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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 arrivalperhaps only a few centuriesmost 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 exterminatedpossibly 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 bridgethere 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 slaughteronly 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 taskany 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.