The Journey of Man: A Genetic Odyssey (19 page)

It is likely that these early central Asians would have stayed in the relatively warm environs of the southern steppes had encroaching desertification not forced them on. Some stayed behind, retreating into the foothills of the Hindu Kush where the water supply from glacial melting, and the number of animals, were sufficient for survival. Most,
though, appear to have followed the migrating herds of game to the north – into the face of the storm, as it were. It is likely that they first reached Siberia during the early part of this period, around 40,000 years ago, when Upper Palaeolithic tools make their appearance in the Altai Mountains. The conditions would have been unimaginably different from those their ancestors had left behind in Africa 10,000 years before. Winter temperatures dropped to –40°C or lower, and much of their time would have been spent hunting for food and keeping warm. But the animals they hunted would have made the difficulties worthwhile.

We saw earlier that one of the defining features of species living at high latitudes is their great size – Bergmann’s rule. The reason is that large animals have less surface area relative to their volume than small ones, and heat is lost through the surface. Shrews must eat constantly to maintain their hyperactive metabolism, in part because their tiny size makes it extremely difficult for them to retain heat. In cold environments, then, there is selection for large animals with slower metabolisms (since the food resources are not as plentiful as they are in warmer regions) – big, lumbering beasts that aren’t particularly clever. This is how natural selection created animals such as the woolly mammoth.

The first Upper Palaeolithic people to encounter a mammoth, probably in southern Siberia or central Asia, must have been more than a little intimidated. While the traditional prey of these consummate hunters would have been perhaps two or three times the size of a human, mammoths were the size of small buses, with intimidating tusks and thick fur. As they watched these odd giants, though, they would have discovered that the mammoth’s great size also made it slow and ungainly. Given the right hunting technique, and the right tools, it was possible to kill them. And once this was done, the meat from one animal would feed a clan for weeks, so it was worth making the effort.

It is also likely that dead mammoth carcasses were scavenged by Upper Palaeolithic humans. On the basis of animal remains at Upper Palaeolithic sites in southern Africa, anthropologist Lewis Binford has suggested that scavenging formed a substantial part of the diet of early human populations. While the relative levels of scavenging and
hunting practised by our early ancestors have been debated by scientists, it is likely that at least some scavenging took place – as is the case in modern hunter-gatherer groups. And with the great mass of meat on a mammoth carcass, they would have been prime scavenging targets for the first Eurasians.

The Eurasian interior was a fairly brutal school for our ancestors. Advanced problem-solving skills would have been critical to their survival, which helps us to understand why it was only after the Great Leap Forward in intellectual capacity that humans were ready to colonize most of the world. During their sojourn on the steppes, modern humans developed highly specialized toolkits, including bone needles that allowed them to sew together animal skins into clothing that provided warmth at temperatures not unlike those on the moon, but still allowed the mobility necessary to hunt game such as reindeer and mammoth successfully. They had to venture beyond sheltering hills and caves, out on to the icy open steppe and tundra, necessitating the development of portable shelters. Their migrations would have taken them far beyond ready sources of the fine-grained stone they used to make weapons, so they had to become more economical in their tool-making. This led them to develop microliths, small stone points (such as arrowheads) that were hafted on to wooden shafts and used as weapons.

The problem-solving intelligence that would have allowed Upper Palaeolithic people to live in the harsh northern Eurasian steppes and hunt enormous game illustrates something that could be called the ‘will to kill’. Survival depended on finding sufficient food resources, whatever the obstacles – and the steppes were a veritable meat locker. It was the necessity of obtaining food that led them into the freezer, but it would take them well beyond central Asia. The Steppe Highway gave them a straight shot to the extreme ends of the continent, and once they had adapted to the harsh conditions a new world lay open to them.

The genetic composition of these first Siberians was a mixture of both central Asian
and
ancestral Eurasian clan lineages. While M45 is the marker that we use to infer the migrations of the early central Asian steppe hunters, there were still many men alive who did not have Y-chromosomes marked with M45 – they would have had unmarked Eurasian M9 Y-chromosomes. This is because new markers do not immediately increase in frequency to the point where all other markers – such as the ancestral M9 lineage – are lost. All of the Y-chromosome markers we study originated in a single man at some point in the past, so their original frequency was one (that individual) divided by the total number of men in the population – a very low frequency in all but the smallest groups. Over time, they become more common primarily due to the effect of genetic drift – the random changes in frequency that characterize all human populations. Thus the earliest people to colonize southern Siberia would have had members of both the central Asian M45 and the older Eurasian M9 clans, although drift appears to have caused them to lose most of their ancestral Middle Eastern chromosomes by this point.

As with the Eurasians who entered India on the other side of the Hindu Kush, some of these Eurasian clan members would have migrated to the north and east, guided in their journey by the Tien Shan mountains. Some of them, perhaps taking advantage of the so-called ‘Dzhungarian Gap’ used thousands of years later by Genghis Khan to invade central Asia, made it into present-day China. It is likely that the majority were migrants along the Steppe Highway further to the north, avoiding the harsh deserts of western China by detouring through southern Siberia. However, make it they did. We know this because they left descendants from another Y-chromosome marker that is almost completely limited to east Asia, and is entirely absent from western Asia and Europe – M175.

Today, M175, which arose on a Eurasian M9 chromosome, is found at highest frequency, around 30 per cent, in Korean populations. Based on absolute dating methods, it appears to be roughly 35,000 years old, coinciding very closely with the appearance of the Upper Palaeolithic
in Korea and Japan. There are several more recently derived markers that have M175 as an ancestor (particularly M122, which will play a significant role in
Chapter 8
), and together these related lineages account for 60–90 per cent of the Y-chromosomes in east Asia today. Like a collection of soup recipes that all have a common ingredient, M175 unites most Asian men living east of the Hindu Kush and Himalayas, defining an east Asian clan.

When these modern humans reached east Asia, they found themselves in an area that had been inhabited by their distant relatives
Homo erectus
for nearly a million years. Dubois’ missing link had relatives in China, called (before being united with their Javanese cousins to the south) Peking Man. But mysteriously, no
erectus
remains from Chinese sites are found after 100,000 years ago – there is a gap in the record until fully modern
Homo sapiens
make their appearance around 40,000 years ago. What caused this hominid gap is unclear, although the likely culprit is – once again – the steadily deteriorating climate. For example, the cave at Zhoukoudian, where many
erectus
remains have been found, is located in north-eastern China, near Beijing – a region that experiences extremely cold winters even today. During the intense cold of the penultimate glaciation, around 250,000 to 150,000 years ago, the climate in northern China would have become much harsher. Consistent with this, no
erectus
remains post-date 250,000 years at Zhoukoudian. It seems likely that the deep freeze drove them away – or even killed them off.

We know that
erectus
didn’t change substantially for 1 million years in east Asia, perhaps the result of stable selection pressures. Isolation from other hominids and a penchant for relatively uniform climatic conditions would have favoured continuity rather than change, and there is no evidence for an
erectus
Great Leap Forward. While some Chinese scientists argue for an evolutionary model known as ‘regional continuity’, in which east Asian
erectus
evolved into a local variant of
Homo sapiens
independently of what was happening in Africa, there is absolutely no genetic evidence for this. Moreover, the genetic results show that there was not even any interbreeding between modern human immigrants to east Asia and
erectus
– if in fact any populations still existed 40,000 years ago that are invisible to today’s archaeologists. In a recent analysis of over 12,000 men from throughout east
Asia, geneticist Li Jin and his colleagues found that every single one traces his ancestry to Africa within the past 50,000 years – because every man has our old friend M168 on his Y-chromosome. Everyone. This result is bad news to those looking for evidence of east Asian regional continuity, since it is impossible to reconcile with any form of local evolution from
erectus
, or even admixture – at least on the male line. East Asian mitochondrial DNA gives the same answer: the thousands of samples that have been tested all trace their ancestry back to Africa. In short, there is no genetic evidence that
Homo erectus
made any contribution to the gene pool of modern east Asians. Rather, Dubois’ ape-man appears to have been an evolutionary dead-end, and he was completely replaced by modern humans.

If the story ended there, it would be very tidy and self-contained. But unfortunately, life is never that simple. In this case, the spanner in the works comes in the form of the presence of our Coastal lineage at high frequency in some east Asian populations. The Coastal lineage is found at a frequency of 50 per cent in Mongolia, and it is common throughout north-east Asia. How it reached this location remains a mystery, but it is likely that the early coastal migrants to south-east Asia gradually moved inland, migrating northward over thousands of years. The M130 chromosomes in the south are older than those in the north, consistent with such a migration. At some point, perhaps 35,000 years ago, they would have met the descendants of the other, main line of migrants – our incoming Eurasians. The presence of both Eurasian and Coastal lineages in east Asian populations attests to the extensive admixture that occurred between them.

The picture that emerges is that east Asia was settled by modern humans from both north and south, like migrational pincers or ‘chopsticks’. The northern route, which was characterized by Eurasian clan members, probably entered around 35,000 years ago from the steppes of southern Siberia. The southern route, which was composed primarily of members of the Coastal clan, was probably in place before this – perhaps as early as 50,000 years ago. The present composition of east Asia still shows evidence of this ancient north-south divide. Luca Cavalli-Sforza, working with Chinese colleagues, examined several dozen non-Y-chromosome polymorphisms in east Asian populations. In their analysis, they saw a clear distinction between the northern and
southern Chinese. Even members of the same ethnic group, such as northern and southern Han, are most closely related to their
geographic
rather than their
ethnic
neighbours; northern Han group with other, non-Han northern populations, and the southerners form a separate group. It seems that the ancient evidence of a two-pronged settlement is still visible in the blood of today’s Chinese.

So, our Middle Eastern clan had made it to the eastern extreme of the continent. Along the way it had acquired additional markers, producing the widespread Eurasian clan, the Indian clan, and the central Asian clan. The mountain ranges of central Asia served as effective barriers to migration 40,000 years ago, as they continue to do today. The effect of this was to produce an isolated east Asian Y-chromosome clan that only occasionally pops up in the west. But while the route to eastern Asia was clear, that to Europe required a more circuitous tour. As we saw, modern Europeans contain rather too many ingredients in their soup to have been the direct descendants of the Middle Eastern clan. The search for the ancestors of the first Europeans is where we are headed next.

When I was a postdoctoral research fellow at Stanford University, my future wife and I lived in San Francisco and commuted down ‘the peninsula’ to Palo Alto. We chose to make this trek every day because we preferred living in the city, with its excitement and fluid mix of people. Our flat was in the Richmond district, the heart of the Russian immigrant community and shoulder-to-shoulder with ‘New Chinatown’ on Clement Street. On the drive home at night I would listen to National Public Radio, which is more or less the American equivalent of BBC Radio Four, in order to pass the time. One evening in the autumn of 1997 as I was driving up 25th Avenue, I heard a news announcement that almost caused me to swerve into an oncoming bus. I pulled over and listened, hanging on every word.

The announcer was reporting that a team of scientists led by Professor Svante Pääbo of the University of Munich had just published the first DNA sequence from a Neanderthal. This was, in a way, one of the Holy Grails of anthropology – a research finding that promised to answer one of the oldest and most contentious questions in the field: had modern Europeans evolved from Neanderthal ancestors, or were the Neanderthals replaced by groups of humans invading from somewhere else?