Out of Eden: The Peopling of the World

by Oppenheimer, Stephen

  The overall distribution of the founding maternal group A shows a clear decline from near 100 per cent in Subarctic North America to generally low frequencies in South America. The more detailed picture of A in the rest of North America is one of great variability, between zero and high frequency. Several studies of ancient DNA in the Americas have revealed peoples completely lacking in Group A, suggesting that the older picture was one of even greater differences. A northern example was the Fremont cultures of Great Salt Lake, while another was in the extinct Fuegan tribes of the tip of South America (discussed earlier), who had only C and D.75

  The distribution of X, as we have seen, is an exaggeration of the A picture in Amerinds, as X is confined to two ethnic groups in the far north of North America. Group D, however, is found at low frequencies in North America but very high frequencies in South America, in particular the equatorial region.

  In summary, the extreme variation seen in the regional frequency of the five founder lines throughout America, could support a view of separate ethnic strands in the original colonization (although there are other explanations).

  A west-coast route into America?

  I have left the distribution of the most interesting mtDNA group, B, to the end. As we have seen, Group B is absent above the 55th parallel but present in North, Central, and South America. This odd southern distribution has often been put forward as proof that B arrived in the Americas along with the other lines and before the ice age. While I agree with these arguments, there is still a question mark over the exact age of Group B. Torroni found B to be significantly younger than A, C, or D; in Forster’s re-analysis, B remained younger in Central America. She is old in South America, where she has a very variable frequency, from 0 to over 70 per cent. While these discrepancies in age and distribution may be just the effects of inadequate sampling, bottlenecks, or drift, there are several other intriguing possibilities. For example, Group B4 is extremely common and diverse in East Asia. So more than one B4 colonization at different times by related twigs might not be obvious in the American genetic record without more detailed sequencing.76

  Alternatively, B4 may indeed have been a separate or last-minute entry to the Americas after A, C, D, and X. This is what the Russian geneticist Yelena Stariovskaya suggests. Her argument is that, apart from the younger age of Group B, she has high frequencies in Central America and in regions where Clovis tools are found, and is absent from the far south of South America. Group B is absent from Ne-Dene-speakers of the Subarctic west coast (see above), but present amongst Amerind speakers farther south. Perhaps to explain her absence from the Subarctic and the problem of the ice barrier, Stariovskaya also suggests that B came in separately along the west coast of North America. Given that there is at least one close B4 match for a Piman Indian B4 in Japan, further complete mtDNA sequencing studies on samples from both sides of the Pacific would be the way to answer these questions (see Figure 7.5).77

  The idea of a coast-hopping fast migratory track from Beringia and down the west coast of the Americas has a respectable vintage and is coming back into fashion. First raised thirty years ago by Knut Fladmark of Simon Fraser University, British Columbia, the idea is an attractive explanation for the rapid colonization of South America. The problem has always been lack of evidence. Most of the beaches such a hypothetical migration would have used are now well beneath the sea. Only coastlines where there is a steep continental drop-off would be less affected by sea-level changes. Two sites on the southern Peruvian coast, Quebrada Jaquay and Quebrada Tacahuay, have just that. The bones of butchered birds and fragments of seashells (clams and mussels), crustaceans, and anchovies tell the story of sophisticated fishers and beachcombers exploiting the South American coastline 11,000 years ago at the same time as the Clovis hunters were valiantly trying to extinguish all northern big game. Evidence is also emerging from the North American west coast of human occupation and beachcombing at the same time.78

  Some of the evidence for the viability of this now-submerged route soon after the LGM comes from another large mammalian omnivore, the bear. UCLA biologist Jennifer Leonard and Alan Cooper, New Zealand specialist in ancient DNA working at Oxford University, have been tracing North American brown bear mtDNA, both modern and ancient, to see what happened to bears in Alaska and the north-west coast at the time of the big freeze.79 There are close mtDNA analogues between brown bears of parts of East Eurasia and the USA, suggesting that Eurasian brown bear mtDNA lines entered Alaska through a preglacial corridor via Beringia. Bears are known to have persisted both in Beringia and on Prince of Wales Island, off south-east Alaska, through the LGM, although like humans, their diversity became reduced.

  Before the LGM there were no brown bears in southern Canada, but they had spread across there by 13,000 years ago. Their mitochondrial group suggests that they arrived there, not from Alaska through the ice corridor, or by re-expansion from Alaska after the melt, but from refuges on west-coast islands. As near extinction loomed, some bear clans must have moved from Alaska down the west coast, where they have survived to the present day. What is key about the brown bears now in the Canadian and US Rockies is that their particular mtDNA clan belongs to a line that was in Alaska 35,000–45,000 years ago and is now extinct there. In other words, they expanded from an ancient Beringian resident population. Since bears and coastal humans have an omnivorous diet that overlaps considerably, the bear story may be pointing us to the route that could have been taken by humans 12,000–15,000 years ago.

  Human remains and artefacts on Prince of Wales Island have been dated to 9,300 years ago and earlier. Some 3,200 km (2,000 miles) to the south, Jon Erlandson has unearthed evidence of beach-combing dating to perhaps 11,600 years ago in Daisy Cave, on San Miguel Island in the Santa Barbara Channel, southern California (see Figure 7.1). Clearly, to cross the 40 km (25 miles) from the mainland to the island, some form of craft must have been used. Radiocarbon tests from a woman’s bones found on nearby Santa Rosa Island give her age as 13,000 years.80

  How early was the coastal route free of ice and open? From 14,000 years ago, it seems. Daryl Fedje of Parks Canada. British Columbia, and Heiner Josenhans of the Geological Survey of Canada have used high-resolution sonar to make a detailed map of the ocean floor off the north-west coast of Canada.81 The area they chose is around the Queen Charlotte Islands, just south of Alaska. Their map shows a new world of former rivers meandering down around flood plains and ancient lakes. The plains would all have been above sea level and free of ice from after 14,000 years ago for a few thousand years – that is, until the continuing sea-level rise drowned the land again. Armed with this new map of the drowned coastline, the researchers went out to collect wood from the flooded forests. They pulled up the stump of a pine tree and other bits of wood from the ocean floor, which they carbon-dated to 12,200 years ago; they even found remains of edible shellfish of the same age. From a slightly younger beach of 10,000 years at 60 metres (200 feet) below sea level they found a stone tool, the earliest tool on the north-west coast of America.

  Putting this all together, it does seem that from 14,000 years ago there was a way for humans to move from Beringia and Alaska and down the west coast of America, completely bypassing the ice caps. There is also evidence that they were there at least 10,000 years ago. Furthermore, the evidence from the west coast of South America shows that the same beachcombers, with the aid of coastal vessels, could have made it all the way down from Beringia by 11,000 years ago.

  The west-coast route gives one geographical explanation for the late spread of Group B as advocated by Yelena Stariovskaya. B4, the American founder line, was also spreading into the islands of eastern Indonesia, presumably by boat and far to the south in the south-west Pacific, around 17,000 years ago (see Chapter 6). The coastal bypass route also has the unexpected potential of making the ice barrier and ice corridor dates irrelevant to the Clovis story. When I visited Knut Fladmark at Simon Frazer University, Vancouver, I was surprised to find he was
a convinced Clovis-first conservative; but the Clovis-first position is to some extent shored up by the bypass.

  Pre-Clovis only

  Can we put this back into the larger genetic picture of the peopling of the Americas? I think we can. In spite of the sparse evidence, I find the story of the alternative west-coast trail compelling as a parallel route of colonization, but not the only one. For Group B4 to have been on the west coast of Canada 12,000–15,000 years ago, it seems to me more likely that – as with the ancestors of the west-coast brown bears – their ancestors were already resident in Beringia with the other lines (A, C, D, and X) before the LGM. The alternative, that B4 raced out of Asia along the coast only just after the LGM, would not have been feasible: there were major coastal barriers, such as the Aleutian ice sheet, to negotiate between the East Asian coast and British Columbia during the LGM and its immediate aftermath.

  So, I still go for the entry of all the ancestors of the Native Americans to Beringia and the Americas before the LGM and before Clovis. The genetic picture for A, B, C, D, and X founders suggests multiple parallel entries into the Americas before the LGM via different routes, by pioneer groups coming ultimately from the north-eastern Eurasian steppe and the east coast of Asia. They may have looked variously like Europeans and like the Ainu and some Pacific islanders. Also at this stage, some, like the ancestors of Wizards Beach Man, looked more like Northern Mongoloids and recent Native Americans. They spread throughout the Americas, perhaps more rapidly via coastal beachcombing. The B4 line (or one particular B4 line) may have entered at this early point and made it down to South America, where, in contrast to her relative youth in North America, she is now as old as the other founders. Then came the ice age. Those left in Beringia north of the ice caps went through the most dreadful privations, but descendants of Group A survived the deep freeze to emerge as skilled fishermen, the ancestors of the Na-Dene and Inuit-Aleut speakers. The B4 group may have sat out the ice age on the west coast and then re-expanded inland, like the brown bears, or there might possibly have been a fresh introduction of Group B from Asia running up, round, and down the Pacific Rim coastline. As far as we know, however, America and Alaska (the remnant of the lost continent of Beringia) had no further significant Asian genetic input from the time of the LGM until the time of Leif Eriksson and the human tidal wave that followed Columbus and his ‘discovery’ of the New World.

  EPILOGUE

  SEVEN MILLION YEARS AGO, cool dry weather devastated the habitat of forest-dwelling ape species. Some time thereafter, the first evolutionary steps were taken towards the two-legged, large-brained creature we call Homo sapiens. Palaeontologists have yet to agree on exactly when anatomical evidence for bipedalism appears in the fossil record, and whether the split with the ancestors of chimps occurred 5 or 7 million years ago, but those steps clearly were taken. While the first walking ape to evolve, Australopithecus, had the same moderately large brain as chimps, this too changed as further genera evolved. With the intensification of the dry cool phase a little over 2 million years ago began a dramatic growth of the brain, which happened only in humans who appeared around that time and their sister-genus Paranthropus. The rapidity of that initial change was never to be repeated.

  Biology and culture: coevolution

  Something new these two new genera of hominids were doing gave them both a special advantage in this period of increasing aridity. The new behavioural resource did not seem to be linked to a specific diet since subsistence differed widely between the two genera, but it selected for, thus presumably benefiting from, a larger brain. During this phase of human evolution our brains grew rapidly while our bodies changed little. The rate of proportionate increase in brain size was at its maximum near the birth of the Homo genus, supporting the implication from Paranthropus that our pre-human ancestors already possessed that unique new behaviour which subsequently drove human evolution. The most obvious candidate ‘unique behaviour’ which would benefit from a large brain is the same one which still separates us from all other living species, namely speech. But, in the absence of prehistoric cassette recorders, the most obvious physical evidence of cultural change was that, from the start, humans fashioned stone tools.

  In spite of the rapid brain enlargement, progress in tool-making was painfully slow for a million years and new technology did not automatically follow the appearance of each new human species. Acheulian-type stone tools were invented by African Homo erectus 1.4 million years ago, but this was long after the ancestor of Asian erectus had left the home continent. Acheulian technology therefore did not enter Eurasia until the next exodus.

  Humans as mammals

  The human story over the last 2.5 million years has been punctuated by great leaps in technology and world exploration separated by long periods of fallow. A common perception is that our ancestors were climbing an evolutionary stairway of achievement and ability, on which each new step had been unavailable to their immediate ancestors. From this Utopian standpoint we are merely the latest in a long line of ever-improving models. This view carries with it the implication that, all along, we have been the masters of our destiny, our limits of colonization set only by the intelligence and resourcefulness of whichever of our species was dominating the planet at any given time. Such an optimistic view of our intelligent self-determination is overstated.

  One of the first humans, Homo erectus, made it rapidly out of Africa to colonize the whole of Eurasia. They were not, however, the first ape to do so, as can be seen from the orang-utans and gibbons that inhabit Southeast Asia. Nor were they the last humans to make the exodus before ourselves. Compared with earlier humans, our recent ancestors’ only additional globetrotting exploits were to have reached the Americas and the Antipodes. Our recurrent expansions were also mirrored by other mammalian genera, and were determined mainly by climate and geography, following the two well-worn paths out of Africa. We differ from other large mammals, but not much from rodents, in the great variety of habitats we now occupy and in the population densities we have achieved. In this context, we differ from rodents only in that, being large, we consume vastly more resources per individual.

  Human movements out of Africa via the northern and southern routes were always determined by the climate cycle and the availability of resources. The grinding glacial cycle not only opened and closed the gates out of Africa but also periodically squeezed local populations through the mangle of near-extinction to produce new, larger-brained humans to stay at home or leave their African birthplace to try their luck elesewhere.

  Our brains stopped growing long ago

  In Africa by 1.2 million years ago the brains of Homo rhodesiense had grown to within 6 per cent of the volume of modern humans. Around 300,000 years ago, the climate-driven brain-growth machine reached a plateau of size 11 per cent above that of today’s people. Since then our brains and bodies have got smaller. The glacial cycles of boom, stress, and bust continued unabated; but except for cosmetic changes in limb proportions, eyebrows, and skull shape, the gross physical evolution of the human genus had by now slowed to a snail’s pace. Perhaps, as with cars, there was a law of diminishing evolutionary return, and it was no longer economical to build models with ever larger engines.

  As anthropologists Sally McBrearty and Alison Brookes argue persuasively, the real physical and behavioural threshold of Homo sapiens was reached at that point. Under this view, Anatomically Modern Humans are merely a later race which developed out of the older, so-called archaic Homo sapiens after another glacial near-extinction in Africa 150,000 years ago. There is evidence that archaic Homo sapiens types themselves also left Africa to colonize Eurasia long before we did.

  Cultural evolution took over

  In the picture McBrearty and Brookes draw, all the discriminating elements of behavioural modernity can be traced back to the African Middle Stone Age. That is not to say there was a technological big bang 300,000 years ago. Their evidence emphasizes the subsequent acceleration in human technology,
first slow, then faster and faster. The early advances were individually rather minor and late to appear, but as more and more knowledge began to be transmitted and accumulated down the generations as compound interest, cultural evolution began to leave genetic evolution far behind. Looked at another way, if cultural evolution really took over from genetic evolution 300,000 years ago, then the differences between us and them are merely cultural and archaic Homo sapiens individuals could well have the intellectual potential to put a man on the Moon if they were living among us today.

  The story of the genes: how it helps

  What can the new genetic tools tell us about ourselves and our ancestors that such perceptive palaeoanthropologists and archaeologists have not already sketched out? The answer is much, as I hope this book has shown. Genetic palaeontology brings clarity to a field of near-medieval confusion. The measurement of skulls and their shapes and the documentation of stone tools and their dates alone lead to an imperfect view of human prehistory. Apart from the paucity of Palaeolithic skeletal remains, there is enormous variation in human skull shape. The use of skull shape as a marker system to determine ethnic relationships has been further confused by the effects of under-nutrition and stunting in traditional agricultural societies and also by unknown proportions of admixture between different groups of humans. This all makes it quite easy to poke holes in reconstructions of the prehistory of human migrations based on such measurements. While stone tools are far more abundant than human bones, they are also a one-sided view and can only tell us about the received culture of their makers, not necessarily about their origins, migration routes, or biology.