The Journey of Man: A Genetic Odyssey

by Spencer Wells

  One of the things that the incoming Upper Palaeolithic Moderns had in their favour was a complex social structure. As we have seen, this probably began as an adaptation to cooperative hunting on the savannahs of east Africa. With their improved toolkits and bands of intelligent, social hunters, modern humans were much more efficient at hunting than the Neanderthals. This can be seen in the Neanderthal remains that have been found, all with extensive evidence of a harsh and physically difficult lifestyle. Most Neanderthals had broken bones, and many had quite extensive injuries that would have made them much less efficient members of the group. What modern humans accomplished with tools and brains, Neanderthals seem to have done with brute force. It was this physically demanding lifestyle that made them relatively short-lived. Few Neanderthals lived to be fifty, and most died in their thirties.

  Neanderthals had always had a very dispersed social structure, with a small number of distinct groups, each with its own local tool-making variants. Some anthropologists have even suggested that different Neanderthal groups may have spoken different languages, which would have contributed to the fragmentation of their population. Whether this is true or not, the dispersed, nuclear nature of the Neanderthal population probably represented an adaptation to the relatively harsh conditions of northern Europe during the last ice age. It allowed them to make use of the resources found over a wide territory, increasing their chances of locating food. It also, inadvertently, probably led to their demise.

  Anthropologist Ezra Zubrow has calculated that a reduction in fertility, or an increase in mortality, of 1 per cent would have led to the extinction of the Neanderthals within 1,000 years. This degree of change is entirely consistent with a model in which Neanderthals were gradually excluded from their food resources by incoming, highly efficient Upper Palaeolithic humans. As they became squeezed into smaller and smaller territories, the Neanderthals would have been less likely to obtain the resources they needed in order to survive. Eventually, their numbers reduced through attrition, they may even have had difficulty finding mates. Admittedly this is all conjecture, but it is entirely consistent with the data on the time of arrival of the first Upper Palaeolithic Europeans, the mitochondrial DNA evidence for their population expansion beginning around 30,000 years ago, and the disappearance of the Neanderthals at the same time.

  One feature of modern human behaviour that may have played a role in giving them an advantage over the Neanderthals was a by-product of the complex behavioural adaptations of modern humans. What probably began as Upper Palaeolithic hunting skills spilled over into complex social networks. This, coupled with their less physical lifestyles, would have given them a longevity advantage over the Neanderthals. Many Upper Palaeolithic people survived into their fifties, well past reproductive age. This gives us another clue as to why the Neanderthals were replaced: old people are good to have around.

  A reliance on teaching and learning, rather than instinct, is one of the things that distinguishes humans from other animals. Most of our early lives are spent learning, and it isn’t until we are well into our twenties that most of us feel that we are in command of sufficient knowledge to be able to synthesize and teach others. The older we get, the more knowledge we accumulate, and the more we can help our offspring to benefit from our experience. Grandparents, like university professors, have ‘been there and done that’ – and, crucially, lived to tell the tale. Having grandparents around also allowed higher fecundity, since (as any new parent can tell you) they can care for children while younger generations go about their lives. This includes continued childbearing – perhaps allowing that small advantage over Neanderthals that led to their extinction. Anthropologist Kristen Hawkes has suggested that grandmothering – the act of a child being cared for by its grandmother – may have played a substantial role in the population expansion of modern humans. Perhaps the small advantage it gave allowed modern humans to drive the Neanderthals to extinction.

  Stepping-stones

  Whatever the causes of their demise, Neanderthals had given up the ghost within a few thousand years of the arrival of modern humans. After 30,000 years ago, the only remains found in Europe are those of fully modern humans – often called Cro-Magnons, after the rock shelter in south-western France where some of the first bones were unearthed in 1868. These early Europeans were much more gracile, and significantly taller, than their Neanderthal neighbours. While Neanderthals were typically only around 165 cm (5 ft 6 in) tall, Cro-Magnons were often over 180 cm (6 feet), with long limbs. To palaeoanthropologists such as Erik Trinkaus, these proportions suggest an origin in a much milder climate. Neanderthals, as long-time residents of the colder regions of Europe, had quite stocky and muscular proportions. The implication is that the Cro-Magnons arrived in Europe from somewhere warmer.

  As we saw, lineages belonging to the Middle Eastern clan – which we would expect to find if there had been a straight shot out of Africa to Europe, via the Middle East – are hardly found at all in Europe. M173, our 30,000-year-old marker, has the advantage of being present at very high frequency in the most isolated European populations (including the Celts and the Basques), and its age corresponds roughly to the inferred date of modern human settlement based on archaeology. Other major Y lineages present in Europe are younger than M173, and thus arrived later, or descended from M173 itself. Thus M173 is the likely marker of the first modern Europeans, defining the European clan. Of course, it is simply the terminal marker in a long line of genealogical descent that traces back to M168 and our African Adam. The penultimate marker, though, actually solves the mystery of where the earliest Europeans came from. This marker, a stepping-stone on the way to M173, is M45 – making Europeans a subset of the central Asian clan.

  As we discussed earlier, the steppelands of 30–40,000 years ago stretched across a vast swathe of the Eurasian landmass. To Upper Palaeolithic hunters, this ecosystem would have been a land of plenty, and migration along it would have allowed modern humans to disperse well to the west, into Europe proper, as well as to the east into Korea and China. During this period, the steppe zone extended well into present-day Germany, and may have reached France. We know from bones that have been found in French caves of 30,000 years ago that reindeer – a species adapted to the cold steppe and tundra of northern Eurasia – were common in France around this time. The climate had opened a window into Europe that allowed these central Asian steppe hunters to enter. As we have seen, they soon took over, dominating the region within a few thousand years.

  It is likely that their sojourn on the steppes had honed their hunting skills, leading to innovations in technology that gave them a greater advantage over the Neanderthals than would have been possible if they had simply shot straight out of Africa. During the thousands of years they spent on the grasslands of central Asia they almost certainly underwent a period of intense cultural adaptation to this difficult environment. This period took the place of the hundreds of thousands of years of Neanderthal biological adaptation – what had given them their short, stocky frames. As recent migrants from tropical Africa, Upper Palaeolithic humans initially would have been ill equipped for life in the northern hemisphere. The central Asian steppes served as their apprenticeship, in a sense – preparing them for life in the most inhospitable environments on the planet. The caves of western Europe must have seemed relatively benign after the howling winds of the frozen Kazak grasslands.

  It is this honing process that may explain why the early Middle Eastern immigrants did not come to dominate Europe. While the mountains and forests of the Balkans would have been a bit of a barrier for a species adapted to the steppes, some early Middle Eastern immigrants clearly did get through. We can speculate that the low frequency of their Y-chromosome lineages belies a population that was not quite ready for the rigours of life in western Europe – but of course it is impossible to say with certainty. What is clear is that most European men, including me, trace their ancestry back to central Asia within the past 35,000 years. And
interestingly, this links us with a small population of Siberian hunters who – just as the last ice age was at its most intense – headed into the frozen tundra of north-eastern Asia.

  The final frontier

  Zaliv Kresta, the Bay of the Cross, is perched on the eastern edge of Russia, 10,000 km from Moscow. For six months of the year it is frozen solid, a mass of sea ice that isolates the small ex-Soviet settlement of Egvekinot from the rest of the world. The only way to reach it is by a two-hour helicopter flight from Anadyr, the nearest city with regular air connections to the outside world. From Egvekinot, it is a further eight-hour trek on military personnel carriers – armoured, with full tracks – inside the Arctic Circle to reach the reindeer herders living there. It feels like one of the most remote places on earth.

  The people who live in this harsh environment, known as the Chukchi, are wonders of adaptation. They have developed a lifestyle that allows them to exist in an environment of unimaginable harshness. When I visited them in November 2001 the temperatures were already plummeting to –50°C at night, and in the depths of winter they can reach –70°. The landscape is an other-worldly tundra, covered in snow and frost from September to June, and there is no edible vegetation. The Chukchi live entirely off of their reindeer and the fish they catch through holes in the icy rivers. They manage to do this with technology that has changed very little over the past few thousand years, sewing their clothes from reindeer skin and sinew, living in tents constructed of hides and wooden poles and migrating with their herds as they search for the succulent lichen tips that provide their only source of nourishment.

  Most of us who live in relative comfort in the modern world find it difficult to imagine how humans could exist in these conditions. And yet they do live – and thrive – in a climate that would probably kill most of us. Of all the hominids that have existed over the past few million years, it is only fully modern humans who have been able to live in the harsh Arctic. The conditions are simply too extreme to allow any mental leeway. Natural selection has favoured only those intellectually capable of surviving in this icy evolutionary laboratory.

  This is certainly why we see evidence of human occupation in the Asian Arctic only after 20,000 years ago. If modern humans reached southern Siberia around 40,000 years ago, as the genetic and archaeological data suggest, it would take them another 20,000 years before they had developed the cultural adaptations necessary to live in the harsh conditions of the Arctic. It is also possible that population pressures, which may have encouraged a northward migration, were not felt until this time. Whatever the reason, the earliest north-eastern Siberian sites, such as that at Dyuktai, south-east of Yakutsk, and Ushki Lake, in Kamchatka, date from after 20,000 years ago. The people living in Siberia during this time appear to have developed a tool-making culture that was distinct from that of populations living further to the south and west, consistent with their highly adapted lifestyle. They were particularly adept at making microliths, small weapon points, by striking both sides into a symmetrical ‘leaf’ shape. Similar types of stone points have also been found in the earliest excavated American sites, suggesting a direct continuity in culture between Siberia and the Americas.

  Anthropologists had assumed for many years that Native Americans and Asians have a common origin. Thomas Jefferson even stated the case in his 1787 book Notes on the State of Virginia:

  … if the two continents of Asia and America be separated at all, it is by only a narrow strait … and the resemblance between the Indians of America and the Eastern inhabitants of Asia, would induce us to conjecture, that the former are the descendants of the latter, or the latter of the former …

  Several anthropological traits – most famously the dental pattern known as sinodonty – are found in north-east Asia and the Americas. By the mid-twentieth century anthropologists such as Carleton Coon had even begun to classify Native Americans as ‘Mongoloid’ in their racial checklists. The problem was that no one knew exactly how long the Native Americans had been living there, and when they had split from their Asian cousins. In the 1950s carbon dating was used to infer an age of 11,000 years for the archaeological site at Clovis, New Mexico. The remains at Clovis contained leaf-shaped stone spear points in the same layer as extinct mammoth bones, which immediately suggested great antiquity to its discoverers. Over the next two decades, sites dating to roughly the same time period were excavated throughout North America. The pattern that seemed to be emerging from the archaeological record was that humans had occupied the Americans no earlier than 12,000 years ago.

  In the 1970s and 1980s, though, three archaeological digs – one in North America and two in South America – turned up evidence for a human presence before Clovis. The Meadowcroft Rockshelter in Pennsylvania yielded artefacts that were originally dated using radiocarbon to roughly 14,000 years ago, pre-dating Clovis by 3,000 years. The care with which Meadowcroft was excavated was impressive, and while the dates for the earliest occupation have been revised downward (to around 12,500 years ago), they are accepted by many anthropologists. The site at Monte Verde in southern Chile yielded similar dates to those at Meadowcroft, roughly 13,000 years ago, although nearby hearths have been estimated to be as old as 33,000 years. The earlier date has not been widely accepted, and thus Monte Verde is thought to date – like Meadowcroft – to around 13,000 years ago.

  The age of the remains at Monte Verde suggests that humans must have been in North America at least several hundred years prior to reaching Chile, and so pushes back the date of settlement a bit. But it was the final site that was the real bombshell. In a 1986 paper in the scientific journal Nature, archaeologist Niede Guidon summed up the find in the title: ‘Carbon-14 dates point to man in the Americas 32,000 years ago’. It was the result of her excavation of the cave of Boqueirão de Pedro Furada in north-eastern Brazil, and it seemed to pull the rug out from under the post-13,000 consensus. Careful examination, though, has failed to confirm Guidon’s results. The charcoal from the site, which provided a radiocarbon date and was thought by Guidon to be the remains of a fireplace, could have been produced by a natural fire. Furthermore, most of the crude stone artefacts discovered there do not look convincingly human in origin – they could easily have resulted from natural breakage. These doubts have led palaeoanthropologist Richard Klein to suggest that ‘Furada may soon join the long list of dubious claims [for early human settlement in the Americas]’.

  In summary, most of the reliable archaeological evidence points to a settlement of the Americas within the past 15,000 years. There is one small problem with this scenario, however: it was at this point that the ice age was at its most intense, and if early humans came from Siberia – as the anthropological and archaeological material suggests – they would have had to traverse the harshest environment on earth just as it reached its nadir. It implies a journey of unimaginable hardship for a species that only recently left its tropical homeland. Surely this would have been impossible? It is at this point that the genetic data provides us with more clues.

  Doug Wallace, a geneticist at Emory University in Atlanta, had helped to pioneer mitochondrial DNA analysis of human populations when he was at Stanford University in the early 1980s. By the time he moved to Emory in the mid-1980s, he had become focused on the origins of Native Americans. In particular, he was trying to use mtDNA as a tool to track the origin of the first Native Americans back to particular populations in Asia. The first major publication of this work, in 1992 with Antonio Torroni, showed that Native Americans could be divided into at least two waves of migration. The earliest led to the settlement of both North and South America, while the later wave of migration left genetic traces only in North America. Their estimates of when these migrations took place varied widely, and could have occurred any time between 6,000 and 34,000 years ago. The results confirmed, though, that Native Americans and north-east Asians shared a recent common mitochondrial ancestry.

  But how do these results fit with our Y-chromosome data? Th
is question was answered in 1996 by Peter Underhill and his colleagues. Underhill found a single nucleotide change on the Y-chromosome, later named M3, that was common throughout the Americas. While their sample of Native Americans was by no means exhaustive, over 90 per cent of South and Central Americans they examined were M3, while around 50 per cent of North Americans had this lineage. Clearly, it was the major Native American Y-chromosome founder, defining the American clan.

  The only problem was that M3 was not found in Asia. This could have been due to its age, which Underhill and his colleagues estimated to be as little as 2,000 years. The age estimate was extremely uncertain, however, as it was made in the early days of Y-chromosome analysis, and the mutation rate of the single microsatellite used to assess M3 diversity (using the same method that we used to date M173 in Europe) was uncertain. Thus it could also have been as much as 30,000 years old. Clearly more work was needed.

  This came in 1999, when Fabricio Santos and Chris Tyler-Smith at Oxford and Tanya Karafet and Mike Hammer at the University of Arizona independently reported that the ancestor of M3 was defined by a marker called 92R7, named for an undefined nucleotide change on the Y-chromosome. They found that 92R7 was present in populations throughout Eurasia, with a distribution from Europe to India. In combination with other nucleotide changes this pinpointed Siberia as the source population for Native Americans, confirming Wallace’s results from mitochondrial DNA. It was difficult to assess the age of the 92R7 lineage, however, as it was so widespread. What was needed was an additional marker on the lineage that would focus attention on the precise populations that could have given rise to the first Native Americans.