A short history of nearly everything

27 ICE TIME

I had a dream, which was not all a dream.

The bright sun was extinguish’d, and the stars

Did wander . . .

– Byron, “Darkness”

IN 1815 on the island of Sumbawa in Indonesia, a handsome and long-quiescent mountain named Tambora exploded spectacularly, killing a hundred thousand people with its blast and associated tsunamis. It was the biggest volcanic explosion in ten thousand years-150 times the size of Mount St. Helens, equivalent to sixty thousand Hiroshima-sized atom bombs.

News didn’t travel terribly fast in those days. In London, The Times ran a small story-actually a letter from a merchant-seven months after the event. But by this time Tambora’s effects were already being felt. Thirty-six cubic miles of smoky ash, dust, and grit had diffused through the atmosphere, obscuring the Sun’s rays and causing the Earth to cool. Sunsets were unusually but blearily colorful, an effect memorably captured by the artist J. M. W. Turner, who could not have been happier, but mostly the world existed under an oppressive, dusky pall. It was this deathly dimness that inspired the Byron lines above.

Spring never came and summer never warmed: 1816 became known as the year without summer. Crops everywhere failed to grow. In Ireland a famine and associated typhoid epidemic killed sixty-five thousand people. In New England, the year became popularly known as Eighteen Hundred and Froze to Death. Morning frosts continued until June and almost no planted seed would grow. Short of fodder, livestock died or had to be prematurely slaughtered. In every way it was a dreadful year-almost certainly the worst for farmers in modern times. Yet globally the temperature fell by only about 1.5 degrees Fahrenheit. Earth’s natural thermostat, as scientists would learn, is an exceedingly delicate instrument.

The nineteenth century was already a chilly time. For two hundred years Europe and North America in particular had experienced a Little Ice Age, as it has become known, which permitted all kinds of wintry events-frost fairs on the Thames, ice-skating races along Dutch canals-that are mostly impossible now. It was a period, in other words, when frigidity was much on people’s minds. So we may perhaps excuse nineteenth-century geologists for being slow to realize that the world they lived in was in fact balmy compared with former epochs, and that much of the land around them had been shaped by crushing glaciers and cold that would wreck even a frost fair.

They knew there was something odd about the past. The European landscape was littered with inexplicable anomalies-the bones of arctic reindeer in the warm south of France, huge rocks stranded in improbable places-and they often came up with inventive but not terribly plausible explanations. One French naturalist named de Luc, trying to explain how granite boulders had come to rest high up on the limestone flanks of the Jura Mountains, suggested that perhaps they had been shot there by compressed air in caverns, like corks out of a popgun. The term for a displaced boulder is an erratic, but in the nineteenth century the expression seemed to apply more often to the theories than to the rocks.

The great British geologist Arthur Hallam has suggested that if James Hutton, the father of geology, had visited Switzerland, he would have seen at once the significance of the carved valleys, the polished striations, the telltale strand lines where rocks had been dumped, and the other abundant clues that point to passing ice sheets. Unfortunately, Hutton was not a traveler. But even with nothing better at his disposal than secondhand accounts, Hutton rejected out of hand the idea that huge boulders had been carried three thousand feet up mountainsides by floods-all the water in the world won’t make a boulder float, he pointed out-and became one of the first to argue for widespread glaciation. Unfortunately his ideas escaped notice, and for another half century most naturalists continued to insist that the gouges on rocks could be attributed to passing carts or even the scrape of hobnailed boots.

Local peasants, uncontaminated by scientific orthodoxy, knew better, however. The naturalist Jean de Charpentier told the story of how in 1834 he was walking along a country lane with a Swiss woodcutter when they got to talking about the rocks along the roadside. The woodcutter matter-of-factly told him that the boulders had come from the Grimsel, a zone of granite some distance away. “When I asked him how he thought that these stones had reached their location, he answered without hesitation: ‘The Grimsel glacier transported them on both sides of the valley, because that glacier extended in the past as far as the town of Bern.’ ”

Charpentier was delighted. He had come to such a view himself, but when he raised the notion at scientific gatherings, it was dismissed. One of Charpentier’s closest friends was another Swiss naturalist, Louis Agassiz, who after some initial skepticism came to embrace, and eventually all but appropriate, the theory.

Agassiz had studied under Cuvier in Paris and now held the post of Professor of Natural History at the College of Neuchâtel in Switzerland. Another friend of Agassiz’s, a botanist named Karl Schimper, was actually the first to coin the term ice age (in German Eiszeit), in 1837, and to propose that there was good evidence to show that ice had once lain heavily across not just the Swiss Alps, but over much of Europe, Asia, and North America. It was a radical notion. He lent Agassiz his notes-then came very much to regret it as Agassiz increasingly got the credit for what Schimper felt, with some legitimacy, was his theory. Charpentier likewise ended up a bitter enemy of his old friend. Alexander von Humboldt, yet another friend, may have had Agassiz at least partly in mind when he observed that there are three stages in scientific discovery: first, people deny that it is true; then they deny that it is important; finally they credit the wrong person.

At all events, Agassiz made the field his own. In his quest to understand the dynamics of glaciation, he went everywhere-deep into dangerous crevasses and up to the summits of the craggiest Alpine peaks, often apparently unaware that he and his team were the first to climb them. Nearly everywhere Agassiz encountered an unyielding reluctance to accept his theories. Humboldt urged him to return to his area of real expertise, fossil fish, and give up this mad obsession with ice, but Agassiz was a man possessed by an idea.

Agassiz’s theory found even less support in Britain, where most naturalists had never seen a glacier and often couldn’t grasp the crushing forces that ice in bulk exerts. “Could scratches and polish just be due to ice?” asked Roderick Murchison in a mocking tone at one meeting, evidently imagining the rocks as covered in a kind of light and glassy rime. To his dying day, he expressed the frankest incredulity at those “ice-mad” geologists who believed that glaciers could account for so much. William Hopkins, a Cambridge professor and leading member of the Geological Society, endorsed this view, arguing that the notion that ice could transport boulders presented “such obvious mechanical absurdities” as to make it unworthy of the society’s attention.

Undaunted, Agassiz traveled tirelessly to promote his theory. In 1840 he read a paper to a meeting of the British Association for the Advancement of Science in Glasgow at which he was openly criticized by the great Charles Lyell. The following year the Geological Society of Edinburgh passed a resolution conceding that there might be some general merit in the theory but that certainly none of it applied to Scotland.

Lyell did eventually come round. His moment of epiphany came when he realized that a moraine, or line of rocks, near his family estate in Scotland, which he had passed hundreds of times, could only be understood if one accepted that a glacier had dropped them there. But having become converted, Lyell then lost his nerve and backed off from public support of the Ice Age idea. It was a frustrating time for Agassiz. His marriage was breaking up, Schimper was hotly accusing him of the theft of his ideas, Charpentier wouldn’t speak to him, and the greatest living geologist offered support of only the most tepid and vacillating kind.

In 1846, Agassiz traveled to America to give a series of lectures and there at last found the esteem he craved. Harvard gave him a professorship and built him a first-rate museum, the Museum of Comparative Zoology. Doubtless it helped that he had settled in New England, where the long winters encouraged a certain sympathy for the idea of interminable periods of cold. It also helped that six years after his arrival the first scientific expedition to Greenland reported that nearly the whole of that semicontinent was covered in an ice sheet just like the ancient one imagined in Agassiz’s theory. At long last, his ideas began to find a real following. The one central defect of Agassiz’s theory was that his ice ages had no cause. But assistance was about to come from an unlikely quarter.

In the 1860s, journals and other learned publications in Britain began to receive papers on hydrostatics, electricity, and other scientific subjects from a James Croll of Anderson’s University in Glasgow. One of the papers, on how variations in Earth’s orbit might have precipitated ice ages, was published in the Philosophical Magazine in 1864 and was recognized at once as a work of the highest standard. So there was some surprise, and perhaps just a touch of embarrassment, when it turned out that Croll was not an academic at the university, but a janitor.

Born in 1821, Croll grew up poor, and his formal education lasted only to the age of thirteen. He worked at a variety of jobs-as a carpenter, insurance salesman, keeper of a temperance hotel-before taking a position as a janitor at Anderson’s (now the University of Strathclyde) in Glasgow. By somehow inducing his brother to do much of his work, he was able to pass many quiet evenings in the university library teaching himself physics, mechanics, astronomy, hydrostatics, and the other fashionable sciences of the day, and gradually began to produce a string of papers, with a particular emphasis on the motions of Earth and their effect on climate.

Croll was the first to suggest that cyclical changes in the shape of Earth’s orbit, from elliptical (which is to say slightly oval) to nearly circular to elliptical again, might explain the onset and retreat of ice ages. No one had ever thought before to consider an astronomical explanation for variations in Earth’s weather. Thanks almost entirely to Croll’s persuasive theory, people in Britain began to become more responsive to the notion that at some former time parts of the Earth had been in the grip of ice. When his ingenuity and aptitude were recognized, Croll was given a job at the Geological Survey of Scotland and widely honored: he was made a fellow of the Royal Society in London and of the New York Academy of Science and given an honorary degree from the University of St. Andrews, among much else.

Unfortunately, just as Agassiz’s theory was at last beginning to find converts in Europe, he was busy taking it into ever more exotic territory in America. He began to find evidence for glaciers practically everywhere he looked, including near the equator. Eventually he became convinced that ice had once covered the whole Earth, extinguishing all life, which God had then re-created. None of the evidence Agassiz cited supported such a view. Nonetheless, in his adopted country his stature grew and grew until he was regarded as only slightly below a deity. When he died in 1873 Harvard felt it necessary to appoint three professors to take his place.

Yet, as sometimes happens, his theories fell swiftly out of fashion. Less than a decade after his death his successor in the chair of geology at Harvard wrote that the “so-called glacial epoch . . . so popular a few years ago among glacial geologists may now be rejected without hesitation.”

Part of the problem was that Croll’s computations suggested that the most recent ice age occurred eighty thousand years ago, whereas the geological evidence increasingly indicated that Earth had undergone some sort of dramatic perturbation much more recently than that. Without a plausible explanation for what might have provoked an ice age, the whole theory fell into abeyance. There it might have remained for some time except that in the early 1900s a Serbian academic named Milutin Milankovitch, who had no background in celestial motions at all-he was a mechanical engineer by training-developed an unexpected interest in the matter. Milankovitch realized that the problem with Croll’s theory was not that it was incorrect but that it was too simple.

As Earth moves through space, it is subject not just to variations in the length and shape of its orbit, but also to rhythmic shifts in its angle of orientation to the Sun-its tilt and pitch and wobble-all affecting the length and intensity of sunlight falling on any patch of land. In particular it is subject to three changes in position, known formally as its obliquity, precession, and eccentricity, over long periods of time. Milankovitch wondered if there might be a relationship between these complex cycles and the comings and goings of ice ages. The difficulty was that the cycles were of widely different lengths-of approximately 20,000, 40,000, and 100,000 years, but varying in each case by up to a few thousand years-which meant that determining their points of intersection over long spans of time involved a nearly endless amount of devoted computation. Essentially Milankovitch had to work out the angle and duration of incoming solar radiation at every latitude on Earth, in every season, for a million years, adjusted for three ever-changing variables.

Happily this was precisely the sort of repetitive toil that suited Milankovitch’s temperament. For the next twenty years, even while on vacation, he worked ceaselessly with pencil and slide rule computing the tables of his cycles-work that now could be completed in a day or two with a computer. The calculations all had to be made in his spare time, but in 1914 Milankovitch suddenly got a great deal of that when World War I broke out and he was arrested owing to his position as a reservist in the Serbian army. He spent most of the next four years under loose house arrest in Budapest, required only to report to the police once a week. The rest of his time was spent working in the library of the Hungarian Academy of Sciences. He was possibly the happiest prisoner of war in history.

The eventual outcome of his diligent scribblings was the 1930 book Mathematical Climatology and the Astronomical Theory of Climatic Changes. Milankovitch was right that there was a relationship between ice ages and planetary wobble, though like most people he assumed that it was a gradual increase in harsh winters that led to these long spells of coldness. It was a Russian-German meteorologist, Wladimir Köppen-father-in-law of our tectonic friend Alfred Wegener-who saw that the process was more subtle, and rather more unnerving, than that.

The cause of ice ages, Köppen decided, is to be found in cool summers, not brutal winters. If summers are too cool to melt all the snow that falls on a given area, more incoming sunlight is bounced back by the reflective surface, exacerbating the cooling effect and encouraging yet more snow to fall. The consequence would tend to be self-perpetuating. As snow accumulated into an ice sheet, the region would grow cooler, prompting more ice to accumulate. As the glaciologist Gwen Schultz has noted: “It is not necessarily the amount of snow that causes ice sheets but the fact that snow, however little, lasts.” It is thought that an ice age could start from a single unseasonal summer. The leftover snow reflects heat and exacerbates the chilling effect. “The process is self-enlarging, unstoppable, and once the ice is really growing it moves,” says McPhee. You have advancing glaciers and an ice age.

In the 1950s, because of imperfect dating technology, scientists were unable to correlate Milankovitch’s carefully worked-out cycles with the supposed dates of ice ages as then perceived, and so Milankovitch and his calculations increasingly fell out of favor. He died in 1958, unable to prove that his cycles were correct. By this time, write John and Mary Gribbin, “you would have been hard pressed to find a geologist or meteorologist who regarded the model as being anything more than an historical curiosity.” Not until the 1970s and the refinement of a potassium-argon method for dating ancient seafloor sediments were his theories finally vindicated.

The Milankovitch cycles alone are not enough to explain cycles of ice ages. Many other factors are involved-not least the disposition of the continents, in particular the presence of landmasses over the poles-but the specifics of these are imperfectly understood. It has been suggested, however, that if you hauled North America, Eurasia, and Greenland just three hundred miles north we would have permanent and inescapable ice ages. We are very lucky, it appears, to get any good weather at all. Even less well understood are the cycles of comparative balminess within ice ages, known as interglacials. It is mildly unnerving to reflect that the whole of meaningful human history-the development of farming, the creation of towns, the rise of mathematics and writing and science and all the rest-has taken place within an atypical patch of fair weather. Previous interglacials have lasted as little as eight thousand years. Our own has already passed its ten thousandth anniversary.

The fact is, we are still very much in an ice age; it’s just a somewhat shrunken one-though less shrunken than many people realize. At the height of the last period of glaciation, around twenty thousand years ago, about 30 percent of the Earth’s land surface was under ice. Ten percent still is-and a further 14 percent is in a state of permafrost. Three-quarters of all the fresh water on Earth is locked up in ice even now, and we have ice caps at both poles-a situation that may be unique in Earth’s history. That there are snowy winters through much of the world and permanent glaciers even in temperate places such as New Zealand may seem quite natural, but in fact it is a most unusual situation for the planet.

For most of its history until fairly recent times the general pattern for Earth was to be hot with no permanent ice anywhere. The current ice age-ice epoch really-started about forty million years ago, and has ranged from murderously bad to not bad at all. Ice ages tend to wipe out evidence of earlier ice ages, so the further back you go the more sketchy the picture grows, but it appears that we have had at least seventeen severe glacial episodes in the last 2.5 million years or so-the period that coincides with the rise of Homo erectus in Africa followed by modern humans. Two commonly cited culprits for the present epoch are the rise of the Himalayas and the formation of the Isthmus of Panama, the first disrupting air flows, the second ocean currents. India, once an island, has pushed two thousand kilometers into the Asian landmass over the last forty-five million years, raising not only the Himalayas, but also the vast Tibetan plateau behind them. The hypothesis is that the higher landscape was not only cooler, but diverted winds in a way that made them flow north and toward North America, making it more susceptible to long-term chills. Then, beginning about five million years ago, Panama rose from the sea, closing the gap between North and South America, disrupting the flows of warming currents between the Pacific and Atlantic, and changing patterns of precipitation across at least half the world. One consequence was a drying out of Africa, which caused apes to climb down out of trees and go looking for a new way of living on the emerging savannas.

At all events, with the oceans and continents arranged as they are now, it appears that ice will be a long-term part of our future. According to John McPhee, about fifty more glacial episodes can be expected, each lasting a hundred thousand years or so, before we can hope for a really long thaw.

Before fifty million years ago, Earth had no regular ice ages, but when we did have them they tended to be colossal. A massive freezing occurred about 2.2 billion years ago, followed by a billion years or so of warmth. Then there was another ice age even larger than the first-so large that some scientists are now referring to the age in which it occurred as the Cryogenian, or super ice age. The condition is more popularly known as Snowball Earth.

“Snowball,” however, barely captures the murderousness of conditions. The theory is that because of a fall in solar radiation of about 6 percent and a dropoff in the production (or retention) of greenhouse gases, Earth essentially lost its ability to hold on to its heat. It became a kind of all-over Antarctica. Temperatures plunged by as much as 80 degrees Fahrenheit. The entire surface of the planet may have frozen solid, with ocean ice up to half a mile thick at higher latitudes and tens of yards thick even in the tropics.

There is a serious problem in all this in that the geological evidence indicates ice everywhere, including around the equator, while the biological evidence suggests just as firmly that there must have been open water somewhere. For one thing, cyanobacteria survived the experience, and they photosynthesize. For that they needed sunlight, but as you will know if you have ever tried to peer through it, ice quickly becomes opaque and after only a few yards would pass on no light at all. Two possibilities have been suggested. One is that a little ocean water did remain exposed (perhaps because of some kind of localized warming at a hot spot); the other is that maybe the ice formed in such a way that it remained translucent-a condition that does sometimes happen in nature.

If Earth did freeze over, then there is the very difficult question of how it ever got warm again. An icy planet should reflect so much heat that it would stay frozen forever. It appears that rescue may have come from our molten interior. Once again, we may be indebted to tectonics for allowing us to be here. The idea is that we were saved by volcanoes, which pushed through the buried surface, pumping out lots of heat and gases that melted the snows and re-formed the atmosphere. Interestingly, the end of this hyper-frigid episode is marked by the Cambrian outburst-the springtime event of life’s history. In fact, it may not have been as tranquil as all that. As Earth warmed, it probably had the wildest weather it has ever experienced, with hurricanes powerful enough to raise waves to the heights of skyscrapers and rainfalls of indescribable intensity.

Throughout all this the tubeworms and clams and other life forms adhering to deep ocean vents undoubtedly went on as if nothing were amiss, but all other life on Earth probably came as close as it ever has to checking out entirely. It was all a long time ago and at this stage we just don’t know.

Compared with a Cryogenian outburst, the ice ages of more recent times seem pretty small scale, but of course they were immensely grand by the standards of anything to be found on Earth today. The Wisconsian ice sheet, which covered much of Europe and North America, was two miles thick in places and marched forward at a rate of about four hundred feet a year. What a thing it must have been to behold. Even at their leading edge, the ice sheets could be nearly half a mile thick. Imagine standing at the base of a wall of ice two thousand feet high. Behind this edge, over an area measuring in the millions of square miles, would be nothing but more ice, with only a few of the tallest mountain summits poking through. Whole continents sagged under the weight of so much ice and even now, twelve thousand years after the glaciers’ withdrawal, are still rising back into place. The ice sheets didn’t just dribble out boulders and long lines of gravelly moraines, but dumped entire landmasses-Long Island and Cape Cod and Nantucket, among others-as they slowly swept along. It’s little wonder that geologists before Agassiz had trouble grasping their monumental capacity to rework landscapes.

If ice sheets advanced again, we have nothing in our armory that could deflect them. In 1964, at Prince William Sound in Alaska, one of the largest glacial fields in North America was hit by the strongest earthquake ever recorded on the continent. It measured 9.2 on the Richter scale. Along the fault line, the land rose by as much as twenty feet. The quake was so violent, in fact, that it made water slosh out of pools in Texas. And what effect did this unparalleled outburst have on the glaciers of Prince William Sound? None at all. They just soaked it up and kept on moving.

For a long time it was thought that we moved into and out of ice ages gradually, over hundreds of thousands of years, but we now know that that has not been the case. Thanks to ice cores from Greenland we have a detailed record of climate for something over a hundred thousand years, and what is found there is not comforting. It shows that for most of its recent history Earth has been nothing like the stable and tranquil place that civilization has known, but rather has lurched violently between periods of warmth and brutal chill.

Toward the end of the last big glaciation, some twelve thousand years ago, Earth began to warm, and quite rapidly, but then abruptly plunged back into bitter cold for a thousand years or so in an event known to science as the Younger Dryas. (The name comes from the arctic plant the dryas, which is one of the first to recolonize land after an ice sheet withdraws. There was also an Older Dryas period, but it wasn’t so sharp.) At the end of this thousand-year onslaught average temperatures leapt again, by as much as seven degrees in twenty years, which doesn’t sound terribly dramatic but is equivalent to exchanging the climate of Scandinavia for that of the Mediterranean in just two decades. Locally, changes have been even more dramatic. Greenland ice cores show the temperatures there changing by as much as fifteen degrees in ten years, drastically altering rainfall patterns and growing conditions. This must have been unsettling enough on a thinly populated planet. Today the consequences would be pretty well unimaginable.

What is most alarming is that we have no idea-none-what natural phenomena could so swiftly rattle Earth’s thermometer. As Elizabeth Kolbert, writing in the New Yorker, has observed: “No known external force, or even any that has been hypothesized, seems capable of yanking the temperature back and forth as violently, and as often, as these cores have shown to be the case.” There seems to be, she adds, “some vast and terrible feedback loop,” probably involving the oceans and disruptions of the normal patterns of ocean circulation, but all this is a long way from being understood.

One theory is that the heavy inflow of meltwater to the seas at the beginning of the Younger Dryas reduced the saltiness (and thus density) of northern oceans, causing the Gulf Stream to swerve to the south, like a driver trying to avoid a collision. Deprived of the Gulf Stream’s warmth, the northern latitudes returned to chilly conditions. But this doesn’t begin to explain why a thousand years later when the Earth warmed once again the Gulf Stream didn’t veer as before. Instead, we were given the period of unusual tranquility known as the Holocene, the time in which we live now.

There is no reason to suppose that this stretch of climatic stability should last much longer. In fact, some authorities believe that we are in for even worse than what went before. It is natural to suppose that global warming would act as a useful counterweight to the Earth’s tendency to plunge back into glacial conditions. However, as Kolbert has pointed out, when you are confronted with a fluctuating and unpredictable climate “the last thing you’d want to do is conduct a vast unsupervised experiment on it.” It has even been suggested, with more plausibility than would at first seem evident, that an ice age might actually be induced by a rise in temperatures. The idea is that a slight warming would enhance evaporation rates and increase cloud cover, leading in the higher latitudes to more persistent accumulations of snow. In fact, global warming could plausibly, if paradoxically, lead to powerful localized cooling in North America and northern Europe.

Climate is the product of so many variables-rising and falling carbon dioxide levels, the shifts of continents, solar activity, the stately wobbles of the Milankovitch cycles-that it is as difficult to comprehend the events of the past as it is to predict those of the future. Much is simply beyond us. Take Antarctica. For at least twenty million years after it settled over the South Pole Antarctica remained covered in plants and free of ice. That simply shouldn’t have been possible.

No less intriguing are the known ranges of some late dinosaurs. The British geologist Stephen Drury notes that forests within 10 degrees latitude of the North Pole were home to great beasts, including Tyrannosaurus rex. “That is bizarre,” he writes, “for such a high latitude is continually dark for three months of the year.” Moreover, there is now evidence that these high latitudes suffered severe winters. Oxygen isotope studies suggest that the climate around Fairbanks, Alaska, was about the same in the late Cretaceous period as it is now. So what was Tyrannosaurus doing there? Either it migrated seasonally over enormous distances or it spent much of the year in snowdrifts in the dark. In Australia-which at that time was more polar in its orientation-a retreat to warmer climes wasn’t possible. How dinosaurs managed to survive in such conditions can only be guessed.

One thought to bear in mind is that if the ice sheets did start to form again for whatever reason, there is a lot more water for them to draw on this time. The Great Lakes, Hudson Bay, the countless lakes of Canada-these weren’t there to fuel the last ice age. They were created by it.

On the other hand, the next phase of our history could see us melting a lot of ice rather than making it. If all the ice sheets melted, sea levels would rise by two hundred feet-the height of a twenty-story building-and every coastal city in the world would be inundated. More likely, at least in the short term, is the collapse of the West Antarctic ice sheet. In the past fifty years the waters around it have warmed by 2.5 degrees centigrade, and collapses have increased dramatically. Because of the underlying geology of the area, a large-scale collapse is all the more possible. If so, sea levels globally would rise-and pretty quickly-by between fifteen and twenty feet on average.

The extraordinary fact is that we don’t know which is more likely, a future offering us eons of perishing frigidity or one giving us equal expanses of steamy heat. Only one thing is certain: we live on a knife edge.

In the long run, incidentally, ice ages are by no means bad news for the planet. They grind up rocks and leave behind new soils of sumptuous richness, and gouge out fresh water lakes that provide abundant nutritive possibilities for hundreds of species of being. They act as a spur to migration and keep the planet dynamic. As Tim Flannery has remarked: “There is only one question you need ask of a continent in order to determine the fate of its people: ‘Did you have a good ice age?’ ” And with that in mind, it’s time to look at a species of ape that truly did.

28 THE MYSTERIOUS BIPED

JUST BEFORE CHRISTMAS 1887, a young Dutch doctor with an un-Dutch name, Marie Eugène François Thomas Dubois, arrived in Sumatra, in the Dutch East Indies, with the intention of finding the earliest human remains on Earth.[46]

Several things were extraordinary about this. To begin with, no one had ever gone looking for ancient human bones before. Everything that had been found to this point had been found accidentally, and nothing in Dubois’s background suggested that he was the ideal candidate to make the process intentional. He was an anatomist by training with no background in paleontology. Nor was there any special reason to suppose that the East Indies would hold early human remains. Logic dictated that if ancient people were to be found at all, it would be on a large and long-populated landmass, not in the comparative fastness of an archipelago. Dubois was driven to the East Indies on nothing stronger than a hunch, the availability of employment, and the knowledge that Sumatra was full of caves, the environment in which most of the important hominid fossils had so far been found. What is most extraordinary in all this-nearly miraculous, really-is that he found what he was looking for.

At the time Dubois conceived his plan to search for a missing link, the human fossil record consisted of very little: five incomplete Neandertal skeletons, one partial jawbone of uncertain provenance, and a half-dozen ice-age humans recently found by railway workers in a cave at a cliff called Cro-Magnon near Les Eyzies, France. Of the Neandertal specimens, the best preserved was sitting unremarked on a shelf in London. It had been found by workers blasting rock from a quarry in Gibraltar in 1848, so its preservation was a wonder, but unfortunately no one yet appreciated what it was. After being briefly described at a meeting of the Gibraltar Scientific Society, it had been sent to the Hunterian Museum in London, where it remained undisturbed but for an occasional light dusting for over half a century. The first formal description of it wasn’t written until 1907, and then by a geologist named William Sollas “with only a passing competency in anatomy.”

So instead the name and credit for the discovery of the first early humans went to the Neander Valley in Germany-not unfittingly, as it happens, for by uncanny coincidence Neander in Greek means “new man.” There in 1856 workmen at another quarry, in a cliff face overlooking the Düssel River, found some curious-looking bones, which they passed to a local schoolteacher, knowing he had an interest in all things natural. To his great credit the teacher, Johann Karl Fuhlrott, saw that he had some new type of human, though quite what it was, and how special, would be matters of dispute for some time.

Many people refused to accept that the Neandertal bones were ancient at all. August Mayer, a professor at the University of Bonn and a man of influence, insisted that the bones were merely those of a Mongolian Cossack soldier who had been wounded while fighting in Germany in 1814 and had crawled into the cave to die. Hearing of this, T. H. Huxley in England drily observed how remarkable it was that the soldier, though mortally wounded, had climbed sixty feet up a cliff, divested himself of his clothing and personal effects, sealed the cave opening, and buried himself under two feet of soil. Another anthropologist, puzzling over the Neandertal’s heavy brow ridge, suggested that it was the result of long-term frowning arising from a poorly healed forearm fracture. (In their eagerness to reject the idea of earlier humans, authorities were often willing to embrace the most singular possibilities. At about the time that Dubois was setting out for Sumatra, a skeleton found in Périgueux was confidently declared to be that of an Eskimo. Quite what an ancient Eskimo was doing in southwest France was never comfortably explained. It was actually an early Cro-Magnon.)

It was against this background that Dubois began his search for ancient human bones. He did no digging himself, but instead used fifty convicts lent by the Dutch authorities. For a year they worked on Sumatra, then transferred to Java. And there in 1891, Dubois-or rather his team, for Dubois himself seldom visited the sites-found a section of ancient human cranium now known as the Trinil skullcap. Though only part of a skull, it showed that the owner had had distinctly nonhuman features but a much larger brain than any ape. Dubois called it Anthropithecus erectus (later changed for technical reasons to Pithecanthropus erectus) and declared it the missing link between apes and humans. It quickly became popularized as “Java Man.” Today we know it as Homo erectus.

The next year Dubois’s workers found a virtually complete thighbone that looked surprisingly modern. In fact, many anthropologists think it is modern, and has nothing to do with Java Man. If it is an erectus bone, it is unlike any other found since. Nonetheless Dubois used the thighbone to deduce-correctly, as it turned out-that Pithecanthropus walked upright. He also produced, with nothing but a scrap of cranium and one tooth, a model of the complete skull, which also proved uncannily accurate.

In 1895, Dubois returned to Europe, expecting a triumphal reception. In fact, he met nearly the opposite reaction. Most scientists disliked both his conclusions and the arrogant manner in which he presented them. The skullcap, they said, was that of an ape, probably a gibbon, and not of any early human. Hoping to bolster his case, in 1897 Dubois allowed a respected anatomist from the University of Strasbourg, Gustav Schwalbe, to make a cast of the skullcap. To Dubois’s dismay, Schwalbe thereupon produced a monograph that received far more sympathetic attention than anything Dubois had written and followed with a lecture tour in which he was celebrated nearly as warmly as if he had dug up the skull himself. Appalled and embittered, Dubois withdrew into an undistinguished position as a professor of geology at the University of Amsterdam and for the next two decades refused to let anyone examine his precious fossils again. He died in 1940 an unhappy man.

Meanwhile, and half a world away, in late 1924 Raymond Dart, the Australian-born head of anatomy at the University of the Witwatersrand in Johannesburg, was sent a small but remarkably complete skull of a child, with an intact face, a lower jaw, and what is known as an endocast-a natural cast of the brain-from a limestone quarry on the edge of the Kalahari Desert at a dusty spot called Taung. Dart could see at once that the Taung skull was not of a Homo erectus like Dubois’s Java Man, but from an earlier, more apelike creature. He placed its age at two million years and dubbed it Australopithecus africanus, or “southern ape man of Africa.” In a report to Nature, Dart called the Taung remains “amazingly human” and suggested the need for an entirely new family, Homo simiadae (“the man-apes”), to accommodate the find.

The authorities were even less favorably disposed to Dart than they had been to Dubois. Nearly everything about his theory-indeed, nearly everything about Dart, it appears-annoyed them. First he had proved himself lamentably presumptuous by conducting the analysis himself rather than calling on the help of more worldly experts in Europe. Even his chosen name, Australopithecus, showed a lack of scholarly application, combining as it did Greek and Latin roots. Above all, his conclusions flew in the face of accepted wisdom. Humans and apes, it was agreed, had split apart at least fifteen million years ago in Asia. If humans had arisen in Africa, why, that would make us Negroid, for goodness sake. It was rather as if someone working today were to announce that he had found the ancestral bones of humans in, say, Missouri. It just didn’t fit with what was known.

Dart’s sole supporter of note was Robert Broom, a Scottish-born physician and paleontologist of considerable intellect and cherishably eccentric nature. It was Broom’s habit, for instance, to do his fieldwork naked when the weather was warm, which was often. He was also known for conducting dubious anatomical experiments on his poorer and more tractable patients. When the patients died, which was also often, he would sometimes bury their bodies in his back garden to dig up for study later.

Broom was an accomplished paleontologist, and since he was also resident in South Africa he was able to examine the Taung skull at first hand. He could see at once that it was as important as Dart supposed and spoke out vigorously on Dart’s behalf, but to no effect. For the next fifty years the received wisdom was that the Taung child was an ape and nothing more. Most textbooks didn’t even mention it. Dart spent five years working up a monograph, but could find no one to publish it. Eventually he gave up the quest to publish altogether (though he did continue hunting for fossils). For years, the skull-today recognized as one of the supreme treasures of anthropology-sat as a paperweight on a colleague’s desk.

At the time Dart made his announcement in 1924, only four categories of ancient hominid were known-Homo heidelbergensis, Homo rhodesiensis, Neandertals, and Dubois’s Java Man-but all that was about to change in a very big way.

First, in China, a gifted Canadian amateur named Davidson Black began to poke around at a place, Dragon Bone Hill, that was locally famous as a hunting ground for old bones. Unfortunately, rather than preserving the bones for study, the Chinese ground them up to make medicines. We can only guess how many priceless Homo erectus bones ended up as a sort of Chinese equivalent of bicarbonate of soda. The site had been much denuded by the time Black arrived, but he found a single fossilized molar and on the basis of that alone quite brilliantly announced the discovery of Sinanthropus pekinensis, which quickly became known as Peking Man.

At Black’s urging, more determined excavations were undertaken and many other bones found. Unfortunately all were lost the day after the Japanese attack on Pearl Harbor in 1941 when a contingent of U.S. Marines, trying to spirit the bones (and themselves) out of the country, was intercepted by the Japanese and imprisoned. Seeing that their crates held nothing but bones, the Japanese soldiers left them at the roadside. It was the last that was ever seen of them.

In the meantime, back on Dubois’s old turf of Java, a team led by Ralph von Koenigswald had found another group of early humans, which became known as the Solo People from the site of their discovery on the Solo River at Ngandong. Koenigswald’s discoveries might have been more impressive still but for a tactical error that was realized too late. He had offered locals ten cents for every piece of hominid bone they could come up with, then discovered to his horror that they had been enthusiastically smashing large pieces into small ones to maximize their income.

In the following years as more bones were found and identified there came a flood of new names-Homo aurignacensis, Australopithecus transvaalensis, Paranthropus crassidens, Zinjanthropus boisei, and scores of others, nearly all involving a new genus type as well as a new species. By the 1950s, the number of named hominid types had risen to comfortably over a hundred. To add to the confusion, individual forms often went by a succession of different names as paleoanthropologists refined, reworked, and squabbled over classifications. Solo People were known variously as Homo soloensis, Homo primigenius asiaticus, Homo neanderthalensis soloensis, Homo sapiens soloensis, Homo erectus erectus, and, finally, plain Homo erectus.

In an attempt to introduce some order, in 1960 F. Clark Howell of the University of Chicago, following the suggestions of Ernst Mayr and others the previous decade, proposed cutting the number of genera to just two-Australopithecus and Homo-and rationalizing many of the species. The Java and Peking men both became Homo erectus. For a time order prevailed in the world of the hominids.[47] It didn’t last.

After about a decade of comparative calm, paleoanthropology embarked on another period of swift and prolific discovery, which hasn’t abated yet. The 1960s produced Homo habilis, thought by some to be the missing link between apes and humans, but thought by others not to be a separate species at all. Then came (among many others) Homo ergaster, Homo louisleakeyi, Homo rudolfensis, Homo microcranus, and Homo antecessor, as well as a raft of australopithecines: A.afarensis, A. praegens, A. ramidus, A. walkeri, A. anamensis, and still others. Altogether, some twenty types of hominid are recognized in the literature today. Unfortunately, almost no two experts recognize the same twenty.

Some continue to observe the two hominid genera suggested by Howell in 1960, but others place some of the australopithecines in a separate genus called Paranthropus, and still others add an earlier group called Ardipithecus. Some put praegens into Australopithecus and some into a new classification, Homo antiquus, but most don’t recognize praegens as a separate species at all. There is no central authority that rules on these things. The only way a name becomes accepted is by consensus, and there is often very little of that.

A big part of the problem, paradoxically, is a shortage of evidence. Since the dawn of time, several billion human (or humanlike) beings have lived, each contributing a little genetic variability to the total human stock. Out of this vast number, the whole of our understanding of human prehistory is based on the remains, often exceedingly fragmentary, of perhaps five thousand individuals. “You could fit it all into the back of a pickup truck if you didn’t mind how much you jumbled everything up,” Ian Tattersall, the bearded and friendly curator of anthropology at the American Museum of Natural History in New York, replied when I asked him the size of the total world archive of hominid and early human bones.

The shortage wouldn’t be so bad if the bones were distributed evenly through time and space, but of course they are not. They appear randomly, often in the most tantalizing fashion. Homo erectus walked the Earth for well over a million years and inhabited territory from the Atlantic edge of Europe to the Pacific side of China, yet if you brought back to life every Homo erectus individual whose existence we can vouch for, they wouldn’t fill a school bus. Homo habilis consists of even less: just two partial skeletons and a number of isolated limb bones. Something as short-lived as our own civilization would almost certainly not be known from the fossil record at all.

“In Europe,” Tattersall offers by way of illustration, “you’ve got hominid skulls in Georgia dated to about 1.7 million years ago, but then you have a gap of almost a million years before the next remains turn up in Spain, right on the other side of the continent, and then you’ve got another 300,000-year gap before you get a Homo heidelbergensis in Germany-and none of them looks terribly much like any of the others.” He smiled. “It’s from these kinds of fragmentary pieces that you’re trying to work out the histories of entire species. It’s quite a tall order. We really have very little idea of the relationships between many ancient species-which led to us and which were evolutionary dead ends. Some probably don’t deserve to be regarded as separate species at all.”

It is the patchiness of the record that makes each new find look so sudden and distinct from all the others. If we had tens of thousands of skeletons distributed at regular intervals through the historical record, there would be appreciably more degrees of shading. Whole new species don’t emerge instantaneously, as the fossil record implies, but gradually out of other, existing species. The closer you go back to a point of divergence, the closer the similarities are, so that it becomes exceedingly difficult, and sometimes impossible, to distinguish a late Homo erectus from an early Homo sapiens, since it is likely to be both and neither. Similar disagreements can often arise over questions of identification from fragmentary remains-deciding, for instance, whether a particular bone represents a female Australopithecus boisei or a male Homo habilis.

With so little to be certain about, scientists often have to make assumptions based on other objects found nearby, and these may be little more than valiant guesses. As Alan Walker and Pat Shipman have drily observed, if you correlate tool discovery with the species of creature most often found nearby, you would have to conclude that early hand tools were mostly made by antelopes.

Perhaps nothing better typifies the confusion than the fragmentary bundle of contradictions that was Homo habilis. Simply put, habilis bones make no sense. When arranged in sequence, they show males and females evolving at different rates and in different directions-the males becoming less apelike and more human with time, while females from the same period appear to be moving away from humanness toward greater apeness. Some authorities don’t believe habilis is a valid category at all. Tattersall and his colleague Jeffrey Schwartz dismiss it as a mere “wastebasket species”-one into which unrelated fossils “could be conveniently swept.” Even those who see habilis as an independent species don’t agree on whether it is of the same genus as us or is from a side branch that never came to anything.

Finally, but perhaps above all, human nature is a factor in all this. Scientists have a natural tendency to interpret finds in the way that most flatters their stature. It is a rare paleontologist indeed who announces that he has found a cache of bones but that they are nothing to get excited about. Or as John Reader understatedly observes in the book Missing Links, “It is remarkable how often the first interpretations of new evidence have confirmed the preconceptions of its discoverer.”

All this leaves ample room for arguments, of course, and nobody likes to argue more than paleoanthropologists. “And of all the disciplines in science, paleoanthropology boasts perhaps the largest share of egos,” say the authors of the recent Java Man-a book, it may be noted, that itself devotes long, wonderfully unselfconscious passages to attacks on the inadequacies of others, in particular the authors’ former close colleague Donald Johanson. Here is a small sampling:

In our years of collaboration at the institute he [Johanson] developed a well-deserved, if unfortunate, reputation for unpredictable and high-decibel personal verbal assaults, sometimes accompanied by the tossing around of books or whatever else came conveniently to hand.

So, bearing in mind that there is little you can say about human prehistory that won’t be disputed by someone somewhere, other than that we most certainly had one, what we think we know about who we are and where we come from is roughly this:

For the first 99.99999 percent of our history as organisms, we were in the same ancestral line as chimpanzees. Virtually nothing is known about the prehistory of chimpanzees, but whatever they were, we were. Then about seven million years ago something major happened. A group of new beings emerged from the tropical forests of Africa and began to move about on the open savanna.

These were the australopithecines, and for the next five million years they would be the world’s dominant hominid species. (Austral is from the Latin for “southern” and has no connection in this context to Australia.) Australopithecines came in several varieties, some slender and gracile, like Raymond Dart’s Taung child, others more sturdy and robust, but all were capable of walking upright. Some of these species existed for well over a million years, others for a more modest few hundred thousand, but it is worth bearing in mind that even the least successful had histories many times longer than we have yet achieved.

The most famous hominid remains in the world are those of a 3.18-million-year-old australopithecine found at Hadar in Ethiopia in 1974 by a team led by Donald Johanson. Formally known as A.L. (for “Afar Locality”) 288-1, the skeleton became more familiarly known as Lucy, after the Beatles song “Lucy in the Sky with Diamonds.” Johanson has never doubted her importance. “She is our earliest ancestor, the missing link between ape and human,” he has said.

Lucy was tiny-just three and a half feet tall. She could walk, though how well is a matter of some dispute. She was evidently a good climber, too. Much else is unknown. Her skull was almost entirely missing, so little could be said with confidence about her brain size, though skull fragments suggested it was small. Most books describe Lucy’s skeleton as being 40 percent complete, though some put it closer to half, and one produced by the American Museum of Natural History describes Lucy as two-thirds complete. The BBC television series Ape Man actually called it “a complete skeleton,” even while showing that it was anything but.

A human body has 206 bones, but many of these are repeated. If you have the left femur from a specimen, you don’t need the right to know its dimensions. Strip out all the redundant bones, and the total you are left with is 120-what is called a half skeleton. Even by this fairly accommodating standard, and even counting the slightest fragment as a full bone, Lucy constituted only 28 percent of a half skeleton (and only about 20 percent of a full one).

In The Wisdom of the Bones, Alan Walker recounts how he once asked Johanson how he had come up with a figure of 40 percent. Johanson breezily replied that he had discounted the 106 bones of the hands and feet-more than half the body’s total, and a fairly important half, too, one would have thought, since Lucy’s principal defining attribute was the use of those hands and feet to deal with a changing world. At all events, rather less is known about Lucy than is generally supposed. It isn’t even actually known that she was a female. Her sex is merely presumed from her diminutive size.

Two years after Lucy’s discovery, at Laetoli in Tanzania Mary Leakey found footprints left by two individuals from-it is thought-the same family of hominids. The prints had been made when two australopithecines had walked through muddy ash following a volcanic eruption. The ash had later hardened, preserving the impressions of their feet for a distance of over twenty-three meters.

The American Museum of Natural History in New York has an absorbing diorama that records the moment of their passing. It depicts life-sized re-creations of a male and a female walking side by side across the ancient African plain. They are hairy and chimplike in dimensions, but have a bearing and gait that suggest humanness. The most striking feature of the display is that the male holds his left arm protectively around the female’s shoulder. It is a tender and affecting gesture, suggestive of close bonding.

The tableau is done with such conviction that it is easy to overlook the consideration that virtually everything above the footprints is imaginary. Almost every external aspect of the two figures-degree of hairiness, facial appendages (whether they had human noses or chimp noses), expressions, skin color, size and shape of the female’s breasts-is necessarily suppositional. We can’t even say that they were a couple. The female figure may in fact have been a child. Nor can we be certain that they were australopithecines. They are assumed to be australopithecines because there are no other known candidates.

I had been told that they were posed like that because during the building of the diorama the female figure kept toppling over, but Ian Tattersall insists with a laugh that the story is untrue. “Obviously we don’t know whether the male had his arm around the female or not, but we do know from the stride measurements that they were walking side by side and close together-close enough to be touching. It was quite an exposed area, so they were probably feeling vulnerable. That’s why we tried to give them slightly worried expressions.”

I asked him if he was troubled about the amount of license that was taken in reconstructing the figures. “It’s always a problem in making re-creations,” he agreed readily enough. “You wouldn’t believe how much discussion can go into deciding details like whether Neandertals had eyebrows or not. It was just the same for the Laetoli figures. We simply can’t know the details of what they looked like, but we can convey their size and posture and make some reasonable assumptions about their probable appearance. If I had it to do again, I think I might have made them just slightly more apelike and less human. These creatures weren’t humans. They were bipedal apes.”

Until very recently it was assumed that we were descended from Lucy and the Laetoli creatures, but now many authorities aren’t so sure. Although certain physical features (the teeth, for instance) suggest a possible link between us, other parts of the australopithecine anatomy are more troubling. In their book Extinct Humans, Tattersall and Schwartz point out that the upper portion of the human femur is very like that of the apes but not of the australopithecines; so if Lucy is in a direct line between apes and modern humans, it means we must have adopted an australopithecine femur for a million years or so, then gone back to an ape femur when we moved on to the next phase of our development. They believe, in fact, that not only was Lucy not our ancestor, she wasn’t even much of a walker.

“Lucy and her kind did not locomote in anything like the modern human fashion,” insists Tattersall. “Only when these hominids had to travel between arboreal habitats would they find themselves walking bipedally, ‘forced’ to do so by their own anatomies.” Johanson doesn’t accept this. “Lucy’s hips and the muscular arrangement of her pelvis,” he has written, “would have made it as hard for her to climb trees as it is for modern humans.”

Matters grew murkier still in 2001 and 2002 when four exceptional new specimens were found. One, discovered by Meave Leakey of the famous fossil-hunting family at Lake Turkana in Kenya and called Kenyanthropus platyops (“Kenyan flat-face”), is from about the same time as Lucy and raises the possibility that it was our ancestor and Lucy was an unsuccessful side branch. Also found in 2001 were Ardipithecus ramidus kadabba, dated at between 5.2 million and 5.8 million years old, and Orrorin tugenensis, thought to be 6 million years old, making it the oldest hominid yet found-but only for a brief while. In the summer of 2002 a French team working in the Djurab Desert of Chad (an area that had never before yielded ancient bones) found a hominid almost 7 million years old, which they labeled Sahelanthropus tchadensis. (Some critics believe that it was not human, but an early ape and therefore should be called Sahelpithecus.) All these were early creatures and quite primitive but they walked upright, and they were doing so far earlier than previously thought.

Bipedalism is a demanding and risky strategy. It means refashioning the pelvis into a full load-bearing instrument. To preserve the required strength, the birth canal must be comparatively narrow. This has two very significant immediate consequences and one longer-term one. First, it means a lot of pain for any birthing mother and a greatly increased danger of fatality to mother and baby both. Moreover to get the baby’s head through such a tight space it must be born while its brain is still small-and while the baby, therefore, is still helpless. This means long-term infant care, which in turn implies solid male-female bonding.

All this is problematic enough when you are the intellectual master of the planet, but when you are a small, vulnerable australopithecine, with a brain about the size of an orange,[48] the risk must have been enormous.

So why did Lucy and her kind come down from the trees and out of the forests? Probably they had no choice. The slow rise of the Isthmus of Panama had cut the flow of waters from the Pacific into the Atlantic, diverting warming currents away from the Arctic and leading to the onset of an exceedingly sharp ice age in northern latitudes. In Africa, this would have produced seasonal drying and cooling, gradually turning jungle into savanna. “It was not so much that Lucy and her like left the forests,” John Gribbin has written, “but that the forests left them.”

But stepping out onto the open savanna also clearly left the early hominids much more exposed. An upright hominid could see better, but could also be seen better. Even now as a species, we are almost preposterously vulnerable in the wild. Nearly every large animal you can care to name is stronger, faster, and toothier than us. Faced with attack, modern humans have only two advantages. We have a good brain, with which we can devise strategies, and we have hands with which we can fling or brandish hurtful objects. We are the only creature that can harm at a distance. We can thus afford to be physically vulnerable.

All the elements would appear to have been in place for the rapid evolution of a potent brain, and yet that seems not to have happened. For over three million years, Lucy and her fellow australopithecines scarcely changed at all. Their brain didn’t grow and there is no sign that they used even the simplest tools. What is stranger still is that we now know that for about a million years they lived alongside other early hominids who did use tools, yet the australopithecines never took advantage of this useful technology that was all around them.

At one point between three and two million years ago, it appears there may have been as many as six hominid types coexisting in Africa. Only one, however, was fated to last: Homo, which emerged from the mists beginning about two million years ago. No one knows quite what the relationship was between australopithecines and Homo, but what is known is that they coexisted for something over a million years before all the australopithecines, robust and gracile alike, vanished mysteriously, and possibly abruptly, over a million years ago. No one knows why they disappeared. “Perhaps,” suggests Matt Ridley, “we ate them.”

Conventionally, the Homo line begins with Homo habilis, a creature about whom we know almost nothing, and concludes with us, Homo sapiens (literally “man the thinker”). In between, and depending on which opinions you value, there have been half a dozen other Homo species: Homo ergaster, Homo neanderthalensis, Homo rudolfensis, Homo heidelbergensis, Homo erectus, and Homo antecessor.

Homo habilis (“handy man”) was named by Louis Leakey and colleagues in 1964 and was so called because it was the first hominid to use tools, albeit very simple ones. It was a fairly primitive creature, much more chimpanzee than human, but its brain was about 50 percent larger than that of Lucy in gross terms and not much less large proportionally, so it was the Einstein of its day. No persuasive reason has ever been adduced for why hominid brains suddenly began to grow two million years ago. For a long time it was assumed that big brains and upright walking were directly related-that the movement out of the forests necessitated cunning new strategies that fed off of or promoted braininess-so it was something of a surprise, after the repeated discoveries of so many bipedal dullards, to realize that there was no apparent connection between them at all.

“There is simply no compelling reason we know of to explain why human brains got large,” says Tattersall. Huge brains are demanding organs: they make up only 2 percent of the body’s mass, but devour 20 percent of its energy. They are also comparatively picky in what they use as fuel. If you never ate another morsel of fat, your brain would not complain because it won’t touch the stuff. It wants glucose instead, and lots of it, even if it means short-changing other organs. As Guy Brown notes: “The body is in constant danger of being depleted by a greedy brain, but cannot afford to let the brain go hungry as that would rapidly lead to death.” A big brain needs more food and more food means increased risk.

Tattersall thinks the rise of a big brain may simply have been an evolutionary accident. He believes with Stephen Jay Gould that if you replayed the tape of life-even if you ran it back only a relatively short way to the dawn of hominids-the chances are “quite unlikely” that modern humans or anything like them would be here now.

“One of the hardest ideas for humans to accept,” he says, “is that we are not the culmination of anything. There is nothing inevitable about our being here. It is part of our vanity as humans that we tend to think of evolution as a process that, in effect, was programmed to produce us. Even anthropologists tended to think this way right up until the 1970s.” Indeed, as recently as 1991, in the popular textbook The Stages of Evolution, C. Loring Brace stuck doggedly to the linear concept, acknowledging just one evolutionary dead end, the robust australopithecines. Everything else represented a straightforward progression-each species of hominid carrying the baton of development so far, then handing it on to a younger, fresher runner. Now, however, it seems certain that many of these early forms followed side trails that didn’t come to anything.

Luckily for us, one did-a group of tool users, which seemed to arise from out of nowhere and overlapped with the shadowy and much disputed Homo habilis. This is Homo erectus, the species discovered by Eugène Dubois in Java in 1891. Depending on which sources you consult, it existed from about 1.8 million years ago to possibly as recently as twenty thousand or so years ago.

According to the Java Man authors, Homo erectus is the dividing line: everything that came before him was apelike in character; everything that came after was humanlike. Homo erectus was the first to hunt, the first to use fire, the first to fashion complex tools, the first to leave evidence of campsites, the first to look after the weak and frail. Compared with all that had gone before, Homo erectus was extremely human in form as well as behavior, its members long-limbed and lean, very strong (much stronger than modern humans), and with the drive and intelligence to spread successfully over huge areas. To other hominids, Homo erectus must have seemed terrifyingly powerful, fleet, and gifted.

Erectus was “the velociraptor of its day,” according to Alan Walker of Penn State University and one of the world’s leading authorities. If you were to look one in the eyes, it might appear superficially to be human, but “you wouldn’t connect. You’d be prey.” According to Walker, it had the body of an adult human but the brain of a baby.

Although erectus had been known about for almost a century it was known only from scattered fragments-not enough to come even close to making one full skeleton. So it wasn’t until an extraordinary discovery in Africa in the 1980s that its importance-or, at the very least, possible importance-as a precursor species for modern humans was fully appreciated. The remote valley of Lake Turkana (formerly Lake Rudolf) in Kenya is now one of the world’s most productive sites for early human remains, but for a very long time no one had thought to look there. It was only because Richard Leakey was on a flight that was diverted over the valley that he realized it might be more promising than had been thought. A team was dispatched to investigate, but at first found nothing. Then late one afternoon Kamoya Kimeu, Leakey’s most renowned fossil hunter, found a small piece of hominid brow on a hill well away from the lake. Such a site was unlikely to yield much, but they dug anyway out of respect for Kimeu’s instincts and to their astonishment found a nearly complete Homo erectus skeleton. It was from a boy aged between about nine and twelve who had died 1.54 million years ago. The skeleton had “an entirely modern body structure,” says Tattersall, in a way that was without precedent. The Turkana boy was “very emphatically one of us.”

Also found at Lake Turkana by Kimeu was KNM-ER 1808, a female 1.7 million years old, which gave scientists their first clue that Homo erectus was more interesting and complex than previously thought. The woman’s bones were deformed and covered in coarse growths, the result of an agonizing condition called hypervitaminosis A, which can come only from eating the liver of a carnivore. This told us first of all that Homo erectus was eating meat. Even more surprising was that the amount of growth showed that she had lived weeks or even months with the disease. Someone had looked after her. It was the first sign of tenderness in hominid evolution.

It was also discovered that Homo erectus skulls contained (or, in the view of some, possibly contained) a Broca’s area, a region of the frontal lobe of the brain associated with speech. Chimps don’t have such a feature. Alan Walker thinks the spinal canal didn’t have the size and complexity to enable speech, that they probably would have communicated about as well as modern chimps. Others, notably Richard Leakey, are convinced they could speak.

For a time, it appears, Homo erectus was the only hominid species on Earth. It was hugely adventurous and spread across the globe with what seems to have been breathtaking rapidity. The fossil evidence, if taken literally, suggests that some members of the species reached Java at about the same time as, or even slightly before, they left Africa. This has led some hopeful scientists to suggest that perhaps modern people arose not in Africa at all, but in Asia-which would be remarkable, not to say miraculous, as no possible precursor species have ever been found anywhere outside Africa. The Asian hominids would have had to appear, as it were, spontaneously. And anyway an Asian beginning would merely reverse the problem of their spread; you would still have to explain how the Java people then got to Africa so quickly.

There are several more plausible alternative explanations for how Homo erectus managed to turn up in Asia so soon after its first appearance in Africa. First, a lot of plus-or-minusing goes into the dating of early human remains. If the actual age of the African bones is at the higher end of the range of estimates or the Javan ones at the lower end, or both, then there is plenty of time for African erects to find their way to Asia. It is also entirely possible that older erectus bones await discovery in Africa. In addition, the Javan dates could be wrong altogether.

Now for the doubts. Some authorities don’t believe that the Turkana finds are Homo erectus at all. The snag, ironically, was that although the Turkana skeletons were admirably extensive, all other erectus fossils are inconclusively fragmentary. As Tattersall and Jeffrey Schwartz note in Extinct Humans, most of the Turkana skeleton “couldn’t be compared with anything else closely related to it because the comparable parts weren’t known!” The Turkana skeletons, they say, look nothing like any Asian Homo erectus and would never have been considered the same species except that they were contemporaries. Some authorities insist on calling the Turkana specimens (and any others from the same period) Homo ergaster. Tattersall and Schwartz don’t believe that goes nearly far enough. They believe it was ergaster “or a reasonably close relative” that spread to Asia from Africa, evolved into Homo erectus, and then died out.

What is certain is that sometime well over a million years ago, some new, comparatively modern, upright beings left Africa and boldly spread out across much of the globe. They possibly did so quite rapidly, increasing their range by as much as twenty-five miles a year on average, all while dealing with mountain ranges, rivers, deserts, and other impediments and adapting to differences in climate and food sources. A particular mystery is how they passed along the west side of the Red Sea, an area of famously punishing aridity now, but even drier in the past. It is a curious irony that the conditions that prompted them to leave Africa would have made it much more difficult to do so. Yet somehow they managed to find their way around every barrier and to thrive in the lands beyond.

And that, I’m afraid, is where all agreement ends. What happened next in the history of human development is a matter of long and rancorous debate, as we shall see in the next chapter.

But it is worth remembering, before we move on, that all of these evolutionary jostlings over five million years, from distant, puzzled australopithecine to fully modern human, produced a creature that is still 98.4 percent genetically indistinguishable from the modern chimpanzee. There is more difference between a zebra and a horse, or between a dolphin and a porpoise, than there is between you and the furry creatures your distant ancestors left behind when they set out to take over the world.

29 THE RESTLESS APE

SOMETIME ABOUT A million and a half years ago, some forgotten genius of the hominid world did an unexpected thing. He (or very possibly she) took one stone and carefully used it to shape another. The result was a simple teardrop-shaped hand axe, but it was the world’s first piece of advanced technology.

It was so superior to existing tools that soon others were following the inventor’s lead and making hand axes of their own. Eventually whole societies existed that seemed to do little else. “They made them in the thousands,” says Ian Tattersall. “There are some places in Africa where you literally can’t move without stepping on them. It’s strange because they are quite intensive objects to make. It was as if they made them for the sheer pleasure of it.”

From a shelf in his sunny workroom Tattersall took down an enormous cast, perhaps a foot and a half long and eight inches wide at its widest point, and handed it to me. It was shaped like a spearhead, but one the size of a stepping-stone. As a fiberglass cast it weighed only a few ounces, but the original, which was found in Tanzania, weighed twenty-five pounds. “It was completely useless as a tool,” Tattersall said. “It would have taken two people to lift it adequately, and even then it would have been exhausting to try to pound anything with it.”

“What was it used for then?”

Tattersall gave a genial shrug, pleased at the mystery of it. “No idea. It must have had some symbolic importance, but we can only guess what.”

The axes became known as Acheulean tools, after St. Acheul, a suburb of Amiens in northern France, where the first examples were found in the nineteenth century, and contrast with the older, simpler tools known as Oldowan, originally found at Olduvai Gorge in Tanzania. In older textbooks, Oldowan tools are usually shown as blunt, rounded, hand-sized stones. In fact, paleoanthropologists now tend to believe that the tool part of Oldowan rocks were the pieces flaked off these larger stones, which could then be used for cutting.

Now here’s the mystery. When early modern humans-the ones who would eventually become us-started to move out of Africa something over a hundred thousand years ago, Acheulean tools were the technology of choice. These early Homo sapiens loved their Acheulean tools, too. They carried them vast distances. Sometimes they even took unshaped rocks with them to make into tools later on. They were, in a word, devoted to the technology. But although Acheulean tools have been found throughout Africa, Europe, and western and central Asia, they have almost never been found in the Far East. This is deeply puzzling.

In the 1940s a Harvard paleontologist named Hallum Movius drew something called the Movius line, dividing the side with Acheulean tools from the one without. The line runs in a southeasterly direction across Europe and the Middle East to the vicinity of modern-day Calcutta and Bangladesh. Beyond the Movius line, across the whole of southeast Asia and into China, only the older, simpler Oldowan tools have been found. We know that Homo sapiens went far beyond this point, so why would they carry an advanced and treasured stone technology to the edge of the Far East and then just abandon it?

“That troubled me for a long time,” recalls Alan Thorne of the Australian National University in Canberra. “The whole of modern anthropology was built round the idea that humans came out of Africa in two waves-a first wave of Homo erectus, which became Java Man and Peking Man and the like, and a later, more advanced wave of Homo sapiens, which displaced the first lot. Yet to accept that you must believe that Homo sapiens got so far with their more modern technology and then, for whatever reason, gave it up. It was all very puzzling, to say the least.”

As it turned out, there would be a great deal else to be puzzled about, and one of the most puzzling findings of all would come from Thorne’s own part of the world, in the outback of Australia. In 1968, a geologist named Jim Bowler was poking around on a long-dried lakebed called Mungo in a parched and lonely corner of western New South Wales when something very unexpected caught his eye. Sticking out of a crescent-shaped sand ridge of a type known as a lunette were some human bones. At the time, it was believed that humans had been in Australia for no more than 8,000 years, but Mungo had been dry for 12,000 years. So what was anyone doing in such an inhospitable place?

The answer, provided by carbon dating, was that the bones’ owner had lived there when Lake Mungo was a much more agreeable habitat, a dozen miles long, full of water and fish, fringed by pleasant groves of casuarina trees. To everyone’s astonishment, the bones turned out to be 23,000 years old. Other bones found nearby were dated to as much as 60,000 years. This was unexpected to the point of seeming practically impossible. At no time since hominids first arose on Earth has Australia not been an island. Any human beings who arrived there must have come by sea, in large enough numbers to start a breeding population, after crossing sixty miles or more of open water without having any way of knowing that a convenient landfall awaited them. Having landed, the Mungo people had then found their way more than two thousand miles inland from Australia’s north coast-the presumed point of entry-which suggests, according to a report in the Proceedings of the National Academy of Sciences, “that people may have first arrived substantially earlier than 60,000 years ago.”

How they got there and why they came are questions that can’t be answered. According to most anthropology texts, there’s no evidence that people could even speak 60,000 years ago, much less engage in the sorts of cooperative efforts necessary to build ocean-worthy craft and colonize island continents.

“There’s just a whole lot we don’t know about the movements of people before recorded history,” Alan Thorne told me when I met him in Canberra. “Do you know that when nineteenth-century anthropologists first got to Papua New Guinea, they found people in the highlands of the interior, in some of the most inaccessible terrain on earth, growing sweet potatoes. Sweet potatoes are native to South America. So how did they get to Papua New Guinea? We don’t know. Don’t have the faintest idea. But what is certain is that people have been moving around with considerable assuredness for longer than traditionally thought, and almost certainly sharing genes as well as information.”

The problem, as ever, is the fossil record. “Very few parts of the world are even vaguely amenable to the long-term preservation of human remains,” says Thorne, a sharp-eyed man with a white goatee and an intent but friendly manner. “If it weren’t for a few productive areas like Hadar and Olduvai in east Africa we’d know frighteningly little. And when you look elsewhere, often we do know frighteningly little. The whole of India has yielded just one ancient human fossil, from about 300,000 years ago. Between Iraq and Vietnam-that’s a distance of some 5,000 kilometers-there have been just two: the one in India and a Neandertal in Uzbekistan.” He grinned. “That’s not a whole hell of a lot to work with. You’re left with the position that you’ve got a few productive areas for human fossils, like the Great Rift Valley in Africa and Mungo here in Australia, and very little in between. It’s not surprising that paleontologists have trouble connecting the dots.”

The traditional theory to explain human movements-and the one still accepted by the majority of people in the field-is that humans dispersed across Eurasia in two waves. The first wave consisted of Homo erectus, who left Africa remarkably quickly-almost as soon as they emerged as a species-beginning nearly two million years ago. Over time, as they settled in different regions, these early erects further evolved into distinctive types-into Java Man and Peking Man in Asia, and Homo heidelbergensis and finally Homo neanderthalensis in Europe.

Then, something over a hundred thousand years ago, a smarter, lither species of creature-the ancestors of every one of us alive today-arose on the African plains and began radiating outward in a second wave. Wherever they went, according to this theory, these new Homo sapiens displaced their duller, less adept predecessors. Quite how they did this has always been a matter of disputation. No signs of slaughter have ever been found, so most authorities believe the newer hominids simply outcompeted the older ones, though other factors may also have contributed. “Perhaps we gave them smallpox,” suggests Tattersall. “There’s no real way of telling. The one certainty is that we are here now and they aren’t.”

These first modern humans are surprisingly shadowy. We know less about ourselves, curiously enough, than about almost any other line of hominids. It is odd indeed, as Tattersall notes, “that the most recent major event in human evolution-the emergence of our own species-is perhaps the most obscure of all.” Nobody can even quite agree where truly modern humans first appear in the fossil record. Many books place their debut at about 120,000 years ago in the form of remains found at the Klasies River Mouth in South Africa, but not everyone accepts that these were fully modern people. Tattersall and Schwartz maintain that “whether any or all of them actually represent our species still awaits definitive clarification.”

The first undisputed appearance of Homo sapiens is in the eastern Mediterranean, around modern-day Israel, where they begin to show up about 100,000 years ago-but even there they are described (by Trinkaus and Shipman) as “odd, difficult-to-classify and poorly known.” Neandertals were already well established in the region and had a type of tool kit known as Mousterian, which the modern humans evidently found worthy enough to borrow. No Neandertal remains have ever been found in north Africa, but their tool kits turn up all over the place. Somebody must have taken them there: modern humans are the only candidate. It is also known that Neandertals and modern humans coexisted in some fashion for tens of thousands of years in the Middle East. “We don’t know if they time-shared the same space or actually lived side by side,” Tattersall says, but the moderns continued happily to use Neandertal tools-hardly convincing evidence of overwhelming superiority. No less curiously, Acheulean tools are found in the Middle East well over a million years ago, but scarcely exist in Europe until just 300,000 years ago. Again, why people who had the technology didn’t take the tools with them is a mystery.

For a long time, it was believed that the Cro-Magnons, as modern humans in Europe became known, drove the Neandertals before them as they advanced across the continent, eventually forcing them to its western margins, where essentially they had no choice but to fall in the sea or go extinct. In fact, it is now known that Cro-Magnons were in the far west of Europe at about the same time they were also coming in from the east. “Europe was a pretty empty place in those days,” Tattersall says. “They may not have encountered each other all that often, even with all their comings and goings.” One curiosity of the Cro-Magnons’ arrival is that it came at a time known to paleoclimatology as the Boutellier interval, when Europe was plunging from a period of relative mildness into yet another long spell of punishing cold. Whatever it was that drew them to Europe, it wasn’t the glorious weather.

In any case, the idea that Neandertals crumpled in the face of competition from newly arrived Cro-Magnons strains against the evidence at least a little. Neandertals were nothing if not tough. For tens of thousands of years they lived through conditions that no modern human outside a few polar scientists and explorers has experienced. During the worst of the ice ages, blizzards with hurricane-force winds were common. Temperatures routinely fell to 50 degrees below zero Fahrenheit. Polar bears padded across the snowy vales of southern England. Neandertals naturally retreated from the worst of it, but even so they will have experienced weather that was at least as bad as a modern Siberian winter. They suffered, to be sure-a Neandertal who lived much past thirty was lucky indeed-but as a species they were magnificently resilient and practically indestructible. They survived for at least a hundred thousand years, and perhaps twice that, over an area stretching from Gibraltar to Uzbekistan, which is a pretty successful run for any species of being.

Quite who they were and what they were like remain matters of disagreement and uncertainty. Right up until the middle of the twentieth century the accepted anthropological view of the Neandertal was that he was dim, stooped, shuffling, and simian-the quintessential caveman. It was only a painful accident that prodded scientists to reconsider this view. In 1947, while doing fieldwork in the Sahara, a Franco-Algerian paleontologist named Camille Arambourg took refuge from the midday sun under the wing of his light airplane. As he sat there, a tire burst from the heat, and the plane tipped suddenly, striking him a painful blow on the upper body. Later in Paris he went for an X-ray of his neck, and noticed that his own vertebrae were aligned exactly like those of the stooped and hulking Neandertal. Either he was physiologically primitive or Neandertal’s posture had been misdescribed. In fact, it was the latter. Neandertal vertebrae were not simian at all. It changed utterly how we viewed Neandertals-but only some of the time, it appears.

It is still commonly held that Neandertals lacked the intelligence or fiber to compete on equal terms with the continent’s slender and more cerebrally nimble newcomers, Homo sapiens. Here is a typical comment from a recent book: “Modern humans neutralized this advantage [the Neandertal’s considerably heartier physique] with better clothing, better fires and better shelter; meanwhile the Neandertals were stuck with an oversize body that required more food to sustain.” In other words, the very factors that had allowed them to survive successfully for a hundred thousand years suddenly became an insuperable handicap.

Above all the issue that is almost never addressed is that Neandertals had brains that were significantly larger than those of modern people-1.8 liters for Neandertals versus 1.4 for modern people, according to one calculation. This is more than the difference between modern Homo sapiens and late Homo erectus, a species we are happy to regard as barely human. The argument put forward is that although our brains were smaller, they were somehow more efficient. I believe I speak the truth when I observe that nowhere else in human evolution is such an argument made.

So why then, you may well ask, if the Neandertals were so stout and adaptable and cerebrally well endowed, are they no longer with us? One possible (but much disputed) answer is that perhaps they are. Alan Thorne is one of the leading proponents of an alternative theory, known as the multiregional hypothesis, which holds that human evolution has been continuous-that just as australopithecines evolved into Homo habilis and Homo heidelbergensis became over time Homo neanderthalensis, so modern Homo sapiens simply emerged from more ancient Homo forms. Homo erectus is, on this view, not a separate species but just a transitional phase. Thus modern Chinese are descended from ancient Homo erectus forebears in China, modern Europeans from ancient European Homo erectus, and so on. “Except that for me there are no Homo erectus,” says Thorne. “I think it’s a term which has outlived its usefulness. For me, Homo erectus is simply an earlier part of us. I believe only one species of humans has ever left Africa, and that species is Homo sapiens.”

Opponents of the multiregional theory reject it, in the first instance, on the grounds that it requires an improbable amount of parallel evolution by hominids throughout the Old World-in Africa, China, Europe, the most distant islands of Indonesia, wherever they appeared. Some also believe that multiregionalism encourages a racist view that anthropology took a very long time to rid itself of. In the early 1960s, a famous anthropologist named Carleton Coon of the University of Pennsylvania suggested that some modern races have different sources of origin, implying that some of us come from more superior stock than others. This hearkened back uncomfortably to earlier beliefs that some modern races such as the African “Bushmen” (properly the Kalahari San) and Australian Aborigines were more primitive than others.

Whatever Coon may personally have felt, the implication for many people was that some races are inherently more advanced, and that some humans could essentially constitute different species. The view, so instinctively offensive now, was widely popularized in many respectable places until fairly recent times. I have before me a popular book published by Time-Life Publications in 1961 called The Epic of Man based on a series of articles in Life magazine. In it you can find such comments as “Rhodesian man . . . lived as recently as 25,000 years ago and may have been an ancestor of the African Negroes. His brain size was close to that of Homo sapiens.” In other words black Africans were recently descended from creatures that were only “close” to Homo sapiens.

Thorne emphatically (and I believe sincerely) dismisses the idea that his theory is in any measure racist and accounts for the uniformity of human evolution by suggesting that there was a lot of movement back and forth between cultures and regions. “There’s no reason to suppose that people only went in one direction,” he says. “People were moving all over the place, and where they met they almost certainly shared genetic material through interbreeding. New arrivals didn’t replace the indigenous populations, they joined them. They became them.” He likens the situation to when explorers like Cook or Magellan encountered remote peoples for the first time. “They weren’t meetings of different species, but of the same species with some physical differences.”

What you actually see in the fossil record, Thorne insists, is a smooth, continuous transition. “There’s a famous skull from Petralona in Greece, dating from about 300,000 years ago, that has been a matter of contention among traditionalists because it seems in some ways Homo erectus but in other ways Homo sapiens. Well, what we say is that this is just what you would expect to find in species that were evolving rather than being displaced.”

One thing that would help to resolve matters would be evidence of interbreeding, but that is not at all easy to prove, or disprove, from fossils. In 1999, archeologists in Portugal found the skeleton of a child about four years old that died 24,500 years ago. The skeleton was modern overall, but with certain archaic, possibly Neandertal, characteristics: unusually sturdy leg bones, teeth bearing a distinctive “shoveling” pattern, and (though not everyone agrees on it) an indentation at the back of the skull called a suprainiac fossa, a feature exclusive to Neandertals. Erik Trinkaus of Washington University in St. Louis, the leading authority on Neandertals, announced the child to be a hybrid: proof that modern humans and Neandertals interbred. Others, however, were troubled that the Neandertal and modern features weren’t more blended. As one critic put it: “If you look at a mule, you don’t have the front end looking like a donkey and the back end looking like a horse.”

Ian Tattersall declared it to be nothing more than “a chunky modern child.” He accepts that there may well have been some “hanky-panky” between Neandertals and moderns, but doesn’t believe it could have resulted in reproductively successful offspring.[49] “I don’t know of any two organisms from any realm of biology that are that different and still in the same species,” he says.

With the fossil record so unhelpful, scientists have turned increasingly to genetic studies, in particular the part known as mitochondrial DNA. Mitochondrial DNA was only discovered in 1964, but by the 1980s some ingenious souls at the University of California at Berkeley had realized that it has two features that lend it a particular convenience as a kind of molecular clock: it is passed on only through the female line, so it doesn’t become scrambled with paternal DNA with each new generation, and it mutates about twenty times faster than normal nuclear DNA, making it easier to detect and follow genetic patterns over time. By tracking the rates of mutation they could work out the genetic history and relationships of whole groups of people.

In 1987, the Berkeley team, led by the late Allan Wilson, did an analysis of mitochondrial DNA from 147 individuals and declared that the rise of anatomically modern humans occurred in Africa within the last 140,000 years and that “all present-day humans are descended from that population.” It was a serious blow to the multiregionalists. But then people began to look a little more closely at the data. One of the most extraordinary points-almost too extraordinary to credit really-was that the “Africans” used in the study were actually African-Americans, whose genes had obviously been subjected to considerable mediation in the past few hundred years. Doubts also soon emerged about the assumed rates of mutations.

By 1992, the study was largely discredited. But the techniques of genetic analysis continued to be refined, and in 1997 scientists from the University of Munich managed to extract and analyze some DNA from the arm bone of the original Neandertal man, and this time the evidence stood up. The Munich study found that the Neandertal DNA was unlike any DNA found on Earth now, strongly indicating that there was no genetic connection between Neandertals and modern humans. Now this really was a blow to multiregionalism.

Then in late 2000 Nature and other publications reported on a Swedish study of the mitochondrial DNA of fifty-three people, which suggested that all modern humans emerged from Africa within the past 100,000 years and came from a breeding stock of no more than 10,000 individuals. Soon afterward, Eric Lander, director of the Whitehead Institute/Massachusetts Institute of Technology Center for Genome Research, announced that modern Europeans, and perhaps people farther afield, are descended from “no more than a few hundred Africans who left their homeland as recently as 25,000 years ago.”

As we have noted elsewhere in the book, modern human beings show remarkably little genetic variability-“there’s more diversity in one social group of fifty-five chimps than in the entire human population,” as one authority has put it-and this would explain why. Because we are recently descended from a small founding population, there hasn’t been time enough or people enough to provide a source of great variability. It seemed a pretty severe blow to multiregionalism. “After this,” a Penn State academic told the Washington Post, “people won’t be too concerned about the multiregional theory, which has very little evidence.”

But all of this overlooked the more or less infinite capacity for surprise offered by the ancient Mungo people of western New South Wales. In early 2001, Thorne and his colleagues at the Australian National University reported that they had recovered DNA from the oldest of the Mungo specimens-now dated at 62,000 years-and that this DNA proved to be “genetically distinct.”

The Mungo Man, according to these findings, was anatomically modern-just like you and me-but carried an extinct genetic lineage. His mitochondrial DNA is no longer found in living humans, as it should be if, like all other modern people, he was descended from people who left Africa in the recent past.

“It turned everything upside down again,” says Thorne with undisguised delight.

Then other even more curious anomalies began to turn up. Rosalind Harding, a population geneticist at the Institute of Biological Anthropology in Oxford, while studying betaglobin genes in modern people, found two variants that are common among Asians and the indigenous people of Australia, but hardly exist in Africa. The variant genes, she is certain, arose more than 200,000 years ago not in Africa, but in east Asia-long before modern Homo sapiens reached the region. The only way to account for them is to say that ancestors of people now living in Asia included archaic hominids-Java Man and the like. Interestingly, this same variant gene-the Java Man gene, so to speak-turns up in modern populations in Oxfordshire.

Confused, I went to see Harding at the institute, which inhabits an old brick villa on Banbury Road in Oxford, in more or less the neighborhood where Bill Clinton spent his student days. Harding is a small and chirpy Australian, from Brisbane originally, with the rare knack for being amused and earnest at the same time.

“Don’t know,” she said at once, grinning, when I asked her how people in Oxfordshire harbored sequences of betaglobin that shouldn’t be there. “On the whole,” she went on more somberly, “the genetic record supports the out-of-Africa hypothesis. But then you find these anomalous clusters, which most geneticists prefer not to talk about. There’s huge amounts of information that would be available to us if only we could understand it, but we don’t yet. We’ve barely begun.” She refused to be drawn out on what the existence of Asian-origin genes in Oxfordshire tells us other than that the situation is clearly complicated. “All we can say at this stage is that it is very untidy and we don’t really know why.”

At the time of our meeting, in early 2002, another Oxford scientist named Bryan Sykes had just produced a popular book called The Seven Daughters of Eve in which, using studies of mitochondrial DNA, he had claimed to be able to trace nearly all living Europeans back to a founding population of just seven women-the daughters of Eve of the title-who lived between 10,000 and 45,000 years ago in the time known to science as the Paleolithic. To each of these women Sykes had given a name-Ursula, Xenia, Jasmine, and so on-and even a detailed personal history. (“Ursula was her mother’s second child. The first had been taken by a leopard when he was only two. . . .”)

When I asked Harding about the book, she smiled broadly but carefully, as if not quite certain where to go with her answer. “Well, I suppose you must give him some credit for helping to popularize a difficult subject,” she said and paused thoughtfully. “And there remains the remote possibility that he’s right.” She laughed, then went on more intently: “Data from any single gene cannot really tell you anything so definitive. If you follow the mitochondrial DNA backwards, it will take you to a certain place-to an Ursula or Tara or whatever. But if you take any other bit of DNA, any gene at all, and trace it back, it will take you someplace else altogether.”

It was a little, I gathered, like following a road randomly out of London and finding that eventually it ends at John O’Groats, and concluding from this that anyone in London must therefore have come from the north of Scotland. They might have come from there, of course, but equally they could have arrived from any of hundreds of other places. In this sense, according to Harding, every gene is a different highway, and we have only barely begun to map the routes. “No single gene is ever going to tell you the whole story,” she said.

So genetic studies aren’t to be trusted?

“Oh you can trust the studies well enough, generally speaking. What you can’t trust are the sweeping conclusions that people often attach to them.”

She thinks out-of-Africa is “probably 95 percent correct,” but adds: “I think both sides have done a bit of a disservice to science by insisting that it must be one thing or the other. Things are likely to turn out to be not so straightforward as either camp would have you believe. The evidence is clearly starting to suggest that there were multiple migrations and dispersals in different parts of the world going in all kinds of directions and generally mixing up the gene pool. That’s never going to be easy to sort out.”

Just at this time, there were also a number of reports questioning the reliability of claims concerning the recovery of very ancient DNA. An academic writing in Nature had noted how a paleontologist, asked by a colleague whether he thought an old skull was varnished or not, had licked its top and announced that it was. “In the process,” noted the Nature article, “large amounts of modern human DNA would have been transferred to the skull,” rendering it useless for future study. I asked Harding about this. “Oh, it would almost certainly have been contaminated already,” she said. “Just handling a bone will contaminate it. Breathing on it will contaminate it. Most of the water in our labs will contaminate it. We are all swimming in foreign DNA. In order to get a reliably clean specimen you have to excavate it in sterile conditions and do the tests on it at the site. It is the trickiest thing in the world not to contaminate a specimen.”

So should such claims be treated dubiously? I asked.

Harding nodded solemnly. “Very,” she said.

If you wish to understand at once why we know as little as we do about human origins, I have the place for you. It is to be found a little beyond the edge of the blue Ngong Hills in Kenya, to the south and west of Nairobi. Drive out of the city on the main highway to Uganda, and there comes a moment of startling glory when the ground falls away and you are presented with a hang glider’s view of boundless, pale green African plain.

This is the Great Rift Valley, which arcs across three thousand miles of east Africa, marking the tectonic rupture that is setting Africa adrift from Asia. Here, perhaps forty miles out of Nairobi, along the baking valley floor, is an ancient site called Olorgesailie, which once stood beside a large and pleasant lake. In 1919, long after the lake had vanished, a geologist named J. W. Gregory was scouting the area for mineral prospects when he came across a stretch of open ground littered with anomalous dark stones that had clearly been shaped by human hand. He had found one of the great sites of Acheulean tool manufacture that Ian Tattersall had told me about.

Unexpectedly in the autumn of 2002 I found myself a visitor to this extraordinary site. I was in Kenya for another purpose altogether, visiting some projects run by the charity CARE International, but my hosts, knowing of my interest in humans for the present volume, had inserted a visit to Olorgesailie into the schedule.

After its discovery by Gregory, Olorgesailie lay undisturbed for over two decades before the famed husband-and-wife team of Louis and Mary Leakey began an excavation that isn’t completed yet. What the Leakeys found was a site stretching to ten acres or so, where tools were made in incalculable numbers for roughly a million years, from about 1.2 million years ago to 200,000 years ago. Today the tool beds are sheltered from the worst of the elements beneath large tin lean-tos and fenced off with chicken wire to discourage opportunistic scavenging by visitors, but otherwise the tools are left just where their creators dropped them and where the Leakeys found them.

Jillani Ngalli, a keen young man from the Kenyan National Museum who had been dispatched to act as guide, told me that the quartz and obsidian rocks from which the axes were made were never found on the valley floor. “They had to carry the stones from there,” he said, nodding at a pair of mountains in the hazy middle distance, in opposite directions from the site: Olorgesailie and Ol Esakut. Each was about ten kilometers, or six miles, away-a long way to carry an armload of stone.

Why the early Olorgesailie people went to such trouble we can only guess, of course. Not only did they lug hefty stones considerable distances to the lakeside, but, perhaps even more remarkably, they then organized the site. The Leakeys’ excavations revealed that there were areas where axes were fashioned and others where blunt axes were brought to be resharpened. Olorgesailie was, in short, a kind of factory; one that stayed in business for a million years.

Various replications have shown that the axes were tricky and labor-intensive objects to make-even with practice, an axe would take hours to fashion-and yet, curiously, they were not particularly good for cutting or chopping or scraping or any of the other tasks to which they were presumably put. So we are left with the position that for a million years-far, far longer than our own species has even been in existence, much less engaged in continuous cooperative efforts-early people came in considerable numbers to this particular site to make extravagantly large numbers of tools that appear to have been rather curiously pointless.

And who were these people? We have no idea actually. We assume they were Homo erectus because there are no other known candidates, which means that at their peak-their peak-the Olorgesailie workers would have had the brains of a modern infant. But there is no physical evidence on which to base a conclusion. Despite over sixty years of searching, no human bone has ever been found in or around the vicinity of Olorgesailie. However much time they spent there shaping rocks, it appears they went elsewhere to die.

“It’s all a mystery,” Jillani Ngalli told me, beaming happily.

The Olorgesailie people disappeared from the scene about 200,000 years ago when the lake dried up and the Rift Valley started to become the hot and challenging place it is today. But by this time their days as a species were already numbered. The world was about to get its first real master race, Homo sapiens. Things would never be the same again.

30 GOOD-BYE

IN THE EARLY 1680s, at just about the time that Edmond Halley and his friends Christopher Wren and Robert Hooke were settling down in a London coffeehouse and embarking on the casual wager that would result eventually in Isaac Newton’s Principia, Henry Cavendish’s weighing of the Earth, and many of the other inspired and commendable undertakings that have occupied us for much of the past four hundred pages, a rather less desirable milestone was being passed on the island of Mauritius, far out in the Indian Ocean some eight hundred miles off the east coast of Madagascar.

There, some forgotten sailor or sailor’s pet was harrying to death the last of the dodos, the famously flightless bird whose dim but trusting nature and lack of leggy zip made it a rather irresistible target for bored young tars on shore leave. Millions of years of peaceful isolation had not prepared it for the erratic and deeply unnerving behavior of human beings.

We don’t know precisely the circumstances, or even year, attending the last moments of the last dodo, so we don’t know which arrived first, a world that contained a Principia or one that had no dodos, but we do know that they happened at more or less the same time. You would be hard pressed, I would submit, to find a better pairing of occurrences to illustrate the divine and felonious nature of the human being-a species of organism that is capable of unpicking the deepest secrets of the heavens while at the same time pounding into extinction, for no purpose at all, a creature that never did us any harm and wasn’t even remotely capable of understanding what we were doing to it as we did it. Indeed, dodos were so spectacularly short on insight, it is reported, that if you wished to find all the dodos in a vicinity you had only to catch one and set it to squawking, and all the others would waddle along to see what was up.

The indignities to the poor dodo didn’t end quite there. In 1755, some seventy years after the last dodo’s death, the director of the Ashmolean Museum in Oxford decided that the institution’s stuffed dodo was becoming unpleasantly musty and ordered it tossed on a bonfire. This was a surprising decision as it was by this time the only dodo in existence, stuffed or otherwise. A passing employee, aghast, tried to rescue the bird but could save only its head and part of one limb.

As a result of this and other departures from common sense, we are not now entirely sure what a living dodo was like. We possess much less information than most people suppose-a handful of crude descriptions by “unscientific voyagers, three or four oil paintings, and a few scattered osseous fragments,” in the somewhat aggrieved words of the nineteenth-century naturalist H. E. Strickland. As Strickland wistfully observed, we have more physical evidence of some ancient sea monsters and lumbering saurapods than we do of a bird that lived into modern times and required nothing of us to survive except our absence.

So what is known of the dodo is this: it lived on Mauritius, was plump but not tasty, and was the biggest-ever member of the pigeon family, though by quite what margin is unknown as its weight was never accurately recorded. Extrapolations from Strickland’s “osseous fragments” and the Ashmolean’s modest remains show that it was a little over two and a half feet tall and about the same distance from beak tip to backside. Being flightless, it nested on the ground, leaving its eggs and chicks tragically easy prey for pigs, dogs, and monkeys brought to the island by outsiders. It was probably extinct by 1683 and was most certainly gone by 1693. Beyond that we know almost nothing except of course that we will not see its like again. We know nothing of its reproductive habits and diet, where it ranged, what sounds it made in tranquility or alarm. We don’t possess a single dodo egg.

From beginning to end our acquaintance with animate dodos lasted just seventy years. That is a breathtakingly scanty period-though it must be said that by this point in our history we did have thousands of years of practice behind us in the matter of irreversible eliminations. Nobody knows quite how destructive human beings are, but it is a fact that over the last fifty thousand years or so wherever we have gone animals have tended to vanish, in often astonishingly large numbers.

In America, thirty genera of large animals-some very large indeed-disappeared practically at a stroke after the arrival of modern humans on the continent between ten and twenty thousand years ago. Altogether North and South America between them lost about three quarters of their big animals once man the hunter arrived with his flint-headed spears and keen organizational capabilities. Europe and Asia, where the animals had had longer to evolve a useful wariness of humans, lost between a third and a half of their big creatures. Australia, for exactly the opposite reasons, lost no less than 95 percent.

Because the early hunter populations were comparatively small and the animal populations truly monumental-as many as ten million mammoth carcasses are thought to lie frozen in the tundra of northern Siberia alone-some authorities think there must be other explanations, possibly involving climate change or some kind of pandemic. As Ross MacPhee of the American Museum of Natural History put it: “There’s no material benefit to hunting dangerous animals more often than you need to-there are only so many mammoth steaks you can eat.” Others believe it may have been almost criminally easy to catch and clobber prey. “In Australia and the Americas,” says Tim Flannery, “the animals probably didn’t know enough to run away.”

Some of the creatures that were lost were singularly spectacular and would take a little managing if they were still around. Imagine ground sloths that could look into an upstairs window, tortoises nearly the size of a small Fiat, monitor lizards twenty feet long basking beside desert highways in Western Australia. Alas, they are gone and we live on a much diminished planet. Today, across the whole world, only four types of really hefty (a metric ton or more) land animals survive: elephants, rhinos, hippos, and giraffes. Not for tens of millions of years has life on Earth been so diminutive and tame.

The question that arises is whether the disappearances of the Stone Age and disappearances of more recent times are in effect part of a single extinction event-whether, in short, humans are inherently bad news for other living things. The sad likelihood is that we may well be. According to the University of Chicago paleontologist David Raup, the background rate of extinction on Earth throughout biological history has been one species lost every four years on average. According to one recent calculation, human-caused extinction now may be running as much as 120,000 times that level.

In the mid-1990s, the Australian naturalist Tim Flannery, now head of the South Australian Museum in Adelaide, became struck by how little we seemed to know about many extinctions, including relatively recent ones. “Wherever you looked, there seemed to be gaps in the records-pieces missing, as with the dodo, or not recorded at all,” he told me when I met him in Melbourne a year or so ago.

Flannery recruited his friend Peter Schouten, an artist and fellow Australian, and together they embarked on a slightly obsessive quest to scour the world’s major collections to find out what was lost, what was left, and what had never been known at all. They spent four years picking through old skins, musty specimens, old drawings, and written descriptions-whatever was available. Schouten made life-sized paintings of every animal they could reasonably re-create, and Flannery wrote the words. The result was an extraordinary book called A Gap in Nature, constituting the most complete-and, it must be said, moving-catalog of animal extinctions from the last three hundred years.

For some animals, records were good, but nobody had done anything much with them, sometimes for years, sometimes forever. Steller’s sea cow, a walrus-like creature related to the dugong, was one of the last really big animals to go extinct. It was truly enormous-an adult could reach lengths of nearly thirty feet and weigh ten tons-but we are acquainted with it only because in 1741 a Russian expedition happened to be shipwrecked on the only place where the creatures still survived in any numbers, the remote and foggy Commander Islands in the Bering Sea.

Happily, the expedition had a naturalist, Georg Steller, who was fascinated by the animal. “He took the most copious notes,” says Flannery. “He even measured the diameter of its whiskers. The only thing he wouldn’t describe was the male genitals-though, for some reason, he was happy enough to do the female’s. He even saved a piece of skin, so we had a good idea of its texture. We weren’t always so lucky.”

The one thing Steller couldn’t do was save the sea cow itself. Already hunted to the brink of extinction, it would be gone altogether within twenty-seven years of Steller’s discovery of it. Many other animals, however, couldn’t be included because too little is known about them. The Darling Downs hopping mouse, Chatham Islands swan, Ascension Island flightless crake, at least five types of large turtle, and many others are forever lost to us except as names.

A great deal of extinction, Flannery and Schouten discovered, hasn’t been cruel or wanton, but just kind of majestically foolish. In 1894, when a lighthouse was built on a lonely rock called Stephens Island, in the tempestuous strait between the North and South Islands of New Zealand, the lighthouse keeper’s cat kept bringing him strange little birds that it had caught. The keeper dutifully sent some specimens to the museum in Wellington. There a curator grew very excited because the bird was a relic species of flightless wrens-the only example of a flightless perching bird ever found anywhere. He set off at once for the island, but by the time he got there the cat had killed them all. Twelve stuffed museum species of the Stephens Island flightless wren are all that now exist.

At least we have those. All too often, it turns out, we are not much better at looking after species after they have gone than we were before they went. Take the case of the lovely Carolina parakeet. Emerald green, with a golden head, it was arguably the most striking and beautiful bird ever to live in North America-parrots don’t usually venture so far north, as you may have noticed-and at its peak it existed in vast numbers, exceeded only by the passenger pigeon. But the Carolina parakeet was also considered a pest by farmers and easily hunted because it flocked tightly and had a peculiar habit of flying up at the sound of gunfire (as you would expect), but then returning almost at once to check on fallen comrades.

In his classic American Omithology, written in the early nineteenth century, Charles Willson Peale describes an occasion in which he repeatedly empties a shotgun into a tree in which they roost:

At each successive discharge, though showers of them fell, yet the affection of the survivors seemed rather to increase; for, after a few circuits around the place, they again alighted near me, looking down on their slaughtered companions with such manifest symptoms of sympathy and concern, as entirely disarmed me.

By the second decade of the twentieth century, the birds had been so relentlessly hunted that only a few remained alive in captivity. The last one, named Inca, died in the Cincinnati Zoo in 1918 (not quite four years after the last passenger pigeon died in the same zoo) and was reverently stuffed. And where would you go to see poor Inca now? Nobody knows. The zoo lost it.

What is both most intriguing and puzzling about the story above is that Peale was a lover of birds, and yet did not hesitate to kill them in large numbers for no better reason than that it interested him to do so. It is a truly astounding fact that for the longest time the people who were most intensely interested in the world’s living things were the ones most likely to extinguish them.

No one represented this position on a larger scale (in every sense) than Lionel Walter Rothschild, the second Baron Rothschild. Scion of the great banking family, Rothschild was a strange and reclusive fellow. He lived his entire life in the nursery wing of his home at Tring, in Buckinghamshire, using the furniture of his childhood-even sleeping in his childhood bed, though eventually he weighed three hundred pounds.

His passion was natural history and he became a devoted accumulator of objects. He sent hordes of trained men-as many as four hundred at a time-to every quarter of the globe to clamber over mountains and hack their way through jungles in the pursuit of new specimens-particularly things that flew. These were crated or boxed up and sent back to Rothschild’s estate at Tring, where he and a battalion of assistants exhaustively logged and analyzed everything that came before them, producing a constant stream of books, papers, and monographs-some twelve hundred in all. Altogether, Rothschild’s natural history factory processed well over two million specimens and added five thousand species of creature to the scientific archive.

Remarkably, Rothschild’s collecting efforts were neither the most extensive nor the most generously funded of the nineteenth century. That title almost certainly belongs to a slightly earlier but also very wealthy British collector named Hugh Cuming, who became so preoccupied with accumulating objects that he built a large oceangoing ship and employed a crew to sail the world full-time, picking up whatever they could find-birds, plants, animals of all types, and especially shells. It was his unrivaled collection of barnacles that passed to Darwin and served as the basis for his seminal study.

However, Rothschild was easily the most scientific collector of his age, though also the most regrettably lethal, for in the 1890s he became interested in Hawaii, perhaps the most temptingly vulnerable environment Earth has yet produced. Millions of years of isolation had allowed Hawaii to evolve 8,800 unique species of animals and plants. Of particular interest to Rothschild were the islands’ colorful and distinctive birds, often consisting of very small populations inhabiting extremely specific ranges.

The tragedy for many Hawaiian birds was that they were not only distinctive, desirable, and rare-a dangerous combination in the best of circumstances-but also often heartbreakingly easy to take. The greater koa finch, an innocuous member of the honeycreeper family, lurked shyly in the canopies of koa trees, but if someone imitated its song it would abandon its cover at once and fly down in a show of welcome. The last of the species vanished in 1896, killed by Rothschild’s ace collector Harry Palmer, five years after the disappearance of its cousin the lesser koa finch, a bird so sublimely rare that only one has ever been seen: the one shot for Rothschild’s collection. Altogether during the decade or so of Rothschild’s most intensive collecting, at least nine species of Hawaiian birds vanished, but it may have been more.

Rothschild was by no means alone in his zeal to capture birds at more or less any cost. Others in fact were more ruthless. In 1907 when a well-known collector named Alanson Bryan realized that he had shot the last three specimens of black mamos, a species of forest bird that had only been discovered the previous decade, he noted that the news filled him with “joy.”

It was, in short, a difficult age to fathom-a time when almost any animal was persecuted if it was deemed the least bit intrusive. In 1890, New York State paid out over one hundred bounties for eastern mountain lions even though it was clear that the much-harassed creatures were on the brink of extinction. Right up until the 1940s many states continued to pay bounties for almost any kind of predatory creature. West Virginia gave out an annual college scholarship to whoever brought in the most dead pests-and “pests” was liberally interpreted to mean almost anything that wasn’t grown on farms or kept as pets.

Perhaps nothing speaks more vividly for the strangeness of the times than the fate of the lovely little Bachman’s warbler. A native of the southern United States, the warbler was famous for its unusually thrilling song, but its population numbers, never robust, gradually dwindled until by the 1930s the warbler vanished altogether and went unseen for many years. Then in 1939, by happy coincidence two separate birding enthusiasts, in widely separated locations, came across lone survivors just two days apart. They both shot the birds, and that was the last that was ever seen of Bachman’s warblers.

The impulse to exterminate was by no means exclusively American. In Australia, bounties were paid on the Tasmanian tiger (properly the thylacine), a doglike creature with distinctive “tiger” stripes across its back, until shortly before the last one died, forlorn and nameless, in a private Hobart zoo in 1936. Go to the Tasmanian Museum today and ask to see the last of this species-the only large carnivorous marsupial to live into modern times-and all they can show you are photographs. The last surviving thylacine was thrown out with the weekly trash.

I mention all this to make the point that if you were designing an organism to look after life in our lonely cosmos, to monitor where it is going and keep a record of where it has been, you wouldn’t choose human beings for the job.

But here’s an extremely salient point: we have been chosen, by fate or Providence or whatever you wish to call it. As far as we can tell, we are the best there is. We may be all there is. It’s an unnerving thought that we may be the living universe’s supreme achievement and its worst nightmare simultaneously.

Because we are so remarkably careless about looking after things, both when alive and when not, we have no idea-really none at all-about how many things have died off permanently, or may soon, or may never, and what role we have played in any part of the process. In 1979, in the book The Sinking Ark, the author Norman Myers suggested that human activities were causing about two extinctions a week on the planet. By the early 1990s he had raised the figure to some six hundred per week. (That’s extinctions of all types-plants, insects, and so on as well as animals.) Others have put the figure even higher-to well over a thousand a week. A United Nations report of 1995, on the other hand, put the total number of known extinctions in the last four hundred years at slightly under 500 for animals and slightly over 650 for plants-while allowing that this was “almost certainly an underestimate,” particularly with regard to tropical species. A few interpreters think most extinction figures are grossly inflated.

The fact is, we don’t know. Don’t have any idea. We don’t know when we started doing many of the things we’ve done. We don’t know what we are doing right now or how our present actions will affect the future. What we do know is that there is only one planet to do it on, and only one species of being capable of making a considered difference. Edward O. Wilson expressed it with unimprovable brevity in The Diversity of Life: “One planet, one experiment.”

If this book has a lesson, it is that we are awfully lucky to be here-and by “we” I mean every living thing. To attain any kind of life in this universe of ours appears to be quite an achievement. As humans we are doubly lucky, of course: We enjoy not only the privilege of existence but also the singular ability to appreciate it and even, in a multitude of ways, to make it better. It is a talent we have only barely begun to grasp.

We have arrived at this position of eminence in a stunningly short time. Behaviorally modern human beings-that is, people who can speak and make art and organize complex activities-have existed for only about 0.0001 percent of Earth’s history. But surviving for even that little while has required a nearly endless string of good fortune.

We really are at the beginning of it all. The trick, of course, is to make sure we never find the end. And that, almost certainly, will require a good deal more than lucky breaks.