The Great Divide

by Peter Watson

  Over this 14,000-year period (28,000–14,000 years ago), as had happened several times before, water was removed from the seas in enormous quantities by evaporation, with the clouds formed being blown over the land by winds, there to fall as precipitation, usually snow, contributing further to the build-up. Eventually, the glaciers held around one-twentieth of the world’s water, and half of that was to be found in the Laurentide Ice Sheet. As a result, the sea eventually dropped by about 125 metres, or 400 feet (not 180, as Johnson said) below where they are now.9 As the waters receded, Asia and North America ‘began to reach for each other like the outstretched arms of God and Adam on the ceiling of the Sistine Chapel. When the finger tips touched, a charge of new life streamed into the Americas.’10 An imaginative analogy but the contact was achieved with geological slowness. Gradually, the shelf widened until, at 18,000–14,000 years ago – the height of glacial activity, or LGM, for Late Glacial Maximum – the shelf between Alaska and Siberia was exposed as dry land for over nine hundred miles, north to south. ‘North America and Asia were joined at the head like great, sprawling Siamese twins (see maps 1, 5 and 7)’.

  There was more to it than a simple land bridge forming. Every time the sea levels fell and rose, Alaska effectively switched continents. During the cold cycles, when the land bridge was exposed, glaciers formed in Canada, and Alaska was cut off from the New World – it became, in effect, the eastern end of Siberia. When the glaciers melted, and sea levels rose, dividing Alaska from Siberia, she became what she is today, the north-western tip of North America.11

  Geological studies, involving drilling cores of both land and ice, show that there have been some sixteen Ice Ages in the last million years alone, separated by ‘inter-glacials’. The Bering Land Bridge would have connected the continents during most of these glacials, when animals – if not humans, who had yet to evolve or reach Siberia – could have switched between the New and Old Worlds.12 The last land bridge is of most interest precisely because humans were around at that time. This period endured from, roughly speaking, twenty-five thousand to fourteen thousand years ago.

  Even without ice, it was a harsh landscape, dry and windy. Loess (windblown glacial silt) built up in great grey dunes. Vegetation was thin, the land little more than a polar desert, a drier version of today’s tundra. Despite this, Beringia – as it came to be called – was home to a variety of animals. Or that is what research shows. Woolly mammoths were probably the biggest of the beasts that roamed the region. Their six-inch-thick hairy coats, hanging in ragged skirts, offered sufficient protection against the cold and the bitter winds. Ground sloths, weighing as much as six thousand pounds, and the long-horned steppe bison, were almost as big. Horses, which evolved in North America, migrated across the land bridge going the other way, into Asia, but they almost certainly had much thicker coats than today’s horses. Various forms of antelope, moose, caribou and sheep – all these inhabited the land bridge during the Ice Age. And so too did huge sabre-toothed tigers with their six-inch-long canines capable of piercing the thick hides of the mammoth and bison, plus giant lions and packs of timber wolves. An ancient form of bear, bigger even than today’s Alaskan grizzly, completed this exotic bestiary (see figure 2).13

  Fig. 2 A comparison of body sizes of the prehistoric bear, Arctodus simus (shaded) and the modern grizzly, Ursus arctos horribilis.

  The identification of this plant and animal wildlife was itself an achievement of scholarship. Eric Hultén, the Swedish botanist referred to earlier, studied the plants of Siberia and Alaska in the 1930s, to produce his Flora of the Aleutian Islands. Hultén was statistically minded and, as well as describing the plants, he noted their distribution, in particular their spread around Canada’s Mackenzie River and Siberia’s Lena River.14 Plotting these distributions on a map, he saw that they spread out in a series of ovals, stretching east-west. Moreover, these ovals, as well as being symmetrical, were also concentric, and the axis of symmetry was always located in a line drawn through the Bering Strait. ‘If they spread a little bit to the east of this line, they also spread a short way to the west of it. Should they spread far east, they also spread far west.’ A consistent picture therefore suggested itself to Hultén. He imagined there must have been at one time a dry landmass ‘stretching from mostly unglaciated Siberia into mostly unglaciated Alaska’. This landmass was isolated from the much colder areas around it and therefore acted as ‘a great biological refugium, a place where northern plants and animals survived extinction and from whence they evidently spread when the glaciers receded’.15 The area was for him ‘a broad highway of biological exchange’, with animal and plant traffic travelling in both directions. Wanting to emphasise the importance of the area, he gave it the name Beringia, after Vitus Bering who was actually Danish-born and therefore a fellow Scandinavian. (Bering had sailed through the strait in 1728 but had not himself reached Alaska until after Fedorov and Grozdev.)

  Hultén’s work was soon built on by Louis Giddings, a Texan archaeologist who, in the 1940s, at Cape Krusenstern, south-east of Point Hope, identified ‘no fewer than 114 beach ridges each parallel to the shore line and extending more than three miles inland’. Not only that, each ridge furnished a series of archaeological discoveries in which the outer ridges (closer to today’s coastline) were older than those further inland. The natural conclusion was that these seafaring cultures had transferred inland by stages as the seas rose and encroached on their dwellings, showing neatly how human habitation and sea level were intimately linked.16

  But the individual who did more than anyone else to advance the scientific understanding of Beringia was David M. Hopkins, a graduate of the University of New Hampshire. Together with William Oquilluk, a famous Iñupiat historian from north Alaska, Hopkins’ first project was a study of fossil mollusc shells, because he understood that their distribution and sedimentation would reveal when the Bering Strait was open and when it was not. The basis for this study was the natural history of the giant snail Neptunea. This has existed in the north Pacific all the way back to the Tertiary geological epoch, some sixty-five million years ago. But there was no evidence of Neptunea in the Atlantic sediments until the early Pleistocene, about a million years ago. What this suggested was that a land bridge blocked marine migration for most of the Tertiary period and was only flooded out at the start of the Pleistocene, allowing Neptunea to move north and east and to reach the Atlantic.17

  Oquilluk introduced Hopkins to many shell deposits, and as a result they collaborated on a seminal paper, published in Science in 1959, dedicated to the Bering Land Bridge. Their conclusion was that the bridge had been in existence throughout most of the Tertiary era (from sixty-five to two million years ago), that the evidence from fossil fishes suggested that a waterway cut through the bridge in the middle of the Eocene (about fifty million years ago), and that it was fully submerged around a million years ago. Since then, the bridge had appeared and been submerged numerous times as the Ice Ages had come and gone. The land bridge disappeared for the last time, they said, some 9,500 years ago.18

  Details of this broad picture have been fleshed out by others. Around the turn of the century, the vertebrate palaeontologist, W.D. Matthews, from the American Museum of Natural History in New York, observed that though mammals migrated across the land bridge in both directions, far more travelled from the Old World to the New than vice versa. Presumably, he said, this had to do with the fact that Eurasia is a much larger landmass than the Americas and extends much further in an east-to-west direction, in north temperate latitudes. This would mean that more populations of Old World animals could evolve separately. Their relatively similar environmental conditions would provoke more varied adaptive strategies, meaning there were more discrete species to take advantage of the land bridge. In North America, heavily glaciated during the Ice Ages, only the relatively small area of Alaska was available for species to evolve.19*

  Charles Repenning, of the US Geological Survey, took Matthews’ reasoni
ng further. Based on a detailed study of fossil mammals, he argued that there were three, and possibly four, ‘surges’ of animal migration across the land bridge into the New World. The first influx, he said, occurred at about twenty million years ago, the types of fossil suggesting that the land bridge then was ‘warm-temperate, humid and forested’. A further surge took place in the middle Pleistocene, roughly 750,000–500,000 years ago, when the bridge consisted mainly of temperate grasslands, though with some forest. Not until the late Pleistocene (125,000–12,000 years ago) did mammal remains suggest arctic conditions, ‘with a flora dominated by tundra, steppe, and the scrubby northern forest called taiga’.

  A very different kind of evidence comes from two colleagues of Hopkins, Joe Creager and Dean McManus, who found the vestiges of an ancient river at the bottom of the Bering Strait. They drilled the seabed south of Cape Thompson, near Point Hope, and recovered core sediments that indicated a ‘brackish-deltaic’ river. Apparently they had struck an ancient estuary formed by a river that linked in with today’s Kobuk and Noatak Rivers of north-west Alaska. Radiocarbon dating put the estuarine remains at twelve to fourteen thousand years ago. This was named the ‘Hope Seavalley’.21

  Although the picture that has been gradually revealed has by and large proved consistent, one important area of disagreement has concerned the vegetation on the land bridge. There are two views. According to one side, the analysis of pollen shows that ancient Beringia was only sparsely populated, by herbaceous tundra at the higher levels and sedge-grass meadows in the lower locations. If so, this would have been sufficient to support only primarily rodents, and does not square at all with the fact that, at the same time, elsewhere in Alaska that was unglaciated during the last Ice Age, fossils of large mammals, especially bison, horse and mammoth, have been found.22 On top of it all, R. Dale Guthrie, now an emeritus professor at the University of Alaska Fairbanks, has observed that the three most common species – bison, horse and mammoth – were gregarious, existing in herds. According to this version, then, Beringia was populated by many large mammals, not just rodents.

  Matters began to be resolved in the 1970s, when Siberian gold miners, digging underground near the Selerikan River, stumbled upon the remains of an ancient horse, still frozen in the earth. The miners informed the Siberian Academy of Sciences who excavated the animal and found it to be ‘a beautifully mummified specimen of a stallion’ which, unlike today’s stallions, had a two-inch thick coat, ‘light yellowish colour underneath and a coffee brown above, with a black mane and a dark streak running all along its spine to its black tail’. Dated to 37,000 years ago, the most interesting thing about the remains, from our point of view, was that its preserved stomach contents were found to be 90 per cent herbaceous material, two-thirds grasses and one-third sedges. Other mummified mammals were discovered subsequently (baby mammoths, for instance), and fossil insects, and the Siberian findings were confirmed, indicating a landscape of dry substrates, but with no trees, generally suitable for ungulates (hoofed mammals).23 On top of this, the hooves of such fossil ungulates as have been found indicate that they lived on firm, dry ground, enabling them to escape their predators by running.24 Even the remains of plant fragments stuck between the teeth of fossil ungulates show that grasses dominated their diet by a wide margin. Beringia at the LGM was essentially a ‘steppe’ environment.

  The final confirmation of the Beringia landscape was discovered by Hopkins himself. In 1974, near a lake at Cape Espenberg on the northern Seward Peninsula, he came across a layer of tephra, or volcanic ash, that was fully one metre deep and had congealed within it masses of twigs and tufts of grass. The nearby lake was in fact a maar or circular lake of the kind produced when a volcano erupts at ground level. Hopkins was aware of previous research which had established that this maar, Devil Mountain, erupted 18,000 years ago, at a time when the land bridge existed. It therefore followed logically that the twigs, roots and grasses congealed in the tephra constituted land bridge vegetation. Examination of this plant material showed it to be a dry meadow and herb-rich tundra, a mix of herbs and grasses, in particular the sedge Kobresia, plus the occasional willow, and a carpet of mosses. This amalgam of Kobresia-dominated vegetation has since been discovered in the stomachs of several preserved mammals and confirms the steppe-type landscape. Hopkins, who died in 2001, held firmly to the view that the Bering Land Bridge could support whole herds of grazing animals, and the predators who sought them out.

  Computer models carried out more recently confirm Hopkins’ claim that areas of Beringia that were submerged by rising sea levels were more productive biologically/nutritionally than the higher areas that remained above the water, that there may have been a climate refuge in central Beringia for a time, perhaps causing a migration ‘bottleneck’, and that climate varied between east and west Beringia, changing the vegetation and animal life dependent on it and forcing people eventually to migrate. In general the climate change would have favoured the wapiti or elk, and contributed to the demise of the mammoth and horse.25

  Importantly, this environment meant that the land bridge would have supported humans.

  ANIMALS WHO HAD NEVER MET HUMANS

  When early man entered Beringia, it was, as we have seen, the eastern extension of Siberia. As sea levels rose around 14500 BC, and Siberia was cut off, Beringia became part of what is now Alaska. And sea levels rose, of course, because the world was warming up, the Ice Age was coming to an end, and glaciers all over the world were beginning to melt. For early man in Alaska, this had two direct consequences. In the first place, as map 7 shows, an ice-free corridor opened up between two enormous glaciers – the Laurentide and the Cordilleran – which covered most of Canada and Northern America down beyond the Great Lakes. Second, as the glaciers retreated, and the weight of ice over the land lessened, the North American continent (as did others) rose and beaches appeared along the coast. In this way, two routes southwards opened up by which means early men and women could move into new territory.

  Opinions differ as to what was the actual route taken. As was mentioned in an earlier chapter, the mtDNA study by Sijia Wang and colleagues showed that genetic diversity is greatest among the Pacific coast Indians, indicating that the seashore was the probable route. On the other hand, Gary Haynes, professor of anthropology at the University of Nevada at Reno, favours the ice-free corridor, he says, partly because, although this passageway would have been cold and sparsely vegetated at first, ‘with ice-blocked lakes and wetlands covering most of it’, migratory birds would have acted as ‘scouts’, ‘beckoning’ humans to follow their movements to the south since the birds returned every year to where they had been. Haynes argues that traversing the coast in small boats would not have worked, since the populations would have been too small to be viable. C. Vance Haynes, at the Department of Geochronology at the University of Arizona at Tucson, imagines as plausible a trek of 5,700 kilometres from the Tanana Valley of Alaska to Anzick in Montana, where the biotic environment would have been suitably rich, a journey, he says, which could have been completed in six to twelve years, or maybe less.26 The new genetic evidence, alongside the continuous existence of kelp beds around the northern Pacific coastal rim, remains persuasive however.

  Then, after a few tentative forays, Gary Haynes says a further move south would have been attempted. The landscape then was very different from now. It was dominated by the massive lakes that developed along the edges of the ice sheets.27 These were expanses quite unlike anything anywhere else, both then and now. Lake Missoula, at the southern edge of the Cordilleran ice sheet, was the size of Lake Ontario today, but was dwarfed by Lake Agassiz, to the west, which lasted from 12000 BC to 8000 BC and was four times the size of what Lake Superior is today, equivalent in area to Ireland or Hungary.

  The geography was unstable, too, in a way that is much less true today. As the ice melted, many shrinking glaciers, or ‘calves’ of glaciers, formed blockages in valleys, keeping lakes dammed behind them. But as
they shrank, there came a point where they simply gave way, and the water was released in unpredictable massive floods, yet another litany of catastrophes to be remembered, possibly, in myth.

  So the northern reaches of North America had their dangers, as between the unstable and inhospitable landscape and the predatory animals. Valerius Geist finds an inverse relation between human sites in North America and the extinction of megafauna, leading him to ask if the great carnivores, especially the prehistoric bear, Arctodus simus, kept humans out of the New World for thousands of years before dying off (see figure 2). Humans are known to have met Arctodus at only one site but are also known to have entered North America along with moose, caribou, timber wolf and glutton, so maybe there is something to this (and confirming that the ice did block humans and other animals for a time). There is evidence that Arctodus was more aggressive than modern bears – it was more often caught in traps whereas modern bears tend to avoid humans. And Native North American tribes traditionally did not like hunting bears.28 They thought they were the animal that most resembled humans, that they were wiser, and could overhear human conversations across vast distances and were, perhaps, shamans themselves inhabiting bear form.29