The rise and fall of the third chimpanzee

Our species is now at the pinnacle of its numbers, its geographic extent, its power, and the fraction of the Earth's productivity that it commands. That is the good news. The bad news is that we are also in the process of reversing all that progress much more rapidly than we created it. Our power threatens our own existence. We do not know whether we shall suddenly blow ourselves up before we would otherwise expire in a slow stew caused by global warming, pollution, habitat destruction, more mouths to feed, less food to feed those mouths, and extermination of other species that form our resource base. Are these dangers really new ones that arose since the Industrial Revolution, as widely assumed? It is a common belief that species in a state of nature live in balance with each other and with their environment. Predators do not exterminate their prey, nor do herbivores overgraze their plants. According to this view, humans are the unique misfit. If this were true, Nature would hold no lessons for us.

There is something to this view, insofar as species do not go extinct under natural conditions as rapidly as we are exterminating them now, except under rare circumstances. Such a rare event was the mass fatality sixty-five million years ago, possibly due to an asteroidal impact, that finished the dinosaurs. Since evolutionary multiplications of species are very slow, natural extinctions obviously must also be slow, otherwise we would have been left with no species long ago. Expressed alternatively, the vulnerable species get eliminated quickly, and what we see persisting in Nature are the robust combinations of species.

That broad conclusion still leaves us with many instructive examples of species exterminating other species. Almost all known cases prove to combine two elements. Firstly, the cases involve species reaching environments where they did not occur before, and where they encounter prey populations that are naive about the threat of those invading predators. By the time that the ecological dust settles and a new equilibrium is reached, some of the new-found prey may have been exterminated. Secondly, the perpetrators of such exterminations prove to be so-called switching predators, which are not specialized to eat only a single prey species but can feed on many different ones. Although the predator exterminates some prey, it survives by switching to others.

Such exterminations often occur when humans intentionally or accidentally transfer a species from one part of the globe to another. Rats, cats, goats, pigs, ants, and even snakes are among these transferred killers. For instance, during the Second World War a tree snake native to the Solomon Islands was accidentally transported on ships or planes to the previously snake-free Pacific island of Guam. This predator has already exterminated or brought to the brink of extinction most of Guam's native forest bird species, which had had no opportunity to evolve behavioural defences against snakes. Yet the snake is in no danger itself despite having virtually eliminated its bird prey, because it can switch to bats, rats, lizards, and other victims. As another example, cats and foxes introduced into Australia by humans have been eating their way through Australia's small native marsupials and rats without endangering themselves, because there remain abundant rabbits and other prey species on which to feed.

We humans furnish the prime example of a switching predator. We eat everything from snails and seaweed to whales, mushrooms, and strawberries. We can overharvest some species to the point of extinction, and then just switch to other food. A wave of extinctions has ensued every time that humans have reached a previously unoccupied part of the globe. The dodo, whose name has become synonymous with extinction, formerly lived on the island of Mauritius, half of whose land and freshwater bird species became extinct following the island's discovery in 1507. Dodos in particular were big, edible, flightless, and easily caught by hungry sailors. Hawaiian bird species similarly died out en masse following Hawaii's discovery by Polynesians 1,500 years ago, as did America's large mammal species after ancestral Indians arrived 11,000 years ago. Extinction waves have also accompanied major improvements of hunting technology in lands long occupied by humans. For example, wild populations of the Arabian oryx, a beautiful antelope of the Near East, survived one million years of human hunting, only to succumb to high-powered rifles in 1972.

Thus, there are numerous animal precedents for our propensity to exterminate individual prey species but to sustain ourselves by switching to others. Is there any precedent for an animal population destroying its entire resource base and eating its way into extinction? This outcome is uncommon, because animal numbers are regulated by many factors that tend automatically to lower birth rates or increase death rates when the animal is numerous, and vice versa when it is rare. For example, mortality due to external factors like predators, diseases, parasites, and starvation tends to increase at high population densities. Responses of the animal itself to high densities also contribute, such as infanticide, postponed breeding, and increased aggression. These responses and external factors generally reduce the animal's population and relieve its pressure on its resources before they can be exhausted.

Nevertheless, some animal populations actually have eaten themselves into extinction. One example involves the progeny of twenty-nine reindeer that were introduced in 1944 to St Matthew Island in the Bering Sea. By 1957 they had multiplied nearly fifty-fold to 1,350, by the year 1963 another four-fold to 6,000. But reindeer depend for food on slow-growing lichens, which on St Matthew had no chance to recover from reindeer grazing, since the animals had nowhere to migrate. When a harsh winter struck in 1963-64, all the animals except forty-one females and one sterile male starved to death, leaving a doomed population on an island littered with thousands of skeletons. A similar example was the introduction of rabbits to Lisianski Island west of Hawaii in the first decade of this century. Within a decade the rabbits had eaten themselves into oblivion by consuming every plant on the island except two morning glories and a tobacco patch. These and other similar examples of ecological suicide all involve populations that suddenly became free of the usual factors regulating their numbers. Rabbits and reindeer are normally subject to predators, and reindeer on continents use migration as a safety valve to leave an area and allow its vegetation to recover. But Lisianski and St Matthew Islands lacked predators, and emigration was impossible, so that the animals bred and ate unchecked. On reflection, it is clear that the entire human species has been equally successful in recently escaping from the former controls on our numbers. We eliminated predation on ourselves long ago; twentieth-century medicine has greatly reduced our mortality from infectious disease; and some of our leading behavioural techniques of population control, such as infanticide, chronic war, and sexual abstinence, have become socially unacceptable. Our population is now doubling about every thirty-five years. Granted, that is not as fast as the St Matthew reindeer, and Island Earth is bigger than St Matthew Island, and some of our resources are more elastic than lichens (though other resources, like oil, are less elastic). Yet the qualitative conclusion remains the same: no population can grow indefinitely.

Thus, our present ecological predicament has familiar animal precursors. Like many switching predators, we exterminate some prey species when we colonize a new environment or acquire new destructive power. Like some animal populations that suddenly escaped their former limits on growth, we risk destroying ourselves by destroying our resource base. What about the view that we were in a state of relative ecological equilibrium until the Industrial Revolution, and that only since then have we begun seriously to exterminate species and overexploit our environment? That Rousseau-esque fantasy will be taken up in the three chapters of Part Five. Chapter Seventeen examines the widespread belief in a former Golden Age, when we supposedly lived as noble savages practising a conservation ethic and in harmony with Nature. In reality, mass extinctions have coincided with each major extension of human lebensraum during the last ten thousand years and possibly much longer. Our direct responsibility for the extinctions is clearest in the case of the most recent expansions, where the evidence is still fresh—Europeans' expansion over the globe since 1492, and the earlier colonization of oceanic islands by Polynesians and the Malagasy. Older expansions such as the first human occupation of the Americas and Australia were also accompanied by mass extinctions, though the trail of evidence has had much more time to fade and so conclusions about cause and effect are necessarily weaker. It is not just the case that the Golden Age was blackened by mass extinctions. While no large human population has eaten itself out of existence, some populations on small islands have done so, and many large populations have damaged their resources to the point of economic collapse. The clearest examples come from isolated cultures, such as the collapses of Easter Island and Anasazi civilizations. But environmental factors also drove the major shifts in Western civilization, including the successive collapses of the Middle Eastern, then Greek, then Roman hegemonies. Self-destructive abuse of our environment, far from being a modern invention, has long been a prime mover of human history. Chapter Eighteen looks more closely at the biggest, most dramatic, and most controversial of these

'Golden Age mass extinctions'. Around 11,000 years ago most of the large mammals of two entire continents, North America and South America, became extinct. Around the same time appears the first unequivocal evidence for human occupation of the Americas, by the ancestors of American Indians. It was the biggest expansion of human territory since Homo erectus spread out of Africa to colonize Europe and Asia a million years ago. The temporal coincidence between the first Americans and the last big American mammals, the lack of mass extinctions elsewhere in the world at that same time, and proofs that some of the now-extinct beasts were hunted have suggested what is termed the New World blitzkrieg hypothesis. According to this interpretation, as the first wave of human hunters multiplied and spread from Canada to Patagonia, they encountered big animals that had never seen humans before, and they exterminated as they marched. However this theory's critics are at least as numerous as its backers; Chapter Eighteen 'will try to make sense of the debate.

The concluding chapter will seek to put approximate numbers on the count of species that we have already driven into extinction. We shall start with the firmest numbers: the species whose extinctions occurred in modern times and were well documented, and for which the search for survivors has been so thorough as to leave no doubt that there are no survivors. Next come estimates of three less certain numbers: the modern species that have not been seen alive for some time and that became extinct before anyone was aware of it; the modern species that have not even been 'discovered' and received a name; and the species that humans exterminated before the rise of modern science. That background will let us appraise the main mechanisms by which we exterminate, and the number of species that we are likely to exterminate within my sons' lifetime—if we proceed at our current rate.

SEVENTEEN THE GOLDEN AGE THAT NEVER WAS

We cling to belief in a Rousseau-esque fantasy that the past was a Golden Age of environmentalism, when people lived in harmony with Nature. In reality, human societies, including those of stone-age farmers and possibly of hunter-gatherers as well, have been undermining their own subsistence by exterminating species and damaging environments for thousands of years. We differ from our supposedly conservationist forebears only in our greater numbers, more potent technology for inflicting damage, and access to written histories from which we refuse to learn.

Every part of the earth is sacred to my people. Every shining pine needle, every sandy shore, every mist in the dark woods, every clearing and humming insect is holy in the memory and experience of my people. . The white man… is a stranger who conies in the night and takes from the land whatever he needs. The earth is not his brother but his enemy. . Continue to contaminate your bed, and you will one night suffocate in your own waste. [From a letter written in 1855 to President Franklin Pierce, by Chief Seattle of the Duwanish tribe of American Indians.]

Environmentalists sickened by the damage that industrial societies are wreaking on the world often look to the past as a Golden Age. When Europeans began to settle America, the air and rivers were pure, the landscape green, the Great Plains teeming with bison. Today we breathe smog, worry about toxic chemicals in our drinking water, pave over the landscape, and rarely see any large wild animal. Worse is surely to come. By the time that my young sons reach retirement age, half of the world's species will be extinct, the air radioactive, and the seas polluted with oil. Undoubtedly, two simple reasons go a long way towards explaining our worsening mess: modern technology has far more power to cause havoc than did the stone axes of the past, and far more people are alive now than ever before. But a third factor may also have contributed, a change in attitudes. Unlike modern city-dwellers, at least some pre-industrial peoples—like the Duwanish, whose chief I quoted—depend on and revere their local environment. Stories abound of how such peoples are in effect practising conservationists. As a New Guinea tribesman once explained to me, 'It's our custom that if a hunter one day kills a pigeon in one direction from the village, he waits a week before hunting for pigeons again, and then goes in the opposite direction. We are only beginning to realize how sophisticated are the conservationist policies of so-called 'primitive' peoples. For instance, well-intentioned foreign experts have made deserts out of large areas of Africa. In those same areas, local herders had thrived for uncounted millennia, by making annual nomadic migrations which ensured that land never became overgrazed.

The nostalgic outlook shared until recently by most of my environmentalist colleagues and myself is part of a human tendency to view the past as a Golden Age in many other respects. A famous exponent of this outlook was the eighteenth-century French philosopher Jean-Jacques Rousseau, whose Discourse on the Origin of Inequality traced our degeneration from the Golden Age to the human misery that Rousseau saw around him. When eighteenth-century European explorers encountered pre-industrial peoples like Polynesians and American Indians, those peoples became idealized in European salons as 'noble savages' living in a continued Golden Age, untouched by-such curses of civilization as religious intolerance, political tyranny, and social inequality. Even now, the days of classical Greece and Rome are widely considered to be the Golden Age of western civilization. Ironically, the Greeks and Romans also saw themselves as degenerates from a past Golden Age. I can still recite half-consciously those lines of the Roman poet Ovid that I memorized in tenth-grade Latin, 'Aurea prima sata est aetas, quae viydice nullo. . ('First came the Golden Age, when men were honest and righteous of their own free will. . ) Ovid went on to contrast those virtues with the rampant treachery and warfare of his own times. I have no doubt that any humans still alive in the radioactive soup of the Twenty-second Century will write equally nostalgically about our own era, which will then seem untroubled by comparison. Given this widespread belief in a Golden Age, some recent discoveries by archaeologists and paleontologists have come as a shock. It is now clear that pre-industrial societies have been exterminating species, destroying habitats, and undermining their own existence for thousands of years. Some of the best documented examples involve Polynesians and American Indians, the very peoples most often cited as exemplars of environmentalism. Needless to say, this revisionist view is hotly contested, not only in the halls of academia but also among lay people in Hawaii, New Zealand, and other areas with large Polynesian or Indian minorities. Are the new 'discoveries' just one more piece of racist pseudo-science by which white settlers seek to justify dispossessing indigenous peoples? How could the discoveries be reconciled with all the evidence for conservationist practices by modern pre-industrial peoples? If the discoveries were true, could we use them as case histories to help us predict the fate that our own environmental policies may bring upon us? Could the recent findings explain some otherwise mysterious collapses of ancient civilizations, like those of Easter Island or the Maya Indians?

Before we can answer these controversial questions, we need to understand the new evidence belying the assumed past Golden Age of environmentalism. Let's first consider evidence for past waves of exterminations, then evidence for past destruction of habitats. When British colonists began to settle New Zealand in the 1800s, they found no native land mammals except bats. That was not surprising, for New Zealand is a remote island lying much too far from the continents for flightless mammals to reach. However, the colonists' ploughs uncovered instead the bones and eggshells of large birds that were then already extinct but that the Maori (the earlier Polynesian settlers of New Zealand) remembered by the name moa. From complete skeletons, some of them evidently recent and still retaining skin and feathers, we have a good idea how moas must have looked alive: they were ostrich-like birds comprising a dozen species, and ranging from little ones 'only' 3 feet high and forty pounds in weight up to giants of 500 pounds and 10 feet tall. Their food habits can be inferred from preserved gizzards containing twigs and leaves of dozens of plant species, showing them to have been herbivores. They thus used to be New Zealand's equivalents of big mammalian herbivores like deer and antelope. While the moas are New Zealand's most famous extinct birds, many others have been described from fossil bones, totalling at least twenty-eight species that disappeared before Europeans arrived. Quite a few besides the moas were big and flightless, including a big duck, a giant coot, and an enormous goose. These flightless birds were descended from normal birds that had flown to New Zealand and that had then evolved to use their expensive wing muscles in a land free of mammalian predators. Others of the vanished birds, such as a pelican, a swan, a giant raven, and colossal eagle, were perfectly capable of flight.

Weighing up to thirty pounds, the eagle was by far the biggest and most powerful bird of prey in the world when it was alive. It dwarfed even the largest hawk now in existence, tropical America's harpy eagle. The New Zealand eagle would have been the sole predator capable of attacking adult moas. Although some moas were nearly twenty times heavier than the eagle, it still could have killed them by taking advantage of the moas' erect two-legged posture, crippling them with an attack on the long legs, then killing them with an attack on the head and long neck, and finally remaining for many days to consume the carcass, just as lions take their time at consuming a giraffe. The eagle's habits may explain the many headless moa skeletons that have been found. Up to this point I have discussed New Zealand's big extinct animals. But fossil-hunters have also discovered the bones of small scampering animals of the size of mice and rats. Scampering or crawling on the ground were at least three species of flightless or weak-flying songbirds, several frogs, giant snails, many giant cricket-like insects up to double the weight of a mouse, and strange mouse-like bats that rolled up their wings and ran. Some of these little animals were completely extinct by the time that Europeans arrived. Others still survived on small offshore islands near New Zealand, but their fossil bones show that they were formerly abundant on the New Zealand mainland. Collectively, all these now-extinct species that had evolved in isolation on New Zealand would have provided New Zealand with the ecological equivalents of the continents' flightless mammals that had never arrived: moas instead of deer, flightless geese and coot instead of rabbits, big crickets and little songbirds and bats instead of mice, and colossal eagles instead of leopards. Fossils and biochemical evidence indicate that the moas' ancestors had reached New Zealand millions of years ago. When and why, after surviving for so long, did the moas finally become extinct? What disaster could have struck so many species as different as crickets, eagles, ducks, and moas? Specifically, were all these strange creatures still alive when the ancestors of the Maoris arrived around 1000 AD?

At the time that I first visited New Zealand in 1966, the received wisdom was that moas had died out because of a change in climate, and that any moa species surviving to greet the Maoris were on their figurative last legs. New Zealanders took it as dogma that Maoris were conservationists and did not exterminate the moas. There is still no doubt that Maoris, like other Polynesians, used stone tools, lived mainly by farming and fishing, and lacked the destructive power of modern industrial societies. At most, it was assumed, Maoris might have given the coup de grace to populations already on the verge of extinction. However, three sets of discoveries have demolished this conviction. Firstly, much of New Zealand was covered with glaciers or cold tundra during the last Ice Age ending about 10,000 years ago. Since then, the New Zealand climate has become much more favourable, with warmer temperatures and the spread of magnificent forests. The last moas died with their gizzards full of food, and enjoying the best climate that they had seen for tens of thousands of years.

Secondly, radiocarbon-dated bird bones from dated Maori archaeological sites prove that all known moa species were still present in abundance when the first Maoris stepped ashore. So were the extinct goose, duck, swan, eagle, and other birds now known only from fossil bones. Within a few centuries, the moas and most of those other birds were extinct. It would have been an incredible coincidence if every individual of dozens of species that had occupied New Zealand for millions of years chose the precise geological moment of human arrival as the occasion to drop dead in synchrony.

Finally, more than a hundred large archaeological sites are known—some of them covering dozens of acres—where Maoris cut up prodigious numbers of moas, cooked them in earth ovens, and discarded the remains. They ate the meat, used the skins for clothing, fashioned bones into fishhooks and jewellery, and blew out the eggs for use as water containers. During the Nineteenth Century moa bones were carted away from these sites by the wagonload. The number of moa skeletons in known Maori moa-hunter sites is estimated to be between 100,000 and 500,000, about ten times the number of moas likely to have been alive in New Zealand at any instant. Maoris must have been slaughtering moas for many generations.

Hence it is now clear that Maoris exterminated moas, at least partly by killing them, partly by robbing their nests of eggs, and probably partly as well by clearing some of the forests in which moas lived. Anyone who has hiked in New Zealand's rugged mountains will initially be incredulous at this thought. Just picture those travel posters of New Zealand's fiordland, with its steep-walled gorges 10,000 feet deep, its 400 inches of annual rainfall, and its cold winters. Even today, full-time professional hunters armed with telescopic rifles and operating from helicopters cannot control the numbers of deer in those mountains. How could the few thousand Maoris living on New Zealand's South Island and Stewart Island, armed only with stone axes and clubs and operating on foot, have hunted down the last moas?

But there would have been a crucial difference between deer and moas. Ueers have been selected for tens of thousands of generations to flee from human hunters, while moas had never seen humans until Maoris arrived. Like the naive animals of the Galapagos Islands today, moas were probably tame enough for a hunter to walk up to one and club it. Unlike deer, moas may have had such low reproductive rates that a few hunters visiting a valley only once every couple of years could kill moas faster than they could breed. That is precisely what is happening today to New Guinea's largest surviving native mammal, a tree kangaroo in the remote Bewani Mountains. In areas settled by people, tree kangaroos are nocturnal, incredibly shy, live in trees, and are far harder to hunt than moas would have been. Despite all that, and despite the very low human population of the Bewanis, the cumulative effects of occasional hunting parties—literally one visit per valley per several years—have sufficed to bring this kangaroo to the verge of extinction. Having seen it happen to tree kangaroos, I now have no difficulty understanding how it happened to moas.

Not only moas, but also all of New Zealand's other extinct bird species, were still alive when Maoris landed. Most were gone a few centuries later. The larger ones—the swan and pelican, the flightless goose and coot—were surely hunted for food. The giant eagle, however, may have been killed by Maoris in self-defence. What do you think happened when that eagle, specialized at crippling and killing two-legged prey between three and ten feet tall, saw its first six-foot-tall Maoris? Even today, Manchurian eagles trained for hunting occasionally kill their human handlers, but the Manchurian birds were mere dwarfs beside New Zealand's giant, which was pre-adapted to become a man-killer.

Surely, though, neither self-defence nor hunting for food explains the rapid disappearance of New Zealand's peculiar crickets, snails, wrens, and bats. Why were so many of those species exterminated, either throughout their range or else everywhere except on some offshore islands?

Deforestation may be part of the answer, but the major reason was the other hunters that Maoris intentionally or accidentally brought with them—rats! Just as moas that evolved in the absence of humans were defenceless against humans, so, too, small insular animals that evolved in the absence of rats were defenceless against rats. We know that the rat species spread by Europeans played a major role in modern exterminations of many bird species on Hawaii and other previously rat-free oceanic islands. For example, when rats finally reached Big South Cape Island off New Zealand in 1962, they exterminated or decimated the populations of eight bird species and a bat within three years. That is why so many New Zealand species are restricted today to rat-free islands, the sole places where they could survive when the tide of rats accompanying the Maoris swept over the New Zealand mainland.

When the Maoris landed, they found an intact New Zealand biota of creatures so strange that we would dismiss them as science-fiction fantasies if we did not have their fossilized bones to convince us of their former existence. The scene was as close as we will ever get to what we might see if we could reach another fertile planet on which life had evolved. Within a short time, much of that community had collapsed in a biological holocaust, and some of the remaining community collapsed in a second holocaust following the arrival of Europeans. The end result is that New Zealand today has about half of the bird species that greeted the Maoris, and many of the survivors are either now at risk of extinction or else confined to islands with few introduced mammalian pests. A few centuries of hunting had sufficed to end millions of years of moa history. Not only on New Zealand but on all other remote Pacific islands where archaeologists have looked recently in Polynesia, bones of many now-extinct bird species have been found at sites of the first settlers, proving there that the bird extinctions and human colonizations were somehow related. From all the main islands of Hawaii, paleontologists Storrs Olson and Helen James of the Smithsonian Institution have identified fossil bird species which disappeared during the Polynesian settlement that began around 500 AD. The fossils include not only small honey-creepers related to species still present but also bizarre flightless geese and ibises with no living close relatives at all. While Hawaii is notorious for its bird extinctions following European settlement, this earlier extinction wave had been unknown until Olson and James began publishing their discoveries in 1982. The known extinctions of Hawaiian birds before Captain Cook's arrival now total the incredible number of at least fifty species, nearly one-tenth of the number of bird species breeding on mainland North America.

That is not to say that all these Hawaiian birds were hunted out of existence. Although geese probably were indeed exterminated by overhunting, like the moas, small songbirds are more likely to have been eliminated by rats that arrived with the first Hawaiians, or else by destruction of forests that Hawaiians cleared for agriculture. Similar discoveries of extinct birds at archaeological sites of early Polynesians have also been made on Tahiti, Fiji, Tonga, New Caledonia, the Marquesas Islands, Chatham Islands, Cook Islands, Solomon Islands, and Bismarck Archipelago.

An especially intriguing collision of birds and Polynesians took place on Henderson Island, an extremely remote speck of land lying in the tropical Pacific Ocean 125 miles east of Pitcairn Island, which is in turn famous for its own isolation. (Recall that Pitcairn is so remote that the mutineers who wrested the H.M.S. Bounty from Captain Bligh lived undetected on Pitcairn for eighteen years until the island was rediscovered.) Henderson consists of jungle-covered coral riddled with crevices and totally unsuitable for agriculture. Naturally, the island is now uninhabited and has been ever since Europeans first saw it in 1606. Henderson has often been cited as one of the world's most pristine habitats, totally unaffected by humans.

It was therefore a big surprise when Olson and fellow paleontologist David Steadman recently identified bones of two large species of pigeons, one smaller pigeon, and three seabirds that had become extinct on Henderson some time between 500 and 800 years ago. The same six species or close relatives had already been found in archaeological sites on several inhabited Polynesian islands, where it was clear how they could have been exterminated by people. The apparent contradiction of birds also being exterminated by humans on uninhabited, seemingly uninhabitable Henderson was solved by the discovery there of former Polynesian sites with hundreds of cultural artifacts, proving that the island had actually been occupied by Polynesians for several centuries. At those same sites, along with the bones of the six bird species that were exterminated on Henderson, were the bones of other bird species that survived, plus many fish.

Those early Polynesian colonists of Henderson evidently subsisted mainly on pigeons, seabirds, and fish until they had decimated the bird populations, at which point they had destroyed their food supply and either starved or else abandoned the island. The Pacific contains at least eleven other 'mystery' islands, besides Henderson, which were uninhabited on European discovery but showed archaeological evidence of former occupation by Polynesians. Some of these islands had been settled for hundreds of years before their human population finally died out or left. All were small or in other respects marginally suitable for agriculture, leaving human settlers heavily dependent on birds and other animals for food. Given the widespread evidence for over-exploitation of wild animals by early Polynesians, not only Henderson but the other mystery islands as well may represent the graveyards of human populations that ruined their own resource base.

Lest I leave the impression that Polynesians were in any way unique as pre-industrial exterminators, let's now jump nearly halfway around the globe to the world's fourth largest island, Madagascar, lying in the Indian Ocean off the coast of Africa. When Portuguese explorers arrived around 1500 AD, they found Madagascar already occupied by people now called the Malagasy. On geographic grounds, you might have expected their language to be related to African languages spoken a mere 200 miles to the west, on the coast of Mozambique. Astonishingly, though, it actually proved to belong to a group of languages spoken on the Indonesian island of Borneo, on the opposite side of the Indian Ocean thousands of miles to the northeast. Physically, the Malagasy range in appearance from typical Indonesians to typical blacks of East Africa. These paradoxes are due to the Malagasy having arrived between 1,000 and 2,000 years ago, as a result of Indonesian traders voyaging around the Indian Ocean coastline to India and eventually to East Africa. In Madagascar they proceeded to build a society based on herding cattle and goats and pigs, farming, and fishing, and linked to the East African coast by Muslim traders. As interesting as Madagascar's people are the wild animals that it has—and those that it lacks. Living in enormous abundance on the nearby African mainland are many species of large and conspicuous beasts that run on the ground and are active by day—the antelopes, ostriches, zebras, baboons, and lions that draw modern tourists to East Africa. None of these animals, and no animals remotely equivalent to them, have occurred on Madagascar in modern times. They were kept out by the 200 miles of sea separating Madagascar from Africa, just as the sea also kept Australia's marsupials from reaching New Zealand. Instead, Madagascar supports two dozen species of small, monkey-like primates called lemurs, weighing only up to twenty pounds and mostly active at night and living in trees.

Various species of rodents, bats, insectivores, and relatives of mongooses also occur, yet the largest still only weighs about twenty-five pounds.

However, littering Madagascar's beaches are proofs of vanished giant birds, in the form of countless eggshells of the size of a soccer ball. Eventually, bones turned up not only of the birds that laid those eggs, but also of a remarkable suite of vanished large mammals and reptiles. The egg-makers were half-a-dozen species of flightless birds up to 10 feet tall and weighing up to 1,000 pounds, like moas and ostriches but more massively built and hence now termed elephant birds. The reptiles were two species of giant land tortoises with shells about a yard long, and formerly very common, as indicated by the abundance of their bones. More diverse than either of these large birds or reptiles were a dozen species of lemurs up to the size of a gorilla, and all larger than or at least as large as the largest surviving lemur species. To judge from the small size of the eye orbits in their skulls, all or most of the extinct lemurs were probably diurnal rather than nocturnal. Some of them evidently lived on the ground like baboons, while others climbed in trees like orangutans and koala bears.

As if all this were not enough, Madagascar also yielded the bones of an extinct 'pygmy' hippopotamus ('only' the size of a cow), an aardvark, and a big mongoose-related carnivore built like a short-legged puma. Taken together, these extinct large animals formerly gave Madagascar the functional equivalents of the surviving large beasts for which tourists still flock to African game parks—just as did New Zealand's moas and other strange birds. The tortoises, elephant birds, and pygmy hippo would have been the herbivores replacing antelope and zebras; the lemurs would have replaced the baboons and great apes; and the mongoose-related carnivore made do for a leopard or scaled-down lion. What happened to all these big extinct mammals, reptiles, and birds? We can be confident that at least some of them were alive to delight the eyes of the first arriving Malagasy, who used elephant bird eggshells as water containers and discarded butchered bones of the pygmy hippo and some of the other species in their rubbish heaps. In addition, the bones of all the other extinct species are known from fossil sites only a few thousand years old. Since they must have evolved and survived for millions of years until then, it is unlikely that all those animals had the foresight to give up the ghost just in those last few moments before hungry humans showed up. In fact, a few may still have been holding out in remote parts of Madagascar when Europeans arrived, since the seventeenth-century French governor Flacourt was given descriptions of an animal suggestive of the gorilla-sized lemur. The elephant birds may have survived long enough to have become known to Arab traders in the Indian Ocean, and to have given rise to the account of the rok (a giant bird) in the tale of Sinbad the Sailor.

Certainly some and probably all of Madagascar's vanished giants were somehow exterminated by the activities of the early Malagasy. It is not hard to understand why the elephant birds became extinct, since their eggshells made such convenient two-gallon jerrycans. While the Malagasy were herders and fisherman rather than big-game hunters, the other big animals would have been easy prey, since they had never seen humans before. Probably, like New Zealand's moas, they were as tame as Antarctic penguins and other creatures that evolved in the absence of humans. A hungry Malagasy could have walked up to one of those tame beasts, clubbed it, and enjoyed a quick barbecue. That is presumably why the easy-to-see, easy-to-catch lemurs big enough to be worth the effort of butchering them—the large, diurnal, terrestrial species—all became extinct, while the small, nocturnal, tree-living ones all survived.

However, unintended by-products of Malagasy activities probably killed more big animals than did hunting. Fires lit to clear forest for pasture and to stimulate growth of new grass each year would have destroyed habitats on which the beasts depended. Grazing cattle and goats also transformed habitats, as well as competing directly with grazing tortoises and elephant birds for food. Introduced dogs and pigs have preyed on ground-dwelling animals, their young, and their _eggs. By the time that the Portuguese arrived, Madagascar's once-abundant elephant birds had all been reduced to eggshells covering the beaches, skeletons in the ground, and vague memories of roks. Madagascar and Polynesia merely provide well-documented examples of the extinction waves that probably unfolded on all large oceanic islands colonized by people before the European expansion of the last 500 years. Like New Zealand and Madagascar, all such islands where life had evolved in the absence of humans used to have unique species of big animals that modern zoologists never saw alive. Mediterranean islands like Crete and Cyprus had pygmy hippos and giant tortoises (just as did Madagascar), as well as dwarf elephants and dwarf deer. The West Indies lost monkeys, ground sloths, a bear-sized rodent, and owls of several sizes: normal, giant, colossal, and titanic. It seems likely that these big birds, mammals, and tortoises too somehow succumbed to the first Mediterranean peoples or American Indians to reach their islands. Nor were birds the only victims. Mammals, lizards, frogs, snails, and even large insects disappeared as well, comprising thousands of species when one adds up all oceanic islands. Olson describes these insular extinctions as 'one of the swiftest and most profound biological catastrophes in the history of the world'. However, we will not be sure that humans were responsible until the bones of the last animals and the remains of the first people have been dated more exactly for other islands, as has already been done for Polynesia and Madagascar.

In addition to these pre-industrial extermination waves on islands, other species may have fallen victim to extermination waves on continents, in the more distant past. About 11,000 years ago, around the probable time that the first ancestors of American Indians reached the New World, most large species of mammals became extinct throughout all of North and South America. The disappearances involved species as varied as lions, horses, giant armadillos, mammoths, and saber-toothed cats. A long-standing debate has raged over whether these big mammals were done in by Indian hunters, or whether they just happened to succumb to climate changes around the same time. I shall explain in the next chapter why I personally think that hunters did it. However, it is much harder to pinpoint dates and causes of events that happened around 11,000 years ago than it is for recent events, like the collision of the Maoris and the moa within the past thousand years. Similarly, within the past 50,000 years Australia lost most of its big mammal species and was colonized by the ancestors of today's Aboriginal Australians, but we are still uncertain whether the second event caused the first. Therefore, although it is now reasonably certain that the first pre-industrial peoples to reach islands wrought havoc among island species, the jury is still out on the question of whether this also happened on continents. From all this evidence that the Golden Age was tarnished by exterminations of species, let's now turn to evidence for destruction of habitats. Three dramatic examples involve famous archaeological puzzles: the giant stone statues of Easter Island, the abandoned pueblos of the American Southwest, and the ruins of Petra.

An aura of mystery has clung to Easter Island ever since it and its Polynesian inhabitants were 'discovered' by the Dutch explorer Jakob Roggeveen in 1722. Lying in the Pacific Ocean 2,300 miles west of Chile, Easter surpasses even Henderson as one of the world's most isolated scraps of land. Hundreds of statues, weighing up to eighty-five tons and up to 37 feet tall, were carved from volcanic quarries, somehow transported several miles, and raised to an upright position on platforms, by people without metal or wheels and with no power source other than human muscle. Even more statues remain unfinished in the quarries, or lie finished but abandoned between the quarries and platforms. The scene today is as if the carvers and movers had suddenly walked off the job, leaving an eerily silent landscape. When Roggeveen arrived, many statues were still standing, though new ones were no longer being carved. By 1840 all the erected statues had been deliberately toppled by the Easter Islanders themselves. How were such huge statues transported and erected, why were they eventually toppled, and why had carving ceased?

The first of those questions was answered when living Easter Islanders showed Thor Heyerdahl how their ancestors had used logs as rollers to transport the statues and then as levers to erect them. The other questions were solved by subsequent archaeological and paleontological studies that revealed Easter's gruesome history. When Polynesians settled Easter around 400 AD, the island was covered by forest that they gradually proceeded to clear, in order to plant gardens and to obtain logs for canoes and for erecting statues. By around 1500 AD the human population had built up to about 7,000 (over 150 per square mile), about a thousand statues had been carved, and at least 324 of those statues had been erected. But—the forest had been destroyed so thoroughly that not a single tree survived.

An immediate result of this self-inflicted ecological disaster was that the islanders no longer had the logs needed to transport and erect statues, so that carving ceased. But deforestation also had two indirect consequences that brought starvation. These were soil erosion, causing lower crop yields, plus lack of timber to build canoes, resulting in less protein available from fishing. As a result, the population was now greater than Easter could support, and island society collapsed in a holocaust of internecine warfare and cannibalism. A warrior class took over; spear-points manufactured in huge quantities came to litter the landscape; the defeated were eaten or enslaved; rival clans pulled down each other's statues; and people took to living in caves for self-protection. What had once been a lush island supporting one of the world's most remarkable civilizations deteriorated into the Easter Island of today: a barren grassland littered with fallen statues, and supporting less than one-third of its former population.

Our second case study of pre-industrial habitat destruction involves the collapse of one of the most advanced Indian civilizations of North America. When Spanish explorers reached the US Southwest, they found gigantic multi-storey dwellings (pueblos) standing uninhabited in the middle of treeless desert. For example, the 650-room dwelling at Chaco Canyon National Monument in New Mexico was five stories high, 670 feet long, and 315 feet wide, making it the largest building ever erected in North America until topped by steel skyscrapers in the late Nineteenth Century. Navajo Indians in the region knew of the vanished builders only as 'Anasazi', meaning 'the Ancient Ones'. Archaeologists subsequently established that construction of the Chaco pueblos began shortly after 900 AD, and that occupation ceased in the Twelfth Century. Why did the Anasazi erect a city in a barren wasteland, of all unpromising places? Where did they obtain their firewood, or the 16-foot-long wooden beams (200,000 of them!), that supported the roofs? Why did they then abandon the city that they had built at such enormous effort?

The conventional view, analogous to the claim that Madagascar's elephant birds and New Zealand's moas died out from natural changes in climate, attributes the abandonment of Chaco Canyon to a drought. However, a different interpretation emerges from the work of paleo-botanists Julio Betancourt, Thomas Van Devender, and their colleagues, who used an ingenious technique to decipher changes in Chaco vegetation through time. Their method relied on the little rodents called packrats, which gather plants and other materials into shelters ('middens') that they eventually abandon after fifty or a hundred years but that remain well preserved under desert conditions. The plants can be identified centuries later, and the midden can be dated by radiocarbon techniques. Thus, each midden is virtually a time capsule of the local vegetation.

By this method, Betancourt and Van Devender were able to reconstruct the following course of events. At the time that the Chaco pueblos were erected, they were not surrounded by barren desert but by pinyon and juniper woodland, with ponderosa pine forests nearby. This discovery at once solves the mystery of where the firewood and timber came from, and disposes of the apparent paradox of an advanced civilization rising from barren desert. As occupation continued at Chaco, however, the woodland and forest were cleared until the environment became the treeless wasteland that it remains today. The Indians were then having to go over ten miles to get firewood, and over twenty-five miles to get pine logs. When the pine forests had been felled, they built an elaborate road system to haul spruce and fir logs from mountain slopes over fifty miles away, relying on nothing more than their own muscle power. In addition, the Anasazi had solved the problems of agriculture in a dry environment by building irrigation systems to concentrate available water into valley bottoms. As deforestation caused progressively increasing erosion and water runoff, and as irrigation channels gradually dug gullies into the ground, the water table may finally have dropped below the level of the Anasazi fields, making irrigation without pumps impossible. Thus, while drought may have made some contribution to the Anasazi abandonment of Chaco Canyon, a self-inflicted ecological disaster was also a major factor. Our remaining example of pro-industrial habitat destruction illuminates the gradual geographic shift in the power centre of ancient western civilizations. Recall that the first centre of power and innovation was the Mideast where so many crucial developments arose- agriculture, animal domestication, writing, imperial states, battle chariots and others. Ascendancy shifted between Assyria, Babylon, Persia and occMionaUy Egypt or Turkey, but remained in or near the Mideast. With the overthrow of the Persian Empire by Alexander the Great, ascendancy moved finally westward, at first to Greece, then to Rome, and later to western and northern Europe. Why did the Mideast, Greece, and Rome in turn lose their primacy? (The transient current importance ot the

Mideast, resting as it does on the single resource of oil, merely emphasizes by contrast the region's modern weakness in other respects.) Why do modern superpowers include the US and Russia, Germany and England, Japan and China, but no longer Greece and Persia? This geographic shift in power is too big and lasting a pattern to have arisen by accident. A plausible hypothesis attributes it to each ancient centre of civilization in turn ruining its resource base. The Mideast and Mediterranean were not always the degraded landscape that they appear today. In ancient times much of this area was a lush, fertile mosaic of wooded hills and fertile valleys. Thousands of years of deforestation, overgrazing, erosion, and valley siltation converted this heartland of Western civilization into the relatively dry, barren, infertile landscape that predominates today. Archaeological surveys of ancient Greece have revealed several cycles of population growth alternating with population crashes and local abandonment of human settlements. In the growth phases, terracing and dams initially protected the landscape until felling of forests, clearing of steep slopes for agriculture, overgrazing by too many livestock, and planting of crops at too short intervals overwhelmed the system. The result each time was massive erosion of the hills, flooding of the valleys, and the collapse of local human society. One such event coincided with (and may have caused) the otherwise mysterious collapse of Greece's glorious Mycenean civilization, after which Greece fell back for several centuries into a dark age of illiteracy.

The support for this view of ancient environmental destruction comes from sources such as contemporary accounts and archaeological evidence. Yet a few sequences of photographs would constitute more decisive tests than all that anecdotal evidence combined. If we had snapshots of the same Greek hillside taken at thousand-year intervals, we could identify the plants, measure the ground cover, and calculate the shift from forest to goat-proof shrubs. We could thereby put numbers on the extent of environmental degradation. Enter middens to the rescue again. While the Mideast does not have packrats, it does have rabbitsized, marmot-like animals called hyraxes that build middens in the same way as packrats. (Surprisingly, the closest living relatives of hyraxes may be elephants.) Three Arizona scientists—"atricia Fall, Cynthia Lindquist, and Steven Falconer—studied hyrax middens at Jordan's famous lost city of Petra, which typifies the paradox °i ancient Western civilization. Petra is now especially familiar to movie-going aficionados of Steven Spielberg and George Lucas, whose turn Indiana Jones and the Last Crusade shows Scan Connery and Harrison Ford searching for the Holy Grail in Petra's magnificent rock tombs and temples amidst the desert sand. Anyone who sees those scenes of Petra must wonder how such a wealthy city could have arisen and supported itself in such a bleak landscape. In fact, there was already a Neolithic village near the site of Petra before 7000 BC, and farming and herding appeared there soon after. Under the Nabataean kingdom, of which it was the capital, Petra thrived as a commercial centre controlling trade between Europe, Arabia, and the Orient. The city grew even larger and richer under Roman, then Byzantine, control. Yet it was subsequently abandoned and so completely forgotten that its ruins were not rediscovered until 1812. What caused Petra's collapse?

Each hyrax midden from Petra yielded remains of up to 100 plant species, and the habitat prevailing when each midden's owner was alive could be calibrated by comparing pollen proportions in the midden with those in modern habitats. From the middens, the following trajectory was reconstructed for the degradation of Petra's environment. Petra lies in an area of dry Mediterranean climate not unlike that of the wooded mountains behind my home in Los Angeles. The original vegetation would have been a woodland dominated by oak and pistachio trees. By Roman and Byzantine times, most of the trees had been felled, and the surroundings had been degraded to an open steppe, as expressed in the fact that only eighteen per cent of midden pollen came from trees, the rest from low plants. (For comparison, trees contribute forty to eighty-five per cent of the pollen in modern Mediterranean forests, eighteen per cent in forest steppes.) By 900 AD, a few centuries after Byzantine control of the Petra area ended, two-thirds of the remaining trees had disappeared. Even shrubs, herbs, and grasses had declined, converting the environment into the desert that we see now. Surviving trees today have their lower branches pruned off by goats and are scattered on goat-proof cliffs or in groves protected from goats.

Juxtaposing these data from hyrax middens with archaeological and literary data yields the following interpretation. Deforestation from Neolithic to Imperial times was driven by the clearing of land for agriculture, browsing by sheep and goats, gathering of firewood, and wood needs for house construction. Even Neolithic houses not only were supported By massive timbers but also consumed up to thirteen tons of firewood per house to make the plaster for the walls and floor. The Imperial population explosion quickened the pace of forest destruction and overgrazing. Elaborate systems of channels, pipes, and cisterns were needed to collect and store water for the orchards and city.

After Byzantine authority collapsed, orchards were abandoned and the population crashed, but land degradation continued as the remaining inhabitants became dependent on intensive grazing. The insatiable goats began to eat their way through the shrubs, herbs, and grasses. The Ottoman government decimated surviving woodlands before the First World War, to obtain the wood needed for the Hejaz Railway. I and many other movie-goers thrilled at the sight of Arab guerrillas led by Lawrence of Arabia (a.k.a. Peter O'Toole) blowing up that railway in widescreen technicolour, without realizing that we were watching the last act in the destruction of Petra's forests. Petra's ravaged landscape today is a metaphor for what happened to the rest of the cradle of Western civilization. The modern surrounds of Petra could no more feed a city that commanded the world's main trade routes than the modern surrounds of Persepolis could feed the capital of a superpower such as the Persian Empire once was. The ruins of those cities, and of Athens and Rome, are monuments to states that destroyed their means of survival. Nor are Western civilizations the only literate societies that committed ecological suicide. The collapse of Classic Maya civilization in Central America, and of Harappan civilization in India's Indus Valley, are other obvious candidates for eco-disasters due to an expanding human population overwhelming its environment. While courses in the history of civilization often dwell on kings and barbarian invasions, deforestation and erosion may in the long run have been more important shapers of human history. These are some of the recent discoveries making the supposed past Golden Age of environmentalism look increasingly mythical. Let's now go back to the larger issues I raised at the outset. Firstly, how can these discoveries of past environmental damage be reconciled with accounts of conservationist practices by so many modern pre-industrial peoples? Obviously, not all species have been exterminated, and not all habitats have been destroyed, so the Golden Age could not have been all black. I suggest the following answer to this paradox. It is still true that small, long-established, egalitarian societies tend to evolve conservationist practices, because they have had plenty of time to get to know their local environment and to perceive their own self-interest. Instead, damage is likely to occur when people suddenly colonize an unfamiliar environment (like the first Maoris and Easter Islanders); or when people advance along a new frontier (like the first Indians to reach America), so that they can just move beyond the frontier when they have damaged the region behind; or when people acquire a new technology whose destructive power they have not had time to appreciate (like modern New Guineans, now devastating pigeon populations with shotguns). Damage is also likely in centralized states that concentrate wealth in the hands of rulers, who are out of touch with their environment. Some species and habitats are more susceptible to damage than others—such as flightless birds that had never seen humans (like moas and elephant birds), or the dry, fragile, unforgiving environments in which both Western civilization and Anasazi civilization arose. Secondly, are there any practical lessons that we can learn from these recent archaeological discoveries? Archaeology is often regarded as a socially irrelevant academic discipline that becomes a prime target for budget cuts whenever money gets tight. In fact, archaeological research is one of the best bargains available to government planners. All over the world, we are launching developments that have great potential for doing irreversible damage, and that are really just more powerful versions of ideas put into operation by past societies. We cannot afford the experiment of developing five counties in five different ways and seeing which four counties get ruined. Instead, it will cost us much less in the long run if we hire archaeologists to find out what happened the last time, than if we go on making the same mistakes again. Here is just one example. The American Southwest has over 100,000 square miles of pinyon and juniper woodland that we are exploiting more and more for firewood. Unfortunately, the US Forest Service has little data available to help it calculate sustainable yields and recovery rates in that woodland. Yet the Anasazi already tried the experiment and miscalculated, with the result that the woodland still has not recovered in Chaco Canyon after over 800 years. Paying some archaeologists to reconstruct Anasazi firewood consumption would be cheaper than committing the same mistake and ruining 100,000 square miles of the US, as we may now be doing. Finally, let's face the touchiest question. Today, environmentalists view people who exterminate species and destroy habitats as morally bad. Industrial societies have jumped at any excuse to denigrate pre-industrial peoples, in order to justify killing them and appropriating their land. Are the purported new finds about moas and Chaco Canyon vegetation just pseudo-scientific racism that in effect is saying, Maoris and Indians dp not deserve fair treatment because they were bad? What has to be remembered is that it has always been hard for humans to know the rate at which they can safely harvest biological resources indefinitely, without depleting them. A significant decline in resources may not be easy to distinguish from a normal year-to-year fluctuation. It is even harder to assess the rate at which new resources are being produced. By the time that the signs of decline are clear enough to convince everybody, it may be too late to save the species or habitat. Thus, pre-industrial peoples who could not sustain their resources were guilty not of moral sins, but of failures to solve a really difficult ecological problem. Those failures were tragic, because they caused a collapse in lifestyle for the people themselves.

Tragic failures become moral sins only if one should have known better from the outset. In that regard there are two big differences between us and eleventh-century Anasazi Indians—those of scientific understanding, and literacy. We know, and they did not know, how to draw graphs that plot sustainable resource population size as a function of resource harvesting rate. We can read about all the ecological disasters of the past; the Anasazi could not. Yet our generation continues to hunt whales and clear tropical rainforest, as if no one had ever hunted moas or cleared pinyon and juniper woodland. The past was still a Golden Age, of ignorance, while the present is an Iron Age of wilful blindness.

From this point of view it is beyond understanding to see modern societies repeating the past's suicidal ecological mismanagement, with much more powerful tools of destruction in the hands of far more people. It is as if we had not already run that particular film many times before in human history, and as if we did not know the inevitable outcome. Shelley's sonnet 'Ozymandias' evokes Persepolis, Tikal, and Easter Island equally well; perhaps it will some day evoke to others the ruins of our own civilization.

I met a traveller from an antique land Who said: Two vast and trunkless legs of stone Stand in the desert. Near them, on the sand, Half sunk, a shattered visage lies, whose frown, And wrinkled lip, and sneer of cold command, Tell that its sculptor well those passions read Which yet survive, stamped on these lifeless things, The hand that mocked them and the heart that fed; And on the pedestal these words appear: 'My name is Ozymandias, king of kings: Look on my works, ye Mighty, and despair! Nothing beside remains. Round the decay Of that colossal wreck, boundless and bare The lone and level sands stretch far away.

EIGHTEEN BLITZKRIEG AND THANKSGIVING IN THE NEW WORLD

The colonization of the Americas eleven thousand years ago by ancestors of today's American Indians was the greatest extension of human lebensraum since Homo erectus emerged from Africa. It may also have been the first of the blitzkriegs against Nature that have since marked every expansion of humans into previously unpeopled areas. Within a short time of human arrival—perhaps only a few centuries—most of the big mammals of North and South America were extinct.

The United States devote two national holidays, Columbus Day and Thanksgiving Day, to celebrating dramatic moments in the European 'discovery' of the New World. No holidays commemorate the much earlier actual discovery by Indians. Yet archaeological excavations suggest that, in drama, that earlier discovery dwarfs the adventures of Christopher Columbus and of the Plymouth Pilgrims. Within perhaps as little as a thousand years after finding a way through an Arctic ice sheet and crossing the present border between the US and Canada, Indians had swept down to the tip of Patagonia and populated two productive and unexplored continents. The Indians' march southwards was the greatest range expansion in the history of Homo sapiens. Nothing remotely like it can ever happen again on our planet.

The sweep southwards was marked by another drama. When Indian hunters arrived, they found the Americas teeming with big mammals that are now extinct: elephant-like mammoths and mastodonts, ground sloths weighing up to three tons, armadillo-like glyptodonts weighing up to one ton, bear-sized beavers, and saber-toothed cats, plus American lions, cheetahs, camels, horses, and many others. Had those beasts survived, today's tourists in Yellowstone National Park would be watching mammoths and lions along with the bears and bison. The question of what happened at that moment of hunters-meet-beasts is still highly controversial among archaeologists and paleontologists. According to the interpretation that seems most plausible to me, the outcome was a blitzkrieg in which the beasts were quickly exterminated—possibly within a mere ten years at any given site. If that view is correct, it would have been the most concentrated extinction of big animals since an asteroid collision (it is believed) knocked off the dinosaurs sixty-five million years ago. It would also have been the first of the series of blitzkriegs that marred our supposed Golden Age of environmental innocence (Chapter Seventeen), and that have remained a human hallmark ever since. That dramatic confrontation came as the finale to a long epic in which humans, spreading out of their centre of origin in Africa, occupied all the other habitable continents. Our African ancestors expanded to Asia and Europe around a million years ago, and from Asia to Australia around 50,000 years ago, leaving North and South America as the last habitable continents still without Homo sapiens.

From Canada to Tierra del Fuego, American Indians today are physically more homogeneous than the inhabitants of any other continent, implying that they arrived too recently to have become very diverse genetically. Even before archaeology uncovered evidence of the first Indians, it was clear that they must have originated from Asia, because modern Indians look similar to Asiatic Mongoloids. Much recent evidence from genetics and anthropology has made that conclusion certain. A glance at a map shows that by far the easiest route from Asia to America is across the Bering Straits separating Siberia from Alaska. The last such land bridge existed (with a few brief interruptions) from about 25,000 to 10,000 years ago.

However, colonization of the New World required more than a land bridge—there had to be people living at the Siberian end of the bridge. Because of its harsh climate the Siberian Arctic, too, was not colonized until late in human history (Chapter Two). Those colonists must have come from the cold temperate zones of Asia or Eastern Europe, as exemplified by stone-age hunters who lived in what is now the Ukraine and who built their houses out of neatly stacked bones of mammoths. By at least 20,000 years ago there were mammoth hunters in the Siberian Arctic as well, and by around 12,000 years ago stone tools similar to those of the Siberian hunters appear in Alaska's archaeological record.

After traversing Siberia and the Bering Straits, the ice-age hunters were still separated by one more barrier from their future hunting grounds in the US: a broad ice cap like that covering

Greenland today, but stretching coast-to-coast across Canada. At intervals during the ice ages a narrow, ice-free, north/south corridor opened through this ice cap, just east of the Rocky Mountains. One such corridor closed around 20,000 years ago, but there had apparently as yet been no human in Alaska waiting to cross it. However, when the corridor next opened around 12,000 years ago, the hunters must have been ready, for their tell-tale stone tools appear soon thereafter not only at the south end of the corridor near Edmonton (Alberta) but also elsewhere south of the ice cap. At that point, hunters met America's elephants and other great beasts, and the drama began. Archaeologists term these pioneering ancestral Indians the Clovis people, since their stone tools were first recognized at an excavation near the town of Clovis, ten miles inside New Mexico from the Texas border. However, Clovis tools or ones similar to them have been found in all forty-eight contiguous states of the US, and from Edmonton in the north to Mexico. Vance Haynes, a University of Arizona archaeologist, has emphasized that the tools are much like those of the earlier Eastern European and Siberian mammoth hunters, with one conspicuous exception: the flattish, two-faced, stone spear-points were 'fluted' on each face as a result of a longitudinal groove having been chipped out to make it easier to bind the stone point to the shaft. It is not clear whether the fluted points were mounted on spears to throw by hand, on darts to hurl by a throwing stick, or on lances to thrust. Somehow, though, the points were propelled into big mammals with such force that the points sometimes snapped in half, or else penetrated bone. Archaeologists have dug up skeletons of mammoths and bison with Clovis points inside the rib cage, including a mammoth from southern Arizona containing a total of eight points. At excavated Clovis sites, mammoths are by far the commonest prey (to judge from their bones), but other victims include bison, mastodonts, tapirs, camels, horses, and bears.

Among the startling discoveries about Clovis people is the speed of their spread. All Clovis sites in the US dated by the most advanced radiocarbon techniques were occupied for only a few centuries, in the periodjust before 11,000 years ago. A human site even at the southern tip of Patagonia is dated at about 10,500 years ago. Thus, within about a millenium of emerging from the ice-free corridor at Edmonton, humans had spread from coast to coast and over the entire length of the New World.

Equally startling is the rapid transformation of Clovis culture. Around 11,000 years ago Clovis points are abruptly replaced by a smaller, more finely made model now known as Folsom points (after a site near Folsom, New Mexico, where they were first identified). The Folsom points are often found associated with bones of an extinct wide-horned bison, never with the mammoths preferred by Clovis hunters.

There may be a simple reason why Folsom hunters switched from mammoths to bison: there were no more mammoths left. There also were no more mastodonts, camels, horses, giant ground sloths, nor several dozen other types of big mammals. In all, North America lost an astonishing seventy-three per cent, and South America eighty per cent, of their genera of big mammals around this time. Many paleontologists do not blame this American extinction spasm on Clovis hunters, since there is no surviving evidence of mass slaughter—only the fossilized bones of a few butchered carcasses here and there. Instead, those paleontologists attribute the extinctions to changes of climate and habitats at the end of the ice ages, just around the time that Clovis hunters arrived. That reasoning puzzles me for several reasons. Ice-free habitats for mammals expanded rather than contracted as glaciers yielded to grass and forest; big American mammals had already survived the ends of at least twenty-two previous ice ages without such an extinction spasm; and there were far fewer extinctions in Europe and Asia when the glaciers of those continents melted around the same time.

If changing climate had been the cause, one might have expected opposite effects on species preferring hot and cold climates. Instead, radiocarbon-dated fossils from the Grand Canyon show that the Shasta ground sloth and Harrington's mountain goat, derived from areas of hot and cold climates respectively, both died out within a century or two of each other, around 11,100 years ago. The sloths were common until just before their sudden extinction. In their softball-sized dung balls, still well-preserved in some southwestern US caves, botanists have identified remains of plants on which the last sloths chomped: the Mormon tea and globe mallow, which still occur around those caves today. It is highly suspicious that both those well-fed sloths and the goats of the Grand Canyon disappeared just after Clovis hunters reached Arizona. Juries have convicted murderers on the grounds of less compelling circumstantial evidence. If climate really was what did in the sloths, we would have to credit those supposedly stupid beasts with unsuspected intelligence, since they all chose to drop dead simultaneously at just the right moment to deceive some twentieth-century scientists into blaming Clovis hunters.

A more plausible explanation of this 'coincidence' is that it really was a case of cause and effect. Paul Martin, a geoscientist at the University of Arizona, describes the dramatic outcome of hunter-meets-elephant as a blitzkrieg'. According to his view, the first hunters to emerge from the ice-free corridor at Edmonton thrived and multiplied, because they found an abundance of tame, casy-to-hunt big mammals. As the mammals were killed off in one area, the hunters and their offspring kept tanning out into new areas that still had abundant mammals, and kept exterminating the mammal populations at the front of their advance. By the time that the hunters' front finally reached the south tip of South America, most of the big mammal species of the New World had been exterminated.

Martin's theory has attracted lots of vigorous criticism, most of it centring on four doubts. Could a band of 100 hunters arriving at Edmonton breed fast enough to populate a hemisphere in a thousand years? Could they spread fast enough to cover the nearly 8,000 miles from Edmonton to Patagonia in that time? Were Clovis hunters really the first people in the New World? And could stone-age hunters really pursue hundreds of millions of big mammals so efficiently that not a single individual survived, while nevertheless leaving little fossil evidence of their hunts? Take first the question of breeding rates. Populations of modern hunter-gatherers on even their best hunting grounds number only about one per square mile. Hence, once the whole western hemisphere had been settled, its population of hunter-gatherers would have been at most ten million, since the New World's area outside of Canada and other areas covered by glaciers in Clovis times is about ten million square miles. In modern instances where colonists have arrived at an uninhabited land (for instance, when the H.M.S-. Bounty mutineers reached Pitcairn Island), their population growth has been as rapid as 3.4 % per year. That growth rate, which corresponds to each couple having four surviving children and a mean generation time of twenty years, would multiply 100 hunters into ten million in only 340 years. Thus, Clovis hunters should easily have been able to multiply to ten million within a millenium.

Could the descendants of the Edmonton pioneers have reached the south tip of South America in a thousand years? The overland straight-line distanced slightly under 8,000 miles, so that they would have to average eight miles a year. That is a trivial task—any fit hunter or huntress could have fulfilled the year's quota in a day and not moved for another 364 days. The quarry from which a Clovis tool was made can often be identified by its local type of stone, and we know in that way that individual tools travelled up to 200 miles. Some of the nineteenth-century Zulu migrations in southern Africa are known to have covered nearly 3,000 miles in a mere fifty years. Were Clovis hunters the first humans to spread south of the Canadian ice sheet? That is a harder question, and it is extremely controversial among archaeologists. Primacy claims for Clovis are inevitably based on negative evidence: there are no unequivocal human remains or artifacts with universally accepted pre-Clovis dates anywhere in the New World south of the former Canadian ice sheet. Mind you, there are dozens of claims of sites with pre-Clovis human evidence, but all or almost all of them are marred by serious questions about whether the material used for radiocarbon dating was contaminated by older carbon, or whether the dated material was really associated with the human remains, or whether the tools supposedly made by humans were just naturally shaped rocks. The two most nearly convincing of those claimed pre-Clovis sites are Meadowcroft Rock Shelter in Pennsylvania, dated to about 16,000 years ago, and the Monte Verde site in Chile, dated to at least 13,000 years ago. Monte Verde is described as having amazingly good preservation of many types of human artifacts, but those results have not yet been published in detail, so they cannot yet be properly evaluated. At Meadowcroft there has been an unresolved debate about whether the radiocarbon dates are in error, especially because the plant and animal species from the site are ones expected to have been living there only much more recently than 16,000 years ago. In contrast, the evidence for Clovis people is undeniable, is to be found in all forty-eight contiguous states, and is accepted by all archaeologists. Evidence for the still earlier settlement of the other habitable continents by more primitive humans is also unequivocal and universally accepted. At one Clovis site after another, you can see a level with Clovis artifacts and bones of numerous large extinct mammal species; immediately above (that is, younger than) the Clovis level, a level with Folsom artifacts but with the bones of not a single large extinct mammal except for bison; and immediately below the Clovis level, levels spanning thousands of years before Clovis times, reflecting benign environmental conditions, and full of the bones of large extinct mammals, but with not a single human artifact. How could people possibly have settled the New World in pre-Clovis times and not left behind the usual trail of abundant evidence that convinces archaeologists, like stone tools, hearths, occupied caves, and occasionally skeletons, with unequivocal radiocarbon dates? How could there have been pre-Clovis people who left no trace of their presence at Clovis sites, despite such favourable living conditions? How could people have arrived at Pennsylvania or Chile from Alaska, as if by helicopter, without leaving good evidence of their presence in all the intervening territory? For these reasons, I find it more plausible that the dates given for Meadowcroft and Monte Verde are somehow wrong, than that they are correct. The Clovis-first interpretation makes good sense, but the pre-Clovis interpretation just does not make sense to me.

The other hotly contested argument over Martin's blitzkrieg theory concerns the supposed over-hunting and extermination of big mammals. It seems hard to imagine how stone-age hunters could kill a mammoth at all, let alone hunt all mammoths to extinction. Even if the hunters could slaughter mammoths, why would they want to? And where are all the skeletons now? Certainly, when we stand under a mammoth skeleton in a museum, the thought of using a stone-tipped spear to attack such a gigantic tusked beast feels utterly suicidal. Yet modern Africans and Asians with equally simple weapons do succeed in killing elephants, often hunting as a group relying on ambush or fire, but sometimes stalking an elephant as a single hunter armed with a spear or poisoned arrow. These modern elephant hunters still rate as amateur dabblers, compared to the mammoth hunters of Clovis times, heirs to hundreds of thousands of years of hunting experience with stone tools. Museum artists like to depict late stone-age hunters as naked brutes risking their lives to hurl boulders at an enraged charging mammoth, with one or two hunters already lying trampled to death on the ground. That is absurd. If any hunters had died in a typical mammoth hunt, mammoths would have exterminated hunters, rather than vice versa. Instead, a more realistic picture is of warmly-clad professionals safely spearing a terrified mammoth ambushed in a narrow stream bed.

Recall also that the big mammals of the New World had probably never seen humans before Clovis hunters, if the hunters indeed were the first people to reach the New World. We know from Antarctica and the Galapagos how tame and unafraid are animals that evolved in the absence of humans. When I visited New Guinea's isolated Foja mountains, which lack any human population, I found the large tree kangaroos so tame that I could walk up to within a few yards of them. Probably the New World's large mammals were equally naive and were killed off before they could have time to evolve a fear of man.

Could Clovis hunters have killed mammoths fast enough to exterminate them? Assume again that an average square mile supports one hunter-gatherer and (by comparison with elephants in Africa today) one mammoth, and that one-quarter of the Clovis population consisted of adult male hunters who each killed a mammoth every two months. That means six mammoths killed per four square miles per year, so the mammoths would have to reproduce their numbers in less than a year to keep up with the killing. Yet modern elephants are slow breeders that take about twenty years to reproduce their numbers, and few other large mammal species breed fast enough to reproduce their numbers in less than three years. It could plausibly have taken Clovis hunters only a few years to exterminate the large mammals locally and to move on to the next area. Archaeologists trying to document the slaughter today are searching for needles in a fossil haystack: a few years' worth of butchered mammoth bones among the bones of all the mammoths that died naturally over hundreds of thousands of years. It is no wonder that so few mammoth carcasses with Clovis points among the ribs have been found.

Why would a Clovis hunter even want to kill a mammoth every two months, when a 5,000-pound mammoth yielding 2,500 pounds of meat would provide ten pounds of meat per day for two months for the hunter, his wife, and two children? Ten pounds may sound like gross gluttony, but it actually approaches the daily meat ration per person on the US frontier in the last century. That is assuming that Clovis hunters really ate all 2,500 pounds of mammoth meat. But to keep the meat for two months would require drying it: would you go to the work of drying a ton of meat, when you could instead just go and kill a fresh mammoth? As Vance Haynes noted, Clovis mammoth kills prove to be only partly butchered, suggesting very wasteful and selective utilization of meat by people living amidst an abundance of game. Some hunting probably was not for meat at all but for ivory, hides, or just machismo. Seals and whales have similarly been hunted in modern times for oil or fur, leaving the meat to rot. In New Guinea fishing villages I often see the discarded carcasses of large sharks, killed only for their fins to make the delicacy of shark's fin soup.

We are all too familiar with the blitzkriegs by which modern European hunters nearly exterminated bison, whales, seals, and many other large animals. Recent archaeological discoveries on islands have shown that such blitzkriegs were an outcome whenever earlier hunters reached a land with animals unused to humans (Chapter Seventeen). New Zealand's giant flightless birds, the moas, were all exterminated by Maori colonists within a few centuries. The Indonesians and Africans who colonized Madagascar 1,500 years ago exterminated other giant flightless birds (the elephant birds), along with a dozen species of primates (the lemurs) ranging up to the size of a gorilla. Polynesian colonists of Hawaii exterminated numerous species of big flightless geese. Since the collision between humans and large naive animals has always ended in an extermination spasm, how could it have been otherwise when Clovis hunters entered a naive New World?

This end, though, would hardly have been foreseen by the first hunters to arrive at Edmonton. It must have been a dramatic moment when, after entering the ice-free corridor from an overpopulated, overhunted Alaska, they emerged to see herds of tame mammoths, camels, and other beasts. In front of them stretched the Great Plains to the horizon. As they began to explore, they must soon have realized (unlike Christopher Columbus and the Plymouth Pilgrims) that there were no people at all in front of them, and that they had truly arrived first at a fertile land. Those Edmonton Pilgrims, too, had cause to celebrate a Thanksgiving Day.

NINETEEN THE SECOND CLOUD

Human society is now at risk of destroying itself by stumbling into either a nuclear holocaust or an environmental holocaust. The latter could arise from mass extinction of species. This chapter estimates how many species we have already exterminated, how many more we are likely to exterminate within the next century, and how mass extinction would affect us. Until our own generation, no one had grounds to worry whether the next human generation would survive or enjoy a planet worth living on. Ours is the first generation to be confronted with these questions about its children's future. We devote much of our lives to training our children to support themselves and to get along with other people. Increasingly, we are asking ourselves whether all those efforts of ours might be wasted.

These concerns arise because of two clouds hanging over us—clouds that would have similar consequences, but that we view very differently. One, the risk of a nuclear holocaust, first revealed itself in the cloud over Hiroshima. Everyone agrees that the risk is real, since there are huge stockpiles of nuclear weapons and since politicians throughout history have occasionally made dumb miscalculations. Everyone agrees that, if a nuclear holocaust does happen, it will be bad for us and might even kill us all. This risk shapes much of current world diplomacy. The only thing about which we disagree is how best to handle it—for instance, whether we should aim for complete or partial nuclear disarmament, nuclear balance, or nuclear superiority.

The other cloud is the risk of an environmental holocaust, of which one often discussed potential cause is the gradual extinction of most of the World's species. In contrast to the case with nuclear holocaust, there is almost complete disagreement about whether the risk of a mass extinction is real and about whether it would really do us much harm if it happened. For instance, one of the most frequently cited estimates is that humans have caused about one per cent of the world's bird species to be, come extinct within the last few centuries. At one extreme, many thoughtful people—especially economists and industrial leaders, but also some biologists and many laypeople—think that that loss of one per cent would have been inconsequential, even if it had really happened. In fact, such people reason that one per cent is a gross overestimate, that most species are superfluous to us, and that it would do us no harm to lose ten times more species. At the opposite extreme, many other thoughtful people—especially conservation biologists and a growing number of laypeople belonging to environmentalist movements—think that the one per cent figure is a gross underestimate, and that mass extinction would undermine the quality or possibility of human life. Obviously, it will make a big difference to our children which of these two extreme views is closer to the truth.

The risks of a nuclear holocaust and of an environmental holocaust constitute the two really pressing questions facing the human race today. Compared to these two clouds, our usual obsessions with cancer, AIDS, and diet pale into insignificance, because those problems do not threaten the survival of the human species. If the nuclear and environmental risks should not materialize, we shall have plenty of leisure time to solve bagatelles like cancer. If we fail to avert those two risks, solving cancer will not have helped us anyway.

How many species have humans really driven into extinction already? How many more are likely to become extinct within our children's lifetimes? If more do become extinct, so what? How much do wrens contribute to our gross national product? Are not all species destined to become extinct sooner dr later? Is the claimed mass extinction crisis an hysterical fantasy, a real risk for the future, or a proven event that is already well underway?

We need to go through three steps if we are to arrive at realistic estimates of the numbers involved in the mass extinction debate. Firstly, let's see how many species have become extinct in modern times (that is, since 1600). Secondly, let's estimate how many other species had become extinct before 1600. As the third step, let's try to predict how many further sp*ecies are likely to become extinct within the lifespans of ourselves, our children, and our grandchildren. Finally, let's ask what difference it all makes to us anyway.

The first step, that of calculating the number of species that have become extinct in modern times, seems easy when one initially thinks about it. Just take some group of plants or animals, count up in a catalogue the total number of species, mark off the ones known to have become extinct since 1600, and add them up. As a group on which to try this exercise, birds have the advantage that they are easy to see and identify, and hoards of bird-watchers watch them. As a result, more is known about them than about any other group of animals.

Approximately 9,000 species of birds exist today. Only one or two previously unknown species are still being discovered each year, so virtually all living birds have already been named. The leading agency concerned with the status of the world's birds—the International Council for Bird Preservation (ICBP)—lists 108 species of birds, plus many additional subspecies, as having become extinct since 1600. Virtually all these cases of extinction were caused in one way or another by humans—more of that later. One hundred and eight is about one per cent of that total number of bird species: 9,000. That is where the one per cent figure I mentioned earlier comes from.

Before we take that as the final word on the number of modern birds that have become extinct, let's understand how the number of 108 was arrived at. The ICBP decides to list a species as extinct only after that bird has been specifically looked for in areas where it was previously known to occur or might have turned up, and after it has not been found for many years. In many cases, birders have watched a population dwindle down to a few individuals and have followed the fates of those last individuals. For example, the most recent subspecies of bird to have become extinct in the US was the dusky seaside sparrow that lived in marshes near Titusville, Florida. As its population shrank due to destruction of the marshes where it lived, wildlife agencies put identification bands on the few remaining sparrows so that they could be individually recognized. When only six remained, they were brought into captivity in order to protect and breed them. Unfortunately, one after another died. The last individual, and with it the subspecies itself, died on 16 June 1987.

Thus, there is no doubt that the dusky seaside sparrow is extinct. Equally little doubt attaches to the many other subspecies and the 108 full species of birds listed as extinct. The full species listed as having vanished m North America since European settlement, and the years in which the last individual of each died, are the great auk (1844), spectacled cormorant (1852), Labrador duck (1875), Carolina parakeet (1914), and passenger pigeon (1914). The great auk also formerly occurred in Europe, but no other European bird species is listed as having become extinct since 1600, though some species have disappeared within Europe while surviving on other continents. What about all those remaining bird species that did not fulfil the BP's rigorous criteria for extinction? Can we be certain that they still exist? For most North American and European birds the answer is 'yes'. Hundreds of thousands of fanatical birdwatchers monitor all bird species on these continents every year. The rarer the species, the more fanatical is the annual search for it. No North American or European bird species could possibly drift into extinction unnoticed. There is only one North American bird species whose current existence is uncertain, the Bachman's warbler, last definitely recorded in 1977, but the ICBP hasn't given up hope for it because of more recent unconfirmed records. (The ivory-billed woodpecker may also be extinct, but the North American population is 'only' a subspecies; a few individuals of the other subspecies of this woodpecker survive in Cuba.) Thus, the number of North American bird species that have suffered extinction since 1600 is surely not less than five nor more than six. Every species but Bachman's warbler can be assigned to one of two categories—those that are 'definitely extinct', or 'definitely in existence'. Similarly, the number of European bird species extinct since 1600 is surely one—not two, not zero, but one.

Consequently we have an exact, unequivocal answer to the question of how many North American and European bird species have become extinct since 1600. If we could be equally definite for other groups of species, our first step in assessing the mass extinction debate would be complete. Unfortunately, this cut-and-dried situation does not apply to other groups of plants and animals, nor does it apply elsewhere in the world—least of all in the tropics, where the overwhelming majority of species lives. Most tropical countries have few or no bird-watchers, and no annual monitoring of birds. Many tropical areas have never again been monitored since they were first explored biologically many years ago. The status of many tropical species is unknown, because no one has seen them again or specifically looked for them since they were discovered. For instance, among the New Guinea birds that I study, Brass's friarbird is known only from eighteen specimens shot at one lagoon on the Idenburg River between 22 March and 29 April 1939. No scientist has revisited that lagoon, so we know nothing about the current status of Brass's friarbird. At least, jve know where to look for that friarbird. Many other species were described from specimens collected by nineteenth-century expeditions that provided only vague indications of the collecting site, such as, 'South America'. Try resolving the status of some rare species when you have only that broad hint where to look! The songs, behaviour, and habitat preferences of such species are unknown. Hence we do not know where to seek them, nor how to identify them if we glimpsed or heard them.

The status of many tropical species cannot be classified either as 'definitely extinct' or 'definitely in existence', but just as 'unknown'. Instead, it becomes a matter of chance which species happens to attract the attention of some ornithologist, becomes the object of a specific search, and hence may be recognized as possibly extinct.

Here is an example. The Solomon Islands are another of my favourite bird-watching areas in the tropical Pacific Ocean, and will be recalled by older Americans and Japanese as the site of some of the fiercest fighting in the Second World War. (Remember Guadalcanal, Henderson Field, President Kennedy's PT boat, the Tokyo Express?) The ICBP lists one Solomon bird species, Meek's crowned pigeon, as extinct. Yet when I tabulated all recent observations of all 164 known Solomon bird species, I noticed that twelve of those 164 species had not been encountered since 1953. Some of those twelve species are surely extinct, because they were formerly abundant and conspicuous. Several Solomon islanders told me that those birds had been exterminated by cats. Twelve species possibly extinct out of 164 still may not sound like much to worry about. However, the Solomons are in much better shape environmentally than most of the remaining tropical world, because they have relatively few people, few bird species, little economic development, and much natural forest. More typical of the tropics is Malaysia, which is rich in species and has had most of its lowland forest cut down. Biological explorers had identified 266 fish species dependent on fresh water in Malaysia's forest rivers. A recent search that lasted four years was able to find only 122 of those 266 species—less than half. The other 144 Malaysian freshwater fish species must either be extinct, rare, or very local. They reached that status before anyone noticed it.

Malaysia is typical of the tropics in the pressure it faces from humans. Fish are typical of all species other than birds, in that they attract only patchy scientific attention. The estimate that Malaysia has already lost (or nearly lost) half of its freshwater fish is therefore a reasonable ballpark figure for the status of plants, invertebrates, and vertebrates other than birds in much of the rest of the tropics.

That is one complication in trying to pinpoint the number of extinctions since 1600: the status of many or most named species is unknown. But there is a further complication. So far, we have been trying to assess the extinction only of those species that had already been discovered and described (named). Could any species have become extinct before they were even described? Of course they could, since sampling procedures suggest that the actual number of the world's species is near thirty million, but less than two million species have been described. Two examples illustrate the certainty of other species becoming extinct before description. Botanist Alwyn Gentry surveyed the plants of an isolated ridge in Ecuador called Centinela, where he found thirty-eight new species confined to that edge. Shortly afterwards, the ridge was logged and those plants were exterminated. On Grand Cayman Island in the Caribbean, zoologist Fred Thompson discovered two new species of land snails confined to forest on a limestone ridge that was completely cleared a few years later for a housing development. The fact that Gentry and Thompson accidentally visited those ridges before rather than after they were cleared means that we have names for those extinct species. But most tropical areas that are being developed are not first surveyed by biologists. There must have been land snails on Centinela, and plants and snails on innumerable other tropical ridges, that we exterminated before we discovered them.

In short, the problem of determining the number of modern species that have become extinct seems at first to be simple and to lead to modest estimates—for example, only five or six extinct bird species in all of North America plus Europe. On reflection, though, we appreciate two reasons why published lists of species known to be extinct must be gross underestimates of the actual numbers involved. Firstly, by definition the published lists consider only named species, whereas the great majority of species (except in well-studied groups like birds) have not even been named. Secondly, outside North America and Europe and except for birds, the published lists consist only of those few named species which some biologist happened to get interested in for one or another reason and found to be extinct. Among all those remaining species of unknown status, many are likely to be extinct or nearly so—for example, about half in the case of Malaysian freshwater fishes.

Now let's move on to the second step in evaluating the mass extinction debate. Our estimates up to this point have concerned only those species exterminated since 1600 AD, when scientific classification of species was beginning. These exterminations have taken place because the world's human population has grown in numbers, reached previously uninhabited areas, and invented increasingly destructive technologies. Did these factors spring up suddenly in 1600, after several million years of human history? Were there no exterminations before 1600? Of course not. Until fifty thousand years ago, humans were confined to Africa plus the warmer areas of Europe and Asia. Between then and 1600 AD our species underwent a massive geographic expansion that took us to Australia and New Guinea around 50,000 years ago, Siberia around 20,000 years ago, most of North and South America around 11,000 years ago, and most of the world's remote oceanic islands only since 2000 BC. We also underwent a massive expansion in numbers, from perhaps a few million people 50,000 years ago to about half a billion in 1600. Our destructiveness also increased, with the development of improved hunting skills in the last 50,000 years (Chapter Two), polished stone tools and agriculture in the last 10,000 years (Chapters Ten and Fourteen), and metal tools in the last 6,000 years.

In every area of the world that paleontologists have studied and that humans first reached within the last 50,000 years, human arrival approximately coincided with massive instances of prehistoric species' extinction waves. For Madagascar, New Zealand, Polynesia, and the Americas I have described those instances in the preceding two chapters. After people reached Australia, that continent lost its giant kangaroos, its 'marsupial lion', and other giant marsupials. Around the time that Indians reached North America 11,000 years ago, it lost lions, cheetahs, native wild horses, mammoths, mastodonts, giant ground sloths, and several dozen other large mammals. Mediterranean islands like Crete and Cyprus lost dwarf elephants and pygmy hippos, while Madagascar lost giant lemurs and flightless elephant birds. New Zealand lost its giant flightless moas, and Hawaii its flightless geese and dozens of smaller birds, when the Polynesians arrived around 1000 and 500 AD, respectively.

Ever since scientists became aware of these prehistoric extinction waves associated with human arrival, they have argued over whether people were the cause or just happened to arrive while animals were succumbing to climate changes. In the case of the extinction waves on Polynesian islands, there is now no reasonable doubt that Polynesian arrival in one way or another caused them. Bird extinction waves and Polynesian arrival coincided within a few centuries at a time when no big climate change was happening, and bones of thousands of roasted moas have been found in Polynesian ovens. The coincidence of timing is equally convincing for Madagascar. But the causes of the earlier extinction waves, especially those in Australia and the Americas, are still being debated. As I explained on America's extinction waves in Chapter Eighteen, the evidence seems to me overwhelming that humans also played a role in those prehistoric cases of extinction outside Polynesia and Madagascar. In each part of the world an extinction wave occurred after the first arrival of humans, but did not occur simultaneously in other areas undergoing similar climate swings, and did not occur in the same area whenever si rnilar climate swings had occurred previously.

Hence I doubt that climate did it. Instead, all of you who have visited Antarctica or the Galapagos Islands know how tame are the animals there, being unaccustomed to humans until recently. Photographers can still walk up to those naive animals as easily as hunters used to. I assume that the first arriving hunters similarly walked up to naive mammoths and moas elsewhere in the world, while rats that came with the first hunters walked up to naive little birds of Hawaii and other islands.

It is not just in those areas of the world previously unoccupied by humans that prehistoric humans probably exterminated species. Within the last 20,000 years species also became extinct in the areas long occupied by humans—in Eurasia, woolly rhinos, mammoths, and giant deer ('Irish elk') died out, and Africa lost its giant buffalo, giant hartebeest, and giant horse. These big beasts may also have been among the victims of prehistoric humans who had already been hunting them for a long time, but who now were able to hunt them with better weapons than ever before. Eurasia's and Africa's big mammals were not unused to humans, but they disappeared for the same two simple reasons that California's grizzly bear, and Britain's bears, wolves, and beavers, succumbed only in recent times, after thousands of years of human persecution. Those reasons were more people, and better weaponry.

Can we at least estimate how many species were involved in these prehistoric extinction waves? No one has ever tried to guess the number of plants, invertebrates, and lizards exterminated by prehistoric habitat destruction, but virtually all oceanic islands explored by paleontologists have yielded remains of recently extinct bird species. Extrapolation to those islands not yet paleontologically explored suggests that about 2,000 bird species—one fifth of all the birds that existed a few thousand years ago—were island species already exterminated prehistorically. That does not include birds that may have been exterminated prehistorically on the continents. Among genera of large mammals, about seventy-three, eighty, and eighty-six per cent respectively became extinct in North America, South America, and Australia at the time of or after human arrival. The remaining step in evaluating the mass extinction debate is to predict the futufe. Is the peak of the extinction wave that we have caused already past, or is most still to come? There are a couple of ways to assess this question.

A simple way is to reason that tomorrow's extinct species will be drawn from today's endangered species. How many species that still exist have populations already reduced to dangerously low levels? The ICBP estimates that at least 1,666 bird species are either endangered or at imminent risk of extinction—almost twenty per cent of the world's surviving birds. I said 'at least 1,666 , because this number is an underestimate for the same reason I mentioned that the ICBP's estimate of extinct species was an underestimate. Both numbers are based just on those species whose status caught a scientist's attention, rather than on a _^reappraisal of the status of all bird species.

The alternative way of predicting what is to come is to understand the mechanisms by which we exterminate species. Extinction of species caused by humans may continue accelerating until human population and technology reach a plateau, but neither shows any signs of plateauing. Our population, which grew ten fold from half a billion in 1600 to over five billion now, is still growing at close to two per cent per year. Every day brings new technological advances for changing the earth and its denizens. There are four main mechanisms by which our growing population exterminates species: by overhunting, species introductions, habitat destruction, and ripple effects. Let's see if these four mechanisms have plateaued.

Overhunting—killing animals faster than they can breed—is the main mechanism by which we have exterminated big animals, from mammoths to California grizzly bears. (The latter appears on the flag of California, the state in which I live, but many of my fellow Californians do not recall that we exterminated our state's symbol long ago.) Have we already killed off all big animals that we might kill off? Obviously not. While the low numbers of whales led to an international ban on whaling for commercial reasons, Japan thereupon announced its decision to triple the rate at which it kills whales 'for scientific reasons'. We have all seen photos of the accelerating slaughter of Africa's elephants and rhinos, for their ivory and horns respectively. At current rates of change, not just elephants and rhinos but most populations of most other large mammals of Africa and Southeast Asia will be extinct outside game parks and zoos in a decade or two. The second mechanism by which we exterminate is through intentionally or accidentally introducing certain species to parts of the world where they did not previously occur. Familiar examples of introduced species now firmly established in the US are Norway rats, European starlings, boll weevils, and the fungi causing Dutch elm disease and chestnut blight. Europe too has acquired introduced species, of which the misnamed Norway rat is an example (it originated in

Asia, not Norway). When species are introduced from one region to another, they often proceed to exterminate some of the new species they encounter, by eating them or causing diseases. The victims evolved in the absence of the introduced Pests and never developed defences against them. American chestnut trees have already been virtually exterminated in this way by chestnut ^b"ght, an Asian fungus to which Asian chestnut trees are resistant. milarly, goats and rats have exterminated many plants and birds on oceanic islands. Have we already spread all possible pests all around the world?

Obviously not; there are many islands still free of goats and Norway rats, and many insects and diseases to try to keep out of many countries by quarantines. The US Department of Agriculture has been trying at great expense, but apparently without success, to forestall the arrival of killer bees and Mediterranean fruit flies. In fact, what will probably prove to be the biggest extinction wave caused by an introduced predator in modern times has just started in Africa's Lake Victoria, home to hundreds of species of remarkable fishes found nowhere else in the world. A large predatory.fish called the Nile perch, intentionally introduced in a misguided effort to establish a new fishery, is now eating its way through the lake's unique fishes. Habitat destruction is the third means by which we exterminate. Most species occur in just a certain type of habitat: marsh warblers live in marshes, while pine warblers live in pine forests. If one drains marshes or cuts forests, one eliminates the species dependent on those habitats just as certainly as if one were to shoot every individual of the species. For example, when all the forest on Cebu Island in the Philippines was logged, nine of the ten birds unique to Cebu became extinct. In the case of habitat destruction, the worst is still to come because we are just starting in earnest to destory tropical rainforests, the world's most species-rich habitats. The rainforests' biological richness is legendary—over 1,500 beetle species living in a single rainforest tree species in Panama, for instance. Rainforests cover only six per cent of the Earth's surface but harbour about half of its species. Each area of rainforest has large numbers of species unique to that area. To mention only some exceptionally rich rainforests now being destroyed, the felling of Brazil's Atlantic forest and Malaysia's lowland forest is already almost complete, and those of Borneo and the Philippines will be mostly logged within the next two decades. By the middle of the next century, the only large tracts of tropical rainforest likely to be still surviving will be in parts of Zaire and the Amazon Basin.

Every species depends on other species for food and for providing its habitat. Thus, species are connected to each other like branching chains of dominoes. Just as toppling one domino in a chain will topple some others, so too the extermination of one species may lead to the loss of others, which may in turn push still others over the brink. This fourth mechanism of extinction may be described as a ripple effect. Nature consists of so many species, connected to each other in such complex ways, that it is virtually impossible to foresee where the ripple effects from the extinction of any particular species may lead.

For example, fifty years ago no one anticipated that the extinction of big predators (jaguars, pumas, and harpy eagles) on Panama's Barro Colorado Island would lead to the extinction there of little antbirds, and to massive changes in the tree species composition of the island's forest. Yet it did so, because the big predators used to eat medium-sized predators like peccaries, monkeys, and coatimundis, and medium-sized seed-eaters like agoutis and pacas. With the disappearance of the big predators, there was a population explosion of the medium-sized predators, which proceeded to eat up the antbirds and their eggs. The medium-sized seed-eaters also exploded in abundance and ate large seeds that had fallen on the ground, thereby suppressing the propagation of tree species producing large seeds and favouring instead the spread of competing tree species with small seeds. That shift in forest tree composition is expected in turn to cause an explosion of mice and rats feeding on small seeds, and then to an explosion in hawks, owls, and ocelots preying on those small rodents. Thus, the extinction of three uncommon species of big predators will have triggered a rippling series of changes in the whole plant and animal community, including the extinction of many other species.

Through these four mechanisms—overhunting, species introductions, habitat destruction, and ripple effects—probably over half of existing species will be extinct or endangered by the middle of the next century, when this year's crop of human babies reaches the age of sixty. Like many fathers today, I often wonder how I will describe to my twin sons, who are now three years old, the world that I grew up in and that they will never see. By the time they would have been old enough to come with me to New Guinea, one of the world's biological treasurehouses where I have worked for the past twenty-five years, most of New Guinea's eastern highlands will be deforested. When one adds the extinction of species we have already caused to that which we are about to cause, it is clear that the current extinction wave is surpassing the asteroid collision that may have wiped out the dinosaurs. Mammals, plants, and many other types of species survived that collision nearly unscathed, while the current wave is affecting everything from leeches and lilies to lions. Thus, the claimed extinction crisis is neither a hysterical fantasy, nor just a serious risk for the future. Instead, it is an event that has already been accelerating for 50,000 years and will start to approach completion in our children's lifetimes.

Let's finally consider two arguments that accept the reality of the extinction crisis but dismiss its significance. Firstly, is extinction not a natural process anyway? If so, why make a big deal about the wave of extinction happening now?

The answer to this first argument is that the current extinction rate caused by humans is far higher than the natural rate. If the estimate that half the world's total of thirty million species will become extinct in the next century is correct, then species are now becoming extinct at a rate of about 150,000 per year, or seventeen per hour. The world's 9,000 bird species are becoming extinct at a rate of at least two per year, but bird species under natural conditions were disappearing at a rate of less than one per century, so the present rate is at least 200 times the normal rate. Dismissing the extinction crisis on the grounds that extinction is natural would be just like dismissing genocide on the grounds that death is the natural fate of all humans.

The second argument is a simple one: so what? We care about our children, not about beetles and snail darters; who cares if ten million beetle species become extinct? The answer to this argument is equally simple. Like all species, we depend on other species for our existence, in many ways. Some of the most obvious ways are that other species produce the oxygen we breathe, absorb the carbon dioxide we exhale, decompose our sewage, provide our food, maintain the fertility of our soil, and provide our wood and paper.

Then could we not preserve only those particular species that we need, and let other species become extinct? Of course not, because the species we need also depend on other species. Just as Panama's antbirds could not have anticipated their need for jaguars, the ecological chain of dominoes is much too complex for us to have figured out which dominoes we can dispense with. For instance, could anyone please answer these three questions. Which ten tree species produce most of the world's paper pulp? For each of those ten tree species, which are the ten bird species that eat most of its insect pests, the ten insect species that pollinate most of its flowers, and the ten animal species that spread most of its seeds? Which other species do these ten birds, insects, and animals depend on? You would have to be able to answer those three impossible questions if you were the president of a timber company trying to figure out which species you could afford to allow to become extinct.

If you are trying to evaluate some proposed development project that would bring in a million dollars but might exterminate a few species, it is still tempting to prefer the certain profit over the uncertain risk. Then consider the following analogy. Suppose someone offers you a million dollars in return for the privilege of painlessly cutting out two ounces of your valuable flesh. You figure that two ounces is only one-thousandth of your body weight, so you will still have nine hundred and ninety-nine thousandths of your body left, which is plenty. That is fine if the two ounces come from your spare body fat and if they will be removed by a skilled surgeon. But what if the surgeon just hacks two ounces from any conveniently accessible part of your body, or does not know which parts are essential? You might then find that the two ounces came from your urethra. If you plan to sell off most of your body, as we now plan to sell off most of our planet's natural habitats, you are certain eventually to lose your urethra.

To conclude, let's place matters in perspective by comparing the two clouds which, as I mentioned at the outset, are hanging over our future. A nuclear holocaust is certain to prove disastrous, but it is not happening now, and it may or may not happen in the future. An environmental holocaust is equally certain to prove disastrous, but it differs in that it is already well underway. It started tens of thousands of years ago, is now causing more damage than ever before, is in fact accelerating, and will climax within about a century if unchecked. The only uncertainties are whether the resulting disaster would strike our children or our grandchildren, and whether we choose to adopt now the many obvious countermeasures.