strong was pushing Weak subtly aside. In some species the runtish Weak might have been allowed to die. But Plesi’s kind bore few young, and in an uncertain and dangerous world, all of them had to be cared for.

But Plesi could not protect her pups forever. They were both weaned now. Though they had learned to seek out the fruit and insects that inhabited this, their birth tree, they must learn to be more adventurous — to move out into the forest, to seek out their own food.

And to do that, they had to learn to jump.

Hesitantly, scrambling at the scaly surface of the branch, Plesi tensed, and leapt.

Plesi was a plesiadapid: she belonged, in fact, to a species that would one day be called carpolestid. Plesi closely resembled her remote grandmother, Purga. Like Purga, she looked something like a small squirrel, with a low-slung body like a large rat’s, and a bushy tail. Though a true primate, Plesi retained Purga’s claws rather than nails, her eyes did not face forward, and her brain was little developed. She still even had the big night-vision eyes that had served Purga so well in the time of the dinosaurs.

The most significant development of primate bodies since Purga’s time was in the teeth; Plesi’s was a species adapted to husk fruit, as would be the possums of Australia, much later. It was a necessary response, if the primates were to find something to eat. Few animals of this time fed off leaves. In an equable world where tropical or paratropical forests spread far from the equator, there was little seasonal variation, and here in Texas the trees did not shed their leaves regularly. In fact, the trees loaded their leaves with toxins and chemicals to make them bitter or poisonous to curious mammalian tongues.

But still, since Purga, there had been little innovation in the primate line — even across two million years. It was the same for many other lineages. Long after the great impact, it was as if the emptied world had been shocked into stasis.

Plesi landed on her target branch without difficulty.

Her two pups were still huddled hesitantly against the tree trunk, and they made the mewling calls of babies. But, though the calls tugged at her, Plesi only raised her head and twitched her snout. She tried to encourage the pups to follow her by nibbling the fruit that clustered on this new tree.

At last the pups reacted. To Plesi’s surprise it was the little one, Weak, who came forward first. She scampered to the end of the branch — nervous, hesitant, but showing good balance. She raised her tail and tensed her muscles — she backed off nervously, preened the fur of her face — and then, at last, she jumped.

She overjudged slightly. She came tumbling out of the air and collided with her mother, making Plesi hiss in protest. But her agile hands and feet soon gripped the lumpy bark, and she was safe. Trembling, Weak scampered to her mother and buried her face in her belly, seeking a nipple that was now dry. Plesi let her suckle, rewarding her with comfort.

But now there was a blur of movement from the other tree. Strong, left behind, suddenly lunged forward, her immature feet slipping on the bark. And — without looking carefully, without trying to use her innate skills to estimate the distance — she leapt into the air.

Fear prickled inside Plesi.

Strong made the branch, but she landed too hard. Immediately she slid backwards. For a heartbeat she hung there, her small hands scrabbling uselessly at the bark, her hind legs waving. And then she fell.

Plesi saw her tumble in the air, wriggling, her white underbelly exposed, her hands and feet clutching at nothing. Even now Strong made the peeping cry of a lost infant. Then she fell into the leaves, and in a moment she was gone, taken by the green below, which swallowed all the forest’s dead.

Plesi clung to her branch, shuddering. It had happened so quickly. One young lost, one runtish weakling left. It was not to be borne. She hissed her defiance into the menacing green.

And, leaving Weak clinging piteously to the trunk of the tree, Plesi began to descend, down toward the green, down to the ground.

At last she reached the lower story of branches, and looked down into an oasis of light.

This was one of the endless forest’s few clearings. Within the last few months, an ancient canopy tree had fallen, eaten from within, wrecked by a random lightning strike. When it had crashed down it had cut a swath through the dense foliage. This clearing would not last long. But for now the plants of the undergrowth, like those hardy survivors, the ground ferns, were taking the opportunity to germinate, and the forest floor here was unusually lush and green. And already saplings were sprouting, beginning a ruthless vegetable race to steal the light and plug that hole in the canopy.

The forest was an oddly static place. The great canopy trees competed with each other to trap as much sunlight as they could. In the gloom of the lower levels, the light was too weak to support growth, and the floor was customarily littered by dead vegetable matter and the bones of any animals or birds unlucky enough to fall. But under the silent ground, seeds and spores abided — waiting centuries, even millennia if necessary, until the day came when chance opened up a gap in the canopy, and the race to live could begin.

Plesi slithered down a buttressing root and reached the ground. Under the broad fronds of a ground fern she scuttled uneasily through a patch of direct sunlight. The solid ground, with no give or sway, felt very strange to her, as peculiar as the shuddering of an earthquake would have felt to a human.

There were other animals here in the clearing, drawn by the prospect of novel pickings. There were frogs, salamanders, and even a few birds, flapping across the air in bright bursts of color, seeking insects and seeds.

And there were mammals.

There were creatures like raccoons but more closely related to the hoofed animals of the future, and scurrying insectivores whose descendants would include the shrews and the hedgehogs. Here was a taeniodont, like a small, fat wombat. It grubbed in the soil, expert at digging out roots and tubers. None of the grubbing creatures in this clearing would have been familiar to a watching human. They were furtive, odd, ungainly, almost reptilian in their behavior, forever looking over their shoulders, like petty thieves expecting the return of the householder.

These mammals were holdovers from the Cretaceous. Then, it had been as if the whole Earth had been a vast city, shaped for the needs of its owners, the dinosaurs. But now the dominant inhabitants were gone, the great buildings erased, and the only creatures left alive were the urban species who had lived in the drains and sewers, subsisting on garbage.

But the recovering Earth had become a very different place from the dreamy Cretaceous. The Earth’s new forests were much more dense now. There were no great herbivores: The sauropods had gone, and the elephants lay far in the future. There were no animals big enough to topple these trees, to smash clearings and corridors and make parklike savannah. In response the vegetation had gone crazy, filling the world with greenery of a density and profusion not seen since the first animals had walked onto the land.

But it was an oddly bare stage. In these thick jungles there were no more predatory dinosaurs — but neither were there yet jaguars, leopards, tigers. Practically all of the forest’s inhabitants were small, tree-dwelling mammals like Plesi. For an extraordinary span of time — for millions of years — the animals would cling to their Cretaceous habits, and no mammal species would grow to even moderately large sizes. They still contented themselves with the darkness and the corners of the empty world, nibbling on insects, eschewing any evolutionary innovations more spectacular than a new set of teeth.

Like long-term prisoners, the survivors of the impact were institutionalized. The dinosaurs were long gone — but for the mammals, habits ingrained over a much longer span, a full hundred and fifty million years of incarceration, were not so easy to give up.

But things were changing.

At last Plesi heard the quiet mewling of her young.

At the edge of the clearing Strong was huddled, pathetically, in a kind of nest of browned fronds. After she had fallen out of the tree and tumbled into the clearing, at least she had had the sense to seek cover. But she was far from safe: a large, scarlet-bellied predatory frog was watching her, an absent curiosity in its blank eyes. When she saw Plesi, Strong dashed forward and fell on her mother. She tried to find Plesi’s nipples, just as her sister had, but Plesi snapped at her, denying her comfort.

Plesi was deeply disturbed. A carpolestid who was strong in the nest but who had no instinct for the trees — who lacked even the sense to keep silent when exposed — had poor survival prospects. Suddenly Strong didn’t look so strong after all. Plesi felt an odd impulse to find a mate, to breed again. For now, though, she merely nipped at Strong’s flank with her sharp incisor teeth, and led the way back toward the tree from which she had descended.

But she had gone no more than a few body lengths when she froze.

The predator’s blank eyes fixed Plesi with lethal calculation.

The predator was an oxyclaenus.

He was a sleek, four-footed, dark-furred animal: long-bodied, stout-legged, he looked like an outsized weasel, though his face and muzzle were more reminiscent of a bear’s. But he was related to neither weasel nor bear. In fact he was an ungulate, an early member of that great family that would one day include the hoofed mammals like pigs, elephants, horses, camels, even the whales and dolphins.

This oxy might have seemed clumsy, slow, even unfinished to an eye used to cheetah or wolf. But his kind had learned to stalk prey through the sparse undergrowth of the endless forest. He could even climb, pursuing his quarry into the lower branches of the trees. In this archaic time, this oxy had little competition.

And, as he looked on Plesi’s timorous, flattened form, two cold questions dominated the oxy’s mind: How will I trap you? And, How good will you be to eat?

Plesi lay flat against the floor, quivering, her whiskers twitching, her small, sharp teeth bared. But she was equipped with instincts honed over a million centuries at the feet of the dinosaurs. And in the cold calculus of her mind, a reassessment of risk was beginning. In this open place she could not hide. She could not reach a tree to clamber out of the oxy’s grasp. Surely if she tried to outrun him he would trap her easily with one of those cruel claws.

Only one option was left.

She arched her back, opened her mouth and hissed, so violently that she spattered the oxy with her spittle.

The oxy flinched at the unexpected aggression of this tiny creature. But she is no threat. The oxy, angered, quickly recovered his composure, and prepared to call Plesi’s bluff.

But Plesi had vanished into the undergrowth. She had never meant to attack the oxy, only to gain a precious second of time. And she had left Strong behind.

The young carpolestid, transfixed by the carnivore’s stare, flattened herself against the ground. The oxy crushed Strong with his paw, snapping the little primate’s spine. Strong, flooded with pain, turned on her attacker, seeking to gouge his flesh with her teeth. In her final moments Strong discovered something like courage. But it did her no good.

The oxy played with the crippled animal for a while. Then he began to feed.

As the world recovered, so its changing conditions shaped its living inhabitants.

The mammals were beginning to experiment with new roles. The ancestors of the true carnivores, which would eventually include the dogs and cats, were still small, ferretlike animals, busy, opportunistic general feeders. But the oxyclaenus had begun to develop the specializations of mammalian predators to follow: vertical legs for sustained speed, strong permanent teeth anchored by double roots and with interlocking cusps designed to shred meat.

It was all part of an ancient pattern. All living things worked to stay alive. They took in nourishment, repaired themselves, grew, avoided predators.

No organism lived forever. The only way to counter the dreadful annihilation of death was reproduction. Through reproduction, genetic information about oneself was passed on to one’s offspring.

But no offspring was identical to its parents. At any moment each species contained the potential for much variation. But all organisms had to exist within a frame of habitability set by their environment — an environment, of weather, land, and living things, which they shaped in turn. As survival was sought with ruthless ferocity, the frame of the environment was filled up; every viable variation of a species that could find room to survive was expressed.

But room was at a premium. And competition for that room was relentless and unending. Many more offspring were born than could possibly survive. The struggle to exist was relentless. The losers were culled by starvation, predation, disease. Those slightly better adapted to their corner of the environment inevitably had a slightly better chance of winning the battle for survival than others — and therefore of passing on genetic information about themselves to subsequent generations.

But the environment could change, as climates adjusted, or as continents collided and species, mixed by migrations over land bridges, found themselves with novel neighbors. As the environment, of climate and of living things, changed, so the requirements of adaptation changed. But the principle of selection continued to operate.

Thus, generation by generation, the populations of organisms tracked the changes in the world. All the variations of a species that worked in the new frame were selected for, and those that were no longer viable disappeared, sinking into the fossil record, or into oblivion altogether. Such turnovers were unending, a perpetual churn. As long as the “required” variation lay within the available genetic spectrum, the changes in the population could be rapid — as rapid as human breeders of domesticated animals and plants would find as they strove for their own ideas of perfection in the creatures in their power. But when the available variation ran out, the changes would stall, until a new mutation came along, a chance event caused, perhaps, by radiation effects, that opened up new possibilities for variation.

This was evolution. That was all there was to it: It was a simple principle, based on simple, obvious laws. But it would shape every species that ever inhabited the Earth, from the birth of life to the last extinction of all, which would take place under a glowering sun, far in the future.

And it was working now.

It was hard.

It was life.

Plesi had made an unspoken bargain with the oxy: Take my child. Spare me. Even as she clambered back through layers of green and into the safety of the trees, seeking her surviving daughter, that dreadful stratagem still echoed in her mind.

That, and a feeling that came from deep within her cells, a thought she might have expressed as: I always knew it was too good to be true. The teeth and claws weren’t gone. They were just hiding. I always knew they’d come back.

Her instinct was right. Two million years after the uneasy truce imposed by the dinosaurs’ death, the mammals had started to prey on each other.

That night Weak, bewildered, terrified herself, watched her mother twitch and growl in her sleep.

I

There was no true morning during these long days of Arctic summer, no authentic night. But as the clouds cleared from the face of the climbing sun, and light and warmth slanted through the trees’ huge leaves, a mist rose from the swampy forest floor, and Noth’s sensitive nostrils filled with the pleasing scent of ripe fruit, rotting vegetation, and the damp fur of his family.

It felt like a morning, like a beginning. A pleasing energy spread through Noth’s young body.

His powerful hind legs folded under him, his fat tail upright, he squirmed along the branch to get closer to his family — his father, his mother, his new twin sisters. Together, the family groomed pleasurably. The nimble fingers of their small black hands combed through fur to pick out bits of bark and fragments of dried baby shit, even a few parasitic insects that made a tasty, blood-filled treat. There was some loose fur, but the adult adapids had already lost most of last year’s winter coat.

Perhaps it was the gathering light that inspired the singing.

It began far away, a thin warbling of intertwined male and female voices, probably just a single mating pair. Soon more voices joined in the duo’s song, a chorus of whooping cries that added counterpoint and harmony to the basic theme.

Noth moved to the end of the branch to hear better. He peered through banks of giant leaves that angled south toward the sun, like so many miniature parasols. You could see a long way. The circumpolar forest was open, and the trees, cypress and beech, were well spaced so their leaves could catch the low Arctic sunlight. There were plenty of broad clearings where clumsy ground-dwelling herbivores rummaged. Noth’s eyes in their mask of black fur were huge — like his remote ancestor Purga’s, well adapted to the dark, but prone to dazzling in the daylight.

The song’s meaning was simple: This is who we are! If you are not kin, stay away, for we are many and strong! If you are kin, come home, come home! But the song’s richness went beyond its utilitarian value. Much of it was random, bubbling, like scat singing. But at its best it was a spontaneous vocal symphony, running on for long minutes, with passages of extraordinary harmonic purity that entranced Noth.

He lifted his muzzle to the sky and called.

Noth was a kind of primate that would be called notharctus, of a class called adapid, descended from the plesiadapids of the early millennia after the comet. He looked much like a small lemur. He had a high conical chest, long and powerful legs, and comparatively short arms with black, grasping hands. His face was small with a pronounced muzzle, an inquisitive nose, and pricked-up ears. And he was equipped with a long, powerful tail, laden with fat, his winter hibernation store. He was a little more than one year old.

Noth’s brain was considerably larger than Plesi’s or Purga’s, and his engagement with the world was correspondingly richer. There was more in Noth’s life than the urgencies of sex and food and pain; there was room for something like joy. And it was a joy he expressed in his song. His mother and father quickly joined in. Even Noth’s infant sisters contributed as best they could, adding their tiny mewling voices to the adults’ cries.

It was noon, and the sun was the highest it would travel today, but it was still low in the sky. Shafts of low green-filtered light slanted through the trees, illuminating the dense, warm mist that rose from the steaming mulch on the floor, and the tree trunks sent shadows striping over the forest floor.

This was Ellesmere, the northernmost part of North America. The summer sun never set, but merely completed circles in the sky, suspended above the horizon, as the broad leaves of the conifer trees drank in the light. This was a place where the shadows were always long, even in high summer. The forest, circling the Earth’s pole, had the air of a vast sylvan cathedral, as if the leaves were fragments of stained glass.

And everywhere the adapids’ voices echoed.

Emboldened, the adapids began to clamber down the branches toward the ground.

Noth was primarily a fruit eater. But he came upon a fat jewel beetle. Its beautiful carapace, metallic blue green, crunched when he bit into it. As he moved he followed the scent marks of his own kind: I came this way. This way is safe… I saw danger here. Teeth! Teeth!… I am of this troop. Kin, come this way. Others, stay away… I am female. Follow this to find me… That last message gave Noth an uncomfortable tightness in his groin. He had scent glands on his wrists and in his armpits. Now he wiped his wrists through his armpits and then drew his forearms across the trunk, using bony spurs on his wrists to embed the scent, and to cut a distinctive curving scar in the bark. The female patch was old; the brief mating season was long over. But instinct prompted him to cover the patch with his own multimedia signature so that no other male would be alerted by it.

Even now, even fourteen million years after the comet, Noth’s body still bore marks of his kind’s long nocturnal ancestry, like the glands for scent marking. His toes were tipped, but not with nails, like a monkey’s, but with grooming claws, like a lemur’s. His watchful eyes were huge, and like Purga he had whiskers to help him feel his way forward. He retained a powerful sense of hearing and smell; he had mobile radar-dish ears. But Noth’s eyes, while wide and capable of good night vision, did not share the dark-loving creatures’ ultimate adaptation, a tapetum, a yellow reflective layer in the eye. His nose, while sensitive, was dry. His upper lip was furry and mobile, making his face more expressive than those of earlier adapid species. His teeth were monkeylike, lacking the tooth comb — a special tooth used for grooming — of his ancestors.

Like every species in the long evolutionary line that led from Purga to the unimaginable future, Noth’s was a species in transition, ladened with the relics of the past, glowing with the promise of the future.

But his body and mind were healthy and vigorous, perfectly adapted to his world. And today he was as happy as it was possible for him to be.

In the canopy above, Noth’s mother was taking care of her infants.

She thought of her two remaining daughters as something like Left and Right, for one preferred the milk of the row of nipples on her left side, and the other — smaller, more easily bullied — had to make do with the right. The notharctus usually produced large litters — and mothers had multiple sets of nipples to support such broods. Noth’s mother had in fact borne quadruplets. But one of the infants had been taken by a bird; another, runtish, had quickly caught an infection and died. Their mother had soon forgotten them.

Now she picked up Right and pushed her against the trunk of the tree, where the infant clung. Parked like this, her brownish fur blending into the background of the tree bark, Right would remain here until her mother returned to feed her. She was able to stay immobile for long hours.

It was a form of protection. The notharctus were deep enough in the forest to be safe from any diving bird of prey, but the pup was vulnerable to the local ground-based predators, especially the miacoids. Ugly animals the size of ferrets, sometime burrow-raiders who scavenged opportunistically from the kills of other predators, the miacoids were an unprepossessing bunch, but nonetheless were the ancestors of the mighty cats and wolves and bears of later times. And they could climb trees.

Now the attentive mother moved along the branch, seeking a comparable place of safety to leave Left. But the stronger child was happy where she was, clinging to her mother’s belly fur. After gentle pushing, her mother gave up. Laden with her daughter’s warm weight, she worked her way down a ladder of branches toward the ground.

On all fours, Noth walked across a thick mulch of leaves.

The trees here were deciduous, every autumn dropping their broad, veined leaves to cover the ground with a thick layer of decaying vegetation. Much of the mat on which Noth walked was made up of last autumn’s leaves, frozen by the winter’s hard cold before they could rot; now the leaves were mulching quickly, and small flies buzzed irritatingly through the misty air. But there were also butterflies, their gaudy wings making splashes of flitting color against the drab ground cover.

Noth moved slowly, seeking food, wary of danger. He wasn’t alone here.

Two fat taeniodonts grubbed their way across the ground, their faces buried in the rotting leaves. They looked like heavy-jawed wombats, and they used their powerful forelimbs to dig into the dirt, seeking roots and tubers. They were followed by an infant, a clumsy bundle pushing at her parents’ legs, struggling through the thick layer of leaves. A paleanodont scuffed for ants and beetles with its long anteater’s snout. And here was a solitary barylambda, a clumsy creature like a ground sloth with powerfully muscled legs and a stubby pointed tail. This creature, scuffling gloomily in the dirt, was the size of a Great Dane — but some of its cousins, in more open country, grew to the size of bison, among the largest animals of their day.

In one corner of the clearing Noth made out the slow movement of a primate, in fact another kind of adapid. But it was quite unlike Noth himself. Like the loris of later times, this slow, ground-loving creature looked more like a lazy bear cub than any primate. It moved slowly across the mush of leaves, making barely a sound, its nose snuffling the ground. This adapid generally stuck to the deeper forest where its slowness was not as disadvantageous as it would be on more open ground. Here, its slow and silent movements made it almost invisible to predators — and to the insect prey it sniffed out acutely.

Noth wrinkled his nose. This adapid used urine as its scent marker; every time it toured its range it would carefully urinate on its hands and feet to leave its signature. As a result, to Noth’s sensitive nose it stank badly.

Noth found a fallen beehive. He inspected it curiously, hesitantly. Hive bees were relatively new arrivals, part of an explosion of new forms of butterflies and beetles and other insects. The hive was abandoned, but there were whole handfuls of delicious honey to be had inside it.

But, before he attacked the honey, Noth listened carefully, sniffing the air. His sensitive nose told him that the others, high in the trees above, were still far away. He ought to be able to devour this food before they reached him. But he shouldn’t. There was a calculation to be made.

Noth was low-ranking among the males in his group. What Noth was expected to do was to call out, letting the rest know he had found food. Then the other males and the females would come, take as much honey as they wanted, and — if Noth was lucky — leave him a little for himself. If he stayed silent and was caught with the honey, he would be severely beaten, and any food left would be taken away, leaving him nothing at all. But on the other hand if he got away with it he might get to eat all the honey, and be spared any punishment…

The choice was made. Soon he was working the honey with his small hands, licking it down as fast as he could, eyes flicking around to check on the others. He had finished the honey and wiped away any traces on his muzzle by the time his mother reached the ground.

She still had her pup, Left, clinging to her belly. She began to scrabble at the floor, her fat-ladened tail held out behind her, silhouetted against the bright shafts of light that pierced the forest’s higher layers. She quickly uncovered more chunks of the fallen hive. Noth made a play of grabbing at the honey, but his mother pushed him away with a sharp shove and fell on it herself.

Noth’s father now tried to join in the bounty, but his mate turned her back on him. Here came two of Noth’s aunts, his mother’s sisters. They immediately rushed to their sister’s side and, with screeches, bared teeth, and handfuls of thrown leaves, drove Noth’s father away. One of them even grabbed a chunk of honeycomb from his hand. Noth’s father fought back, but, like most adult males, he was outsized by any one of the females, and his struggles were futile.

It was always the way. The females were the center of notharctus society. Powerful clans of sisters, mothers, aunts, and nieces, together for life, excluded the males. All this was a behavioral fossil: The dominance of females over males, and the tendency of male-female pairings to endure after mating, were more common in nocturnal species than those able to live in the light. This powerful matriarchy was making sure that the sisters had first call on the best of the food, before any male.

Noth took his own exclusion calmly. After all, the taste of illicit honey still lingered in his mouth. He loped away in search of more food.

Purga and Plesi had lived isolated lives, usually as females with pups, or as half of a mating pair. Solitary foraging was a better strategy for nocturnal creatures; not being part of a noisy group made it easier to hide from night hunters, who would wait in silent ambush for their prey.

But animals active by day did better to keep to groups, with more eyes and ears on the alert to spot attackers. The notharctus had even evolved alarm calls and scents to warn each other of different classes of predators — birds of prey, ground predators, snakes — each of which required a different defensive response. And if you were part of a group there was always the chance that the predator would take the next guy, not you. It was a cold-blooded lottery that paid off often enough to be worthwhile adapting for.

But there were disadvantages to group living: mainly, if there were large numbers of you, there was increased competition for food. As that competition resolved itself, the inevitable result was social complexity — and the size of the adapids’ brains had increased so that they were capable of handling that complexity. Then, of course, they were forced to become even more efficient at searching for food to fuel those big brains.

It was the way of the future. As primate societies became ever more complex, a kind of cognitive arms race would continue, increasing smartness fueled by increasing social complications.

But Noth wasn’t that smart. When he had found the honey, Noth had applied a simple behavioral rule: Call out if the big ones are close by. Don’t call if they aren’t. The rule gave Noth a good chance of getting away with maximum food and minimal beating. It didn’t always work, but often enough to be worth trying.

It looked as if he had lied about the honey. But Noth was incapable of telling genuine lies — planting a false belief in the minds of others — for he had no real understanding that others had beliefs at all, let alone that their beliefs could be different from his, or that his actions could shape those beliefs. The peekaboo game played with human infants — if you want to hide, just cover your eyes; if you can’t see them, they can’t see you — would have fooled him every time.

Noth was one of the most intelligent creatures on the planet. But his intelligence was specialized. He was a great deal smarter concerning problems about the others of his kind — where they were, their potential for threat or support, the hierarchies they formed — than about anything else in his environment. He couldn’t, for example, associate snake tracks with the possibility that he might stumble on a snake. And though his behavior looked complex and subtle, he obeyed rules as rigidly as if they had been programmed into a tribe of robots.

And still the notharctus spent much of their lives as solitary foragers, just as Purga had. It was visible in the way they moved: They were aware of each other, avoided each other, huddled for protection, but they did not move together. They were like natural loners forced to cooperate, uncomfortably imprisoned by necessity with others.

As Noth worked the forest floor, a troop of dark little creatures scurried by nervously. They had ratlike incisors, and a humble verminous look compared to Noth and his family, their black-and-white fur patchy and filthy. These little primates were plesiadapids: all but identical to Purga, even though she had died more than fourteen million years before. They were a relic of the past.

One plesi came too close, snuffling in its comparative blindness; Noth deigned to spit a seed at it; the seed hit the scuttling creature in the eye, and it flinched.

A lithe body, low-slung, slim, darted from the shade of the trees. Looking like a hyena, this was a mesonychid.

Noth and his family cleared off the ground quickly.

The plesi froze. But it was hopelessly exposed on this open forest floor.

The mesonychid hurled itself forward. The plesi squirmed and rolled, hissing. But the meso’s teeth had already taken a chunk out of its hind leg. Now more of the meso’s pack, scenting blood, came jostling toward the site of the attack.

The mesonychid was a kind of condylarth, a diverse group of animals related to the ancestors of hoofed animals. The meso was not an expert killer or a meat specialist but, like a bear or wolverine, it was an opportunistic feeder. All the condylarths were doomed to extinction ten million years before the age of mankind. But for now they were in their pomp, top predators of the world forest.

The other inhabitants of the forest floor reacted in their different ways. The lorislike adapid had a shield of thickened skin over bony bumps on its back, beneath which it now tucked its head. The big, dull barylambda concluded it was under no threat even from a pack of these small hunters; like the hyenas of later ages, the mesos were primarily scavengers and rarely attacked an animal much bigger than themselves. The taeniodont, however, decided that caution was called for; pompously it trotted away, its gaping mouth showing its high teeth.

Meanwhile the plesi fought on, inflicting scratches and bites on its assailants. One of the mesos was left whining, the tendons of its right hind leg badly ripped, blood leaking from torn flesh. But at last the plesi succumbed to their teeth and weight. The mesos formed a loose circle around their victim, their slim bodies and waving tails clustered around their meal like maggots around a wound. The rising stink of blood, and the fouler stench of panic shit and stomach contents, overwhelmed Noth’s sensitive nose.

Though some of the ancient plesiadapids had specialized, learning how to husk fruit like opossums or to live off the gum of trees, they remained primarily insect eaters. But now they faced competition from other insectivores, the ancestors of hedgehogs and shrews — and from their own descendant forms like the notharctus. Already the early-form plesis had become extinct across much of North America, surviving only in fringe areas like this marginally habitable polar forest, where the endless days did not suit bodies and habits shaped during the nights of the Cretaceous. Soon the last of them would be gone.

Noth, high in the cathedral calm of the trees, could see the family as they climbed up toward him, their lithe limbs working smoothly. But something disturbed him: a shift in the light, a sudden coldness. As clouds crowded past the sun, the great forest-spanning buttresses of light were dissolving. Noth felt cold, and his fur bristled. Rain began to fall: heavy misshapen drops that clattered against the trees’ broad leaves and pounded like artillery shells into the mud below.

It was because of the onset of the rain, and the overwhelming stink of the bloody deaths below, that Noth did not detect the approach of Solo.

Hidden in a patch of shadow, his scent blowing downwind, Solo saw the notharctus troop scurrying to safety.

And he saw Noth’s mother with her infant.

She was a fertile, healthy female: that was what the presence of the infant told him about her. But there was a mate with her, and since she already had a pup, she was unlikely to come into heat again this season. Neither of those factors were an obstacle to Solo. He waited until Noth’s family had settled on a branch, calming down, out of immediate danger.

Solo, at three years old, was a mature, powerful male notharctus. And he was something of a freak.

Most males roamed the forests in small bands, seeking out the larger, more sedentary troops of females where they might find a chance to mate. Not Solo. Solo preferred to travel alone. He was larger and more powerful than almost all the females he had encountered in his travels in this polar forest. Again, in this Solo was unusual; the average adult male was smaller than the average female.

And he had learned to use his strength to get what he wanted.

With a lithe swing Solo dropped down to the branch and stood upright before Noth’s mother. He looked unbalanced, for his hind legs were comparatively massive, his forearms short and slender, and he held his long tail up in the air so that it hooked over his head. But he was tall, and very still, and very intimidating.

Noth’s mother could smell this huge stranger: not kin. She immediately panicked. She hissed and pushed Left behind her.

Noth’s father came forward. He raised himself up on his hind legs and faced the intruder. Moving with fast jerky gestures he rubbed his genital glands against the foliage around him, and swept his tail over his forearms so that the horny spurs above his wrist glands combed through his tail fur and impregnated it with his scent. Then he waved the lustily stinking tail above his head at the intruder. In the scent-dominated world of the notharctus, it was an awesome display. Get away. This is my place. This is my troop, my young. Get away.

There was nothing sentimental in the father’s behavior. Producing healthy offspring that survived to breeding age was the only purpose of this father’s life; he was preparing to take on the intruder solely through a selfish drive to see his own heritage preserved.

Usually this game of malodorous bluff would have continued until one or the other of the males backed down, without physical contact. But again Solo was unusual. He did not respond with any form of display, save for a cold stare at the other’s feverish posturing.

Noth’s father was unnerved by the newcomer’s eerie stillness. He faltered, his scent glands drying, his tail drooping.

Then Solo struck.

With teeth bared he lunged at Noth’s father, slamming into his chest. Noth’s father fell back, squealing. Solo dropped to all fours and fell on him, biting into his chest through a layer of fur. Noth’s father screamed and scurried out of sight. He was only slightly injured, but his spirit was broken.

Now Solo turned on the females. The aunts could easily have resisted Solo, if they had combined their efforts. But they scrambled out of Solo’s way. Solo’s assault had disturbed them as much as its victim. They had never seen anything like it. All of them were mothers; all thought immediately of the infants they had left parked in the high branches.

Solo ignored them too. With a carnivore’s steely movements he advanced on Noth’s mother, his principal target.

She hissed, she showed her teeth, she even kicked at him with her powerful hind legs. But he resisted her blows easily, walked through her kicking — and took the unresisting, baffled infant from her grasp. He bit quickly into the pup’s throat, opening up the flesh, and rummaged there until he had ripped open the infant’s trachea. It was over in heartbeats. He dropped the quivering scrap into the forest below, where mesonychids, alerted by the scent of fresh blood, ran forward with their eerie uncaninelike barking. His mouth and hands bloodied, Solo turned to Noth’s mother. Of course she would not be fertile yet, perhaps not for some weeks, but he could mark her with his scent, make her his own, and repel the attentions of other males.

There was nothing truly cruel in Solo. If her pups were killed, it was possible Noth’s mother would come into heat again before the end of the summer — and if Solo covered her then, he could generate more offspring through her. So, for Solo, infanticide was a good tactic.

Solo’s brutal strategy wouldn’t have been sustainable for everybody. Notharctus males were not equipped to fight. They lacked the canine teeth that later primate species would use to inflict damage on rivals. And this polar forest was a marginal environment where true fights were literally a waste of energy, a squandering of scarce resources, which was why the ritual stink fights had evolved. But for Solo, the exception, it was a strategy that worked, over and over, and which had won him many mates — and which had generated many offspring, scattered through the forest, whose veins ran with Solo’s blood.

But it wasn’t going to work this time.

Noth’s mother, marked by the killer’s scent, gazed down into the green void below. She had lost her baby — just as Purga, her remote grandmother, had once endured. But, considerably more intelligent than Purga had been, she was much more acutely aware of her pain.

Blackness filled her. She lunged at Solo, her small limbs flailing, mouth gaping. Startled, he darted back.

She lunged past him. And she fell.

Noth saw his mother fall into the pit where his infant sister had fallen before. Immediately her twisting form was lost under the slick, writhing bodies of the mesos.

Noth had been weaned a few weeks after he had been born. Soon would have come a time when he would have wandered from the troop. His link to his mother was tenuous. And yet he felt a loss as powerful as if his mother’s breast had been ripped from his mouth.

And still the rain fell, harder all the time.

Noth, shivering, crawled through the branches. With the wind low, the rain fell in massive drops that pounded exposed flesh and hammered against the trees’ broad leaves.

Following lingering traces of his mother’s scent, he found his baby sister. She still clung motionless to the tree trunk where her mother had parked her — where she would have clung, probably, until she starved. Noth sniffed her damp fur. He huddled up close and wrapped his arms around her. She was a tiny shivering mass against his belly fur, but he was sheltering her from the rain.

He was drawn to stay with her. She smelled of family; she shared much of his genetic inheritance, and therefore he had a stake in any offspring she might one day have.

But the rain fell through a night and a day, as the sun continued its purposeless dance around the sky. The forest floor became sodden, and glimmering pools, laden with floating leaf debris, began to cover the ground, hiding gnawed and scattered bones.

And the continuing rain washed away the last traces of the scent markers of Noth’s troop from the trees. Noth and his sister were lost.

II

As the endless day wore on, as the sun wheeled through its meaningless cycles, Noth and Right stumbled through the forest’s branches.

They had already been lost for a week. They had found none of their own kind. But here in the forest canopy there were many adapids, cousins of the notharctus. Many of them were smaller than Noth. He would glimpse their glowing eyes, like eerie yellow pits, peering out of a shadowed nook. These miniature insect hunters looked more like mice. Some of them scuttled along branches, racing from shaded cover to cover. But one made a spectacular upright leap from tree to tree, its powerful hind legs dangling, its paws reaching. Its membranous ears swiveling like a bat’s, it caught an insect, plucking it out of the air in its jaws in midjump.

One solitary little creature clung to the rotten bark of an ancient tree. It had a scruffy black coat, batlike ears, and prominent front teeth, and it tapped patiently at the wood with a claw-tipped finger, its large ears swiveling. When it heard a larva burrowing under the bark, it ripped off the bark with its teeth and plunged in a peculiarly long middle finger to hook the larva and deliver it to its gaping, greedy mouth. This was a primate that had learned to live like a bird, like a woodpecker.

Once Noth blundered into a giant, slothlike creature hanging upside down from a thick branch, its primate’s hands locked around the wood. This monster’s head swiveled to inspect Noth and Right, its eyes blank. Its mouth chewed slowly, crammed full of the fat deciduous leaves that were its principal diet. Its kind had been driven to larger sizes by the need to accommodate a gut big enough to break down the cellulose in the leaves’ cell walls. The sloth-thing’s face was oddly immobile, static, limited in its expressiveness. The social life of this gloomy hanging creature was unexciting; its slow metabolism, and lack of spare energy to devote to social activities, saw to that.

The world had warmed steadily since the terrible impact. Waves of vegetation had migrated away from the equator, until tropical rain forest eventually covered all of Africa and South America, North America to what would become the Canadian border, China, Europe as far north as France, and much of Australia. Even at the poles there were jungles.

North America was still joined by mighty land bridges to Europe and Asia, while the southern continents lay in a great band below the equator, like scattered islands. India and Africa were both migrating north, but for now the Tethys Sea still girdled the equator, a mighty current that spread warmth around the belly of the planet. The Tethys was like a river through Eden.

In response to the great warming, the children of Plesi and other mammals had at last thrown off their past. It was as if the Earth’s inheritors had finally realized that the empty planet offered them a lot more than just another kind of grub to chew. While the reptilian survivors, the lizards, crocodiles, and turtles, clung on largely unchanged, soon the foundations of the successful mammalian lineages of the future would be laid down.

Plesi, like Purga, had been a low-slung crawler, with the typical mammalian four-footed head-down body stance. But her primate descendants grew larger, with more powerful hind limbs to support upright bodies and heads. Meanwhile the primates’ eyes had moved forward to the front of their faces. This would give them three-dimensional vision, enabling them to judge their increasingly long leaps, and to triangulate on the prey insects and small reptiles that still formed part of their diet. And as they explored different ways to make a living, the primates would fan out into many different forms.

There was no design in this: no sense of improvement, of purpose. All that was happening was that each organism was struggling to preserve itself, its offspring, and its kin. But as the environment slowly changed, so through relentless selection did the species that inhabited it. It was not a process fueled by life, but by death: the elimination of the less well adapted, the endless culling of inappropriate possibilities. But the potential of an unseen future was no consolation to those who lived through the relentless culling.

Many of the adapids had become too specialized. This comfortable planet-swaddling warmth would not last forever. In cooler times in the future, as the forests became sparse and seasonal differences became more pronounced, it wouldn’t seem so smart to be a fussy eater. Extinctions would follow, as they always had.

Meanwhile, amid this clutter of exotic primates, the siblings found no notharctus.

Exploring the forest floor, Noth found a plant with podded fruit, a kind of pea. He broke open a few pods and let his sister feed.

A kind of anteater, a meter long, approached a pillarlike ants’ nest. It fell on the nest, wielding its powerful arm and shoulder muscles. As though it wielded a pickaxe, all its force was concentrated on a single point, the tip of its strongly flexed middle finger. The ants swarmed — they were huge, each some ten centimeters long — and the anteater quickly ingested them with its long, sticky tongue before the soldiers could unite in defense. The anteater was a descendant of South American stock, which had wandered here over temporary land bridges many generations before.

Noth and Right watched, wide-eyed. But as he kept an eye on the anteater, concern gnawed at Noth’s unconscious.

He had tried to keep them both feeding, to fatten up their tails with the winter storage that would see them through the long months of hibernation to come. That was just as his innate programming instructed. But they weren’t eating enough. Isolated from the support of the troop, he was having to spend much too much of his time watching for predators.

He could have gone back. Like all his species — the mobile males more than the sedentary females — he kept track of his position by dead reckoning, integrating time, space, and the angle of the slanting sunlight. It was an ability that helped him find scattered sources of food and water. If he needed to Noth could find his way back “home,” to the stand of trees that had been the center of his troop’s range. But he never heard the distinctive warbling song of his troop; his rudimentary decision-making machinery pressed him to keep searching for a troop that would accept him and his sister.

Meanwhile, though the sun still circled endlessly above the horizon, much of the daylight was tinged with the red of sunset, and here on the forest floor brown spores clung to the fern fronds. Autumn was coming. And then there would be winter. They were underfed, and time was running out.

Right became distressed, as she so often did. She dropped the pea pods and folded over on herself, rocking, keening softly, her hands over her small face. Noth took her in his arms and carried her to the crook of a branch, where he began to groom her. He worked carefully through the sparse fur on her back, neck, head, and belly, removing dirt, bits of leaf, and dried feces, untangling knots, picking out parasites that were attempting to feast on her young skin.

Right quickly calmed. The grooming’s mixture of pleasure, attention, and mild pain flooded her system with endorphins, her body’s natural opiates. Before she grew much older she would be addicted, literally, to this pleasurable scratching — as her brother already was. Noth badly missed the strong, nipping caress of adult fingers on his back.

But Noth was worried about her, on deep levels he could not understand.

Right’s bewildering grief served a purpose. It was a signal to her that she had suffered a loss, that there was a hole in her world that she must fix. And though Noth was not capable of true empathy — if you didn’t really understand that other people had minds and thoughts and feelings like yours, you couldn’t possibly be empathetic — still the signs of grief in his sister triggered a kind of protectiveness in him. He wanted to put the world right for his sister: The instinct to help the orphaned went very deep.

But in the end obsessive grief was maladaptive. If Right was unable to recover, in the end there would be nothing he could do for her. He would have to abandon her, and then she would surely die.

As day followed day, the sun, at the lowest point of its arc in the sky, began to slip beneath the southern horizon. At first the brief nights were like twilight, and on clear nights purple-red curtains of light climbed into the tall sky. But quickly the sun’s excursions into invisibility became longer, and there were increasing intervals when stars shone in a deepening blue. Soon true darkness would return to the polar forest.

The weather quickly became colder and drier. Rainfall was scarce now, and on some days the warmth of the sun barely seemed to penetrate the lingering mist. Already many of the birds of the forest canopy had departed, skein after skein of them flitting over the sky to the warmer lands to the south, watched by uncomprehending primate eyes.

Noth became exhausted, ragged, and his dreams were full of flashing teeth and biting claws, visions of his scrap of a sister taken by gigantic mouths.

Now their biggest problem was thirst. It had been so long since the last rain that the treetops were becoming parched. And already the trees were starting to shed; the last leaves were withered and brown. Soon Noth was reduced to licking the bark each morning for the cold dew.

At length, driven by their thirst, the siblings went in search of ground water. Near the closest large lake they scurried down a tree trunk, eyes wide.

Approaching the water, the primates crept past a pair of what looked like miniature deer. The size of small dogs with long, trailing tails, these fast, solitary runners, browsing on leaves and fallen fruit, were ancestors of the mighty artiodactyl family, which would one day include pigs, sheep, cattle, reindeer, antelope, giraffes, and camels. Right disturbed a frog, which hopped away, croaking in protest. She cowered back, eyes wide at its strangeness. Soon they saw more amphibians, frogs and toads and salamanders. Birds crowded the bushes, raising shrill cries that filled the dank air.

Noth was uneasy. The shore was too crowded: Noth and Right were not the only thirsty creatures in this shivering jungle.

A meter-long creature like a long-tailed kangaroo ran past; this was a leptictidium, a hunter of small animals and insects. Exploring the ground with its mobile nose, it disturbed a pholidocercus, a spiky-haired ancestor of the hedgehogs, that indignantly hopped away like a rabbit. Here was a close-packed herd of horses. They were tiny: no larger than terriers, with perfectly formed equine heads. Shyly these exquisite little creatures picked their way through the undergrowth. They walked on pads, like cats, and on each foot they had several hoofed toes. Their genus had emerged in Africa only a few million years earlier. A rough growl from an impatient carnivore startled the little horses, making them stir into sudden flight.

Through this exotic crowd the two primates proceeded cautiously, moving in scurries, in fits and starts.

The water itself was a languid sheet, dense with matted vegetation, dead reeds, and algal blooms. In places ice had already formed in thin gray slices. But on the open water birds waded, ancestors of flamingos and avocets, and huge water lilies rested languidly on the surface.

Over the open water a spider was suspended on a thread of silk, and huge ants flew, each as large as a human hand, on their way to found new nests. Through this crowd of insects flapped a family of delicate bats. Recently evolved, as huge and fragile as paper kites, the new flying mammals snapped at the insects. Primitive bony fish broke the surface and gulped at the aerial fodder, as did a twisting eel.

The primates found a place far enough from any of the predators to be able to drink unhindered. They bent and plunged their muzzles into the chill water, sucking it up gratefully.

The largest animals of all wallowed at the muddy fringes of the lake.

A pair of uintatheres stood side by side. These great animals looked like gargantuan rhinos, each with a set of six bony horns on its head and long upper canine teeth like a saber-toothed cat’s. Their thick hides were coated with mud, which helped keep them cool and kept off insects. They cropped placidly on the soft vegetation of the lake bottom, sucking at water stained green by algae, while a fat youngster, more agile and lively, played around his parents’ legs, barging their tree-trunk knees with a head ladened with stubby, unformed tusks. Noth watched their huge feet fearfully. Closer to the shore there walked a family of moeritherium. No more than a meter tall, the adults moved through the water with a stately calm, rumbling reassurance to each other, while their round-bodied infants splashed at their feet. They worked the lake bottom vegetation efficiently with their long noses. These were among the first proboscideans, the ancestors of elephants and mammoths. They were still more piglike than elephantine, but they were already clever and social animals.

Around the herbivorous herds circled carnivores. These were mostly creodonts; they looked like foxes and wolverines. And there was one pack of hoofed predators — like carnivorous horses — bizarre, terrifying creatures with no analogies in human times.

Many of these creatures looked slow and lumbering, oddly ill-formed, the results of nature’s first experiments in producing large herbivores and predators from the mammalian stock that had survived the dinosaur extinction. Open grasslands still lay millions of years in the future, along with the fleet, long-legged, graceful herbivorous forms that would adapt to their open lush spaces, and the cleverer, faster carnivores that would arise to prey on them. When that happened most of the species around Noth would succumb to extinction. But the orders that would be familiar to humans — the true primates, the hoofed animals, the rodents and bats, the deer and the horses — had already made their entrance on the stage.

And there was no more complex and crowded an ecology anywhere on Earth right now than here on Ellesmere Island. This place was a pivot on the great migratory routes up through the Americas and over the roof of the world to Europe, Asia, and Africa. Here, pangolins from Asia, carnivores from North America, hoofed creatures from Africa, European insectivores like ancestral hedgehogs, and even anteaters from South America mingled and competed.

Suddenly Noth pulled back his head.

From inside the water two primates were looking out at him, a burly male and a small female. He could not smell the male, could not tell if he was kin or stranger. He screeched, baring his teeth. The male primate bared his teeth in response.

Enraged, Noth got to his feet and displayed his musk glands to the stranger in the water — who displayed back, angering him further — and then he stamped at the water until the reflected notharctus was gone.

Noth could recognize others of his species, could distinguish them as male or female, and as kin or not kin. But he could not recognize himself, for his mind did not contain the ability to look inward. All his life he would feel threatened by any such chance reflection.

A sleek form burst from the water itself and came lurching up on clumsy flippered limbs onto the rocky platform. Noth and Right stumbled back. Over a snout like a crocodile’s the newcomer gazed at the two baffled primates.

This ambulocetus was a relation of the hyena-like mesonychids. Like an otter, it was covered in sleek black fur, and it had large, powerful back legs equipped with toes ten centimeters long. Ages ago this animal’s ancestors had returned to the water, seeking a better living, and selection had begun its relentless molding. Already the ambulocetus looked more aquatic than terrestrial.

Soon its kind would take permanently to the oceans. Its skull and neck would become shorter, and the nose migrate backward, while its ears would close so that sound would have to pass through a layer of fat. Its legs would morph at last into fins, with more bones added, the fingers and toes becoming shrunken and useless, at last disappearing. When it reached the vast spaces of the Pacific and Atlantic, it would begin to grow — ultimately becoming as large compared to its present form as a human was to a mouse — but those mighty seagoing descendants would still retain within their bodies, like fossils of bone and molecular traces, vestiges of the creatures they had once been.

The walking whale stared uncomprehendingly at the two timid primates. Deciding this crowded shore wasn’t such a good place to bask after all, it flexed its spine and swam gracefully away.

As the light faded, Noth and Right retreated to the shelter of the trees. But the branches were now all but bare, and cover was hard to find. They huddled in a branch’s crook.

The herbivores splashed out of the water, family groups calling to each other. And the predators began to call, harsh doglike barks and leonine growls echoing through the sparse forest.

As the chill settled deeper Noth felt torpor steal over him. But he felt cold, stuck here like this with only his baby sister, cold away from the huddle of his troop.

And then, to his surprise, he was startled awake by a powerful musk scent.

Suddenly there were notharctus all around. They were on the branches above and below him, huddled shapes with their legs drawn up beneath them and their long, fat tails dangling. Their scent told him this was his kind, but not his kin. He had not detected their scent markings before; in fact the markings were sealed in by layers of frost. But the strange notharctus had noticed him.

Two powerful females gathered closely, drawn by the scent of an infant. One, who he thought of as Biggest, pushed aside the other — who was merely Big — to get a closer look at Right.

Noth’s mind churned. He knew that it was vital that they be accepted by this new group. So he reached for the female closest to him, Big, and began, tentatively, to dig his fingers into the fur at the back of her legs. Big responded to his grooming, stretching out her legs with pleasure.

But when Biggest saw what was going on she hooted and slapped them both. Noth cowered, trembling.

Noth was bright enough to understand his own place on the social ladder — in this case, down on the bottom rung. But his social mentality had its limits. Just as he could not detect the beliefs and desires of others, so he was not smart enough to form judgments about the relative ranking of others in a group. He had got it wrong: Biggest outranked Big, and she expected this new male to pay her attention first.

So Noth waited as Biggest played with the drowsy Right. But at least she did not drive him away. And at length Biggest let Noth approach her and groom her own dense, rank-smelling fur.

III

Every day was shorter than the last, every night longer. Soon there were just a few hours of bright daylight, and the intervals between the darknesses were lit only by a pink-gray twilight.

The forest was all but silent now. Most of the birds and the large herbivore herds had long gone, migrated south to warmer, easier climes, taking their dinning cries with them. The buzzing insect swarms of high summer were a memory, leaving only larvae or deep-buried eggs, sleeping dreamlessly. The big deciduous trees had already dropped their broad leaves, which lay in a thick litter on the ground, welded together by the persistent frost. The bare trunks and leafless branches would show no signs of life until the sun returned in a few months’ time. Beneath them, plants like the ground fern had died back to their roots and rhizomes, soon to be sealed into the earth under a lid of frost and snow.

The species here — derived from ancestral stock adapted to the balmy conditions of the tropics — had had to make ferocious adjustments to survive the extreme conditions at the pole. Every plant, wherever it lived, depended on sunlight for energy and growth, and during the endless days of summer the vegetation had lapped up the light with broad, angled leaves. But now there approached a season when for months there would be no light but that of the Moon and stars, useless for growth: If the plants had kept on growing and respiring they would have burned up their energy store. So the flora were heading for a vegetable hibernation, each according to its own strategy.

Even the plants were sleeping.

The notharctus troop was thirty strong, and they had huddled in the branches of a big conifer. They looked like big furry fruit, their hands and feet clinging to the branches as they slept, their faces buried in their chests, their backs exposed to the cold. Frost sparkled on their new winter coats, and where a muzzle showed breath steamed, glowing blue white.

Noth slept away the lengthening nights, his fur bristling, immersed in the body heat of the others of the troop. Sometimes he dreamed. He saw his mother fall into the jaws of the mesos. Or he was alone in an open space surrounded by hard-eyed predators. Or he was like a pup again, pushed out of a troop by adults bigger and stronger than he was, excluded by rules of which he had no innate understanding. But sometimes the dreams faded, and he fell into a kind of torpor, a blankness that prefigured the long months of hibernation to come.

Once he woke in the night shivering, his muscles involuntarily burning energy to keep him alive.

The sleeping world was full of light: the Moon was high and full and the forest glowed blue white and black. Long, sharp shadows striped the littered floor, and the vertical trunks of the leafless trees gave the scene an eerie geometrical precision. But the tangled branches higher up were a more complex and dismal sight, bone bare and glimmering with frost, a harsh contrast to the warm green glow of the leaves of high summer.

In its way it was a beautiful scene, and Noth’s wide archaic eyes served him well, revealing to him detail and subtle colorations that would have been invisible to any human. But all Noth perceived was a lack: a lack of light, of warmth, of food — and a lack of kin in this group of strangers, save for his sister, whose still-growing body was buried somewhere in the huddling troop. And he knew on a deep cellular level that the true winter had yet to begin, long, drawn-out months of a kind of slow agony as his body consumed itself in order to keep him alive.

He squirmed across the branch, trying to force his way deeper into the group. Each of the adults knew that it was in everybody’s long-term interests that she should take her turn at the edge of the group, briefly suffering the cold in order to shelter the rest; it wouldn’t help to have outliers die of frostbite. But still Noth’s lowly rank worked against him, and when the other males picked up his scent they sleepily combined to push him back out of the huddle, so he finished up almost as exposed as when he had started.

He lifted his muzzle and puffed out a breath, hooting mournfully.

These primates could draw no comfort from those around them. Noth found pleasure in grooming — but only in his own physical sensations, and in the effect it had on others’ behavior toward him, not in how others felt. The other notharctus were simply a part of his environment, like the conifer trees and podocarps, the foragers and predators and prey: nothing to do with him.

These huddling notharctus, despite their physical closeness, were each lonelier than any human would ever be. Noth was forever locked inside the prison of his head, forced to endure his miseries and fears alone.

The morning dawned clear, but a freezing mist lay over the forest. Even though the sun grew bright there was little heat to be had from its rays.

The notharctus stretched limbs stiff with cold and long hours of immobility. Cautiously, watchfully, they headed down to the ground. On the forest floor they scattered slowly. The senior females moved around the edge of this loose clearing, using their wrists, armpits, and genitals to renew scent markers.

Noth picked through the frozen mulch. The dead leaves were of no use to him, but he had learned to burrow under places where the leaf litter was particularly thick. The mulching leaves could trap moisture and keep the frost away; here there was dew to lap up, and unfrozen ground to dig in search of tubers, roots, or even the rhizomes of hardy ferns.

A series of hooting cries broke out, startlingly loud, echoing through the forest. Noth looked up, whiskers twitching.

There was a commotion around a stand of podocarp. Noth saw that a group of notharctus, strange females with a scattering of pups, had come out of the forest. They were approaching the podocarp.

Biggest and some of the other females dashed forward. The troop’s big dominant male — who Noth thought of as something like the Emperor — joined in the females’ charge. Soon they were all displaying ferociously, hooting and scraping musk over their long tails. The strange females cowered back, but they responded in kind. The forest briefly filled up with the cacophony of the argument.

The female clans, the heart of the notharctus’ society, were fiercely territorial. These strange females had ignored the scent markers left by Big and the others, bright warning signs in a notharctus’ sensorium. At this time of year food was becoming short; in the final scramble to stoke up their bodies’ stores for the rigors of winter, a rich stand of podocarp was worth fighting for.

The females, with babies clinging to their fur, went further in their wars than their males were prepared to. They quickly escalated the confrontation to lunges and feints and even slashes with canine teeth. The females fought like knife fighters.

But it wasn’t going to work. Though not one notharctus laid a paw on another, the display by Biggest and the rest overwhelmed the newcomers. They backed off toward the long gray-brown shadows of the deeper forest — though not before one older pup had lunged forward, cheekily sunk his teeth into a cold-wizened fruit, and run off with his bounty before he could be stopped.

Suddenly aware of the vulnerability of their treasure, the females closed around the podocarp now, munching greedily at the fruit. Some of the older, more powerful males, including the Emperor, were soon feeding alongside Biggest and the rest. Noth, with the other young males, circled the feeding group, waiting his turn at whatever would be left.

He dared not challenge the Emperor.

Male notharctus had their own complex and different social structure, overlaying that of the females. And it was all about mating, which was the most important thing — the only thing. The Emperor had a large territory, including the ranges of many female groups. He would aim to mate with all the females in his territory, and so maximize his chances of propagating his genes. He would scent mark females to repel other suitors. And he would fight fiercely to keep other strong males away from his wide empire, just as Noth’s father had fought to exclude Solo.

The Emperor had done well to hold on to his wide-ranging fiefdom for more than two years. But like all of his short-lived kind, he was aging quickly. Even Noth, the lowliest newcomer, made endless automatic computations of the Emperor’s strength and fitness; the drive to mate, to produce offspring, to see his own line go on, was as strong in Noth as in any of the males here. Soon the Emperor would surely meet a challenge he could not withstand.

But for now, Noth was in no position to challenge the Emperor or any of the stronger males in the loose pecking order above him. And he could see that the supply of podocarp fruit was dwindling rapidly.

With a frustrated hoot he hurried over the forest floor and scampered briskly into a tree. The branches, slippery with residual frost, dew, and lichen, were all but bare of leaves and fruit. But it might still be possible to find caches of nuts or seeds, stashed away by providential forest creatures.

He came to a hollow in an aging tree trunk. In its dank, rotting interior, he saw the gleam of nutshells. He reached in with his small, agile hands and hauled out one of the nuts. The shell was round, seamless, complete. When he rattled it he could hear the kernel inside, and saliva spurted into his mouth. But when he bit into the shell his teeth slid over the smooth, hard surface. Irritated, he tried again.

There was an almighty hissing. He hooted, dropped the nut, and scuttled to a higher branch.

A creature the size of a large domestic cat came scrambling clumsily toward the nut cache. It raised its head to Noth and hissed again, showing a pink mouth with powerful upper and lower incisors. Satisfied that it had driven off the raider, it dug out one of its stored nuts and, with a clench of its powerful jaws, cracked the shell. Soon it was nibbling purposefully, widening the hole it had made. At last it reached the nut’s kernel — Noth, tucked behind the tree trunk, was almost overwhelmed by the sudden sweet aroma — and fed noisily.

This ailuravus looked something like a rudimentary squirrel, with a mouselike face. It had a long bushy tail, the purpose of which was to slow its fall, parachute-like, every time it tumbled out of a tree, as it frequently did. Although it was clumsier than the notharctus as it moved about in the trees, lacking a primate’s grasping hands and feet, it was more than big enough to have fought off Noth.

The ailuravus was one of the first rodents. That vast, enduring family had emerged a few million years earlier in Asia, and had since migrated around the world. This small encounter was a skirmish at the start of an epochal conflict for resources between the primates and the rodents.

And the rodents were already winning.

They were beating primates to the food, for one thing. Noth would have needed a nutcracker to eat hazelnuts or brazil nuts, and a millstone to process grains like wheat and barley. But the rodents, with their ferocious, ever-growing incisors, could break through the toughest nut and grain seed coats. Soon they would begin to consume the fruits of the best trees before they were even ripe.

Not only that, the rodents outbred the primates by a large factor. This ailu could produce several litters within a single year. Many of its young would fall to starvation, competition with their siblings, or predation by birds and carnivores. But it was enough that some should survive to continue the line, and for the ailu each of its young represented little investment — unlike the notharctus who bred just once a year and for whom the loss of a single pup was a significant disaster. And the rodents’ vast litters incidentally offered up much raw material to the blind sculptors of natural selection; their evolutionary rate was ferocious.

Even though primates like Noth were much smarter than rodents like the ailu, his kind could not compete.

It wasn’t just the plesiadapids that were becoming rare in North America. It was no coincidence that Noth’s kind had been pushed up into this marginal polar forest. In the future Noth’s line would migrate further, passing over the roof of the world to Europe and thence to Asia and Africa, adapting and reshaping as they went. But in North America the rodent victory would, within another few million years, be complete. A new ecology would arise, populated by gophers, squirrels, pack rats, marmots, field mice, and chipmunks. There would be no primates in North America: none at all, not for another fifty-one million years, not until human hunters, very distant descendants of the notharctus, came walking over the Bering Strait from Asia.

When the rodent was done feeding, Noth crept cautiously out of his hiding place. With his agile hands he sought out the scraps of kernel the ailu had dropped, and crammed them into his mouth without shame.

For a few hours a day the southern sky still grew bright. But the sun made its cycles beneath the horizon now. Almost all of the lakes were frozen over, and the trees were laden with frost, some of it gleaming in thick lacy shards where mist had frozen out on spiderwebs. The notharctus’ movements through the trees and over the silent forest floor were sluggish and dull. But it didn’t matter; the forest could offer them little more food this autumn.

There came a last clear day, when layers of red-tinged cloud stacked up against a violet southern sky, and the purple-green aurora rolled like a vast curtain over the stars.

The notharctus hurried to the ground and began to dig in places where the soil had been kept unfrozen by layers of leaves or under the roots of trees. Tonight would be the hardest frost of the winter so far, and they all knew it was time to get under cover. So the primates dug, building burrows in which Purga would have felt comfortable. It was as if the brief interval in the trees had been nothing but a dream of freedom.

In the deepest dark, Noth pushed his way through tunnels quickly worn smooth by the passage of primate bodies and over a floor littered with loose fur. At last his powerful nose guided him to Right.

Gently Noth sniffed his sister. She was already dormant, curled tightly up with her tail wrapped around her, close to the belly of Big. She had grown during their months with Biggest’s troop, but Right would always be small, always retain traces of the runt who had been bullied by her now-dead twin. Still her winter fur seemed sleek, healthy, and free of knots and dirt, and her tail was fat with the store that should sustain her through the winter.

Noth felt a kind of satisfaction. Given their dreadful start to the summer, they had both beaten the survival odds better than expected. With no pups of his own, this was still the only kin Noth had — his entire genetic future depended on Right — but for now there was nothing more he could do for her.

In darkness, immersed in the scents and subtle noises of his kind, Noth snuggled as close as he could to his sister. He shut his eyes, and was soon asleep.

Briefly he dreamed, of fragments of summer light, of long shadows, of his mother’s fall from the trees. And then, as his body shut down, his mind dissolved.

IV

The sun’s rays, almost horizontal, shone like searchlights into the forest. Above the slowly melting ponds a chill mist hovered, shining in elaborate pink-gray swirls, pointlessly beautiful. From the gaunt tree trunks immense black shadows stretched north. But the first leaves were budding on the bare branches, tiny green plates hanging almost vertically to catch the sun’s light. The leaves were already at work: The days of spring and summer were so short that these hardy vegetable servants had to gather every droplet of light they could.

It was just a glimpse, a dawn that would last no more than a few minutes. But it was the first time for several months that the disk of the sun had shown.

The forest was quiet. The great herbivorous migrants were still hundreds of kilometers to the south; it was weeks yet before they would come back, seeking their summer feeding grounds, and even the birds had yet to arrive. But Noth was already awake, already out, working.

Fresh from his burrow he was gaunt, his tail flaccid and drained of fat. His fur, ragged and stained yellow by urine, hung around him in a cloud, lit up by the sun, making him look twice his true size. There was still little fodder to be had in the trees, so he had to scurry over the littered, frosty ground. After the winter’s chill, it was as if nobody had ever lived here, and everywhere he moved he marked rocks and tree trunks with his musk.

All around him, in grim competition, the males of the troop were foraging. They were all adults: Even those born less than a year ago were approaching their full size, while relative veterans like the Emperor himself, approaching his third birthday, moved more stiffly than last year. After their winter of starving sleep, all of them looked sick, and the lingering cold bit hard through their loose fur and into their fat-deprived bodies.

There were risks in moving around so early. In the burrows, the females still slept, consuming the last of their winter stores. The predators were already active — and as food was still scarce, early-bird primates made a tempting target. If one of the males did find an unexpected cache of food he was quickly surrounded by snapping, jealous rivals, making the empty forest echo to their hoots and yips.

But Noth had no choice but to risk the cold. The days of breeding were approaching, a time of ferocious competition for the males. Noth’s body knew that the sooner it laid down a store of strength and energy for the battles to come, the better chance he would have of finding a mate. He had to accept the risks.

Navigating with a blurred recollection of the landscape map he had built up last season, Noth made his way to the largest of the nearby lakes.

The lake was still mostly frozen, covered by a lid of gray ice littered with loose, hard-grained snow. A pair of ducklike birds, early immigrants, padded over the ice, pecking hopefully at its surface. Beneath the gray Noth could see the chill blue of older ice, a lens of deep-frozen material that had failed to melt through last summer, and would likewise fail to melt this year.

Close to the water’s edge he passed a gray-white bundle. It was a mesonychid. Like the Arctic fox of later times, it endured the winter above ground. But in a sudden cold spell during the winter this meso had become lost in a blizzard, and, succumbing to exposure, had died, here at the shore of the lake. Its body had quickly frozen, and for now appeared perfectly preserved. But as it thawed the bacteria and insects had begun feasting: Noth could detect the sweet stench of decay. Saliva spurted into his mouth. The half-frozen meat would be good, and maggots were a salty treat. But his thirst outweighed his hunger.

Near the lake’s shallow, muddy shore the ice was thin and cracked, and Noth could smell dank open water. The water was greenish, already full of life, and littered by grayish chunks of old ice cover. Noth dipped his muzzle into the water and drank, straining out the worst of the mucous slime between his teeth.

He could see that the open water bulged with clusters of small clear gray spheres: the spawn of the lake’s amphibian inhabitants, laid down as early as possible. And closer by, in the shallows at his feet, Noth made out tiny wriggling black forms: the first tadpoles. He ran his hands through the water, letting the slime cling to his palms, and crammed the slippery harvest into his mouth.

With a flexing strain his bowels moved, and watery shit pooled beneath him.

But now the surface of the water broke, the ice cracking with sharp reports. Something huge was coming out of the lake. Noth scurried back to the cover of the nearest trees, eyes wide.

Like Noth, the crocodile had woken early, disturbed from its slumber by the brightness of the day. As it rose from the lake, bits of ice tumbled from its back. With a single graceful motion it clamped its jaws on the frozen meso: frost crackled, bones crunched. Then the croc slid backward into the water, dragging the carcass effortlessly, making barely a sound.

The crocodile was hungry.

Before the comet the largest animals in each of the world’s ecologies had been reptilian: the plesiosaurs and ichthyosaurs in the oceans, the dinosaurs on land, and the crocodilians in fresh water. The disaster had wiped away these great families, and in their empty realms they would soon be replaced by functionally equivalent mammals — all save the crocodiles.

The freshwater environment had always been a difficult place to live. While the supply of plant material on land and in the sea was pretty reliable in space and time, freshwater environments were very variable. Erosion, abrasion, silting, flood, drought, and extremes of water quality were all hazards.

But the crocodiles — and other enduring freshwater species like turtles — were resilient. Some learned to walk overland in search of water. Others could take to the sea. Or they would just bury themselves eight or ten meters deep in the mud, and wait for the next cloudburst. And as for food, even during the worst of the killings on land and in the sea, they would subsist off the nutrients that continued to leach off the corpse-littered land, a “brown” food chain that persisted long after the green, growing things and the creatures that browsed them had died.

In this way the crocodiles had survived across a hundred and fifty million years, through extraterrestrial impacts, glaciation pulses, sea level changes, tectonic upheavals, and competition from successive dynasties of animals.

After all this time they were still capable of evolutionary novelty. Briefly, after the comet impact, the top predators around the water courses had been crocodile cousins with long legs and hooflike claws. They had been a nightmare, running predatory crocodiles capable of chasing down animals as large as small horses. Crocodiles had even adapted to survive here at the pole, where the sun didn’t shine for months on end; they would simply wait out the winter months in deep hibernation.

Unlike the dinosaurs, unlike the plesiosaurs, the crocodiles would not be forced out of their freshwater niches by upstart mammals: not now, not ever.

Noth had lost the meso carcass, but some scraps of flesh and crushed maggots were smeared over the ground where it had lain. Hungrily he licked at the frozen ground.

At last the days of breeding arrived.

The females of the troop gathered in the branches of one tall conifer. They were feeding on ripe young fruit, cramming their bodies with the resources they would need to survive the drain of motherhood to come. The females were loosely marshaled by the more senior among them, including Big and Biggest. Right was among them. She had survived her first winter. She was filling out quickly, and when her scrappy winter fur had blown away she had emerged as a small but elegantly built adult, ready to mate.

The Emperor himself was among his female subjects. He moved from one to the other, heroically humping. Already he had been accepted by Biggest twice, and had deflowered an unprotesting Right. Now he was taking Big. She was bent over, clinging to a low branch, her head tucked between her knees, her tail uplifted. The Emperor was behind her, arms wrapped around her waist, hips thrusting with a rapidity born of exhaustion and urgency.

This was the day toward which the Emperor had worked all year, and now was the time for him to spend all of his authority and energy by covering as many of the females as possible.

But the Emperor was already tiring. And this female troop was only one of several in the wider territory he commanded.

In this ferociously seasonal place, baby rearing had to be squeezed into a drastically short period, so that offspring were produced when food was abundant and their new mothers could eat enough to produce plenty of milk. Any female who mated outside the breeding season was unlikely to see her offspring survive to adulthood. And any male who missed the chance to mate with a fertile female would have to endure a whole year of hardship, danger, and privation before getting another chance.

For the notharctus, the breeding season was just forty-eight hours long. It was a frantic time.

Today, the start of the females’ simultaneous estrus, the air was full of an invisible pheromone cloud, and there were males everywhere, helplessly drawn, erections poking out of their fur. Every male had prepared since the return of the sun, feeding to build up his strength, practicing spectacular tree swings and engaging in mock battles: They had been like athletes preparing for a contest. It was impossible for the Emperor to keep them all away, and there was intense competition. Today the hierarchy of males was stressed to the point of collapse.

The stress on the females would come later, during pregnancy and nursing, when the fast-growing fetus or newborn pup demanded the mother find a stream of high-energy food — and she must eat well at a time when almost every other adult female was nursing too. It was the heavy cost of reproduction that had led to the general dominance of females over males, and it was the reason why the females always got the best of the food.

It was the same all over the forest. Every notharctus troop was hitting its brief mating season simultaneously, the timing dictated by the invisible chemical scents that permeated the air for kilometers around. For today and tomorrow, the forest was filled with primate lust: a tremendous clamor of battling males, pheromone-laden females, and frantically thrusting hips.

Noth, pursuing another young male he thought of as Rival, hurled himself through a loose stand of conifers. He swung one-armed on spindly branches. On each dip the earth tipped up like a vast bowl, dead leaves and new green ferns and the dull forms of snuffling ground feeders fleeing under him.

He approached a gap between two tall trees. On the far side he saw Rival, standing upright, genitals pinkly visible, rubbing his scent markers on the bark. Rival barked a contemptuous challenge.

Without hesitating, Noth took a final huge swing. The branch flexed and hurled him on a smooth parabola high into the air. For a few heartbeats he flew, tail held high, hands and feet held out before him ready to grasp.

His head was filled with the stink of estrus. He had had an erection since he had woken this morning. Even now, as he sailed from tree to tree, his penis stuck out before him, pink and solid. He had yet to succeed in battling his way through the crowding males to get to a receptive female, and he felt as if his belly would burst if he didn’t succeed soon. But even as he was consumed with inchoate lust, still he relished the power of his lithe body as he hurled it through the forest domain for which it was exquisitely adapted.

Noth had never felt so alive.

Noth landed in Rival’s tree, just where he had aimed. He grasped the branches with faultless positioning of his hands and feet. But immediately Rival was on him.

Facing each other they stood upright, their spindly erections poking out. Noth, tail held erect, stalked toward Rival, vigorously rubbing his groin against the tree bark, chattering and barking. Rival responded in kind. It was a ritualized encounter, each of them responding to the other’s movements in a kind of dance: tail waft followed by groin rub, wrist spread provoking a spitting glare.

Soon the air was filled with their angry stink. They came close enough for Noth to feel the tips of the other’s bristling fur, and Rival’s spittle sprayed his face.

Rival was about the same age as Noth, about the same size. He had joined the troop a little earlier than Noth and his sister. To him, Noth had invaded a troop that he had come to regard as “his.” Noth and Rival were too similar, like brothers, too close to be anything but enemies.

Rival was marginally bigger and heavier than Noth, and if anything he had done better in the early-season feeding. But Noth’s difficult year had forged an inner toughness, and he stood his ground.

Psychology won out. Rival subsided suddenly, his display collapsing. He turned his back on Noth and, briefly, symbolically, displayed his pink backside in a curt gesture of submission.

Noth hooted, relishing his moment. Briskly he rubbed his wrists over Rival’s back, marking his victory with his scent, and released a stream of urine. Then he let Rival slink away along the branch toward a cluster of berries.

Rival would come to no harm. He would skulk alone in his tree for a time, perhaps feeding, withdrawing for a while from the fray. But his chances of mating were for a few hours reduced. Noth’s urine would make him briefly sterile; it would even reduce his ability to make the special trilling calls used by the males to attract females.

For Noth it was a valid strategy. Today it was impossible for any male, however heroically he tried, to cover all the females. But he could reduce the number of competing males with such sensory intimidation.

With Rival defeated, Noth’s penis throbbed anew; soon he would at last attain the satisfaction he craved. With fast, vigorous swings he hurled himself through the branches, across the forest toward the place the females clustered.

But he was not aware of the grim battle taking place there.

Still immersed in his females, the Emperor finished yet another mating. His penis raw and dangling, he stalked among the females, cuffing and snapping at any male he could reach.

And suddenly he found himself facing Solo.

The aging Emperor hauled himself upright, bared his teeth, and let his glands pump out still more of his potent musk. Hair bristling, muzzle working, he was a magnificent sight, enough to intimidate any other male.

Any but Solo.

Solo had spent a comfortable winter in a burrow with a female band not far from here. As soon as the light had returned he had joined in the early feeding, rapidly building his body to the peak of strength and power he had enjoyed last year.

And he had begun his roaming. Already today he had planted offspring in half a dozen females throughout the forest. Now he had come to take more — once he had eliminated the opposition.

Solo lunged at the Emperor, ramming his scarred muzzle into his belly.

The Emperor was knocked flat on his back on the branch, winded, and might have fallen from the tree if his quick-working primate hands had not scrabbled at the bark. He was as much shocked by the sudden physical assault as hurt. Save for cuffs and slaps by food-monopolizing females, and occasional inadvertent blows from other males, nobody had ever deliberately hurt him in his life.

But it was not over.

With a bound, almost graceful for a creature his size, Solo jumped on the Emperor. He sat on the older male’s chest, compressing the Emperor’s fragile ribs. The Emperor screamed. He chuffed and panted, and he beat at Solo’s back. If he had used all his strength he might yet have driven the other off. But to injure another went against his instincts, and his punches were weak, his blows ineffective.

He had missed his chance.

Solo bent forward and pushed his muzzle into the Emperor’s crotch. He teased aside fur that was stiff with semen and the vaginal fluids of several females. With a brisk, practiced lunge, he bit into the Emperor’s scrotal sac, severing one testicle.

The Emperor howled, thrashing. Blood gushed, mingling with the mating fluids on his fur.

Solo climbed easily away. With a single firm motion of his foot, he pushed the Emperor off the branch. The older male’s body went crashing through the foliage beneath, plummeting toward the ground. Then Solo spat out the bloody testicle, letting it fall into the green below.

Solo advanced on Right, Noth’s sister, one of the youngest of the females. He fingered his rapidly swelling penis, preparing to take her.

But now here was Noth, young, eager, horny, plummeting out of the air to land at Solo’s feet. Solo turned like a tank turret to face this new challenger.

Noth hadn’t known Solo was here. But he remembered him.

Noth was a creature of the here and now. He had no real conception of yesterday or tomorrow, and his memory was not arranged in an orderly narrative; it was more like a corridor of vivid images, rendered in sight and scent. But the powerful stink of Solo brought images flooding back, shards and glimpses of that dreadful day in another part of the forest, his mother’s despairing howl as she fell into the pit of teeth.

Conflicting impulses surged through him. He should display, stink-fight — or else he should show his submission to this powerful creature, just as Rival had submitted to him.

But Solo didn’t fit. He didn’t obey any of the unwritten rules that governed the notharctus’ fragile society. He had just mutilated the troop’s dominant male. Solo would surely not be satisfied with a symbolic victory. Solo, huge, still, meant to injure him, if not kill him.

And here was Right, Noth’s only kin, cowering in the foliage at Solo’s feet. Here were the females with whom he had lived for half a year, and whose swelling pudenda had filled him with anticipatory lust for days, weeks — and here was this monster, Solo, who had destroyed everything he had grown up with.

He stood upright and howled.

Solo, startled, hesitated.

Noth’s wrists and crotch itched with musk. He performed a frantic, one-second display, an accelerated demonstration of his power and youth. Then, blindly, not understanding what he was doing, he lowered his head and barged at Solo’s midriff. With a gasping hoot, Solo was knocked backward, finishing on his back in a clump of foliage.

If he had followed up, Noth could have capitalized on his surprise attack. But he had never fought a physical fight in his life. And Solo, with the instincts of an experienced fighter, twisted and slammed his knee against Noth’s temple. Noth went down face first and instinctively scrabbled for a hold. An immense mass crashed into his back, crushing him against the bark. And now Noth felt Solo’s incisors sink into the soft flesh of his neck. He screamed at the sharp pain. He twisted and thrashed. He could not shake off Solo — but the vigor of his movements tipped them both off the narrow branch.

Hooting, with Solo’s teeth ripping at his flesh, Noth found himself plummeting through layers of foliage and twigs.

They crashed to the ground, their fall scarcely cushioned by the rotting leaf cover. But Solo was shaken free, his clenched jaw giving one last rip at Noth’s shoulder. Solo made his own display of aggression. He roared, an ugly, unstructured noise. He stood upright and hammered his small fists into the detritus at his feet; bits of leaf flew everywhere, surrounding him in a loose, sunlit cloud.

It was a battle of two small creatures. But much larger animals, watching timidly, backed away from Solo’s ferocity.

It was a one-sided contest. Solo advanced on Noth, stalking out of the settling leaf fragments. Noth watched, not even displaying, as if hypnotized. He looked down in horror at his shoulder, where a flap of skin hung loose and blood soaked into his fur.

But now a burly mass came flying at Solo. It was the Emperor. Even as the blood continued to gush from his ragged scrotum, the big notharctus slammed feetfirst into Solo’s back, knocking him flat, facedown in the debris.

This time Noth did not hesitate. He threw himself at Solo and began to pummel his back and shoulders with feet, hands, and muzzle. The Emperor joined in — and so did more of the males, until Solo was immersed beneath a blanket of hooting, jostling, inexperienced assailants. Any one of them Solo would have defeated — but not all of them together. Under the rain of inexpertly aimed blows, it was impossible even for him to rise.

At last he burrowed like a taeniodont through the forest floor detritus, out from under the clamoring pack. By the time the ragged army had noticed he was gone, that their punches and kicks were either landing in the dirt or on each other, Solo was limping away.

Aching, battered, Noth clambered back into the tree. When he got there he found the females grooming each other calmly, picking bits of dried semen out of their crotch hair, as if the combat beneath had never taken place. The Emperor was sitting quietly with the female Biggest. His flow of blood had stilled — but his copulatory campaign was suspended forever.

And here was Rival, vigorously covering Right. Noth saw his sister’s face buried in her own chest hair, low squeaks of pleasure emanating from her throat. Noth felt an oddly warm glow. He was not driven by jealousy of other males over his sister — even of this male whom he had bested, and who had apparently recovered very quickly. A deeper biochemical part of him recognized that with his sister pregnant, the line would go on: the shining unbroken molecular thread that stretched from Purga, through this moment lit by the low polar sun, on into unimaginable futures.

He heard a remote lowing. It was the call of a moeritherium, the matriarch of a migrant herd, walking slowly up from the south. With the return of the herds, summer had truly come again. And from all over the forest came a high keening: It was the song of the notharctus, a song of loneliness and wonder.

In just a few years Noth’s life would be over. Soon his kind would be gone too, their descendants transmuted into new forms; and soon, as Earth cooled from this midsummer peak, even the polar forest would shrivel and die. But for now — bloodied, panting, his fur covered with leaf mold — this was Noth’s moment, his day in the light.

The huge female Big approached him. He trilled gently. With a glance into his eyes, she turned her back and presented to him. Noth entered her quickly, and his world dissolved into unthinking pleasure.

I

Here, close to its final oceanic destination, the mighty river pushed sluggishly between walls of lush, moist forest. There were many meanders and oxbow lakes, which, cut off from the flow, had turned into stagnant marshes and ponds. It was as if the river were exhausted, its long journey done — but this river was draining the heart of a continent.

And this late summer there had been much rain. The river was high, and it spilled over onto land where the water table was already near the surface. The dense, muddy water contained fragments of eroded rock, mud, and living things. There were even rafts of tangled branches and bits of vegetation drifting like unruly schooners down the river’s tremendous length, relics that had already traveled thousands of kilometers from their point of origin.

High above the water, in the forest’s cacophonous upper story, the anthros were making their daily destructive procession.

They were like monkeys. Running along branches, using their powerful arms to swing from tree to tree, they stripped off fruit, ripped open palm fronds, and tore away great swaths of bark to get at insects. Crowds of females moved and worked together, occasionally stopping for a moment’s grooming. There were mothers with infants clinging to their backs and bellies, supported by clusters of aunts. Males, larger, wider-ranging, made loose alliances that merged and fragmented constantly as they competed for food, status, and access to the females.

More than thirty anthros worked here. They were clever, efficient foragers, and where they passed, they laid waste. It was a joyful, clamoring racket of feeding, cooperating, and challenging.

Temporarily alone, Roamer was swinging from one thick branch to the next. Though she was high above the ground, she had no fear of falling; she was in her element here, her body and mind exquisitely adapted for the conditions of this tangled forest canopy.

Bordering the sea, to the west, there were dense mangrove swamps. But here, inland, the ancient forest was rich and diverse, full of tall trees with flaring buttresses: papaws, cashews, fan palms. Most of the trees were fruit bearing and rich in resin and oils. It was a comfortable, rich place to live. But it was a relic of a world that was vanishing, for a great cooling had gripped the Earth since Noth’s time, and the once global and beneficent forests had shrunk back to scraps and fragments.

Roamer found a palm nut. She settled on a branch to inspect it. A caterpillar, fat and green, crawled over its surface. She licked off the caterpillar and chewed it slowly.

The troop moved noisily through the canopy around her. Alone or not, she knew exactly where everybody else was. In the long years since Noth’s time the primates had become still more intensely social: To the anthros, other anthros had become more interesting than mere things — the most interesting objects in the world. Roamer was as aware of the rest of her troop as if they were a series of Chinese lanterns stuck in the foliage, diminishing the rest of the world to a dull, mute grayness.

Roamer belonged to no species that would ever be labeled by humans. She looked something like a capuchin, the organ-grinder monkey that would one day roam the forests of South America, and was about that size. She weighed a couple of kilograms, and she was covered in dense black fur topped by white shoulders, neck, and face; she looked like she was wearing a nun’s wimple. Her arms and legs were lithe and symmetrical, much more so than Noth’s: It was a body plan typical of the inhabitant of an open forest canopy. Her nose was flat, her nostrils small and protruding sideways, more like the monkeys of a later South America than those of Africa.

She looked like a monkey, but she was no monkey. Remote descendants of Noth’s adapids, her kind was a type of primate called anthropoid — ancestral to both monkeys and apes, for that great schism in the family of primates had yet to occur.

Nearly twenty million years after the death of Noth, the grooming claws of notharctus feet had been replaced on Roamer’s body by nails. Her eyes were smaller than Noth’s, capable of a wide, three-dimensional field of view past her shorter muzzle, and each of her eyes was supported by a solid cup of bone; Noth’s had been protected by a mere ring of bone, and his vision could even be disturbed by his own cheek muscles when chewing. And Roamer had lost many of Noth’s ancestral relics of the times of night foraging. Her reliance on smell had diminished, to be replaced by a greater dependence on sight.

From Right’s grandchildren had sprung a great diffusing army. They had migrated down through the Old World to inhabit the dense tropical forests of Asia, and here in Africa. And as they had migrated, so they had flourished, diversified, and changed. But the line of Old World anthropoids would not continue through Roamer. Roamer could not know that she would never see her mother again — and her fate was to be far more strange than anything that had befallen her immediate ancestors.

The whiteness of Roamer’s fur made her face seem sketchy, unformed, and oddly wistful. But she had a youthful prettiness. In fact, she was three years old, still a year short of her menarche. A juvenile female independent of spirit, not yet fully absorbed into the troop’s hierarchies and alliances, she retained something of the solitary instincts of more distant ancestors. She liked to keep herself to herself. Besides, the group wasn’t a particularly happy bunch right now.

The last few years had been times of plenty, and the troop’s numbers had expanded. There had been a baby boom, of which Roamer was a part. But growth brought problems. There was too much competition for food, for one thing. Every day there were squabbles.

And then there was the grooming. In a small group there was time to groom everybody. It all helped to maintain relationships and cement alliances. When a group got too big, there just wasn’t the time to do that. So cliques were forming, subgroups fragmenting out whose members groomed each other exclusively, ignoring the rest. Already some of the cliques were traveling separately during the day, although they would still come together to sleep.

Eventually all of this would become too intense. The grooming cliques would fission off, and the group would split up. But the new, smaller groups each had to be large enough to offer protection against predators — the main purpose of these daytime bands in the first place — so it would be a long time yet, perhaps even years, before any fission was permanent. It happened all the time, an inevitable consequence of the growing sizes of primate communities. But it meant there was a lot of squabbling to be done.

So Roamer was happy to get away from all the bickering for a while.

The bug thoroughly masticated, Roamer inspected her palm nut. She knew that the kernel was delicious to eat, but her hands and teeth were not strong enough to break open the shell. So she began to pound the shell against the branch.

She became aware of two bright eyes watching her, and a slim, rust-colored body clinging to a branch. She was not alarmed. This was a crowder, a type of primate closely related to Roamer’s kind but smaller, more slender — and a lot less smart. Beyond its slim form Roamer made out many more of its kind, clinging to the branches of this tree and the next, arrayed through the forest’s green-lit world. The crowder was not competing for Roamer’s nut, and was certainly not threatening her; all the little primate wanted was Roamer’s leavings.

Roamer was mostly a fruit eater. But the crowders, like their common adapid ancestors, relied heavily on the caterpillars and grubs they snatched from the branches, and they had sharp, narrow teeth to process their insect prey. They lived in great crowded mobile colonies of fifty or more. This gave them a defense against predators and other primates. Even a troop of anthros would have had trouble driving off one of these agile, coordinated mobs.

But Roamer was a lot smarter than any crowder.

It would be tens of millions of years before any primate used anything that could be called a true tool. Much of Roamer’s intelligence was of a specialized kind, designed to enable her to cope with the fast-shifting intricacies of her social life. But Roamer was clever at understanding the natural environment around her and manipulating it to get what she wanted. Smashing a nut against a tree trunk was hardly advanced engineering, but it required her to plan one or two steps ahead, a precursor of much greater inventiveness in ages to come. And such nut smashing was a cognitive leap beyond the grasp of any crowder, which was why the crowders were hanging around now.

Roamer heard a rustling far below. She clung to her branch, peering down into the green gloom.

She could see the litter of the forest floor, and a shadowy shape moving through the trees with a rustle of feathers, tentative pecks at the ground. It was a flightless bird, something like a cassowary. And when she tracked back the way the bird had come to the middle of the clearing, Roamer made out a rounded, polished gleam.

Eggs. There were ten of them, nestling in the bird’s crude nest, each of them a reservoir of yolk as big as Roamer’s head. In the stillness of noon, with her mate away, the bird had left her nest briefly unguarded, taking the chance that no harm would come to it while she briefly assuaged her hunger. She was unfortunate that Roamer’s sharp eyes had detected the nest so quickly.

Roamer hesitated for a heartbeat. If she went after the eggs she would be taking a risk. Her nut cracking had already delayed her long enough for the troop to move away, and it would be bad to get lost. And the bird itself was a menace. A stalking monster, it was one of the last representatives of a twenty-million-year dynasty. After the comet, around the world, the land mammals had at first remained small, crammed into the dense forests — but some birds had grown large, and flightless monsters like this had briefly contested the role of top predator. Released from the weight limitations of flight they had become heavily built, muscular, and monstrously powerful, with beaks that could snap a backbone. But they had been out of time: When the mammalian herbivores grew large, so did mammal carnivores, and the birds could not compete.

The eggs were there, right below Roamer. She could take them easily.

If she had been older, more integrated into the group, her decision might have been different. But as it was she slid down the tree’s rough bark toward the ground, her small mouth already moist with anticipation. It was this moment of decision that caused a great divergence in her own life — and the destiny of the greater family of primates in the future.

She had dropped the remains of her nut kernel. Behind her the little crowder, its patient wait over, fell on the sweet fragments. But in an instant more of its fellows came swarming over the branch to steal its prize.

As she climbed down the tree Roamer disturbed a troop of screechers. These primates were very small, with manes of fine silky hair and bizarre white moustaches. Startled by her passing they chattered and scurried away into the deeper recesses of the foliage, almost birdlike in the speed of their movements and the brightness of their furry “plumage.”

Screechers made a living by digging into tree bark with their bottom teeth to make the gum flow. When they were done with a hole, they urinated into it to deter others from feeding there. There were many species of these little creatures, each specializing in the gum of one particular tree, and they were differentiated by their hairstyles. With their extravagant fur and trilling calls they made the forest canopy a place of color, life, and noise.

On the ground was still another form of primate. This was a potbelly, a solitary male. He was four times Roamer’s size, his bulky body coated in thick black fur. He sat squat, steadily pulling leaves off a bush and cramming them between his powerful jaws. His muzzle was stained black: he had been chewing charcoal from a lightning-struck stump, a supplement which neutralized the toxins in his leafy diet.

As Roamer dropped lightly to the floor he glared at her, his mouth a ferocious downturn, and let out a roar. She glanced around nervously, fearing his call might have attracted the attention of the careless mother bird.

Roamer was under no threat from the potbelly. He had an enormous stomach with an enlarged lower intestine within which his low-nutrition food could be partially fermented. To let this mighty organic factory work effectively, he had to remain motionless for three-quarters of his time. This close she could hear the endless rumbling of his huge ungainly stomach. He was remarkably clean, though; given his lifestyle, he had to be sanitary, like a sewer rat. As she moved away from his precious patch of forest floor the potbelly subsided into a sulky silence.

The forest clearing was cluttered. Grasslands were still rare. In the absence of grass, the ground cover was rarely less than a meter tall, a clutter of low shrubs and bushes including aloe, cactus, and succulents. Most spectacular of all were giant thistlelike plants strewn, in their season, with psychedelically colored flowers. Such spectacles graced most of Earth’s landmasses in this era, but it was an assemblage that would be unusual in human times; it was something like the fynbos flora of southern Africa.

To reach the bird’s nest, Roamer would have to leave the cover of the trees. But the open sky today seemed very bright — bright and washed-out white — and there was a peculiar electric stink to the air. She would be exposed out there; she hesitated, uneasy.

Clinging to the edge of the forest, she tried to work her way closer to the eggs.

She skirted a marshy area, part of the mighty river’s floodplain. She could see the water: Clogged with scummy vegetation, it glimmered, utterly flat, under a high sun. But there was a smell of salt in the air. Here, not far upstream of the river’s delta, she was close to the ocean, and occasional floods and high tides had laden the soils with brine, making the vegetation sparse.

Animals moved through the clearing, seeking the open water. In low scrub a group of gazellelike stenomylus cropped, moving in a tight, nervous cluster and peering about anxiously as they chewed. They were trailed by a smaller herd of cainotheres, like small, long-eared antelope. Other deerlike browsers worked through the forest itself. But the stenomylus were not gazelles but a kind of camel — as were the cainotheres, with their oddly rabbitlike heads.

Close to the shore clustered a family of bulky herbivores reminiscent of rhinos. These were not true rhinos, and the sad curve of their upper lips gave a clue to their ancestry: They were actually arsinoetheres, creatures related to elephants. In the water itself wallowed a mating pair of metamynodons, very like hippos; wading birds stepped cautiously away from their clumsy passion. The metamynodons were actually more closely related to rhinos than were the arsinoetheres.

Where herbivores gathered, so predators and scavengers came to watch with their calculating eyes, as they had always done. The strange protorhinos and camel-gazelles were followed by cautious packs of bear-dogs — amphicyonids, predators and scavengers, walking like bears with their feet flat on the ground.

So it went. For a human observer it would have been like a fever dream — a bear like a dog, a camel like an antelope — shapes familiar if seen through half-closed eyes, and yet eerily different in detail. The great mammal families had still to find the roles they would occupy later.

But this age could boast its champions. At the forest’s edge Roamer saw a shadow moving through the trees, immense, lumbering, menacing. This was a magistatherium. It walked four-footed, like a bear — but it was immense, twice the size of a Kodiak bear. Its canine teeth, five centimeters thick at the root, were twice the size of a tyrannosaur’s. And, like the tyrannosaurs, it was an ambush hunter. For now it ruled these African forests — and it would prove to be the largest carnivorous mammal ever to live on land. But its shearing teeth, essential tools for a meat eater, came in pairs, unlike those of the true carnivores of the future, and more prone to damage. That slight design flaw would eventually doom the magistatherium to extinction.

Meanwhile, through the largest of the pools cruised the stippled back of a crocodile. She didn’t care about any of this strangeness. As long as you were stupid enough to approach the crocodile’s domain, as long as you had flesh that filled the belly and bones that crunched in the mouth, you could be any shape you pleased: Your fate would be the same.

At last Roamer came close enough to the nest. She dashed out of cover, attracting blank stares from the rooting herbivores, and reached her eggs.

The nest was partially covered by fallen fern fronds, and so she had some shelter to work in. With saliva flooding her mouth she picked up the first egg — and was baffled. Her hands slid over the egg’s smooth surface, finding nothing to rip or tear. When she squeezed the egg against her chest, she did no better; the thick shell was too tough. There was no branch nearby against which she could smash the eggs. She tried cramming the whole egg into her mouth to bring her powerful back teeth into play, but her tiny lips could not reach around more than a fraction of its volume.

The trouble was, her mother had always cracked eggs open for her. Without her mother she had no idea what to do.

The light in the sky seemed to grow brighter, and a wind picked up suddenly, ruffling the surfaces of the ponds and scattering brown fronds across the ground. She felt a rising sense of panic; she was a long way from her troop. She dropped the egg back into the nest and reached for another.

But suddenly the sweet, sickly smell of yolk reached her nose. The egg she’d dropped, falling against the others in the nest, had broken. She jammed her hands into the jagged crack and pushed her face into the sweet yellow goo, and was crunching on half-formed bones. But when she took another egg, she couldn’t remember how she had opened the first. She fingered the egg and tried to bite it, starting the whole trial-and-error process over.

Dropping eggs onto each other was how her mother had opened them before. But even if her mother had been here to demonstrate how to do it, Roamer would not have learned the technique, for Roamer was not capable of reading another’s intentions, and so she couldn’t imitate. Psychology was beyond the anthros, and every generation had to figure everything out from scratch from basic raw materials and situations. It made for slow learning. Still, Roamer soon got into another egg.

She was so intent on the food she wasn’t aware of the lustful eyes that studied her.

Before she broke into a third egg the rain started. It seemed to come out of nowhere, huge droplets falling out of a blank, bright sky.

A great wind swept over the marshes. Wading birds took flight, heading west toward the ocean, away from the approaching storm. The big herbivores turned to face the rain, stoic misery in their posture. The crocodile slid beneath the surface of its pond, preparing to wait out the storm in the changeless depths of its murky empire.

And now clouds fled across the sun, and darkness closed in like a lid. To the east, at the center of the continent where the storm had brewed, thunder clattered. It was a storm of a ferocity that lashed the area only a few times in a decade.

Roamer cowered in the wreckage of the nest, her fur already plastered to her body. The droplets hammered into the ground around her, battering the dead vegetation and digging tiny pits into the clay. She had never known anything like it. She had always ridden out storms in the comparative shelter of the trees, whose foliage diffused and deadened the falling water. But now she was lost, stranded out in the open, suddenly aware how far she had come from her troop. If a predator had found her in those few heartbeats, then she might have lost her life.

But as it was, she had been found by one of her own kind: an anthro, a large male. He dropped to the sodden ground before her and sat still, studying her.

Startled, whimpering, she approached him cautiously. Perhaps he was one of the males who dominated her own troop — the loose, fissioning band she thought of as a kind of composite father — but he was not, she quickly saw. His face, the white fur beaten down with the rain, was strange, and a peculiar patterning of coloration gave him white drips down his black-furred belly, almost like blood.

This male — Whiteblood — was twice her size, and a stranger. And strangers were always bad news. She screeched and scrabbled backward.

But she was too late. He reached out his right hand and grabbed the scruff of her neck. She twisted and fought, but he lifted her easily, as if she were a piece of fruit.

Then he hauled her without ceremony back into the forest.

Whiteblood had spotted Roamer — a juvenile female wandering alone, an unusual opportunity. He had stalked her carefully, a fruit eater moving like an experienced hunter. And now the cover of the storm had given him the opportunity he needed to take her. Whiteblood had his own problems — and he thought Roamer might be part of the answer.

Like their notharctus ancestors, anthro females lived in tight supportive groups. But in this seasonless tropical forest, perpetually abundant, there was no need for their breeding cycles to be synchronized. Life was much more flexible, with different females coming into estrus at different times.

That made it easier for a small group of males — even a single male, sometimes — to monopolize a female group. Unlike the notharctus Emperor, it wasn’t necessary for an anthro male to try to cover all his females in a single day, or to face the impossible task of keeping other males away. Instead it was enough that he kept rivals away from the small number of females who were fertile at any given time.

Though they were physically larger, anthro males did not “own” the females, or dominate them excessively. But the males, bound to the female group by a genetic loyalty — in a promiscuous group there was always a chance that any child born might be yours — would work to protect the group from outsiders and predators. For their part the females were generally content with the loose satellite male communities that accreted around them. Males were occasionally useful, obviously necessary, rarely troublesome.

But recently, for Whiteblood’s troop, things had gone wrong.

Ten of the twenty-three females in the group had gone into estrus simultaneously. Soon other males had been attracted, drawn by the scent of blood and pheromones. Suddenly there weren’t enough females to go around. It had been an unstable situation, intensely competitive. Already there had been bloody battles. There was a danger the group might fission altogether.

So Whiteblood had gone out hunting females. Juveniles were the preferred target: young and small enough to be handled easily, foolish enough to be easy to separate from their home groups. Of course it meant waiting a year or more before a child like Roamer could be mated. But Whiteblood was prepared to wait: His mind was complex enough for him to act now in the prospect of reward later.

For Whiteblood the situation was quite logical. But for Roamer it was a nightmare.

Suddenly they were swinging and running at a ferocious rate. Whiteblood kept hold of her scruff, seeming to find her no trouble to haul. Roamer had never moved in these great bounds, swoops, and leaps: Her mother and the other females, more sedentary than the males, moved much more cautiously than this. And she was being carried a long way; she could smell muddy water, for they were approaching the bank of the river itself.

And meanwhile the rain clattered down, pelting through the leaves and turning the air into a gray misty murk. Her fur was sodden and water ran into her eyes, making it impossible to see. Far below them, water ran across the sodden ground, rivulets gathering into streams that washed red-brown mud into the already swollen river. It was as if forest and river were merging, dissolving into each other under the storm’s power.

Her panic intensified. She struggled to get free of Whiteblood’s grasping hand. All she got for her troubles were cuffs on the back of her head, hard enough to make her squeal.

At last they reached Whiteblood’s home range. Most of the troop, males, females, and infants, had clustered together in a single tree, a low, broad mango. They sat in rows on the branches, huddled together in sodden misery. But when the males saw what Whiteblood had brought back, they hooted and slapped the branches.

Whiteblood, without ceremony, thrust Roamer at a group of females. One female started poking hard at Roamer’s face, belly, and genitals. Roamer slapped her hand away, hooting in protest. But the female came back for more, and now more of them crowded around her, striving to get close to the newcomer. Their curiosity was a mixture of the anthros’ usual fascination with someone new, and a kind of rivalry over this potential competitor, a new recruit in the ever-shifting hierarchies.

For Roamer everything was bewildering: the sheets of lightning flashing over the purple sky, the hammering rain on her face, the roar of water below, the damp-fur, unfamiliar stink of the females and young around her. Surrounded by open pink mouths and questing fingers, she was overwhelmed. Struggling to escape, she lunged forward, and found herself briefly dangling over the branch.

And she looked down on strangeness.

Two indricotheres were lurking under the tree. These great creatures were a kind of hornless rhino. Looking like meaty giraffes, they had long legs, supple necks, and hides like those of elephants. They were oddly graceful in a slow-moving way, even if they did mass as much as three times as an African elephant — and so huge they were unused to being threatened by anything. Even now they reached up their thick necks and horselike faces to crop at the tree’s soaking foliage.

But they were in danger. Muddy water flowed over the ground, washing around the indricotheres’ legs, as if the tree and the indricotheres alike stood in the river itself.

At last a great sheet of muddy soil broke away from the riverbank, right next to the tree’s shallow roots, and slid without ceremony into the river. One mighty indricothere lowed, its great flat elephantine feet scrabbling at a ground suddenly turned into a slippery, treacherous slope — and then it fell, fifteen tons of meat flying, its neck twisting and long tail working. It hit the water with a tremendous splash, and in an instant it was gone, swept away into the voracious river.

The second indricothere lowed its loss. But it too was in peril as the ground continued to dissolve under the water’s relentless probing, and the bereft animal lumbered backward to safety.

But the tree itself was in trouble. Its roots had been exposed by the sudden erosion of the flash flood, and further undermined by the river’s assault on its bank. The trunk creaked once, and shuddered.

And then, with a series of explosive cracks, the roots gave way. The tree began to topple toward the water. Like fruit from a shaken branch, primates of all sizes tumbled out of the tree and fell screaming into the turbulent water.

Roamer howled and clung to her branch as the tree tipped nightmarishly, all the way into the river.

The first few minutes were the worst.

Close to the riverbank the water was at its most turbulent, torn between the fast-flowing current and friction with the land. In this mighty torrent even the great mango tree was like a twig tossed in a brook. It bucked and creaked and twisted. First its foliage slammed into the water, then its roots, clogged with mud and rocks, would claw toward the sky. Roamer was rolled and dunked, plunged into cloudy brown water that forced its way into her mouth and nose, then carried into the air again.

At last the tree slid away from the turbulence near the bank and drifted into the center of the river, where its rocking and twisting quickly damped out.

Roamer found herself stuck underwater. She looked up through muddy murk at a glimmering surface littered with leaves and twigs. Already her mouth and throat were filling up, and panic overwhelmed her. With a bubbling scream she scrambled up through the tangled, broken foliage, clambering toward the light.

She broke through the surface. Light, noise, and the battering rain assaulted her senses. She hauled herself out of the water and lay flat on a branch.

The tree was floating branches first down the river. Its tangled, ripped roots reached up toward the lowering, lightning-strewn sky. Roamer raised her head, peering around for other anthros. It was not easy to recognize them through the thick rain-filled air, so battered and sodden were they, but she made out Whiteblood, the burly male who had abducted her, a couple of other males — and a female with an infant that had somehow hung onto her back, a little bundle of soaked, miserable fur.

Even though she was just as battered and half drowned as before, Roamer felt suddenly better. If she had been left alone it would have been the most unbearable thing of all; the presence of others was comforting. But still, these others were not her family, not her troop.

More displaced vegetation coursed over the surface of the river, clustering along its spine where the water ran deepest. There were more trees and bushes, some of them washed by this precursor of the Congo thousands of kilometers downstream from the very different lands in the center of the continent. There were animals here too. Some of them clung to the floating foliage, like the anthros. She saw the flitting, nervous forms of a couple of crowders, and even a potbelly, sitting squat on the trunk of a walnut. The potbelly, a female, had found a stable place to sit, and the rain didn’t bother her. She had already resumed her usual habit of feeding on leaves conveniently delivered to her clutching hands and feet.

But not all the animals in this gruesome assemblage had made it here alive. There was a whole family of fat, piglike anthracotheres, all of them drowned, stuck in the branches of a broken palm like meaty fruit. And the huge indricothere that had been washed into the river just before the fall of the mango was here too, a great carcass drifting in the water, long neck lolling back and powerful legs splayed, just another bit of floating detritus jammed in with the rest.

Gradually, as the river broadened, the subtle currents shoved these fragments together, foliage and roots tangling, and a makeshift raft assembled itself. The animals stared at one another, and at the churning river, as their crude vessel drifted on.

Roamer could see the forest, growing thick and green on shallow riverbank slopes of eroded sandstone. The trees were mangos, palms, a kind of primitive banana. Branches hung low over the water, and lianas and vines looped over the tangled terraces. Her arms ached for a branch to swing from, a way she could climb from here to there. But the forest was separated from her by churning water — and as the vegetable raft continued to sail downstream, those tempting banks receded further, and the familiar forest gave way to the mangroves that dominated the coastal areas.

The rain wasn’t done yet. It actually fell harder. Fat droplets hurled themselves out of the leaden sky. The water was stippled with craters that disappeared as soon as they were formed. A white-noise harshness flooded her ears, so that it was as if she were lost in a kind of huge bubble of water, water below and around her, with only this broken mango to cling to. Moaning, chilled to the bone, Roamer burrowed into the branches of the mango and huddled, alone, waiting for everything to go away, and for her to be returned to the world she knew, of trees and fruit and anthros.

That, however, was never going to happen.

The storm, heavy as it was, blew itself out quickly. Roamer saw finger-thin shafts of light pushing into her shelter of foliage. The rain noise had gone, to be replaced by the eerily soft lapping of water.

She struggled out of the branches and clambered on top of the tree. The sun was strong, as if the air had been cleared, and she felt its warmth sink deep into her fur, drying it quickly. For a heartbeat she luxuriated in the warmth and dryness.

But there was no forest here: only this fallen tree and its cluster of broken companions, drifting over a gray-brown sheet of water. There weren’t even any riverbanks. On three sides of the tree, all she could see was water, all the way to a knife-sharp horizon. But when she looked back the way the raft had drifted, she spotted land: a line of crowded green and brown, striped over the eastern horizon.

A line that was receding.

The raft of debris had been washed out to sea, out into the widening Atlantic, anthros, potbelly, crowders, and all.

II

After the days of Noth the geometry of the restless world had continued to evolve, and it continued to shape the destinies of the hapless creatures who rode the continental rafts.

The two great cracks that had doomed ancient Pangaea — the east-west Tethys Sea, and the north-south Atlantic Ocean — closed and opened respectively. Africa was undergoing a slow collision with Europe. Meanwhile India was drifting north to crash into Asia, and the Himalayan Mountains were being thrust into the air. But immediately after the young mountains were born, the rain and the glaciers had begun their work, gouging and eroding, washing the mountains back to the sea: On this turbulent planet, rock flowed like water, and mountain ranges rose and fell like dreams. But as the continents closed, the Edenic flow of the Tethys was doomed, though fragments of the shrinking ocean would survive as the Black, Caspian, and Aral Seas, and in the west as the Mediterranean.

As the Tethys died there was a great drying, right across the belly of the world. Once there had been mangrove forests in the Sahara. Now a great belt of semiarid scrub spread around the old track of the Tethys, across North America, southern Eurasia, and northern Africa.

Meanwhile, the huge land bridge that had closed off the northern Atlantic, spanning from North America to northern Europe via Greenland and Britain, was being severed, and the Atlantic reached up to the Arctic Ocean. As the ancient east-west ocean passage was being closed, so a new channel from south to north was opening.

Thus ocean currents were reshaped.

The oceans were great reservoirs of energy, restless, unstable, mobile. And all the oceans were laced with currents, great invisible Niles that dwarfed any river on land. The currents were driven by the sun’s heat and the Earth’s rotation; the top few meters of the oceans stored as much energy as the whole of the atmosphere.

Now the huge equatorial currents that had once rolled around the Tethys belt were disrupted. But already the great flows that would dominate the widening Atlantic were in place: A precursor of the Gulf Stream flowed, a mighty river sixty kilometers across, running south to north with the force of three hundred Amazons.

But this change in circulation patterns would reconstruct the planet’s climate. For while equatorial currents promoted warming, north-south interpolar currents provoked a vast refrigeration.

To make matters worse, Antarctica had settled over the Earth’s southern pole. Now its great ice cap had begun to gather, for the first time in two hundred million years. Vast, cold circumpolar ocean currents gathered in the southern seas, feeding the great northward currents of the Atlantic.

It was a crucial change: the start of a mighty planetary cooling, a downturn of the graph, that would persist to human times and well beyond.

All over the planet, the old climate belts shrank toward the equator. Tropical vegetation types survived only in the equatorial latitudes. In the north, a new kind of ecology appeared, a temperate woodland of mixed conifers and deciduous trees. Vast swaths of it covered the northern lands, stretching across North America, Europe, and Asia from the tropic to the Arctic.

This climatic collapse triggered a new dying — what paleobiologists would later call the Great Cut. It was a drawn-out, multiple event. In the ocean the plankton population crashed repeatedly. Many species of gastropods and bivalves disappeared.

And on the land, after thirty million years of comfortable success, the mammals suffered their first mass extinction ever. Mammalian history was cut in half. The exotic assemblages of Noth’s times finally succumbed. But new, larger herbivores began to evolve, with heavy-duty ridged teeth able to cope with the new, coarser vegetation typical of seasonal woodland. By Roamer’s time the first proboscideans, properly equipped with trunks and tusks, were already walking the African plains. The trunk, unparalleled for muscular flexibility save for an octopus’s arm, was used for stuffing the animal’s mouth with the vast quantity of food it needed. These deinotheres had stubby trunks, and odd, downward-curving tusks that they used for stripping the bark from trees. But, unlike their moeritherium ancestors, they looked like elephants, and some already grew as tall as the African elephants of later times.

And this was a time of success for the horses. The descendants of the timid creatures of Noth’s forest world had diversified into many woodland browser types — some of them as large as gazelles, but with tougher teeth than their ancestors had had to take leaves rather than soft fruit — along with longer-legged plains animals slowly adapting to a diet of grass. Most of the horses now had three toes on both their front and back feet, but some plains-living runners were starting to lose their side toes, and were putting all their weight on their central toes. But as the forests shrank this diversity was already falling; soon many of the forest species would disappear. The rodents, too, were diversifying, with the appearance of the first gophers, beavers, dormice and hamsters, a great diversity of squirrels — and the first rats.

But the new conditions were not kind to the primates. Their natural habitat, the tropical forests, had shriveled back to the southern tropics. Many of the primate families had gone extinct. Fruit eaters like Roamer lingered only in the tropical woodlands of Africa and southern Asia, clinging to the year-long food supply these forests still provided. By the time Roamer was born there were no primates left north of the tropics, and — since the rise of the rodents — none in the Americas at all: not a single species.

But that was soon to change.

The sea around Roamer was a sheet of gunmetal gray across which waves rippled, languid as mercury. Roamer was in an utterly baffling place: a sketchy, elemental two-dimensional environment, static yet full of mysterious churning motion, that could not have been more different from the forest.

She felt nervous climbing around on top of the vegetation. She expected some ferocious aerial predator to bite into her skull at any moment. And as she moved she could feel the uneasy raft shift under her, its loosely tangled components rustling with the slow breathing of the sea. It felt as if the whole thing might disintegrate at any moment.

There were just six anthros: three males, two females — including Roamer — and the infant who still clung sleepily to the fur of its mother. These were the only survivors of Whiteblood’s troop.

The anthros sat on a tangle of branches, eyeing one another. It was time to form provisional hierarchies.

For the two females the priorities were clear enough.

The other female, the mother, was a burly individual more than a decade old. This child was her fourth and — though she could not know it — now her only surviving offspring. Her most noticeable characteristic was a fur-free patch of scar tissue on one shoulder where she had once been burned in a forest fire. The infant, clinging to Patch’s chest, was tiny, small even for its age, just a scrap of fur. Patch, the mother, studied Roamer dismissively. Roamer was small, young, and a stranger, not even remote kin. And, as a nursing mother, Patch would always have priority. So she turned her broad back on Roamer and began to stroke her infant, Scrap.

Roamer knew what she had to do. She scuttled over the branches to Patch, and dug her fingers into fur that was still moist and began to comb out tangles and bits of debris. When she probed at Patch’s skin, she found knots of muscle, and places which made Patch wince to be touched.

As Roamer’s strong fingers worked, Patch relaxed slowly. Patch, like all of them, had been battered by her precipitate removal from the forest, and was stressed by her sudden dumping into this extraordinary emptiness and the loss of her family. It was as if she could, for a moment, under the magic of the other’s touch, forget where she was. Even the infant, Scrap, seemed soothed by the contact between the two females.

Roamer herself was calmed by the simple, repetitive actions of the grooming, and by the subtle social bond she was building up with Patch.

The males’ negotiations were more dramatic.

Whiteblood found himself facing two younger males, brothers, in fact. One had a peculiar crest of snow-white hair that stuck up around his eyes, making him look permanently surprised, and the other had a habit of using his left arm predominantly over his right, so much so that the muscles on his left side were much more heavily developed than those on the right, like those of a left-handed tennis player.

Both Crest and Left were smaller and weaker than Whiteblood, and, younger, they had not outranked him back in the forest. But now Whiteblood had lost all of his allies, and together these two might defeat him.

So, without hesitation, he launched into a display. He stood upright, shakily, hooted and shrieked, and threw handfuls of leaves. Then he turned around, spread his backside and blew shit through moist fur.

Left was immediately intimidated. He shrank back, arms folded around himself.

Crest was more defiant, and answered Whiteblood’s display with a shrieking tantrum of his own. But he was outsized by Whiteblood and, without the support of his brother, could not hope to best the older male. When Whiteblood began to cuff him about the head and neck, Crest quickly backed down, tumbling onto his back and spreading his arms and legs like an infant, showing his submission. All of this was halted only when an incautious stamp plunged Whiteblood’s leg through the foliage and into the cold water. He yelped, pulled back his leg, and sat with legs folded beneath him, subdued.

But he had done enough. The brothers approached him now, their heads bent and postures humble. A brief interval of frantic mutual grooming ensured the new hierarchy was reinforced, and the three males started to pick bits of shit out of each other’s fur.

The rough-and-ready communities of Noth had been like street gangs, held together by not much more than brute force and dominance, with each individual aware of little more than her own place in the pecking order. But by now the advantages of social living had driven primate societies to baroque intricacy, and had spurred the development of new types of mind.

Group living required a lot of social knowledge: knowing who was doing what to whom, how your own actions fit in with this, who you had to groom and when, to make your life easier. The larger the group, the greater the number of relationships you had to keep track of, and as those relationships changed constantly, you needed still more computational capacity to handle it all. By allowing their group living to develop to such extremes of complexity, primates continued to get relentlessly smarter.

Not all primates, though.

Through all this the big potbelly had sat on the comfortable branch she had found, methodically stripping it of leaves. She had no interest in the peculiar displays and hairy fiddling of the anthros.

Even among her own kind the potbelly knew little of the society of others. She ignored other females and let herself be bothered by males only when she felt the urge to mate — which, in fact, was on her now. When they were in season anthros like Patch and Roamer showed sexual swellings on their rumps. That would have been of little use to a creature who spent most of her time sitting on her backside, so on the potbelly’s chest pinkish blisters had swollen brightly in an unmistakable hourglass shape. But as there was no male potbelly around, nobody was doing anything about it.

Not that the potbelly cared much. She didn’t understand where she was and what had become of her any more than the anthros did, but it didn’t trouble her. She could see there were plenty of leaves on this fallen tree to last her through the day. She had no real idea that there could be such a thing as a tomorrow different from today, that it might not find her in an endless forest full of nutritious leaves.

Already the anthros were starting to feel hungry; their low-nutrition diet worked through their systems quickly. They broke up their grooming circles and spread out over the branches of the fallen mango. The tree had lost much of its fruit, along with most of its inhabitants, when it fell from the bank. But Crest, one of the brothers, quickly turned up a cluster of fruit that had gotten lodged in an angle of branch and trunk. He hooted to summon the others.

The new miniature society worked efficiently. Though Crest managed to grab one piece of fruit for himself, he was quickly pushed away by Whiteblood. But Whiteblood was in turn usurped by Patch. Though she was not much more than two-thirds of Whiteblood’s size, the infant clinging to her chest was like a badge of authority. Whiteblood took one fruit and, grumbling, moved back, giving way to Patch.

While this was going on Roamer, like the brothers, knew that she would get no nearer to the fruit until the dominant ones had taken what they wanted.

Alone, she walked carefully, all four limbs grasping, toward the edge of the raft, where the tangle of branches was a little looser. The two terrified crowders, huddled together, skittered away as she approached. Through the foliage she could see murky brown water, littered with bits of wood and leaf, rippling languidly. The sun glimmered in a hundred places, shining through gaps in the cover of the fallen tree, and the dancing light was entrancing, distracting.

Roamer was hungry, but she was also thirsty. She dipped her hand cautiously into the water — it was cool — and scooped up a mouthful. The water was mildly salty — not bitterly so, for even so far from land the river’s powerful outflow diluted the ocean’s brine. But as she drank the taste of salt began to build up in her mouth, and she spat out her last mouthful.

Hungry, bored, the brothers came to inspect her as she drank, head bent down into the foliage, arm outstretched, buttocks raised. They sniffed her curiously, but they could smell how young she was, too young to mate.

When the older ones were done, Roamer and the others fell on the fruit.

With their bellies full for now, the anthros were calming down. But already the haphazard raft had drifted out of sight of the land, already the anthros had eaten much of the fruit from the drowned mango tree. And already the potbelly, complacently munching, had stripped half the branches of their leaves.

And none of them had seen the pale gray triangle that slid silently through the water, not meters away.

The shark circled the crude, disintegrating raft. Alerted by the feeding frenzy as the drowned inhabitants of the riverbank forest were washed out to the waiting mouths of the ocean, the shark had been attracted by the scent of stale blood that leaked from the indricothere carcass. But now it sensed motion on the tangled foliage that floated overhead. It circled, calculating, patient.

The shark was not as intelligent as its parallels on land. But then it was not much like an animal at all. The bones of its back were not bone, but tough cartilage that gave the shark better flexibility than more advanced fish. Its jaw was cartilage too, in which were loosely attached teeth, serrated like steak knives, perfect for shearing flesh. Its projecting snout looked crude, but it cut through the water with the precision of a submarine’s engineering, and it was equipped with nostrils that could detect minute traces of blood. Beneath the snout was a special organ with extraordinary sensitivity to vibration, enabling it to sense the struggles of a frightened animal across immense distances. Behind its small head, the shark’s entire body was made of muscle, designed for power, for forward drive. It was like a battering ram.

Sharks had already been the ocean’s top predators for three hundred million years. They had endured through the great extinctions, while families of land predators had come and gone. They had seen off competition from new classes of animals, some much younger, like the true fish. Over that vast period of time, the sharks’ body design had barely modified, for there was no need.

The shark was relentless, unable to be deflected by guile, prepared to keep on attacking as long as its senses were appropriately stimulated. It was a machine designed for killing.

The shark could sense the great mass of dead meat drifting at the heart of this raft, but it could also hear the scurrying of live animals on its surface. The dead thing could wait.

Time to attack. It went in headfirst, its jaws open. The shark had no eyelids. But to protect its eyes, it rolled them back, so that they turned white, in the last instant before it struck.

Patch was the first to see the approaching fin, to glimpse the white torpedo body gliding through the water toward the raft, to look into the white eyes. She had never seen such a thing before, but her instincts yelled that this sleek form spelled trouble. She ran over the loose foliage to the raft’s far side.

The other anthros were panicking. The two crowders were squalling like tiny birds, running and leaping this way and that. Only the potbelly sat placidly on its branch, munching another handful of leaves.

Scrap, separated from her mother, didn’t react.

Patch was terrified. She had expected her infant to follow her to the far side of the raft. But the infant hadn’t seen the approaching peril. A human mother would have been able to visualize her child’s point of view, understand that the child might not be able to see everything she saw. That transference of understanding was beyond Patch; in that respect, just like Noth, she was like a very young human child herself, imagining that every creature in the world saw what she saw, had the same beliefs she did.

The shark rammed its blunt nose up through the loose foliage. To Roamer this eruption of a gaping mouth from under the world was a nightmarish vision. She hooted and ran helplessly, unable to escape the raft’s confines.

The infant was lucky. As the raft shuddered under the shark’s assault she lodged in an angle of branch and trunk. Her mother lurched across the spinning raft, leaping over the gaping hole the shark had ripped, and snatched up the child.

But the shark came again. This time it drove its wedge-shaped nose between two of the great trunks that formed the raft’s crude structure. The trunks separated, a great lane of leaf-strewn water opening up between them. One of the crowders fell, squeaking, into the widening gap.

The shark’s mouth was like a cavern opening up before it. The crowder’s pinprick mind was snuffed out in a second. The shark was barely aware of taking the tiny warm morsel. Its work was barely begun.

The anthros screamed and ran to the edge of the raft, getting as far from the rift as they could — but they cowered back from the desolate ocean beyond.

Whiteblood saw that the fat, complacent potbelly sat where she had always sat, on her leafy branch, that ridiculous red swelling blazoned across her chest — even though the shark’s vandalism had opened up the ocean right before her. In this instant of ultimate stress, new circuits closed in Whiteblood’s inventive mind. It was a chain of logic beyond all but the brightest of his kind. But then, on average, every generation of anthros was just a little brighter than the last.

Whiteblood took a flying leap. Both his feet rammed into the potbelly’s back. She was pitched precipitately into the sea.

This fat struggling creature was what the shark had been waiting for. It bit into its prey, in the middle of its torso. The shark’s whole body flexed as it shook the potbelly, and its jagged-edged teeth tore a lump out of the hapless creature. Then, closing through a cloud of diffusing blood, it waited for its victim to bleed to death.

The potbelly was utterly bewildered, suddenly immersed in water, overwhelmed by stunning pain. But her brain flooded with chemicals, and the centers of her functional mind closed down, granting her a sort of peace in this bloody darkness.

Whiteblood sat panting over the scene of his assault, where nothing remained of the potbelly but a pile of thin, ill-smelling shit, and handfuls of crushed leaves. Gradually the gap in the raft closed, as if it were healing itself. The anthros cowered, too stressed even to groom.

And the sun climbed down into the western sky, in the direction they helplessly sailed.

III

Days and nights, nights and days. There was no noise save the creaking of the branches, the soft lapping of the wavelets.

The nights revealed a crushing sky from which Roamer wanted to cower.

But the light of day, under the glaring sun or gray lids of cloud, showed nothing but the elemental sea. There was no forest, no land, no hills. She could smell nothing but salt, and her ears brought her no calls of birds or primates, no herbivorous lowing. The river’s outflow had dispersed now into the greater ocean, and even the other fragments of debris washed down by that torrential storm had dispersed, sailing over the horizon to their own mindless destinies.

The raft itself was diminished.

The anthracothere corpses stuck in the branches of the mango tree had long since slithered away. The last crowder had gone too. Perhaps it had fallen into the sea. The great indricothere had swollen as the bacteria of its huge gut ate their way out toward the light. But the invisible mouths of the sea had been at work on the indricothere, eating into it from beneath. As its meat was steadily stripped away, the huge corpse had imploded, at last sliding beneath the sea.

The anthros had long since eaten all the fruit.

They tried to eat the tree’s leaves, and at first they would be rewarded at least by a mouthful of pleasing moisture that would, for a few heartbeats, ease their thirst. But the tree, uprooted, was dead, and its remaining leaves were shriveling. And, unlike the wretched potbelly, the anthros could not digest such coarse fare, and they lost still more fluid in the watery shit that erupted from their backsides.

Roamer was a small animal built for a life in the nourishing embrace of the forest, where food and water were always plentiful. Unlike a human, whose body was adapted to survive long periods in the open, her body carried very little fat, a human’s main fuel reserve. Things got bad quickly. Soon Roamer’s saliva became thick and tasted foul. Her tongue clung to the roof of her mouth. Her head and neck were very painful, for her skin was shrinking as it dried. Her voice was cracked, and she seemed to have a hard, painful lump in her throat that wouldn’t dislodge no matter how many times she tried to swallow. She and the other anthros would have suffered even more, in fact, if not for the overcast skies that mostly spared them from the glare of the sun.

Sometimes Roamer dreamed. The dead mango would suddenly sprout, its roots reaching out like primate fingers to bury themselves in the unforgiving ocean-soil, the leaves would grow green and wave like grooming hands, and fruit would bloom, huge clusters of it. She would reach for the fruit, even crack it and bury her face in the clear water that mysteriously filled each husk. And here would come her mother and her sisters, fat and full of vigor, ready to groom her.

But then the water would evaporate, as if drying in the harsh sun, and she would find she was gnawing nothing more than a bit of bark or a handful of dead leaves.

Patch came into estrus.

Whiteblood, as the top male of this little lost community, was quick to claim his rights. With nothing else to do and nowhere to go, Whiteblood and Patch coupled frequently — sometimes too often, and the bout would be a perfunctory matter of a few dry thrusts.

In normal times subordinates like the brothers would probably have been able to mate Patch in these early days of her estrus. Whiteblood, with plenty of potential mates to choose from, would have excluded them only when Patch’s peak of fertility approached and the best chance of impregnating her arrived.

This would have been in Patch’s interests too. Her swelling was there to advertise Patch’s fertility to as many males as possible. For one thing, the resulting competition kept the quality of her suitors high without requiring any effort from her. And if all the males in the group mated with her at some time, none of them could be sure who exactly was the father of an infant — so any male tempted to murder an infant to speed up a female’s fertility cycle ran the risk of killing his own offspring. The swellings, her very public estrus, were thus a way for Patch to control the males around her at minimal cost to herself, and to reduce the risk of infanticide.

But on this tiny raft there was only one adult female, and Whiteblood wasn’t about to share. Crest and Left looked on, sitting side by side, chewing on leaves, their comical erections sticking out of their fur. They could stare all they liked at Patch’s refulgent swelling. But every time either of them approached Patch, let alone touched her for the most tentative grooming, Whiteblood would fly into a fury, displaying and attacking the perpetrator.

As for Roamer, she would always be subordinate to Patch, always a stranger. But in these stripped-down conditions she had quickly grown as close to Patch as to one of her own sisters.

While Whiteblood and Patch were coupling, Roamer would often take Scrap. After the first few days Scrap had accepted Roamer as an honorary aunt. The infant’s tiny face was bald and her fur was olive-colored, quite different from her mother’s; it was a color that triggered protective feelings in Roamer, and even in the males. Sometimes Scrap would play alone, clambering clumsily over the matted branches, but more often she wanted to cling to Roamer’s chest or back, or to be held in Roamer’s arms.

Sharing the load of child rearing was common among anthros — although it was usually only kin who would be allowed to serve as child minders.

Anthro infants grew much more slowly than had the pups of Noth’s era because of the time it took their larger brains to develop. Though they were well developed at birth compared to human infants, with open eyes and the ability to cling to their mothers’ fur, anthro pups were uncoordinated, weak, and utterly dependent on their mothers for food. It was as if Scrap had been born prematurely and was completing her growth outside her mother’s womb.

This put a lot of pressure on Patch. For eighteen months an anthro mother had to juggle the daily demands of survival with the need to care for her infant — and she had to keep up grooming time with her sisters, peers, and potential mates. Even before her stranding on this raft, all these pressures had left Patch exhausted. But the society of females around her provided her with a ready supply of would-be aunts and nannies to take the infant away and give her a break. Roamer’s amateur aunting was helpful to Patch, and besides it gave Roamer a lot of pleasure. It was a kind of training for her own future as a mother. But also it let her indulge in a lot of grooming.

They all missed grooming. It was the most difficult thing about this oceanic imprisonment. Even now Whiteblood was showing signs of overgrooming by his two acolytes; parts of his head and neck had been rubbed raw. So Roamer was happy to indulge the infant with long hours of gentle fur pulling, finger combing, and tickling.

But as the days went by the infant, perpetually hungry and thirsty, became increasingly unhappy. Scrap would wander around the raft, and even pester the males. Sometimes she would throw tantrums, tearing at the leaves or her mother’s fur or racing precariously around the raft in her tiny fury.

All of which served to wear out Patch further, and irritate everybody else.

So it went, day after long day. The anthros, trapped together on this sliver of dryness in an immense ocean, were continually, intensely aware of one another. If there had been more space, they could have gotten away from the infant’s annoying scampering. If there had been more of them, the younger males’ jealousy of Whiteblood would not have mattered; they could have easily found more receptive females, and relieved their tension with furtive matings out of Whiteblood’s sight.

But there was no larger group to soak up their tensions, no forest into which to escape — and no food but dry leaves, no water but the ocean’s brine.

One featureless day it all came to a head.

Scrap threw yet another tantrum. She hurled herself around the raft, coming perilously close to the patiently waiting ocean, ripping at leaves and bark, making throaty cries. She had grown skinny, the flesh hanging off her tiny belly, her fur bedraggled.

This time, the males did not slap her away. Instead they watched her, all three of them, with a kind of calculation.

At last Patch retrieved Scrap. She clutched the infant to her chest and let her suckle, though there was no milk to be had.

Whiteblood moved toward Patch. Generally he approached her alone — but this time the bigger of the brothers, Crest, followed him, the spray of fur over his eyes gleaming in the harsh sun. With Whiteblood sitting alongside him, Crest began to groom Patch. Gradually his fingers worked their way toward her belly and genitals. It was a clear precursor to an attempt at mating.

Patch looked startled and pulled away, Scrap clinging to her belly. But Whiteblood stroked her back, soothing her, until she settled and let Crest approach her again. Though Crest continually cast nervous glances at him, Whiteblood did not intervene.

Slumped against the crook of a branch, Roamer stared at the males, baffled by their behavior in a way Noth could never have been. As the minds of the primates became steadily more elaborate, it was as if a sense of self was diffusing outward, from the solitary Purga to her increasingly social descendants. All this enabled the anthros to develop new, complex, subtle alliances and hierarchies — and to practice new deceptions. Noth had had a firm understanding of his own place in the hierarchies and alliances of his society. The anthros could go one step beyond this: Roamer understood her own rank as junior to Patch, but she also understood the relative ranking of others. She knew that a senior like Whiteblood should not be allowing Crest to behave like this, as if encouraging him to mate with “his” female.

At last Crest moved behind Patch and placed his hands on her hips. Patch gave in to the inevitable. Presenting her pink rump to Crest, she pulled the sleepy pup from her chest and held her out to Roamer.

But Whiteblood leapt forward. With the precision of the tree-dwelling primate he was, Whiteblood grabbed the infant from Patch’s hands. Then he scampered over to Left, carrying the infant by her scruff, quickly followed by a nervous Crest.

Patch seemed baffled by what had happened. She stared at Whiteblood, her rump still raised to her vanished suitor.

The males had formed a tight huddle, their furry backs making a wall. Roamer saw how Whiteblood cradled Scrap, almost as if nursing her.

The infant kicked her tiny legs and gurgled, gazing up at Whiteblood. Then Whiteblood put his hand over her scalp.

Suddenly Patch understood. She howled and hurled herself forward.

But the brothers turned to meet her. Both of these immature males outsized Patch. Though they were nervous about showing hostility to a senior female, they easily kept her at bay with slaps and hoots.

Whiteblood closed his hand. Roamer heard the crunch of bone — a sound like a potbelly biting into a crisp leaf. The infant kicked convulsively, and then was limp. Whiteblood looked down on the little body for a heartbeat, his expression complex as he stared at the olive-colored face, now twisted in final pain. And then the males fell on the tiny body. A bite at the neck and the head was soon severed; Whiteblood pulled the limbs this way and that until cartilage snapped and bones cracked. But it was not meat the males wanted most but blood, the blood that poured from the child’s severed neck. They drank greedily of the warm liquid, until their mouths and teeth were stained bright red.

Patch howled, displayed, rampaged around the raft tearing at branches and dying leaves, and beat at the males’ stolid backs. The raft shuddered and rocked, and Roamer clung to her branch nervously. But it made no difference.

Whiteblood had not lied, not really. Like Noth before him, he was unable to imagine what others were thinking, and therefore couldn’t plant false beliefs in their heads — not quite. But anthros were very smart socially, and they had a good problem-solving faculty when facing new challenges. Whiteblood, a kind of genius, had managed to put these facets of his intelligence together to come up with the ploy that had succeeded in stealing Scrap from her mother.

With a final hoarse cry, Patch threw herself against the mango trunk and pulled broken foliage around her in a kind of nest. And still the males fed, to the sound of slurping tongues and bones crunching between teeth.

Her head full of the stink of blood, Roamer made her way to the edge of the raft, where dead branches trailed in the water like fingers.

The murky ocean water was like a thin soup, full of life. The upper sunlit layers were thick with a rich algal plankton, a crowded microscopic ecology. The plankton was like a forest in the ocean, but a forest stripped of the superstructure of leaves, twigs, branches, and trunks, leaving only the tiny green chlorophyll-bearing cells of the forest canopy floating in their nutrient-rich bath. Though the ecological structure of the plankton had remained unchanged for half a billion years, the species within it had come and gone, falling prey to variation and extinction like any other; just as on land this ocean-spanning domain was like a long-running play whose actors changed repeatedly.

A jellyfish wafted by. A plankton-grazer itself, it was a translucent sac, pulsing with a slow, languid dilation and contraction. It was strewn with silvery fronds, tentacles that contained the stinging cells with which it would paralyze its planktonic food.

Compared to most animals the jellyfish was a crude creature. It had a simple radial symmetry, and lacked substance and tissue organization. It didn’t even have blood. But its form was very ancient. Once the ocean had been full of creatures more or less like the jellyfish. They had anchored themselves to the seafloor, turning the ocean into a forest of stinging tentacles. They did not need to be more active; they were untroubled by predators or grazers, as there had not been enough oxygen in the environment to fuel such dangerous monsters.

Roamer was baffled by the sea. To her water was something that came in ponds and rivers and cupped leaves, a fresh, salt-free stuff that you drank whenever you were safe enough to do so. Nothing in her experience or her innate neural programming had prepared her for suspension over a great inverted sky through which drifted such bizarre creatures as the jellyfish.

And she was thirsty, terribly thirsty. Her hand reached down, dipped into that murky soup, and lifted a palmful of water to her mouth. She had forgotten that she had done this not an hour ago, forgotten the bitterness of the brine.

The males had done feeding, she saw. They had fallen into a kind of stupor in the day’s continuing heat. Of Patch, all that could be seen was a single foot, toes curled, that protruded from her lonely nest.

Cautiously, Roamer made her way to the place where the infant had been slaughtered. Blood stained the branches, smeared by the licking of anthro tongues. Roamer picked through the leaves carefully. She found nothing of the infant save a scattering of thin fur — and one perfect little hand, severed at the wrist. She grabbed the hand and retreated to a corner of the raft, as far from the others as she could get.

The hand was limp, relaxed, as if it belonged to a sleeping infant. Briefly Roamer ran it over her chest and remembered how Scrap would pull at her fur.

But Scrap was gone.

Roamer bit into the flesh of the forefinger, close to the knuckle. The meat was soft, irritating her dry palate. With a fast, jerking pull she stripped the flesh off the bone. She repeated that with the other fingers, then munched on the bare flesh of the palm. When the hand had been reduced to little more than skeletal, with a few scraps of cartilage and flesh still hanging off it, she bit through the tiny clattering bones, but there was only a dribble of marrow.

She dumped the bone fragments into the endless ocean. She glimpsed tiny silvery fish quickly clustering, before the bones sank out of sight into the greater deep.

Patch stayed in her nest of leaves for two days, barely moving. The males lay immobile in an untidy heap, occasionally picking at each other’s increasingly sparse fur.

Roamer moved listlessly around the tree, seeking relief. Her mouth no longer generated saliva. Her tongue had hardened into a lump without sensation or mobility, like a stone in her mouth. She couldn’t cry out or call; all she could make was a formless groaning. She even found herself picking at the dried shit left behind by the potbelly, seeking moisture, maybe a few nut kernels embedded in the waste. But the leaf eater’s dung was thin and dry. She sank into misery, exhausted, drifting between sleep and wakefulness.

On the third day after Scrap’s death, Patch stirred. Roamer watched listlessly.

Patch scrambled up to all fours. Dizzy, her fluid balance ruined by her long inactivity, she stumbled — and Roamer saw her grab at her belly. She was pregnant by Whiteblood, a pregnancy that was draining still more reserves from her depleted body. But she raised herself up and, doggedly, approached the males.

Crest sat upright as Patch approached, nervous, as if expecting an attack. Roamer could see his blackened tongue protruding from his mouth. His facial fur was still stained brown by Scrap’s blood.

But Patch settled beside him and began running her fingers through his fur. The grooming was only a partial success. All their bodies had lost fur, and their skin was broken by ulcers and lesions that would not heal; as she worked she broke open scabs and probed at bruises. But he submitted, welcoming the attention despite the pain.

And then she moved away a little, turned her back, and presented her rump to him. She was hardly looking her best. Her fur was ragged, her skin broken, and her swelling had all but vanished, days earlier than it should have. But still, as she pressed her rump into his chest, Crest responded; a spindly erection soon poked out from his matted belly fur.

Now, at last, Whiteblood took notice of this violation of the hierarchy. This was not like his own deception; this was not acceptable. He lurched upright, uttering an incoherent roar around his ruined tongue. Crest backed away.

But Patch immediately attacked Whiteblood, ramming her head into his chest and beating him about the temples with her fists. He fell back, startled. Patch hurried back to the other males and made perfunctory presentations of her rump to them, uttering rasping hoots. And then she threw herself back on Whiteblood.

Subtly, alliances shifted, dominance dissolved. Without even looking at each other the brothers came to a quick decision. They joined in Patch’s attack on Whiteblood. Whiteblood began to fight back, snapping and warding off the blows that rained down on him.

It was a grotesque battle, waged by four badly depleted creatures. The blows and kicks were soft and delivered in an eerie slow motion. And it was waged in a silence broken only by gasps of weariness or pain: There was none of the screeching and hooting that would normally have accompanied an attack by two juniors on a dominant male.

And yet it was deadly. For, under Patch’s leadership, the brothers herded Whiteblood step by step toward the lip of the raft.

It was Patch who delivered the final blow: another ram to Whiteblood’s belly, made with a hoarse, wrenching roar. Whiteblood toppled backward and fell through the raft’s loose fringe of branches and into the water. He bobbed, splashed, and spluttered, his fur immediately becoming soaked and impeding his movements. He looked back at the raft, mewling like an infant around his blackened tongue.

Crest and Left were confused. They had not meant to kill Whiteblood; few dominance battles among the anthros ended lethally.

Roamer felt an odd pang of regret. There had been few enough of them already. Her instincts warned her that too small a pool of potential mates was a bad thing. But it was too late for that.

Whiteblood weakened rapidly. Soon the effort of keeping his mouth and nostrils above the water proved too much for him, and his struggling stopped. The shark, attracted by the blood that leaked from Whiteblood’s stale wounds, took his body in a single bite.

After that, the suffering got worse. As the softly creaking raft drifted over the great unforgiving shield of the ocean, as these small creatures rapidly depleted their reserves, it could only get worse.

Roamer’s limbs had swollen. The stretched skin ached continually and cracked easily. Her tongue squeezed past her jaws, as if her mouth were crammed with a great lump of dry dung. Her eyelids had cracked, and it felt as if she were weeping, but when she touched her fur she found blood leaking from her eyeballs.

She was undergoing a living mummification.

At last, one morning, she heard a cry, high and feeble, like a bird’s.

She pushed away her covering of leaves and sat upright. The world turned yellow, and there was an odd ringing in her ears. It was hard to see anything; her vision was a blur, and when she tried to blink her eyes she got no relief, for her body could spare no moisture.

Still, she made out two anthros — Patch, Crest — sitting side by side over a dark, huddled form. Perhaps it was food. Painfully she pushed her way forward to join them.

It was Left, lying flat, his limbs splayed.

The sucking heat of the sun had done its work well. There was barely any of his white fur left on his head or neck. His flesh had shrunk on his bones. Roamer could see the shape of his skull, of the fine bones of his hands and feet and pelvis. His naked skin had turned purple and gray, and it was covered with huge blotches and streaks. His lips had shriveled to thin strips of blackened tissue, exposing teeth and cracked gums. The rest of his face was black and dry, as if burned. The flesh around his nose had withered, so his two small sideways-pointing nostrils were stretched, exposing the black lining of his nostrils. His eyelids had shriveled too, exposing his eyes in an unblinking, sightless stare at the sun. The conjunctiva that surrounded his eyes, exposed, had turned black as charcoal. He had been scrabbling at the bark, helplessly seeking food, and had cut his hands and feet. But there was no trace of blood; the cuts were like scratches in cured leather.

But he was still conscious, emitting dry, wistful cries. He moved his head gently and spread the fingers of his stronger left hand.

In the end, starved of input, striving to keep its vital systems running as long as possible, Left’s body had consumed itself. Once its fat was gone, muscle had begun to be absorbed, a process that soon resulted in damage to the internal organs — which, badly deteriorated, were beginning to close down.

But in these last moments, Left was in no pain. Even the sensations of hunger and thirst had ceased.

Roamer watched, dizzy, bemused. It was like watching an animated skeleton.

Left’s last eerie calls faded to silence. His fingers remained outstretched, frozen forever in his final gesture. His shrunken stomach growled, and a final noxious belch passed through his lifeless lips.

Roamer looked dully at the others. They were heaps of bone and damaged flesh, not much better off than Left, barely recognizable as anthros at all. They made no effort to groom, to make any kind of contact. It was as if the sun had baked away everything that made them anthros, had stripped them of the gains made painfully in thirty million years of evolution.

Roamer turned away and limped painfully back to her patch of soiled leaves, seeking cover.

She lay passively, shifting only to relieve the pain of suppurating sores. Her mind seemed empty, free of curiosity. She existed in a dull reptilian blankness. She would cram her mouth full of bark and dry leaves, but the dead stuff only scratched her broken flesh.

And she kept thinking about Left’s corpse.

She got up slowly and crossed to Left’s body. His chest had split open, a postmortem wound opened up by the drying of his skin. The stench, oddly, wasn’t too bad. On this brine desert, the processes of decay that would, in the forests, have quickly absorbed Left’s body were largely absent, and the slow mummification that had begun while he was still alive had continued.

Cautiously she pushed her hand into the wound. She touched ribs, already dried. She tugged at the flesh over his chest. It peeled back easily, exposing his rib cage.

There was barely any muscle tissue left on the body. There was no fat either, only traces of a translucent, sticky substance. Within Left’s body cavity she could see his organs, his heart, liver, kidneys. They had been shrunken; they were like hard, blackened fruit.

Fruit, yes.

Roamer pushed her hand into the chest cavity. The rib cage split open with a crack, exposing the meaty fruit within. She closed her hand around his blackened heart. It came away easily, with a soft ripping noise.

She sat with the heart, and, as if it were no more exotic than a peculiar variety of mango, bit into it. The meat was lean, fibrous, and it resisted teeth that wobbled loose in her jaw. But soon she was tearing into the organ, and was rewarded with a little fluid, blood from its core that had not yet dried.

Instead of easing her hunger pangs, the meat only served to inflame Roamer’s atavistic drive to eat. Saliva flowed in her mouth again, and digestive juices pumped painfully through her stomach. She vomited out her first mouthfuls, losing them to the sea, but she persisted until the hard, fibrous meat stayed down.

Left’s eyes, milky white and opaque, still stared sightlessly at the sun that had killed him; and his left hand was curled in its final gesture.

Patch had stirred now. She came loping cautiously toward Roamer. Her skin was a tight sack to which only a few clumps of her once-beautiful black fur still clung. Curious, she rummaged through Left’s open chest. She came away with the liver, which she rapidly devoured.

Meanwhile Crest had not moved. Showing no signs of interest in the fate of his brother, he lay on his side, limbs splayed. He might have been dead, but Roamer made out a subtle movement, a slow rise and fall of his chest, slow as the ocean’s swell, the last of his strength invested in keeping him breathing.

Instinct worked in Roamer now. Patch had been made pregnant by Whiteblood — but perhaps her body had destroyed the fetus by now, absorbing it like its own muscle and fat to keep itself functioning. Two females, alone, had nothing but their own deaths to look forward to. So Crest, the last male, must be preserved.

Roamer returned to the body and plucked out a kidney, another hard knot of blackened, shriveled meat. She carried it to Crest and pushed the meat into his shriveled mouth. At last he stirred. With a gesture as feeble as an infant’s he reached up and took the lump of meat, and began to gnaw it slowly.

The food, such as it was, only made them more hungry, for it lacked the fat they needed to enable them to digest it properly. Still, all three of the survivors returned to the body over and over again, emptying the body cavity, gnawing flesh from limbs, ribs, pelvis, back. When they were done, only scattered bones remained — bones and a skull, from which eyeballs still stared at the sun.

After that the three anthros returned to their solitary corners. If they had been human, now that the taboo of devouring the flesh of their own kind had been broken, a kind of cruel mathematics would have begun to work in their minds. Another death, after all, would have provided the survivors with more food — and reduced the number who would share it.

It was, perhaps, a mercy that none of the anthros were able to plan that far ahead.

IV

The raft jolted under her. It was a sharper motion than the broad, slow swell of the sea. But she was beyond curiosity, and she lay passively in the raft’s rough cradling, knotted branches poking into her thin flesh.

She was in pain constantly now. Her bones felt as if they were working their way out through her skin, which was like one giant ulcer. She could barely close her dried eyelids. Her memory was a disorganized hall of images: the feel of her sister’s strong, grooming fingers, the warm, safe scent of her mother’s milk, the brazen cries of the males who believed they owned them all. But then her soft dreams would be shattered by the irruption of great slavering jaws from the floor of the world…

Now came another jolt, a rustle of the dry timber around her. She heard the noise of waves breaking, quite different from the languorous lapping of the deep ocean.

Birds clattered overhead.

She peered up. They were the first birds she had seen since she had been washed from the land. They were brilliant white, and they wheeled high above her.

Something moved on her chest. It felt like fingers, tentatively scratching: perhaps someone was trying to groom her. With an immense effort she lifted her head. It lolled, her skin tight like a mask, her tongue a block of wood in her mouth. She had difficulty focusing her bleeding eyes.

Something was crawling over her: a flat orange shape with many segmented legs and big raised claws. She yelped, a thin, dry sound, and brushed her arm over her chest. The crab scuttled away, indignant.

With nostrils baked black as tar, she could smell something new. Water. And not the stinking brine of the sea, but fresh water.

She lifted an arm and grabbed at the foliage. Every scrap of her raw flesh, as scabs and blisters pulled and broke, was a source of lancing pain. With an immense heave she managed to get herself upright, her feet under her, her legs folded. Her head lolled, too heavy for her neck. It took her more energy yet to raise it, to squint through her broken eyes.

Green.

She saw green, a great horizontal slab of it, running from horizon to horizon. It was the first green she had seen since the last of the mango’s leaves had curled and browned. After so many days of blue and gray, of nothing but sky and sea, the green seemed vibrantly bright, so bright it almost hurt her eyes, beautiful beyond imagining, and just looking at it seemed to strengthen her.

She levered herself forward, half crawling. The mango’s dead foliage pricked and cut her, but there was no blood to flow, nothing but dozens of tiny sources of pain.

She reached the edge of the raft. No ocean, no water. She saw a shallow beach of coarse, young sand, stretching up a short rise to the foot of a sparse forest. Birds, bright blue and orange, flittered through the tops of the trees, piping brightly.

Her first impression could have been summarized as I am home. But she was wrong.

She pulled herself over the branches and half fell onto the sand. It was hot, very hot, and it burned her exposed skin. She mewled, pulled herself up, and limped forward, as if she had grown very old, up the beach toward the forest.

At the forest edge was an undergrowth of low ferns and blessed shade. Taller trees towered above her. On their branches were clusters of a red fruit she didn’t recognize. Her mouth was too dry to salivate, but her tongue clicked against her teeth.

She glanced back the way she had come. The mango tree and its raft of vegetation was just a scrap of driftwood, broken, rotten, seaweed clinging to it, now washed up on this shore. She could see the unmoving form of an anthro — Patch or Crest — lying inert on the broken, salt-crusted foliage. And beyond the raft the sea rolled, huge, eternal, blue gray, reaching as far as she could see to a horizon of chilling geometric perfection.

Now there was a crashing tread, a great snapping of foliage. Roamer shrank back.

A giant form emerged from the forest, like a tank rolling through the undergrowth. Huge, squat, under a great bony dome of a shell, it looked like a giant tortoise — or perhaps an armored elephant — a great plated body supported by four stumpy legs. Behind it a tail swung carelessly, tipped by a spiky club. And as its small reinforced head pushed out into the light, armored eyelids blinked. This tremendous ankylosaur-like creature was a glyptodont. Roamer had never seen anything like it in Africa.

But then, this wasn’t Africa.

The giant armored monster lumbered away. Cautiously Roamer followed the glyptodont deeper into the forest. She came to a clearing, surrounded by a wall of tall, imposing trees. The floor was carpeted by aloes. Experimentally Roamer nibbled at a leaf. It was succulent, but bitter.

She moved further forward, and found the glimmer of still water. It turned out to be a shallow, reed-choked freshwater pond. At its shore browsed a pair of huge animals. They grazed on the plants at the pond’s edge with snouts like spatulas. They looked like hippos, but were actually immense rodents.

The pond was on the fringe of a broader plain. And there, dimly visible now, much stranger mysteries awaited Roamer. There were creatures that might have been horses, camel, deer, and smaller animals, like hoofed pigs. Alongside them moved a small family of dinomyids: bulky, bearlike grazers. They were giant rodents, the extravagant relations of dormice and rats. There were predators here too, creatures who ran in packs like dogs — but they were marsupials, only distantly related to placental counterparts elsewhere, shaped by convergent evolution, similarly adapted for a similar role.

From a green shadow near Roamer, a head turned, startling her. The head was upside down. Two black eyes peered at her dimly. Above the head was a huge brown-furred body, dangling from limbs that clutched a branch above. This was a sloth, a kind of megatherium.

Cautiously, Roamer crept forward, at last, to the pool. The water was muddy, greenish, warm. But when she plunged her face into it, it was the most delicious thing she had ever tasted. She sucked down great mouthfuls. Soon her shrunken belly was full, and agonizing pains shot through her, as if she were being torn apart from inside. She fell forward, crying out, and threw up almost all she had drunk. But she pushed her face back into the water and drank again.

This brackish pond was actually a sinkhole. Fifty meters deep, it had been caused by groundwater dissolving the underlying limestone. There were many such sinkholes in the area, aligned along great, deep-buried faults in the rocks.

Seen from the air, the sinkholes would have formed a huge semicircle some hundred and fifty kilometers across. The arc of sinkholes marked a boundary fault of the ancient, long-buried Chicxulub crater, the rest of which stretched under the Gulf of Mexico’s shallow waters and sediments. This was the Yucatan Peninsula.

Expelled by an African river, riding westward currents, Roamer’s raft had crossed the Atlantic.

Nowhere on Earth was truly isolated.

Everywhere was connected by the ocean currents, some of which covered as much as a hundred kilometers a day. The great currents were like conveyor belts that bore flotsam around the world. In later times, inhabitants of Easter Island would burn logs of American redwood, washed ashore after a journey of five thousand kilometers. People living on coral atolls in the deep Pacific would make tools from stones embedded in the roots of stranded trees.

With the flotsam traveled animals. Some insects rode the surface of the water itself. Other creatures swam: Westward currents could carry leatherback turtles across the Pacific from their feeding ranges near Ascension Island to breeding grounds in the Caribbean.

And some animals rode across the oceans on impromptu rafts — oceanic odysseys undertaken not by choice or design, but by the vicissitudes of chance, just as had befallen Roamer.

The Atlantic, which had been widening since the shattering of Pangaea, was still much narrower than in human times: no more than five hundred kilometers wide at its narrowest point. It was not an impossible distance, a traverse that could be survived even by fragile forest creatures like Roamer, with luck. Such crossings were improbable. But they were possible, given the outflows of mighty rivers, the narrow oceans, perhaps the help of hurricane winds.

On the longest of timescales, over millions of years, the workings of chance defied human intuition. Humans were equipped with a subjective consciousness of risk and improbability suitable for creatures with a lifespan of less than a century or so. Events that came much less frequently than that — such as asteroid impacts — were placed, in human minds, in the category not of rare, but of never. But the impacts happened even so, and to a creature with a lifespan of, say, ten million years, would not have seemed so improbable at all.

Given enough time even such unlikely events as ocean crossings from Africa to South America would inevitably occur, over and again, and would shape the destiny of life.

Thus it was now. In the trees that towered above Roamer there was not a single primate — not one, not in all the continent, for her remote cousins, other children of Purga, had succumbed to extinction here millions of years ago, beaten by the rodents’ competitive pressure.

So, in this place where a world had ended, where differently evolved creatures foraged through different forests, a new life was starting, a new line of Purga’s great family. From just three survivors — given enough time, and the slow plastic working of their genetic material — would radiate a whole spectrum of new species.

By any standards the New World monkeys would be successful. But on this crowded jungle continent, the fate of Roamer’s grandchildren would be quite different from those of her sister’s in Africa. There, the primates, molded cataclysmically by the shifting climate, would rapidly develop new forms. There, Purga’s line would continue — through the apes — its slow shaping toward humanity. Even the later monkeys who Roamer so resembled would diversify away from the forest, finding ways to live in savannah, mountain plateaus, and even deserts.

Here it would be different. On a more equable continent, it would always be too tempting to stay in the vast rain forests.

Roamer’s grandchildren would never leave the trees. They would never grow much smarter than they were now. And they would play no part in the future destiny of mankind — save as pets, or prey, or objects of scientific curiosity.

But all that lay in the unimaginable future.

Roamer already felt remarkably revived by her brief time in the green and the water she had drunk. She looked around. In the undergrowth she saw a splash of red, and she stumbled that way. She found a fruit, unfamiliar, but fat and soft-skinned. She bit into it. As she munched on the flesh, juice burst out and dribbled over her fur. It was the cleanest, sweetest thing she had ever tasted.

I

The burrowers worked through the tough, scrubby grass that clung to the dunes. There were many, many of them. They were so crowded they looked like a ground-covering carpet of squirming brown-gray fur.

Dig spotted a dense patch of ferns on a little headland overlooking the ocean. The foraging crowd seemed a little less dense there, so she headed that way. In the shelter of the fern patch, she picked apart fronds with her agile, five-fingered hands, and she nibbled on brown spores.

At three years old Dig was already one of the oldest of the burrowers. She was just a few centimeters long. She was fat and round and coated with thick layers of brown fur, the better to retain her body’s heat. She looked something like a lemming. But she was no lemming. She was a primate.

From here she could see the ocean. The sun hung low in the northern sky, over the endless, impassable water. As polar autumn drew on, the sun spent more than half of each day beneath the horizon. And already, far from the land, great sheets of pack ice had gathered. Closer to shore Dig could see slushier gray ice forming in great sheets that rippled over the water’s muscular swell. Her body knew what these things meant. The light-filled days of summer were a blurred memory; soon she would have to endure the winter months of continual darkness.

On one pack ice plate she saw a bloody stain, smeared over the gleaming surface, and an unidentifiable mound of inert flesh. Birds wheeled overhead, cawing, waiting their turn at the bloody pickings. And a shadow slid through the water, long, powerful. A huge snout pushed out of the chill water to take a share of the kill.

The seagoing carnivore was an amphibian, a descendant of a form called koolasuchus. Four meters long, it looked like a monstrous predatory frog. The frog was a relic of much more ancient times, when amphibians had dominated the world. In tropical climes, its ancestors had been outcompeted by the crocodiles, whom they closely resembled in size and form; the great amphibians had already been in decline when dinosaurs first appeared on the Earth, but they had clung on in the cooler waters of the poles.

Even from this distance, tucked under her ferns, Dig shuddered.

Suddenly a squat, feathered form came bursting from the tundra plain. The scrambling burrowers scattered in panic, and Dig cowered. The new arrival ran upright on long, powerful legs, and its hands, barely visible against thick white feathers, were grasping and equipped with cruel claws. This creature ran out into the water and splashed its way out to the ice floe. There it began to compete with the amphibian for scraps of the carcass, just as in later times Arctic foxes would try to steal the kills of polar bears.

This battling white-feathered predator looked like a flightless bird. It wasn’t. It was a descendant of the velociraptors of the Cretaceous era.

On Antarctica, fifty-five million years after the comet impact, there were dinosaurs.

Dig made her way inland, away from the bloody scene at the shore. She moved cautiously, sticking to cover. Here and there she saw white feathers, discarded by the raptor in its haste to reach the kill on the ice.

As she clambered over the last dune, she could see the shape of the landscape.

It was a broad plain of green and brown, littered here and there by the blue of water. The grass was still thick, though it had begun to die back, and where it had not yet been cropped to the ground, it was turning golden brown. Most of the flowers had gone, for there were no insects to attract; but here and there bright, pretty blooms like saxifrage still lingered. Around the glimmering freshwater ponds, animals crowded, seeking drink. But the ponds were already gray with surface ice.

It was a classic tundra scene, part of a belt of such landscape that still encircled the continent.

And, over this tundra, dinosaurs walked.

A few kilometers to the southwest, Dig saw what looked like a dark cloud washing over the ground. It was a herd of muttas. Their breath created great clouds of steam that hung in the chill air. They were dinosaurs, huge herbivores. From a distance they looked like tuskless mammoths. But closer in it could be seen that they retained classic dinosaur features: Their hind legs were more powerful than their forelegs, they had powerful balancing tails, they behaved in an oddly skittish and nervous way, more like birds than any huge mammal — and sometimes they would rear up on their hind legs and bellow with the ferocity of a tyrannosaur.

The muttas were descended from muttaburrasaurs, beefy Jurassic herbivores that had once feasted on cycads, ferns, and conifers. As the cold had descended on Antarctica, the muttas had learned to subsist on coarse tundra produce. Their bodies had become squat and round, and they had developed a thick coat made up of multiple layers of dark-brown, scaly feathers. Gradually they became large, migrating tundra herbivores, a role later occupied elsewhere by animals like caribou and musk oxen — and mammoths. Their mournful hooting, made with inflatable skin sacs on their great horny snouts, echoed from the walls of ice to the south.

Once the muttas had migrated all over this continent, taking advantage of the short, rich summer. But as the ice had spread the muttas’ numbers had been much diminished, and now, somewhat forlornly, the remaining herds wandered around the narrowing tundra fringe between ice and sea.

This mutta herd was being stalked by a solitary hunter.

Standing stock-still, the dwarf allosaur inspected the mutta herd. It looked like a golden, feathered statue. The allo was a dwarfed relic of a family of creatures long extinct elsewhere — a direct descendant, in fact, of the Jurassic lion who had killed Stego. But the herd was wary of the allo and stayed tightly bunched, their young at the center. This allo’s movements were slow, as if it had been drugged. Its hunting had already been successful; with its store of fat laid in, its metabolism was already slowing as the air’s chill gathered. Soon the allo would dig out its customary winter den in a snowbank, after the manner of polar bears.

Female allos laid their eggs toward the end of winter, and hatched them out inside their snowy dens, where they would be safe. For the mammals of Antarctica, spring was made more interesting by the possibility that from any snowbank there might suddenly erupt a clutch of ravenous allosaur chicks, snapping and squabbling in pursuit of their first meal.

Now there was a commotion among a throng of burrowers, not far from Dig, and the cold breeze off the ice cap brought her a sharp, meaty scent. Eggs.

She ran as hard as she could through the ferns and the long grass, for once reckless of her own safety.

The nest contained dinosaur eggs: the eggs of a mutta. This was an unusual find so late in the season, and far away from the muttas’ usual nesting sites. Perhaps these eggs had been laid by a sick or injured mother. There were burrowers already at work here, and in amongst the squabbling crowd there were a few bulkier steropodons: clumsy, black-haired, oddly primitive-looking, these creatures were descended from mammals that had inhabited the southern continent since Jurassic times.

Dig was able to force her way into the nest before it was utterly destroyed. Soon her face and hands were coated with sticky yolk. But the competition for the eggs quickly degenerated into a ferocious battle. There were many, many burrowers here on the tundra this autumn, many more than last year. And Dig was smart enough to be worried by the burrowers’ overcrowding on a deep, gnawing level.

There was no simple cause for rises in numbers like this. The burrowers were locked into intricate ecological cycles involving the abundance of the vegetation and insects they browsed, and the carnivores who preyed on them in turn. At times of excess bodies it was the burrowers’ instinct to get away, to strike out blindly over the green land in search of empty places to establish new burrows. Many of them fell to predation, but that was the way of things: Enough of them would survive.

At least, that was how it had happened in the past. But now, as the ice advanced and the tundra shriveled back, there was nowhere left to go that wasn’t already colonized. And so there were always great crowds like this, and you always had to fight.

Of course, it was bad for the mutta who had laid these eggs. The muttas hatched their eggs on the ground, as their ancestors had always done, which made them vulnerable to opportunistic predators like the burrowers. Indeed, the key cause of the decline in the muttas’ numbers was the increasing competition for the protein locked up in their huge eggs. Giant mammalian herbivores, like mammoths or caribou, would have fared better here, as their young would have been safer at such a crucial moment in their lives. But the muttas, stranded like the rest when Antarctica had sailed away from the other continents, had had no choice in the matter.

Suddenly a claw came sweeping out of the sky. With an instinct more than two hundred million years old, Dig flattened herself against the ground, while burrowers squeaked and scrambled over each other.

The claw grabbed a small, immature burrower and popped her whole into a gaping mouth. Again the claw burned through the air, grasping as if frustrated. But the mammals had scattered. And after a time Dig heard the unmistakable sound of chewing, as a toothed beak crushed one mutta embryo after another.

This bandit was a leaellyn. Another dinosaur, it looked like an athletic chicken. Not equipped to be effective hunters of large prey, the leaellyns were mainly opportunistic scavengers. For this leaellyn, like the mammals, a mutta egg this late in the season was a rare treat.

As the leaellyn fed, Dig tried to lie still to avoid the killer’s attention. But she was hungry. It had been a short, poor summer, and it had been impossible for her to fatten herself up as much as she needed to face the privations of winter. And the leaellyn was taking the eggs — taking all of her eggs.

Anger and desperation at last overrode caution. She raised herself up on her hind legs, hissing, her paws spread.

The leaellyn, blood and yolk smeared around its mouth, flinched back, startled by this sudden apparition. But, its small reptilian mind soon told it, this was not a threat to a leaellyn. In fact, this warm furry ball, for all its unusual posture, was good to eat, better than embryos and yolk.

The leaellyn opened its mouth and leaned forward.

Dig evaded and escaped. But she had to abandon the nest, and hunger burned inside her belly.

Dig could have traced her lineage back to Plesi, the little carpolestid who had inhabited the warming world a few million years after the fall of the Devil’s Tail. Plesi’s offspring had wandered the planet, using land bridges, islands, and rafts to cross from one island continent to another. One branch of the ancient family had crossed a land bridge between South America and Antarctica at a time when the southern continent had yet to settle over the pole.

And here it had encountered dinosaurs.

Even during the warm Cretaceous the dinosaurs of Antarctica had had to endure long months of polar darkness. So those chance survivors who had lived through the global catastrophe here had been well equipped to endure the comet winter that had followed while their contemporaries in the warmer latitudes had perished.

But the continents had drifted further apart, fragments of the wreckage of the ancient supercontinent still separating. Antarctica had spun away from the other pieces of southern Pangaea, soon traveling so far that no land bridge, no rafting was possible. And as the world recovered from the impact, the flora and fauna of Antarctica had begun to explore their own unique evolutionary destiny. Here the ancient game of dinosaur versus mammal had been granted a long, drawn-out coda — and here, still, thanks to the twin ferocity of the dinosaurs and the cold, the mammals had remained trapped in their humiliating Cretaceous niches.

But at last Antarctica had settled over the southern pole, and the great ice cap had slowly grown.

The days grew short, the crimson sun arcing only briefly over the horizon. The ground hardened with frost. Many plant species died back to ground level, their spores waiting out the return of the summer’s brief warmth.

There was little fresh snow. In fact, much of the continent was technically a semidesert: What snow did fall came as hard crystalline flakes that rested on the ground like rock until the wind gathered it into banks and drifts.

But the snow, sparse as it was, was essential for the burrowers.

Those who had survived the summer and autumn began to dig into the snow drifts, constructing intricate tunnel systems beneath the hard-crusted upper layers. The tunnels were elaborate, humid, nivean cities, the walls hardened by the passage of many small, warm bodies, the air filled with a warm, damp-fur smell. The burrows were not exactly warm inside, but the temperature never dropped below freezing.

Outside, auroras flapped silently across the star-stained winter sky.

The leaellyn who had stolen the eggs from Dig was one of a pack, mostly siblings, who had hunted together in a small group centered on one dominant breeding pair. In the winter, as they felt their customary cold-weather torpor come on, the leaellyn pack huddled together.

The leaellyns were descended from small, agile herbivorous dinosaurs that had once swarmed in great nervous clans over the floor of the Antarctic forest. In those days the leaellyns could grow as large as an adult human, and they had big eyes well adapted to the darkness of the polar forests. But with the great chill the leaellyns had become dwarfed, fatter, and covered with scaly feathers for insulation.

And, as the megayears had worn away, they had learned to eat meat.

As the cold deepened, the pack members slid into unconsciousness. Their metabolisms slowed to a crawl, astonishingly slow, just enough to keep their flesh from freezing. It was an ancient strategy, shaped by millions of years of habitation in these polar regions, and it had always proven effective.

But not this time. For there had never been a winter as cold as this. In the worst of it the leaellyn group was overwhelmed by a storm. The savage wind took away too much of their body heat. Ice formed inside the leaellyns’ flesh, shattering the structure of their cells; gradually the frostbite extended cold daggers deep into their small bodies.

But the leaellyns felt no pain. Their slumber was a silent, dreamless, reptilian sleep, deeper than any mammal would ever know, and it segued smoothly into death.

Every year the summers were shorter, the onset of winter harder. Each spring the great ice cap that lay over the center of the continent, a place where nothing could live, advanced a little further. Once there had been tall trees here: conifers, tree ferns, and the ancient podocarps, with clusters of heavy fruit at their bases. It had been a forest where Noth would have been at home. But now those trees existed only as seams of coal buried deep beneath Dig’s feet, long since felled by the cold. It had been many millions of years since any of Dig’s ancestors had climbed off the ground.

The primates of Antarctica had had to become adapted to the cold. They could not grow larger; competition with the dinosaurs saw to that. But they developed layers of insulation, fat and fur, designed to trap their body heat. Dig’s feet were kept so cold there was little temperature difference between them and the ground, and little heat was lost. Cold blood coming up into her torso from her feet was pushed through blood vessels containing warm blood that ran the other way. So the descending blood was cooled before it ever got to her feet. The fat in her legs and feet was a special kind, made of shorter hydrocarbon chains, with a low melting point. Otherwise it would have hardened, like chilled butter. And so on.

For all her cold adaptations Dig was still a primate. She still retained the agile hands and strong forearms of her ancestry. And, though her brain was much diminished from her ancestors’ — in this straitened environment a big brain was an expensive luxury, and animals were no smarter than they needed to be — she was smarter than any lemming.

But the climate was getting colder yet. And every year the remnant animals and plants were crowded into an increasingly narrow strip of tundra close to the coast.

The endgame was approaching.

Dig found herself laboring for breath.

In a sudden panic, she scrabbled at the snow above her, hands evolved for climbing trees now digging their way through a roof of snow.

She pushed her way out of the burrow and into a thin spring light, shockingly bright. A gush of fetid air followed her, steaming in the cold — fetid, and laden with the stink of death.

She was a bony bundle of urine-stained skin and fur on a vast, virgin snowscape. The sun was high enough above the horizon to hang like a yellow lantern in a purple-blue sky. Spring was advanced, then. But nothing moved: no birds, no raptors, no dwarf allo chicks erupting from their wintry caves. No other burrower emerged onto the snow; not one of her own kin followed her.

She began to work her way down the bank of snow. She moved stiffly, her joints painful, a ravening hunger in her belly, a thirst in her throat. The long hibernation had used up about a quarter of her body mass. And she was shivering.

Shivering was a great failure of her body’s cold-resistant systems, a last-resort option to generate body heat with muscular movement that burned up huge amounts of energy. Shivering shouldn’t be happening.

Something was wrong.

She reached the bare ground that fringed the sea. The soil was icebound, still as hard as a rock. And despite the lateness of the season nothing grew here, not yet; spores and seeds still lay dormant in the ground.

She came across a group of leaellynasaurs. In the cold, they had intertwined their limbs and necks until they had formed a kind of interlocked, feathery sculpture. Instinctively she flattened herself against the snow.

But the leaellyns were no threat. They were dead, locked in their final embrace. If Dig had pushed, the assemblage might have toppled over, frozen feathers breaking off like icicles.

She hurried on, leaving the leaellyns to their final sleep.

She reached a little headland that overlooked the ocean. She had stood in this place at the end of last summer, under a small stand of fern, watching raptor and frog battle. Now, even the fern’s spores were locked inside the bare ground, and there was nothing to eat. Before her the sea was a plain of unbroken white, all the way to the horizon. She quailed before the lifeless geometry: a horizon sharp as a blade, flat white below, empty blue dome above.

Only at the shore was there a break from the monotony. Here the sea’s relentless swell had broken the ice, and here, even now, life swarmed. Dig could see tiny crustaceans thrusting through the surface waters, gorging themselves on plankton. And jellyfish, small and large, pulsed through this havoc, all but translucent, lacy, delicate creatures that rode the swell of the water.

Even here, at the extremes of the Earth, the endless sea teemed with life, as it always had. But there was nothing for Dig.

As the great global cooling downturn continued, so the great clamp of the ice tightened with each passing year. The unique assemblage of animals and plants, trapped on this immense, isolated raft, had nowhere to go. And in the end, evolution could offer no defense against the ice’s final victory.

It was a gruesome extinction event, hidden from the rest of the planet, drawn out over millions of years. An entire biota was being frozen to death. When the animals and plants were all gone, the monstrous ice sheet that sat squat over the continent’s heart would extend further, sending glaciers to grind their way through the rock until the ice’s lifeless abstraction met the sea itself. And though the deeper fossils and coal beds of ancient times would survive, there would be no trace left to say that Dig’s world of tundra, and the unique life that had inhabited it, had ever existed.

Dispirited, she turned away and set off over the frozen ground, seeking food.

I

As light leaked into the sky, Capo woke. Lying in his treetop nest he yawned, his lips spreading wide to expose his thick gums, and he stretched his long, furry limbs. Then he cupped his balls in one hand and scratched them comfortably.

Capo looked something like a chimpanzee, but there were not yet any chimps in the world. He was an ape, though. In the long years since the death of Roamer, the burgeoning families of primates had diverged, and Capo’s line had split off from the monkeys some twenty million years ago. And yet — still some five million years before the rise of true humans — the great age of the apes had already come and gone.

Capo squinted into the sky. The sky was gray blue and free of clouds. It would be another long, hot, sunny day.

And a good day. He rubbed his penis thoughtfully. His morning erection felt tight, as it always did. Some of the most troublesome of the subordinate males had sloped off into the deeper forest a few days ago. It ought to be weeks before they were back, weeks of relative calm and order. Easy work for Capo.

In the stillness of morning, sound carried far. Lying here, his thoughts rambling, Capo could hear a distant roaring, like the endless grumble of some vast, wounded beast. It came roughly from the west. He listened for a few heartbeats, and his hairs prickled at the sullen majesty of the never-ending, baffling rumble; it was a sound of awesome power. But there was never anything there, nothing to see. It had been there in the background all his life, unchanging, incomprehensible — and remote enough not to matter.

He felt a nagging unease, but not about the noise. A vague concern crept up on him in such reflective moments.

Capo was more than forty years old. His body bore the scars of many battles and the patchy baldness of endless grooming. He was old enough and smart enough to remember many seasons — not as a linear narrative but in glimpses, shards, like vivid scenes cut out of a movie and jumbled up. And on a deep level he knew that the world was not as it had been in the past. Things were changing, and not necessarily for the better.

But there was nothing to be done about it.

Languorously he rolled over onto his belly. His nest was just a mass of thin branches folded over and kept in place by his weight. Through its loose structure he could make out the troop scattered through the tree’s foliage, primates roosting like birds. With a soft grunt he let his bladder go. The piss sprayed messily from his penis, still half erect, and rained down into the tree.

It splashed Leaf, one of the senior females, who had been asleep on her back with her infant clutching her belly fur. She woke with a start, wiping thick urine from her eyes, and hooted her protest.

His time of reflection over, the last of his erection shriveling, Capo sat up and vaulted out of his nest.

Time to go to work. A great black-brown ball of fur, he proceeded to crash through the tree. He smashed nests, punched and kicked their occupants, and screeched and leapt. He kept this up until the whole tree was a leafy bedlam, and there was no possibility of anybody staying asleep, or not being aware of Capo’s dominant presence.

He made one very satisfactory hard landing right in the middle of the nest of Finger, a stocky younger male with a very agile brain and hands. Finger curled up, chattering, and tried to lift his backside in a gesture of submission. But Capo targeted the backside with one well-aimed kick, and Finger, shrieking, went tumbling down through the foliage toward the ground. It was high time Finger was taught a lesson; he had been getting too cocky for Capo’s liking.

At length Capo reached the ground, fur bristling, panting hard. He was on the edge of a small clearing centered on a clogged, marshy pond. He still wasn’t done with his display. He threw himself back and forth around the line of trees, drumming with open palms on tree trunks, ripping off thin branches and shaking them so their leaves cascaded around him, screeching and hooting the while.

Finger had picked himself up from where he had plummeted. Limping slightly, he crawled into the shade of a low palm and cowered away from his master’s display. Other males hopped and hooted in backside-kissing support. One or two females were already up. They kept out of Capo’s way, but otherwise maintained their own morning routines.

As he finished his display, Capo spotted Howl, a female with a peculiarly high-pitched way of calling. She was crouching at the base of an acacia, picking lumps off a morel and popping them into her mouth. Howl was not yet pubescent, but she wasn’t far off. Spying the tight pucker of her genitals, Capo immediately hardened.

Still bristling, panting a little, he swaggered over to Howl, lifted her hips, and smoothly entered her. Her passage was pleasingly tight, and Capo’s supporters hooted and growled, drumming on the ground, urging on their hero. Howl did not resist, adjusting her posture to accommodate him. But as he thrust, she continued to pluck bits of morel, not much interested.

Capo withdrew from Howl before he ejaculated: too early in the day for that. But as a coup de grâce he turned his back on his cowering subordinates, bent over, and ejected a spray of shit that showered over them. Then he threw himself flat on the grass, arms akimbo, and allowed a few of his more favored subordinates to come close and begin the day’s grooming.

Thus the great boss, the capo di capo of this troop — the progenitor of mankind, the ancestor of Socrates and Newton and Napoleon — had begun his day in suitable splendor.

The next priority was to fill his belly.

Capo selected one of his subordinates — Frond, a tall, sinewy, nervous creature — and, hooting loudly, delivered a series of slaps and blows to the cowering creature’s head.

Frond quickly got the message. His assignment was to lead the troop’s daily forage in search of food and water. He selected a direction — east, as it happened, into the light of the rising sun — and, his gait a mixture of clumsy knuckle-walking and upright sprinting, he ran back and forth along a trail that led that way, glancing back at Capo for approval.

Capo had no reason not to choose that direction. With a swagger, his big knuckles punching into the soft ground, he set out after Frond. The rest of the troop quickly formed up behind him, males and females alike, infants clinging to their mothers’ bellies.

The troop worked its way through the trees at the forest fringe, foraging systematically, after their fashion. Mostly they sought fruit, though they were prepared to take insects and even meat if it was available. The males noisily postured and competed, but the females moved more calmly. The smallest infants stayed with their mothers, though older youngsters rolled and wrestled.

As they worked their endless way through the forest, the friendships of the females quietly endured. The truth of Capo’s society was that the females were its foundation. The females stuck to their kinship groups and shared the food they found — a practice that made good genetic sense, as your aunt and nieces and sisters shared your own heritage. As for the males, they just went where the females went, their dominance battles a kind of showy superstructure, signifying very little of true importance for the troop.

With a moist dick and pleasantly aching fists, and the prospect of a belly soon to be filled, Capo ought to have been as happy as he could be. Life was good, here in the forest. For Capo, top of the heap, it could hardly get any better. But still that bit of unease lingered.

Unfortunately for Capo’s mood, the pickings that morning were poor. They were forced to keep moving.

They came across other animals, here in the forest. There were okapi — short-necked giraffes — and pygmy hippos and dwarf forest proboscideans. It was an ancient fauna clinging to the conservatism of forest ways. And there were other primates too. They passed a pair of giants: huge, broad-shouldered, silver-haired creatures who sat massively on the ground, feeding on the leaves they plucked from the trees.

They were like the potbellies of Roamer’s day. Capo’s forebears had developed a new kind of teeth, the better to cope with their fruit diet: Capo had large incisors for biting, necessary for fruit, whereas his molars were small. These leaf eaters’ teeth were the other way around; leaves didn’t need much biting but took a lot of chewing. Closely related to the gigantopithecines of Asia, these great beasts, weighing a quarter of a ton each, were among the largest primates who would ever live. But the giants were rare in Africa now.

They were not in direct competition with Capo’s troop, who, lacking the giants’ immense multiple fermenting stomachs, could not feed on leaves. Still, it bothered Capo to have to divert his course to avoid these silent, patient, statuesque creatures. Not wishing to lose face, Capo knuckle-walked up to the larger of the giants — a male — and displayed, fur bristling, running in circles, drumming on the ground. The leaf eater watched, impassive and incurious. Even sitting down he towered over Capo.

Honor satisfied, Capo skirted the giants and moved on.

It wasn’t long before the morning march came to an end, as the troop ran out of trees.

Here was the root of Capo’s unease. This shrinking, half-flooded patch of forest was not as abundant a home as it used to be. It was just an island, in fact, in a greater, more open world.

Peering out of the trees, he glimpsed that world, still emerging from a misty dawn.

This scrap of forest lay in the palm of an extensive, glimmering plain. The land was like a park, a mix of open green plains and patches of forest. Much of the forest was palms and acacias, but there was some mixed woodland, both conifers and deciduous trees — walnut, oak, elm, birch, juniper.

What would most have surprised Roamer, Capo’s distant great-aunt, was the nature of the ground cover that stretched over those open green areas. It was grass: hardy, resistant, now spreading with slow, unheralded triumph across the world.

And on the plain there were many, many lakes, ponds, marshes. Mist rose everywhere, the sun’s early heat filling the air with moisture. A great river, having spilled from the southern highland, curled lazily over the plain. Around its banks stretched extensive floodplains, some of them marshy or sheets of open water. The land was like a full sponge, brimming with water. Some of the trees were dying, their roots in some cases actually standing in shallow water. The forest remnants, already shrunken by the world’s continuing cooling and drying, were being drowned.

This soggy plain stretched to the north as far as Capo’s eyes could see. But off to the south the land climbed to an immense wall notched by the outflow of that mighty river. Before that great ridge was a more barren area littered with wide, bone-white sheets of salt, on some of which stood small, stagnant-looking lakes.

There was a bellow from the north, and Capo turned back that way. The animals of the plain were going about their business. In the distance Capo could see what looked like a herd of wild, overgrown pigs rooting in the long grasses. Their low-slung gray-brown bodies made them look like huge slugs. They were not pigs or hippos; they were anthracotheres, a holdover from much more ancient times.

Two huge chalicotheres worked their way slowly across the plain, plucking at shrubs with their huge paws. They picked only fresh shoots, and put them into their mouths, delicate as pandas. The taller, the male, was nearly three meters high at the shoulder. They had bulky bodies and stocky hind legs, but their forelegs were long and surprisingly graceful. But, because of their long claws, they could not put their front feet on the ground, and walked on their knuckles. In their bodies they looked a little like huge, short-haired gorillas, but they had long equine heads. These ancient animals were cousins of the horses. Once they had been widespread, but now the shrubs on which they depended were becoming scarce; this species was the last of the chalicothere kind.

Closer to hand, the apes could hear a steady, noisy rustling. Hesitant, they peered out. A family of a kind of elephant was working at the trees at the forest clump’s edge, using their trunks to pull away branches and cram foliage into their mouths. These were gomphotheres, massive creatures. Each had four tusks, a pair protruding from both upper and lower jaws, giving its face the look of a forklift.

This was the heyday of the proboscideans. The very successful elephantine body plan had spun off a whole range of species across the world. In North America the mastodons would survive until humans arrived. Another family was the shovel-tuskers like these gomphotheres, with their hugely expanded and flattened lower tusks. And, walking through Africa and southern Asia, there were the stegodons, with long, straight tusks. They were the ancestors of the true elephants and the mammoths, who had yet to appear.

The sound of the gomphotheres’ calls, carrying far in the cold morning air and echoing deep into the infrasonic, was eerie. These particular proboscideans were omnivorous. They were scarcely fleet-footed hunters. But on the whole a meat-eating elephant was best avoided.

That was when Frond, the spindly male, unexpectedly knuckle-walked out of the forest’s shade and into grass tall enough to come up to his shoulders. The grass waved around him, stirred by a breeze, languid waves crossing the empty acres.

Hesitantly Frond got to his hind legs. For a heartbeat he stood upright, peering out into a world beyond the primates’ reach, out into the green emptiness where animals walked, the antelopes, elephants, and chalicotheres grazing the abundant grass.

Then he dropped back to all fours and scuttled back into the forest’s shadows, his nerve gone.

Capo gave him a sound beating about the head for taking such a risk. Then he led his troop back into the deeper forest.

Capo hauled himself up an acacia tree, seeking fruit and flowers. Capo climbed steadily. He used a kind of shimmying style, pulling himself up with his arms while gripping the tree trunk with his feet to provide a platform.

It was a feat Roamer could not have achieved — or indeed any monkey. Capo’s apes had flat chests, short legs, and long arms. They had achieved greater flexibility by moving their shoulder blades to the backs of their bodies, which enabled Capo to reach up above his head. All this was equipment for hauling oneself up a tree trunk. Where Roamer had spent much of her life running along branches, Capo was a climber.

And this re-engineering for climbing had had another side effect, easily visible in Capo’s long, narrow body. Working vertically, with a new bone structure and system of balance, Capo was already preadapted to walking on two feet. Sometimes he did this in the trees, holding on to branches for balance, trying to reach the highest fruit — and sometimes his kind would stand up out in the open, as Frond had demonstrated.

As their bodies had been redesigned, the apes had become smarter.

In these tropical climes fruit trees rarely fruited simultaneously. Even when you found a fruiting tree, you might have a long way to travel to the next. So the apes needed to spend much of each day searching for patchy resources, foraging alone or in small groups, collecting together again to sleep in the treetop refuges. This basic architecture of food gathering had shaped their social lives. For one thing they needed to understand their environment very well if they were to find the food they needed.

And, given the way they lived their lives, their bonding was loose. They could split and recombine, forming special relationships with other members of the community, even though they might not see them for weeks at a time. Keeping track of a multileveled, fissile social complexity required increasing smartness. As the apes juggled their relationships, it was as if they were living through a soap opera — but it was a social maelstrom that honed their developing minds.

In the first years after the great split of the archaic anthropoid stock into apes and monkeys, the apes had become the Old World’s dominant primates. Though shrinking climate belts restricted them to the middle latitudes, there was plenty of room for them in a continuous band of forest that had spanned the whole of Africa and stretched across Eurasia from China to Spain. Following this green corridor the apes had walked out of Africa and spread through the Old World forests. In fact, they had migrated alongside the proboscideans.

At their peak there were more than sixty ape species. They had ranged from cat-sized to the size of a young elephant. The largest, like the giants, were leaf eaters, the midsized — those the size of Capo — took fruit, but the smallest, weighing under a kilogram or so, were insectivores, like their remote ancestors. The smaller the animal, the faster its metabolism and the higher the quality of the food it demanded. But there was room for everybody. It had been an age of apes, a mighty anthropoid empire.

Sadly for them it hadn’t lasted.

As the world continued to cool and dry, the great forest belts had shriveled into isolated islands, like this one. The vanishing of forest connections between Africa and Eurasia had isolated the Asian ape populations, which would develop independently of events in Africa, into the orangutan and its relatives. With the reduced ranges had come a dwindling of numbers. Most ape species had, in fact, already long gone extinct.

And then had come the rise of a new competitor.

Capo reached a clump of foliage where, he knew, this particular acacia had an especially productive patch of flowers. But he found the spiny branches already stripped. When he pried them aside he was met by a small, startled black face, fringed by white fur and a gray topknot. It was a monkey — like a vervet — and juice dribbled from its small mouth. It peered into Capo’s eyes, squealed, and shot out of sight before he could do anything about it.

Capo rested for a while, scratching his cheek thoughtfully.

Monkeys were a pest. Their great advantage was that they were able to eat unripe fruit. Their bodies manufactured an enzyme to neutralize the toxic chemicals used by the trees to protect their fruit until their seeds were ready to germinate. The apes could not match this. So the monkeys were able to strip the trees before the apes even arrived. They were even moving out into the grasslands, feeding off the nutlike seeds to be found there. To the apes, the monkeys were as tough a competition as the rodents had always been.

High over Capo’s head, a slim form moved, swinging gracefully and purposefully. It was a gibbon. It raced through its forest canopy at extraordinary speed. It used its body as a pendulum to gain momentum, and, like a child on a fairground swing, it pumped its legs up and down to build up its speed.

The gibbon’s body was a kind of extreme version of the apes’ long-armed, flat-chested design. The ball-and-socket joints in its shoulders and wrists had been freed up so that the gibbon could hang from its arms and twist its body through a full circle. With its low weight and extreme flexibility, the gibbon could hang from the outermost branches of the highest trees, and it was able to reach the fruits that grew at the end of the thinnest branches, safe from even tree-climbing predators. And, able to hang upside down from branches, it could reach goodies out of the grasp of other apes, who were too heavy to climb so high, and even the monkeys, who ran along the tops of the branches.

Capo peered up at the gibbon with a kind of envy for a grace, speed, and skill he could not match. But, magnificent though it was, the gibbon was not a triumph for the apes but a relic, forced by the competition it had lost to the monkeys to eke out its living on the ecological margins.

Vaguely disappointed, still hungry, Capo moved on.

At length Capo found another of his favorite resources, a stand of oil palms. The nuts of this tree had rich, oily flesh — but they were enclosed in a particularly hard outer case that rendered them immune to most animals, even the clever fingers of monkeys. But not to apes.

Capo hurled handfuls of the nuts down to the ground, then clambered down after them. He collected the nuts together, carried them to the roots of an acacia he knew, and hid them under a heap of dried palm fronds.

Then he worked his way out toward the perimeter of the forest, to where he had stashed his hammer-stones. These were cobbles that fit neatly into the palm of his hand. He selected one and headed back to his nut stash.

On his way back he passed the adolescent Howl. Briefly he considered mating her again, but Capo’s attention once a day was enough of an honor for any female.

Anyhow she was sitting with an infant, an odd-looking male with a peculiarly elongated upper lip: Elephant. He was actually one of Capo’s sons. He was sitting on the ground clutching his stomach and moaning loudly. Perhaps he had a worm, or some other parasite. Howl was moaning along with him, as if some of the pain had transferred to her body. She was plucking bristly leaves and making the youngster swallow them; the leaves contained compounds that were toxic to many parasites.

And there were Finger and Frond, he saw, grubbing their way along the forest floor. The young males were aiming for a little light thievery, it seemed to Capo — in fact, he realized angrily, they had their eyes on Capo’s own heap of fronds.

Capo contained his impatience. He sat under a tree, dropped his hammer-stone, picked up a stick and began to work methodically to clean out the spaces between his toes. He knew that if he made a dash for his palm nuts the others would get there first and pilfer the nuts. By loitering like this, he was making Frond and Finger believe that no nuts had been hidden at all.

Unlike Roamer, Capo was able to read the intentions of others. And Capo understood that others could have beliefs different from his own, that his actions could affect others’ beliefs. It was a capability that even made a limited kind of empathy possible: Howl really had been sharing the suffering of Elephant. But it also made possible ever more elaborate modes of deceit and treachery. He was able, in a sense, to read minds.

This new ability had even made him self-aware, in a new way. The best way to model the contents of another’s mind was to be able to study your own: If I saw what she sees, if I believed what she does, what would I do? It was an inward look, a reflection: the birth of consciousness. If Capo had been shown his face in a mirror he would have known it was him, not another ape in a window. His were the first animals since the hunters of Pangaea to have achieved such sophistication.

At last Frond and Finger moved away from the stash. Capo grabbed his hammer-stone and descended on his palm nuts. Capo would deliver beatings to the two of them later anyhow, on principle; they would never quite understand why.

He brushed aside the concealing fronds to expose his favorite anvil stone, a flat rock embedded in the ground. To protect his backside he spread some broad leaves over the moist ground. He sat down, legs tucked up to his chest. He set a palm nut on the anvil, holding it steady with finger and forefinger — and then brought down the hammer, snatching his fingers out of the way at the last moment. The nut rolled a little and squirted sideways unbroken; Capo retrieved it and tried again. It was a tricky procedure that took a lot of coordination. But it took Capo only three goes before he had cracked the first nut and was chewing out its flesh with his teeth.

Twenty-seven million years after Roamer and her habit of slamming nuts against branches, this was the height of technology on Earth.

Capo worked steadily on the nuts, losing himself in the tricky little procedure, pushing out of his mind the obscure worries that niggled him. It was high morning now, and for a time he felt content, satisfied in the knowledge that he had gotten enough food to stave off hunger pains for a few hours at least.

Elephant, drawn by the nuts’ rich smell, came to see what was happening. This youngster’s stomach problem had evidently been eased by Howl’s rough-and-ready bush medicine — or perhaps he had been faking it, to get some attention — and he was starting to feel hungry. He made out bits of nutshell scattered around the anvil stone, and even a few scraps of kernel. The youngster snatched these up and crammed them into his mouth.

Capo, grandly, let this pass.

Now Leaf came by with her infant clinging to her back.

Capo dropped his hammer-rock and reached for Leaf. Gently he began to groom her belly, an attention to which she submitted gracefully. Leaf, a big, gentle creature, was one of his favorite females. In fact she was favored by all of the troop’s males, and they would compete for grooming time with her.

But that wasn’t Capo’s way. Very soon his lumpy penis had sprouted from his fur, and Leaf had had all the grooming she was going to get. Leaf carefully lifted the infant from her back and put her down on the ground. Then she lay back and let Capo enter her. She arched her back as he thrust, so that her head was upside down, her weight balanced on her skull. These apes often mated face to face like this. Empathy again: They could share each others’ pleasure in grooming or mating.

Capo and Leaf were close. Though mating was promiscuous, sometimes Capo and Leaf would take themselves off into the forest for days on end — just the two of them — and during such safaris of tenderness, previsioning the sexual privacy of later kinds, most of Leaf’s children by Capo had been conceived, including Elephant.

What Capo and Leaf felt for each other at such moments as this was nothing like human love. Each of the apes remained locked inside a wordless prison; their “language” still wasn’t much more sophisticated than a cry of pain. But they were among the least lonely creatures on the planet, the least lonely who had ever lived.

Meanwhile young Elephant pored over Capo’s tool kit. He started tapping nut against cobble, cobble against anvil.

Capo’s apes, as they grew from infancy, had much to learn about their environment. They needed to learn where to find water and food, how to use occasional tools to get at the food, how to apply their simple bush medicine. They had been driven to live this way, in fact, because of competition from the monkeys: They had to figure out how to extract food the monkeys couldn’t steal, and that took smarts.

But there was no schooling here. It wasn’t that Elephant was trying to figure out what Capo had been doing. But by experimenting, using trial and error and the tools the adults left lying around — all the time driven by the lure of the delicious palm nuts — Elephant would eventually learn how to smash nuts for himself. It would take him three more years before he got it right. Elephant had to figure everything out from scratch himself, as if repeating in his own lifetime the whole intellectual progression of the species.

On and on he pounded at the shells, as if he were the first ape ever to try this trick.

Capo brought himself to a slow, shuddering orgasm, his first of the day. He withdrew from Leaf and rolled onto his back, rather unjustifiably proud of himself, and allowed her to groom him, picking knots from the fur of his belly.

But now his peace of mind was disturbed by a sudden cacophony from deeper into the forest: hooting cries, drumming, the rustle of large bodies clambering and swinging.

Capo sat up. In his world it wasn’t good to have too much excitement in which he wasn’t involved. He vaulted over a tree stump, drummed on a branch, routinely cuffed Elephant about the head, and loped off toward the source of the noise.

A group of young males were hunting a monkey.

To Capo’s eye it looked like the little vervetlike creature he had disturbed munching on acacia flowers earlier. Now it sat cowering at the top of a young palm.

The hunters had spread out around the base of the tree, and were clambering stealthily up neighboring trees. Others, Frond and Finger among them, had gathered around to see the excitement. It was these spectators who were making all the noise; the hunters themselves moved with stealth and silence. But to the monkey the din was terrifying and disorienting.

Capo was unpleasantly surprised when he saw who the hunters were. They were the rowdy young males who had loped off not days before on a foraging trip to another part of the forest clump. Their informal leader, a burly creature called Boulder, had given Capo some trouble in the past with his rebellious ways, and Capo had been happy to see him go: Let him blow off steam, make a few mistakes, even get hurt, and he would soon defer to Capo’s authority once more.

But Boulder had been away just days, where Capo had expected weeks to pass. And from the look of his bristling aggression, his jaunt hadn’t made him any calmer.

Capo was worried by the hunt, too. Monkey hunting usually happened only when other food was scarce, such as during periods of drought. Why hunt now?

One of the clambering apes made a sudden leap. Cluttering, the monkey jumped the other way — straight into the arms of a waiting hunter. The watching apes hooted and barked. The hunter swung the screaming monkey around and slammed its skull against a tree trunk. Its cries were cut off immediately. Then the hunter hurled the limp carcass to the ground, its smashed head making a bright red splash in the forest’s green murk.

That was Capo’s moment. He vaulted past Boulder to be first onto the body. He grabbed up the still-warm scrap, got hold of one ankle and twisted, hard, ripping the little limb loose at the knee.

But, to his astonishment, Boulder challenged him. The burly male leapt at him feetfirst, ramming him in the chest. Capo fell sprawling, an ache spreading along his rib cage, the breath knocked out of him. Boulder deliberately picked up the monkey limb and bit into it, blood spurting over his mouth. All the apes were madly excited now, and they hooted and drummed and scrambled over one another.

Ignoring the pain in his chest Capo leapt to his feet with a roar. He couldn’t let Boulder get away with this. He scrambled up into the lower branches of a tree, drummed ferociously, hooted loudly enough to disturb birds that roosted high above him, and vaulted back to the ground. He let anger surge through him so that he bristled, and a proud pink-purple erection stuck out before him: a nice touch that, his trademark.

But Boulder kept his nerve. With the monkey limb wielded in his hand like a club, he began his own display, his stamping, leaping, and drumming just as impressive as Capo’s.

Capo knew he couldn’t afford to lose this one. If he did, given Boulder’s circle of blood-stained hunters, he might lose not just his status but his very life.

With an agility that belied his years, he leapt forward, knocked Boulder flat, and sat on his chest. Then he began to batter Boulder about the head and chest as hard as he could. Boulder fought back. But, save for youth, Capo had all the advantages: surprise, experience, and authority. Boulder couldn’t shift Capo’s weight, and he couldn’t bring his own powerful arms and legs fully into play.

Gradually, Capo saw, he was winning the battle in the minds of the rest of the troop, which was just as important as subduing Boulder. The young male’s followers seemed to have melted away into the trees, and the whoops of excitement and approval Capo heard now seemed to be directed at him.

But even as he battled to subdue Boulder, a slow deduction worked through Capo’s roomy mind.

He thought of the dying trees he had glimpsed beyond the fringe of the forest island, the speedy return of Boulder and his wanderers, their apparent hunger, their need to hunt.

Boulder had found nowhere to go. The forest patch was shrinking.

That had been true all of Capo’s life, and now it was becoming unavoidable. There was no longer enough room for them here. If he tried to keep the group here, the tension between them, as they competed for dwindling resources, would become too intense.

They would have to move.

At last Boulder gave in. He lay limp under Capo, cupped the older male’s buttocks, and even briefly stroked his still-erect penis, all gestures of submission. To drive home his point Capo kept battering at Boulder’s head for long minutes. Then he clambered off the prone younger male. Still bristling, he made his way into the forest, where he could afford to limp and massage the pain in his chest and nobody could see how he hurt.

Behind him the others fell on the vervet. Their stomachs could not digest flesh well, and later they would pick through their feces for lumps of meat to eat again. It was a digestive system that was going to have to improve, if the descendants of these rummaging creatures were to prosper on the savannah.

II

Since Roamer’s time, grass had remade the world.

The great epochal cooling of the Earth continued. As water was locked up in the Antarctic ice cap, sea levels diminished, and inland seas shrank or became landlocked. But with more continental landmass exposed, there was less sea to buffer the climate from extremes of heat and cold, and the weathering rock drew carbon dioxide from the air, making it less able to retain the sun’s heat. Cooler and drier: The planet had developed a vast feedback mechanism, driving its surface to still more arid, chilly conditions.

Meanwhile tectonic collisions created new mountain ranges: the Andes of South America, and the Himalayas of Asia. These new uplifts cast gigantic rain shadows across the continents; the Sahara Desert would soon be born in such a shadow. In the new desiccation, great belts of broad-leaved deciduous woodland spread from south and north toward the equator.

And the grasses spread.

Grass plants — huddling in their great crowds, able to rely on fertilization by windblown pollen — might have been designed for the new open, dry conditions. Grass was able to subsist on the sporadic rainfall that now fell, whereas most trees, with their roots delving ever deeper into the ground, found only dryness and could not compete. But the real secret of grass lay in its stems. The leaves of most plants grew from the tips of shoots, but not grass’s. Grass blades grew from underground stems. So grass could be cropped by a hungry animal, right down to the ground, without losing its power to regenerate.

These unspectacular properties had enabled grass to take over a world, and to feed it.

The new grass-eating herbivores developed specialized ruminant guts able to digest the grassy fodder over long periods and hence extract the maximum nutrient from it, and teeth able to withstand the abrasive effect of silica grains in grass blades. Many herbivores learned to migrate, because of the seasonality of the rainfall. These new mammals were larger than their archaic ancestors, lean and long-legged with specialized feet and a reduced number of toes to help them walk and run long distances and at speed. And meanwhile there was a sharp rise in the types of rodents, like voles and field mice, able to eat grass seeds.

New carnivores rose, too, equipped to feast on the new herds of large herbivores. But the rules of the ancient game had changed. In the sparse cover of a grassland, predators could see prey from long distances — and vice versa. So predators and prey began a metabolic arms race, with the emphasis on speed and endurance; they developed long legs and quick reactions.

A new kind of landscape began to spread — especially on the eastern side of the continents that were sheltered from the predominantly westerly winds and the rain they carried: open, grass-covered plains marked by scattered scraps of bush and woodland. And in turn animals who adapted to the new vegetation were rewarded with a guaranteed food source that could spread across hundreds of kilometers.

But their specializations, and the stability of the grasslands, would lock in the grazers to the grasses, the predators to their prey, establishing a close codependency. In this period the deer, cows, pigs, dogs, and rabbits looked little different from their equivalents of human times five million years later — although many of them would have looked surprisingly large; they would later be outcompeted by their smaller, faster cousins.

Meanwhile the opening up of land bridges, caused by the falling sea levels, led to a great crisscross migration of animals. Three kinds of elephants — high-browsing deinotheres, omnivorous gomphotheres, and browsing mastodonts — crossed from Africa to Asia. Along with them traveled the apes, cousins of Capo. And in the other direction came rodents and insectivores, cats, rhinos, mouse deer, pigs, and primitive types of giraffe and antelope.

There were some exotica, especially on the islands and the separated continents. In South America the largest rodents that ever lived were flourishing; there was a kind of guinea pig as large as a hippo. In Australia, the first kangaroos appeared. And what would later be considered tropical animals could be found in North America, Europe, and Asia: In England, the Thames was broad and swampy, and hippos and elephants basked on its floodplain. The world had cooled greatly since Noth’s time, but it still wasn’t cold; the deepest chill would afflict later ages.

But still the drying continued. Soon the older mosaic of grassland and woodland able to support a wide variety of animals lingered only in the equator-straddling Africa; elsewhere the grasslands opened up into arid plains, the savannah, steppe, and pampas. In these coarser, simplified conditions, many species fell away.

This intense evolutionary drama was driven by the endless shifts in Earth’s climate — and the animals and plants were as helpless as bits of flux on a great terrestrial forge.

The next morning there was no luxurious ball scratching. As soon as he woke, Capo sat up, hooted softly at the pain of yesterday’s lesions and bruises, and voided his bladder and bowels in a fast, efficient movement, ignoring the chitters of protest from below.

He vaulted from his nest and began to shimmy down the tree. Just as yesterday he roused the troop by crashing into their nests, hooting, kicking, and slapping. But today Capo wasn’t interested in displaying; this morning his purpose was not dominance but leadership.

His determination was still strong in his mind. The troop had to move. Where they should go wasn’t part of his unsophisticated decision making yet. But what was very clear in his head was the pressure of yesterday, his competition with Boulder, what he had sensed of the overcrowding of this little patch of forest.

The troop gathered together on the ground, more than forty of them, including infants clinging to their mothers’ bellies or backs. They were sleepy, wary, scratching themselves and stretching. No sooner had Capo gotten them gathered, of course, than they were drifting apart again, plucking at bits of grass and moss on the ground, reaching for low-growing figs and other fruit. Even among the males he saw reserve, rivalry, resentment; they might resist him just to make their own points in the endless plays for dominance. And as for the females, they were a law unto themselves, for all of Capo’s noise and violence.

How was he going to be able to lead this lot anywhere?

He wasn’t conscious all the time, as a human was. He was conscious intermittently. He was only truly aware of his own thoughts, of himself, when thinking about others in the troop, because that was the primary purpose of consciousness, to model the thinking of others. He wasn’t conscious in the same way about other domains of his life, like food-gathering or even tool using: those were unconscious actions, as peripheral to his awareness as breathing or the working of his legs and arms when he climbed. His thinking was not like a human’s; it was simplified, compartmentalized.

His mind was a sophisticated machine, basically evolved to handle complex social situations. And he had a good innate understanding of his environment. He had a kind of database in his head of the resources he needed to stay alive and where they could be found. He was even good at dead reckoning navigation, and could easily compute good shortcut courses from one site to another. It was his environmental awareness that had prompted his concern about the shrinking forest patch.

It was hard for him to put together the elements of this puzzle: the danger posed by the shrinking forest, what he needed to do with his troop. But the danger was very real to him, and every instinct screamed at him to get away from here. The troop had to follow him. It was as simple as that; he knew it deep in the fibers of his being. If they stayed here they would surely die.

So he roared to get his blood flowing, and threw himself into the most energetic display he could. He raced up and down among the troop, slapping, punching, and kicking. He tore branches from the trees and waved them over his head to make himself look even bigger. He swung and vaulted over branches and trunks, drummed ferociously on the ground, and — as a climactic gesture to reinforce his victory of yesterday — he threw Boulder to the ground and shoved his own puckered anus in the younger male’s face. It was a magnificent spectacle, as good as any Capo had mounted even in his younger days. Males whooped, females flinched, infants cried, and Capo allowed himself a glimmer of pride in his work.

But then he tried to lead them away, toward the fringe of the forest. He walked backward, shaking branches and running back and forth.

They stared. Suddenly he was behaving like a submissive junior male. So he displayed again, drumming, vaulting, and hooting, and went back to the follow-me routine.

At last one of them moved. It was Frond, the spindly young male. He took a couple of tentative knuckle-walk steps. Capo responded with a chattering cry and threw himself at Frond, rewarding him with a burst of intense grooming. Now more came forward: Finger, a few more of the junior males, eager to be groomed in turn. But Capo noticed that Boulder aimed a sly kick at Frond’s backside.

And then, to Capo’s intense relief, here came Leaf, her infant riding on her back, knuckle-walking grandly if a bit stiffly. Where this most senior female came, others followed, including Howl, the near-pubescent youngster.

But not all the females followed — and not all the males. Boulder stayed behind, sitting squat under a tree with his legs ostentatiously crossed under him. Other males gathered around him. Capo displayed at them furiously. But they huddled and groomed each other as if Capo no longer existed. It was a deliberate snub. If he wanted to maintain his position, Capo was going to have to break up this knot of rebellion, perhaps even face down Boulder once more.

But, almost to his own surprise, he gave up his displaying and stood back, panting.

In his heart he knew he had lost them, that he had pushed them too hard, that his troop was fissioning. Those who chose to follow him would find their way, with him, to a new destiny — a destiny he himself couldn’t yet imagine. Those who stayed behind would just have to take their chances.

He loped quickly away, out of the heart of the forest and toward the daylight, without looking back — although he was unable to resist a final valedictory liquid fart in the direction of the rebels.

In the end about half the males and rather more of the females stayed behind. It was a drastic diminishing of Capo’s domain. And as he walked toward the bright light of the plain he could hear the whoops and howling of the males. The battles over the new hierarchy had already begun.

At the forest’s fringe, on the edge of emptiness, Capo paused.

Just as yesterday, gomphotheres grazed on damaged, half-drowned trees. To the north the grassy plain stretched to its misty horizon, littered with glimmering lakes and marshes, herbivore herds passing like shadows. To the south, beyond a kilometer or so, the ground gleamed white as bone. The salt pan would be a difficult place to cross. But Capo could see how the land rose, up toward a green plateau, where — it seemed to his poor eyes, adapted for the short focuses of the forest — a thick blanket of trees lay draped over the rock.

South, then, across the dry land, to the new forest on the plateau. Without glancing back to see if the others were following, he set off on knuckles and feet, pushing through grass that waved around him, shoulder-high.

The land rose, quickly becoming drier.

There were some trees here, but they were just thin-trunked pines clinging to arid ground, with none of the comforting density and moistness of the forest clump. So there was little shelter to be had from the high sun. Capo was soon panting hard, baking inside his thick fur, his knuckles and feet rubbed raw. He could not sweat, and his knuckle-walking gait, effective for clambering around the complex, crowded environment of the forest, was inefficient here.

And Capo, a creature of the forest, was intimidated by this great sweep of openness. He hooted softly and longed to cower, to hold his arms over his head, to hurl himself into the nearest tree.

There were animals to be seen, scattered over the dry plain: There, were deer, some species of dog, and a family of grubbing animals like spiky-furred pigs. The larger animals were very few. But as Capo blundered on, many smaller creatures scampered away underfoot: lizards, rodents, even primitive rabbits.

The twenty or so of the troop who had followed him toiled painfully up the slope after him. They moved slowly, for they stopped frequently to feed, drink, groom, play, argue. This migration was more like a slow walk made by easily distracted children. But it was not in Capo’s instincts to hurry them. They were what they were.

Capo crested a shallow, eroded hill. From here he looked back across the wet, glistening landscape with its islands of forest and crowding herbivores. But when he looked ahead, to the south, he could see the great dryness they approached. It was a broad, high, dry valley, scattered with thin trees and bits of vegetation. It was kept arid by an accident of geology which had left it cupped in a great subterranean bowl of rock, barren of springs, shadowed from rainfall.

It was an intimidating sight; the valley was exposed, utterly open. And yet he must cross it.

And from here, now that there was no forest to soak up the noise, he could make out that great, mysterious roaring from the west. The remote noise sounded like the groaning cry of some huge, pained, angry beast, or like the thunderous hoofs of some great herbivorous herd. But when he looked to the west he could see no dust clouds, no black wash of animal bodies. There was nothing but the roaring, continuing just as it had all his life.

He began to clamber down the rocky slope, still heading south.

The ground became bare. Still trees clung to life here, their roots wormed into faults in the rock. But these pines were sparse, their leaves spiky, jealous of their water. He stopped under one of these trees. Its branches and leaves offered him virtually no shade. He could find no fruit, and the leaves he plucked were sharp and dry in his mouth. He made a grab for a small mouselike creature with long, levered hind legs; his mouth watered at the thought of biting into its soft wet body, its small bones crunching in his mouth. But here on this rocky ground he was clumsy and noisy, and the mouse thing evaded him easily.

Now the ground changed again, becoming a broad slope of broken stone that spread out before him, a road leading to the depths of the dry valley. The going got even harder as Capo slid and slipped on the loose rubble. Hot, thirsty, hungry, scared, he hooted his protest and threw bits of the rubble around, tramping and kicking it. But the land was not to be intimidated even by Capo’s mighty displays.

Meanwhile the chasma watched the ragged group of anthropoids as they struggled down the uneven, treacherous slope.

She had never seen creatures like this before. With a predator’s cold interest, she made unconscious calculations of their speed, strength, and meat yield, and began categorizing the individuals — here was one who seemed wounded and limped a little; here was an infant, clasped tightly to its mother’s chest; here was a juvenile straying, foolishly, from the tight group.

This chasmaporthetes was actually a kind of hyena. But, long legged, slim, she looked more like a cheetah. She did not have all of the true cats’ suppleness and speed, not quite; her kind had more adapting to do in the fleet conditions of this emerging world of grass. But her range was huge in this barren valley. She was the top predator here, and she was well equipped for her grisly work.

To her, the apes were new meat on the savannah. She waited, her eyes glowing like captive stars.

At last, exhausted, Capo gave up. He slumped to the ground. One by one, what was left of his troop joined him. By the time they had all arrived, the sun had started to set, filling the sky with fire and casting long, stark shadows along the floor of this gravel-littered bowl.

A kind of dull indecision raged within Capo. They shouldn’t stay here, out in the open; his body longed to climb a tree trunk, to pull together branches to make a cozy, warm, safe nest. But there were no trees here, no security to be had. On the other hand they couldn’t cross the valley floor in the dark. And they were all hungry, thirsty, exhausted.

He didn’t know what to do. So he did nothing.

The troop began to disperse, following their own instincts. Finger picked up a cobble-shaped, palm-sized rock, perhaps hoping to use it in some future nut-cracking project. But a scorpion scuttled out from beneath the rock, and Finger fled, hooting.

Frond was sitting alone with his back to the rest of the group, assiduously working at something. Capo, suspicious, loped up as quietly as he could on this loose, scattered gravel.

Frond had found a termite mound. He was sitting before it, clumsily poking sticks into it. When he saw Capo he cowered, screeching. Capo delivered brisk, perfunctory blows to his head and shoulders, as Frond would have expected. He should have hooted to the rest on discovering this bounty.

Capo ripped open a shrub. All of its branches were spindly and bent, and when he stripped a branch by passing it through his mouth, the hard, spiky leaves hurt his lips. But it would have to do. He sat alongside Frond. He pushed his stick into a crevice in the mound, and worked it until it had slid in deep. It was not ideal; the stick was too short and bent to be truly effective, but it would have to do. He jiggled it around, waiting patiently. Then he withdrew the stick, centimeter by centimeter. To the stick clung soldier termites, sent to defend the colony from this invader. Capo took great care not to dislodge this cargo. Then he swept the stick through his mouth, enjoying a mouthful of sweet, moist flesh.

When they saw what was going on the rest of the troop crowded around, the older ones making their own fishing sticks. Very quickly a rough pecking order established itself, lubricated by kicks, punches, hoots, and sly grooming. The more senior male and females alike got closest to the mound while the young, who didn’t understand what was happening anyhow, were excluded. Capo didn’t care. He just concentrated on holding his own position close to the mound while working assiduously at the termites.

The termites were antique creatures whose complex society was the result of their own long evolutionary story. This mound was ancient, built of the mud that had pooled here when infrequent rainstorms caused temporary floods. Its rock-hard carapace protected the termites from the attentions of most animals, but not these apes.

Capo’s use of tools — the termite-fishing sticks, the hammer-stones, the leaves he would chew to a sponge to extract water from hollows, even the fine toothpicklike sticks he sometimes used to perform crude dentistry — seemed sophisticated. He knew what he wanted to achieve; he knew what kind of tool he needed to achieve it. He would memorize the location of his favorite tools, like his hammer-stones, and made subtle decisions about using them — for instance trading off the distance he had to carry a hammer against its weight. And it wasn’t a case of just picking up a handy rock, found by chance; he modified some of his tools, like this termite-fishing stick.

And yet he was not like a human craftsman. His modifications were slight: his tools, abandoned after use, would have been hard to distinguish from the products of the inanimate world. The actions he used to make the tools were part of his normal repertoire, like biting, leaf stripping, stone throwing. Nobody had invented wholly new actions, like a potter’s clay throwing or a wood carver’s whittling. He used each tool for one use, and one use only; it never occurred to him that a termite fishing-stick might also be used as a toothpick. He did not improve his tools, once he had found a design that worked. And if — by some chance — he had in the course of his life happened upon a new kind of tool, however successful a design it was, its use would have spread only very slowly through his community, perhaps taking generations to reach every member. Coaching, the notion that the contents of someone else’s mind might be shaped by rehearsal and demonstration, had yet to be discovered.

So Capo’s tool kit was staggeringly limited, and very conservative. Capo’s ancestors, five million years gone, creatures of a different species, had used tools of only fractionally less sophistication. Capo wasn’t even aware he was using tools.

And yet here was Capo, working assiduously, knowing what he wanted, selecting materials to achieve his goal, making and shaping the world around him, the cleverest so far of all of Purga’s long line of descendants. It was as if a slow fire were smoldering in his eyes, his mind, his hands, a fire that would soon burn much more brightly.

As the sun slid beyond the horizon at the valley’s end the apes huddled closer. Deeply unhappy, they pushed, jostled, and slapped, hooting and screeching at each other. This wasn’t their place. They had no weapons to defend themselves, no fire to keep the animals at bay. They didn’t even have the instinct to keep silent at sunset, the hour of predators. All they had was the protection of each other, of their numbers — the hope that another would be taken, not me.

Capo made sure he was right at the center of the band, surrounded by the burly bodies of the other adults.

The young male called Elephant didn’t have as powerful an instinct for self-preservation. And his mother, lost somewhere in the middle of the huddle, was too concerned with her newest child, a female; right now Elephant was a low priority. He was unlucky to be just the wrong age: too old to be defended by the adults, too young to fight for a place at the center, away from the danger.

He soon found himself pushed out to the fringe of the group. Still, he tried to settle down. He found a place close to Finger, a cousin. This ground was hard and bony, unlike the soft roosts he was used to, but by squirming he managed to make himself a bowl-shaped hollow. He pressed his belly against Finger’s back.

He was too young even to understand the danger he was in. He slept uneasily.

Later, in the dark, he was woken by a soft pricking at his shoulder. It was almost gentle, like a grooming. He squirmed a little, burrowing closer to Finger’s back. But then he felt breath on his cheek, heard a purring growl like a rock rolling down a hillside, smelled a breath that stank of meat. Instantly awake, his heart hammering, he screeched and convulsed.

His shoulder was ripped, painfully. He found himself dragged backward, like a branch torn off a tree. He caught a final glimpse of the troop — they were awake, panicking, hooting, scrambling over each other to get away. Then a starlit sky whirled around him, and he was slammed into the ground hard enough to knock the breath out of him.

A form moved over him, sleek, silhouetted against the blue-black sky. He felt a hard-muscled chest press against his, almost lovingly. There was fur with a scent of burning, breath like blood, and two yellow eyes that shone over him.

Then the bites came, to his legs, over one of his kidneys. They were sharp, almost clinical stabs, and he convulsed with the fiery pain. He screeched and rolled, tried to run. But his legs collapsed, his hamstrings cut. Now came those prickings at his neck again. He was lifted up by the scruff, lifted right off the ground, and he could feel sharp teeth working inside his skin. At first he struggled, scrabbling at the gravel with his hands, but his efforts only brought more pain as the flesh at his neck was torn further.

He gave up. Hanging passively from the chasma’s mouth, his head and damaged legs clattering against the uneven ground, his thoughts dissolved. He could no longer hear the hooting cries of his troop. He was alone now, alone with the pain and the iron stink of his own blood, and the steady, patient padding of the chasma’s footsteps.

Perhaps he was unconscious for a while.

He was dropped on the ground. He did not land hard, but all his wounds flared with pain. Mewling, he pushed at the ground. It was littered with rubble like the place he had come from, but was covered in fur, and the stink of chasmas.

And now small shapes bounded around him, black on black, fast moving, a little clumsy. He felt the brush of whiskers on his fur, tiny nips at his ankles and wrists. They were chasma pups. He hooted his defiance, and swung a fist blindly. He connected with a hot little bundle that was knocked off its feet, yowling.

There was a short, barking roar: the mother chasma. In sudden panic, he tried to crawl.

The pups yapped excitedly as they completed their short chase. And now the biting started in earnest, digging into his back, buttocks, belly. He rolled onto his back, lifting his legs to his chest and flapping at the air. But the pups were fast, furious, and dogged; soon one of them had dug her teeth into his cheek, applying all her small weight to ripping open his face.

Again the mother roared, scattering the pups. Again Elephant tried to flee. Again the pups caught him and inflicted a dozen more tiny, debilitating wounds.

If not for her pups, the chasma would have killed Elephant quickly. She was giving them the chance to chase down a prey animal and knock it over. When they were older, they would be able to finish off prey themselves, ripping it apart; later still she would release some of her prey almost unharmed and allow the pups to finish the hunt. It was a kind of learning by opportunity. This was no more human-style teaching than what occurred among the apes: it was an innate behavior evolved in this clever carnivorous species to enable the young to acquire the skills they would need when hunting alone.

And as the lesson went on Elephant was still conscious, a spark of terror and longing buried in a broken shred of blood, flesh, and gristle. The boldest of the pups even fed on the tongue that dangled from his broken jaw.

But the pups were too young to finish off Elephant alone.

At last the mother took over. As her great jaw closed around his skull — as he felt a prickle of biting teeth around his scalp, like a crown of thorns — the last thing Elephant heard was that remote purring growl.

When the morning came, everyone knew that Elephant had been taken.

Capo peered with fascination at the scuffed, hair-strewn gravel patch where Elephant had briefly struggled, at the line of bloody paw marks, already dried to brown, that led away into the distance. He felt a vague regret at the loss of Elephant. It seemed baffling that he would never again see that clumsy youth with his stiff, awkward attempts at grooming, his clumsy fumbling as he tried to figure out how to get the flesh out of an oil palm nut.

But before the day was done, only Elephant’s mother would remember him. And when she was dead in her turn, there would be nothing to say he had ever existed, and he would be gone into the final blank darkness that had swallowed up all of his ancestors, every one.

Elephant had paid the price of the troop’s survival. Capo felt a cold relief. Without hesitation, without even performing the follow-me display, Capo moved down the slope and out onto the salt flats.

III

The next day they had to cross the salt. Under a washed-out blue-white sky the pan spread almost to Capo’s horizon, where hills, trees, and marshes crowded. It was as if this gray sheet were a flaw in the world.

The salt, lying over hard, grayish mud, was broadly flat, but the surface had texture, streaked here and there by swooping concentric lines that crowded to central knots. In one place an underground spring had caused the salt to billow up in great blocks that the apes had to clamber over.

But nothing grew, here on the salt. There weren’t even any tracks. Nothing moved save the apes, no rabbits or rodents, not even an insect. The wind moaned across this hard mineral stage, nowhere broken by the rustle of bushes and trees, the hiss of grass.

But still Capo kept on, for there was nothing else to do.

It took hours to cross the salt pan. But at last, his feet and hands aching, Capo found himself reluctantly climbing a ridge. At the crest of the ridge there was a belt of forest — even if it was a dense, uncomfortable-looking kind of forest.

Capo hesitated, facing the forest. He was overheated; his legs and feet were bleeding from a dozen small lesions. Then he pushed forward awkwardly and entered the forest’s green gloom.

The ground was hidden by a tangle of roots, branches, moss, and leaves. Wild celery grew in clumps everywhere. Although it was around noon, the air here was cold, made damp by a faint mist like a morning fog. The tree trunks were clammy, and thick lichen and moss left uncomfortable green streaks on his palms. The dampness seemed to dig through his fur. But after the aridity of the salt pan he relished the close, comforting tangle of green around him, and he devoured the leaves, fruit, and fungi he was able to pluck from the ground around him. And he felt safe from predators. Surely there was nothing that could strike at the hungry, weary band in this green density.

But now he saw hulking brown-black shapes just ahead, dimly visible through the tangled green. He froze.

A huge arm reached out to a branch wider than Capo’s thigh. Muscles worked in a great mound of shoulder, and the branch was snapped in two as easily as Capo might snap off a twig to clean his teeth. Giant fingers plucked leaves from the nearby branches and pushed them steadily into immense jaws. The whole head worked as the big animal chewed, heavy muscles working the skull and jaw together.

The nearest creature was an ape, as Capo was, a male — and yet unlike Capo. The big male watched the odd, scrawny little apes without curiosity. He looked powerful, threatening. But he didn’t move. The male, and a small clan of females and infants, did nothing but sit around and feed on leaves and the wild celery that carpeted the forest floor.

This was a gorilla: a remote cousin of Capo’s. His kind had split off from the broader lineages of apes a million years ago. The split had come in a period when another forest had fragmented, isolating the populations it supported. As their habitat shrank to the mountaintops, these apes had turned to a diet of leaves, endlessly abundant even here, and became huge enough to resist the cold — yet they remained oddly graceful, able to move silently through this dense forest.

Though populations of gorillas would later adapt back to lowland conditions, learning to climb trees and subsist off fruit, in a sense their evolutionary story was already over. They had become specialized in their environments, learning to eat food that was so well-defended — covered in hooks, spikes, and stings — that no other creatures competed for it. They could eat nettles, for example, with an elaborate maneuver that involved stripping leaves from a stem, folding in the stinging leaf edges, and popping the whole packet into their mouths.

Sitting in their montane islands, lazily eating their leaves, they would survive almost unchanged until human times, when the final extinction would overtake them all.

When he was sure the gorillas were no threat, Capo crept away, leading the others onward through the forest.

At last Capo emerged from the far side of the forested ridge.

They had at last clambered out of the arid lowland basin. When he looked south across the plateau he had reached, he faced a rocky, rubble-strewn valley that scoured its way down to lower ground. But there, beyond the valley, he could see the land he had hoped to find: higher than the plain he had left behind, but well watered, glistening with lakes, coated green by grass, and studded with pockets of forest. The shadowy forms of a great herd of herbivores — proboscideans, perhaps — drifted with stately grandeur across the lush plain.

With a hoot of triumph Capo capered, vaulted over rocks, drummed on the stony ground, and shit explosively, spraying the dry boulders with his stink.

His followers responded to Capo’s display only listlessly. They were hungry and dreadfully thirsty. Capo was exhausted himself. But he displayed anyway, obeying a sound instinct that every triumph, however small, should be celebrated.

But now he had climbed so high that the remote, persistent growling from the west had grown louder. Dimly curious, Capo turned and looked that way.

From this elevated place he could see a long way. He made out a remote turbulence, a white billowing. It seemed to hover above the ground like a boiling cloud. He was actually seeing a kind of mirage, a very remote vision carried to him by refraction in the warming air. But the billowing steam clouds were real, though their suspension above the ground was not.

What he was glimpsing was the Strait of Gibraltar, where even now the mightiest waterfall in Earth’s history — with the power and volume of a thousand Niagaras — was thundering over shattered cliffs and into an empty ocean basin. Once the plain from which Capo had climbed had been covered by water two kilometers deep, for it was the floor of the Mediterranean.

Capo had been born in the basin that lay between the coast of Africa, to the south, and Spain, to the north. In fact, he was not very far from the place where a clever dinosaur called Listener, long ago, had stood at the shore of Pangaea and gazed out on the mighty Tethys Sea. Now he had climbed out of the basin to reach Africa proper. But if Listener had seen the birth of the Tethys, Capo was witnessing something like its death. As the ocean levels dropped, this last fragment of the Tethys had become dammed at Gibraltar. Landlocked, the great ocean had evaporated — until at last it emptied, leaving behind a great valley in places five kilometers deep, littered with salt pans.

But as the climate oscillated, the sea level rose again, and Atlantic waters broke through the Gibraltar barrier. Now, the ocean was refilling. But Capo had nothing to fear of giant waves cascading from the west, for even a thousand Niagaras could not refill an ocean overnight. The Gibraltar waters suffused the great basin more gradually, creating great rivers. The old seafloor turned slowly into sodden marshland, where the vegetation slowly died, before the waters rose so high they covered over the ground altogether.

But after each refilling the global ocean levels would drop again, and once again the Mediterranean would evaporate. This would happen as many as fifteen times over the million years bracketing Capo’s brief life. The Mediterranean would be left with a complex seabed geology, with layers of silt sandwiching salt pans laid down in the successive dryings.

But this trapped ocean’s dryings were having a profound effect on the area Capo lived in — and on Capo’s kind. Before the great dryings, the Sahara region had been densely forested and well watered, and home to many species of apes. But with the climatic pump of the dryings, and in the lengthening rain shadow cast by the more remote Himalayas, the Sahara was becoming increasingly arid. The old forests were breaking up. And with them the communities of apes were splintering, each fragmentary population embarking on its own journey to a new evolutionary destiny — or extinction.

But the great rumbling, the blurred vision of Gibraltar, was too remote to have any meaning for Capo. He turned away, and stumbled down onto the plain.

At last Capo moved off bare rock on to vegetation. He relished the green softness of the grass under his knuckles as he loped forward. As the others tumbled after him they rolled and sprawled, pulling up the long grass around them, relishing the delicious contrast with the hard lifeless rock.

But they weren’t home yet. A stretch of a few hundred meters of open savannah, studded with thorn bushes, separated them from the nearest forest clump — and the plain was not unoccupied.

A group of hyenas worked at a fallen carcass. Bulky, round, it might have been an infant gomphothere, perhaps felled by a chasma. The hyenas snapped and growled at each other as they worked at the scavenged meat, their heads buried in the creature’s stomach, their sleek bodies writhing industriously.

As Capo cowered in the grass, Frond and Finger came up alongside him. They hooted softly, and gave Capo’s backside a perfunctory groom, picking out bits of dust and rock. The younger males were cursorily acknowledging his authority. But Capo could tell they were impatient. Weary, thirsty, hungry, thoroughly spooked by the trek across the openness, they, like the rest of the troop, longed to reach the shelter and provision of the trees. And that was corroding Capo’s hold on them. The tension between the three males was powerful, toxic.

But it was a confrontation conducted in near silence, as the three of them kept their presence concealed from the hyenas.

While Capo still hesitated, it was Frond who made the move. He took one, two tentative shuffles forward. He received a hefty clout on the back of his head from Capo for his defiance. But Frond just bared his teeth, and moved out of reach.

The tall grass stems waved languorously at Frond’s passing, as if he were swimming through a sea of vegetation. And now Frond got up on to his hind legs, poking his head, shoulders, and upper torso out of the grass so he could see better. He was a slim shadow, upright, like a sapling.

The hyenas were still intent on their baby elephant. Frond ducked back into the grass and continued on his way.

At last he reached the nearest stand of trees. Capo, with a mixture of resentment and relief, saw him climb up a tall palm tree, his legs and arms working in synchrony, like components of a smoothly oiled machine. When Frond had reached the top of the palm he hooted softly, calling the others. Then he began plucking nuts from the palm and throwing them down to the ground.

One by one, led by Finger and the senior female, Leaf, the apes scurried through the grass toward the forest pocket.

They were not troubled by the hyenas, though many of the scavengers scented the vulnerable apes. They were fortunate that in the bloody calculations of the hyenas’ small minds, the lure of the immediately available meat outweighed the attraction of attacking these dusty, ragged-looking primates.

Capo tried to make the best of it. He slapped and punched the other males as they loped along, as if the whole thing had been his idea, as if he were directing them in their short migration. The males submitted to his blows, but he sensed a tension about them, a subtle lack of deference that made him uneasy.

On entering the forest, the apes fanned out.

Capo pushed through a bank of slim young trees to find a marshy lake: flat green-blue water surrounded by the comforting green and brown of forest. He hurried down to the water’s edge, pushed his muzzle into the cool liquid, and began to drink.

As the apes reached the water, some of them waded into it, walking upright until they were waist deep. They used their fingers to strain blue-green algae from the water and gobbled it down: a way of feeding that was another little gift of bipedalism. Several youngsters dove headlong into the water and started scraping the accumulated dust out of their fur; they made a terrific hooting and splashing. A flock of birds had been drifting in peace at the heart of the lake, but now they took fright, and clattered thunderously into the sky.

But some of the younger males had gathered together at the water’s edge, Frond and Finger among them. Frond had found a cobble that might serve as a hammer-stone; he was toying with it experimentally. And every now and then the males cast sly glances toward Capo. Their body language was redolent of conspiracy.

Capo pursed his lips and blew a soft raspberry.

He was very smart at working through social problems. He knew what the younger males were thinking. He had brought them to safety, but that wasn’t good enough: his performance as they had crossed that last grassy barrier had not convinced anyone. To restore his authority he was going to have to do some impressive displaying. He could rip down some branches and start stalking around the water’s edge, for instance; the foliage, the water, and the light would make for a powerful show. And then there would be hard battles to be won.

But perhaps now wasn’t the time.

He watched mothers gently bathing their infants, younger males wrestling almost politely as their limbs and skin recovered from the heat and aridity of the salt pan. Later. Let them get over the trek, before business as usual was resumed.

And besides, truth be told, he didn’t feel up to a great new war right now. His limbs ached, his skin was sore and covered in scrapes and lesions, and his gut, used to a continual flow of food and water, rumbled at the stop-start treatment it had endured. He was tired. He rubbed his eyes and yawned, allowing himself an explosive belch. Time enough later for the hard work of life, of being Capo. For now he needed to rest.

With that excuse lodged in his mind, he turned away from the water and loped into the forest.

He quickly found a kapok tree filled with large ripe fruits. The kapok, though, was armed with long sharp thorns to defend its fruit. So he tore two smooth branches from the tree and placed one under each foot, gripping the branches with his toes. Then, clinging to the branches with his feet, he climbed the tree, marching over the thorns as if they didn’t exist. The action of climbing made his limbs glow with the accustomed pleasure, their ancient design fulfilled; if he never took another step on the ground in his life he would have been content.

When he had reached a patch dense with fruit, he pulled off another branch and set it down over the thorns. Sitting on his impromptu saddle, he began to feed.

From here he could see that this forest clump had grown up around an oxbow lake, cast off by a river that wound its way back into the deeper country to the south, across this rich, vegetated Sahara. In the future this great Nile-like artery would be dislodged by tectonic shifting from its present course, and would curl around to the south, no longer crossing the Sahara. Eventually it would outflow into the Bight of Benin in western Africa, and humans would know it as the Niger: Even rivers were molded by time, as the land rose and fell, as the mountains grew and shrank away like dreams.

But for now this river was a great green corridor into the interior of the country. The troop could work that way, following the forest, penetrating deeper, moving away from the coast.

A piercing hoot echoed through the forest. It was a cry with only one meaning: Danger is here. Capo spat out a mouthful of fruit and scrambled down to the ground.

Before he got to the lake he knew what the problem was. He could smell them. And as he looked more carefully he could see the signs of their passing: bits of fruit skin, dumped even under this kapok, what looked like nests high in the taller trees.

Others.

They came swarming out of the trees and the undergrowth. There were many of them, bewilderingly many — fifty, sixty — more than Capo’s troop had ever numbered. Their males came toward the water’s edge. They were all displaying ferociously, fur bristling, drumming on roots and branches and hurling themselves through the low branches of the trees.

After all they had endured to get here, this patch of forest was not empty. Capo’s heart sank, heavy with a sense of failure.

But Capo’s troop was responding. Weak as they were, fur too damp to bristle effectively, nevertheless the males and even a couple of the older females were displaying as best they could. Capo threw himself forward to the front row of his troop and immediately began his own display, summoning up all his long experience to create as spectacular and intimidating a show as possible.

The two troops lined up; two walls of shrieking, posturing apes faced each other. They were the same species, and they looked indistinguishable, one from the other. But they could smell the differences between them: on the one hand the subtle, familiar savor of kin, and on the other the sharper stink of strangers. There was true xenophobic hatred in these displays, an authenticity in the threat they conveyed. Here was the other side of these clever animals’ social bonds: If you were locked into a group, then everybody else became your enemy, just because they weren’t you.

But Capo was scared. He quickly realized that these others were showing no signs of backing down. Indeed, their displays were becoming more ferocious, and those big lead males were steadily advancing on his troop.

Capo knew how it would go. It would not be an all-out war. The strongest would go first, the males and senior females; the infants would probably provide some sweet flesh for the bellies of these strangers. One by one. It would be a slow, bloody killing, but it would continue until it was complete. Such systematic slaughter was a horror new to the world, a horror only these apes, of all the Earth’s animals, were smart enough to conceive of and see through.

They couldn’t stay here, Capo knew. Maybe they could go on, resume the trek across the plain; maybe Capo could yet lead his troop somewhere empty, somewhere safe.

But in his deepest gut he intuitively knew the truth. In this world of shrinking forests, the surviving animals had already crammed themselves into all the remaining islands of the old vegetation. And that was why the others would fight so hard to exclude them. There were already too many of them for this dwindling patch — and they had nowhere else to go either.

There was nowhere safe to go, but no choice but to leave.

With much foot scuffing and branch waving, he began the subtle dance that indicated he wanted to lead his troop away from this place — back to the edge of the forest, back to the savannah. One or two of the females responded. Intimidated by these ferocious others, realizing how hopeless their situation was, Leaf and the others gathered up their infants and prepared to follow. Even Frond, one of the defiant young males, turned in confusion.

But Finger would not accept it.

He had been slamming a hammer-stone against an exposed root, adding its powerful noise to his display. Now, with a sudden, terrifying surge, he turned away from the others and launched a ferocious assault on Capo. He slammed into Capo’s back, knocking him flat, and he pounded his leader’s head with his fists. Then he rolled away and threw himself with equal vigor at the largest of the others’ males. Suddenly the noise, already high, became cacophonous, and the air filled with the stink of blood and panic shit.

Capo rolled on to his back and sat up, his neck aching. The other males subtly moved away, even as they hooted and yelled.

Finger was not faring well. He had managed to pin the big male to the ground. But now more of the others were throwing themselves into the melee. Soon they had hold of Finger. They hauled him away from his opponent, holding his limbs and head as if he were a hunted monkey; already blood streamed from bite-inflicted gashes in his skin. And then they threw him to the ground. But his cries soon became gurgles, drowned in blood, and Capo heard the grisly rip of flesh, the cracking of bone, the snapping of ligaments.

But Finger’s attack had had a profound effect. If anyone was going to attack these others, it should have been Capo. Capo knew he had already lost. He would be lucky to survive the day: If these others did not kill him, then his own former subordinates would.

Capo, though shamed and beaten, resumed his calling dance, trying to get his troop to come away. There was nothing else he could do.

They didn’t all respond, even now. Some of them, spitting fear and defiance, dispersed into the forest to seek their own destinies. He would never see them again.

The young female Howl glanced at her troop with wide, fearful eyes — and then made directly for the others. She would suffer a beating at the hands of the females, but maybe she would be attractive enough to the other males to be allowed to live, especially if she managed to become pregnant quickly through the hard matings she would have to endure.

Those who remained with Capo at last began moving, back toward the fringe of the forest — but only when Frond echoed Capo’s dance.

Capo understood, of course. They were following Frond, not him.

They came back to the fringe of the forest. They were not pursued, not for now. They picked at leaves and scraps of fruit, dismayed, uncertain.

Capo was depressed to be back where he had started. He could even see the corpse of that infant gomphothere, still lying on the ground. He clambered into a tree away from the others, and built an impromptu nest.

Now that Finger was dead, he wasn’t sure who would emerge as his main challenger. Frond, perhaps? It was possible Capo could continue to maintain a powerful position by forming an alliance with one male against the other. He might no longer be the boss of bosses, but like a kingmaker his backing would be crucial, and he would continue to enjoy many of the privileges that came with power, notably mating privileges. Maybe he could even work his way back to the top that way. His subtle mind thought further, considering shifting alliances, treacheries…

His thoughts dissolved. He felt overwhelmed by the journey he had made, the crashing disappointment that had waited at the end of it. Nothing seemed to matter anymore, not even the intricate political games that had won him so much in the past.

The others seemed to sense his mood. They avoided him, not coming to groom, not even looking at him. His gruesome defeat had been postponed by the death of Finger, but its sad process was still under way. Capo’s day was done, his life nearly over. All his swagger was gone.

But now Leaf came to him. She clambered into his nest alongside him, and, gently, began to groom him, as she had when they were both young and the world was bright and rich and full of possibility.

Frond wasn’t interested in Capo, one way or the other. He had something else on his mind.

He knuckle-walked a few paces out into the sunlit green. There he got to his hind legs once more. As always he was unsteady on his feet. But the elevation of his head gave him a platform from which to view the land, check on any predators or other dangers around.

Frond ducked back into the grass, and made his way cautiously to the gomphothere corpse. As he approached, carrion birds screeched their protest but flapped away. The scavengers had done their work well: The body looked as if it had exploded, with limbs and ribs lying scattered on the ground, bloody bone gleaming, and an eyeless, fleshless head peering back at him accusingly, spadelike tusks lying broken and gnawed. He rooted through the scraps of skin and bits of hyena-chewed flesh, but there was little to be had; the scavenging machinery of the savannah had worked thoroughly to consume the proboscidean’s flesh. The hyenas had even destroyed the soft ribs. But he found a thigh bone, long, thick, terminating at either end in huge, bulging lumps. It was unbroken. He tapped it experimentally against another bone; it sounded hollow.

He found a cobble in the dirt, the right size to fit into his fist. He raised the cobble and smashed it into the bone. The bone split, and rich, delicious marrow began to leak out. It was a resource that had been beyond the reach of the dogs and carrion birds, beyond their teeth and beaks. But now it was not beyond Frond. He raised the bone and began to suck down the marrow greedily.

The others who had driven Capo and his troop out of the forest would stay there, clinging to what they had. Such groups would eventually give rise to the chimpanzees, who would differ little from this ancestral stock. They would survive, even prosper: As the desert spread and the forests retreated to their last redoubts around the equator, the great rivers would provide corridors for the chimps to use to migrate into Africa’s interior.

But the descendants of Capo’s troop were now marching toward a very different destiny. This unremarkable troop of apes, stranded by the disappearance of their forest, would find there was a way to make a living out here. But leaving an ecology to which they had been adapting for millions of years was hard: As long as the apes couldn’t walk or run over long distances, while they couldn’t sweat, while they couldn’t even digest meat, many, many would die. But some would survive: just a few, but that was enough.

Frond had finished the marrow. But there were plenty more bones to be broken. He stood up again. He looked back to his troop, hooting to call them over.

Then he turned back to the savannah. He was bipedal, tool wielding, meat eating, xenophobic, hierarchical, combative, competitive — all of which he had brought from the forest — and yet he was imbued with the best qualities of his ancestors, with Purga’s doggedness, Noth’s exuberance, Roamer’s courage, even Capo’s vision. Full of the possibilities of the future, laden with the relics of the past, the young male, standing upright, gazed at the open plain.