by Bryan Sykes
Incredibly, even though the African clans are easily the most ancient in the world, we are still able to reconstruct the genetic relationships among them. Thus we probe the ancestors of the ancestors. At last, my dream of building a complete maternal genealogy for the whole of humanity was coming true. One by one the clans converge until there is only one ancestor, the mother of all of Africa and of the rest of the world. Her existence was predicted in the original scientific paper on mitochondrial DNA and human evolution in 1987. Immediately she was dubbed ‘Mitochondrial Eve’ – hardly a convincing African name. She lies at the root of all the maternal ancestries of every one of the six billion people in the world. We are all her direct maternal descendants. But, just as Ursula and the others were not the only women alive at the time, nor was Eve. Estimates of the size of the human population one hundred and fifty thousand years ago are bound to be not much more than guesswork, but it may have been in the order of one or two thousand individuals. Of these, only Eve’s maternal lineage survives unbroken right through to the present day. The others withered away. But they, like Eve, would also have had maternal ancestors; so there is another woman even further back who was the ancestral mother of Eve and her contemporaries. She in turn would not have been alone, and another ancestral mother becomes a logical inevitability. This line of thought goes on and on, becoming increasingly pointless as we reach back millions of years to the very beginnings of our species and the species from which we ourselves evolved. The dashed line on Figure 7 indicates this even deeper genealogy through which our species, Homo sapiens, is connected to the other, extinct, humans, the Neanderthals and Homo erectus, and eventually back to the common ancestor of humans and other primates.
For our purposes we need only go back in time as far as Mitochondrial Eve. The genetics tells us very clearly that modern humans had their origins in Africa within the last hundred and fifty thousand years. At some point, about a hundred thousand years ago, modern humans began to spread out of Africa to begin the eventual colonization of the rest of the world. Incredible as it may seem, we can tell from the genetic reconstructions that this settlement of the rest of the world involved only one of the thirteen African clans. It could not have been a massive movement of people. Had hundreds or thousands of people moved out, then it would follow that several African clans would have been found in the gene pool of the rest of the world. But that is not the case. Only one clan, which I have called the clan of Lara, was involved. It is theoretically possible from the mitochondrial DNA evidence that only one modern human female, one woman, left Africa, and that from this one woman all of us in the rest of the world can claim direct maternal descent. I think this highly unlikely, since she would have had contemporaries in her foraging band. But the numbers must have been very small. This was no mass exodus. Lara herself was not in the party. She probably lived in Kenya or Ethiopia; certainly in Africa. We know this because many Africans today are members of her clan. So she must have lived her life in Africa, unaware of her gift to the world, while it was her descendants that began to move out. Even so, it is a quite astonishing conclusion that the whole of the rest of the world can trace their maternal ancestry directly back to Lara. She is truly the mitochondrial Eve of the rest of the world.
All the evidence points to the Near East as the jumping-off point for the colonization of the rest of the world by modern humans. It was the only land route out of Africa, across Sinai. The only other possibility was to cross the Straits of Gibraltar at the entrance to the Mediterranean between north Africa and Spain. This is a deep channel which was never a land bridge, even when sea levels were at their lowest. Even so, the Straits of Gibraltar are only 15 kilometres across at their narrowest point, and the high Rock of Gibraltar is easily visible from the African side. But neither the archaeology nor the genetics suggest this route was taken.
There is good fossil evidence in Israel that Homo sapiens had reached the Near East at least one hundred thousand years ago. In this book we have traced the faltering spread of our species to the north and west into Europe, which finally succeeded only fifty thousand years ago. What held them up in the Near East for at least fifty thousand years before that? Europe was already inhabited by Neanderthals, physically adapted to the cold and experienced in the mechanics of making a living by hunting the large animals of the tundra. Homo sapiens in the Near East would have needed some advantage, however slight, over the Neanderthal occupants to make any headway. The long period spent in the Near East would have seen the slow developments in technology and, more important, in social interactions, that equipped them eventually to establish a permanent foothold in Europe.
The colonization of northern Asia was probably delayed for the same reasons. It too was a land dominated by steppe and tundra, running in a wide and uninterrupted ribbon from the Ukraine in the west to the high plateaux of Mongolia in the east. Archaeological sites in Mongolia dated to thirty-five thousand years ago witness the arrival of hunting bands with sophisticated flint arrow points in this bleak terrain at about the same time that modern humans were beginning to dominate the plains of western Europe. Their lives would have run along similar lines to the early Europeans we have already encountered, dominated by the seasonal migrations of the tundra animals and the fight to survive the unforgiving winters. We understand very little about the mitochondrial genetics of this vast region because it has not been widely sampled, but we do know enough to be able to be absolutely sure that it was from here that the colonization of the Americas was launched.
Four mitochondrial clans dominate the genetics of native Americans. All four have easily reconstructed and obvious genetic links with people living in Siberia or north–central Asia today. If they went by land, then their route into the Americas can only have been via Alaska. We have enough information about the sea-level changes over the past hundred thousand years to know that there were two periods when there was a continuous land bridge between Siberia and Alaska. The first bridge was formed fifty thousand years ago and lasted for about twelve thousand years. The second coincided with the last Great Ice Age, when the land was above sea level between twenty-five and thirteen thousand years ago.
There is a fierce controversy about when America was first colonized. Did the first people arrive across the earlier land bridge or the later one? There are two early archaeological sites in South America which have been used in the past to support the earlier date. One is at an open shelter at Pedro Furada in Brazil known for its rock painting. Flakes of paint have been found in the earth below the rocks at levels which have been dated to seventeen thousand years ago. But there is controversy about whether the flakes fell off the wall at that time or much more recently, working their way down into the lower levels through the action of worms or other creatures that disturbed the soil. The second site is at Monteverde in northern Chile, where fragments of wood, possibly part of a shelter, have been found at levels originally dated at thirty thousand years ago, although this has now been revised to a later date by the archaeologist who excavated the site. No human remains have been found at either Pedro Furada or Monteverde, and a big question hangs over the authenticity of both sites.
Perhaps the greatest evidence against the earlier date for the colonization of the Americas is that one would expect the population, in a land full of game and without prior human occupation, to explode, leaving abundant evidence all over the place. It is not as if nobody has looked. American archaeologists have worked hard to find it; but without success. However, there is plenty of evidence of a continuous settlement after twelve thousand years ago, with hundreds of sites scattered all across both North and South America.
The genetic evidence from modern native Americans also favours the later crossing. The accumulation of mutations in native Americans within each of the four clans has given all of them ages that fall well within the last thirteen thousand years. Reconstructions from modern Siberian and Mongolian patterns show very clearly that the clans were already established and se
parate from each other well before they reached America. The same applies to the rare fifth clan, that of Xenia, to which about 1 per cent of native Americans belong. As we have already seen, that clan had its origins on the borders of Europe and Asia.
The genetics fits well with the later land crossing from Siberia into western Alaska, just as the Ice Age was waning and the sea levels had begun to rise once again. But getting into Alaska was not the end of the story. Northern America was covered by two huge ice sheets. One enveloped the Rockies and the high mountains of southern Alaska; the other covered the whole of Canada. At the height of the last Ice Age, when sea levels were low enough to expose the land bridge from Siberia, these two great ice sheets fused to seal off access to the interior. The first Americans were faced with a dilemma. If it was cold enough to cross into Alaska by land, it was also too cold to get past the ice sheets on the other side. Alternatively, if it was warm enough to get through the ice sheets, by then the land bridge would be flooded. There had to be a period when the first Americans were stranded in western Alaska. Eventually the two ice sheets withdrew sufficiently to create a narrow corridor between them. This was no verdant valley, but a harsh passage though which the pioneers advanced little by little. At last the corridor opened out into the rich expanses of the Great Plains which were teeming with game. It must have been a wonderful and welcome sight to those first pioneers who had struggled through the ice corridor. From there on, the way was open for the rapid colonization of the whole of North and South America and, judging by the dates of the abundant archaeological sites, this was accomplished at record speed within only a thousand years.
The genetics agrees well with this scenario – except in one detail, namely that one of the four clans, the clan of Ina, is virtually absent from the modern inhabitants of Siberia and Alaska. It is found in South and Central America, and is still abundant in native Americans as far north as Vancouver Island on the north-west Pacific coast, but no further. Intriguingly, this same clan is also the one that is closely associated with the colonization of the Polynesian islands from south-east Asia. As we saw in an earlier chapter, the detailed sequences of Polynesian and native American members of this widespread clan are sufficiently different to rule out a maritime colonization of the Americas from Asia directly across the Pacific via Polynesia. However, the curious absence of this clan from the present-day inhabitants of Siberia and Alaska suggests to me that we may be seeing the genetic echo of a second seaborne colonization that took the coastal route north up the coast of Asia and down the Pacific coast of North America. The rapid sea-level rises which flooded so much of south-east Asia would have given a great incentive to find new land. Could it be that the same maritime migration that ultimately led to the colonization of the remote Pacific islands also led a different branch of this remarkable clan to seek new land to the north – a journey which led them through polar waters and eventually to the temperate lands of Central America? What a voyage that would have been.
People from the Asian mainland also crossed to Japan at about the same time that they first reached America. One of the major questions in Japanese prehistory is the degree to which the modern population can trace its genetic roots to these earlier Jomon settlers, who are believed to have reached Japan about twelve thousand years ago, or to the much later Yayoi and subsequent migrations from Korea in the last two and a half thousand years. This issue has familiar parallels to the question of the composition of the gene pool of modern Europe and whether most Europeans trace their ancestry to the original hunter–gatherers or to the more recently arrived farmers from the Near East. We were able to settle this dispute using mitochondrial DNA. Could the same be done in Japan?
Comparatively little work has been done in Japan but there are hopeful signs that genetics will be able to decide this question. In addition to the Japanese on the central islands of Honshu, Shikoku and Kyushu, anthropologists recognize two other contemporary ethnic groups: the Ainu of Hokkaido in the north, and the Ryukyuans who live mainly on the southernmost island of Okinawa. One theory is that the Ainu and Ryukyuans are the descendants of the original Jomon settlers who occupied the whole of Japan and were then displaced from the central islands to Hokkaido in the north and Okinawa in the south by the arrival of the Yayoi from Korea. What little work has been carried out in Japan agrees in part with this idea, to the extent that it shows modern Japanese from the central islands sharing many more mitochondrial types with modern Koreans than do the Ainu and Ryukyuans. However, it also shows that the Ainu and Ryukyuans do not have very many shared types in common either. Age estimates, similar to the ones we did for the main clusters in Europe, show that both the Ainu and Ryukyuans have accumulated distinct mutations over the past twelve thousand years – which does suggest they are both the descendants of the original Jomon, but also that they have not been in close contact with each other since that time.
Although the majority of modern Japanese now live in Honshu, Shikoku and Kyushu, they do share many mitochondrial DNA sequences with modern-day Koreans and so trace their maternal ancestry back to the Yayoi and subsequent migrations. Many other Japanese are also the maternal descendants of the Jomon and have their closest maternal relatives among the Ainu and the Ryukyuans. Whereas there is no doubt that the genetics confirms that the impact of the Yayoi settlers from mainland Asia was very substantial, far more so than that of the Near Eastern farmers in Europe, still it was not completely overwhelming. Much more needs to be done in Japan; but there is no doubt that mitochondrial DNA shows that modern Japanese are a mixture of Jomon and Yayoi and once again confirms that there is no such thing as a genetically pure classification into different races.
Both America and Japan were first reached by descendants of the hunting bands that had adapted to survive in the harsh conditions of the Asian tundra. This was a very different world from the one their ancestors knew in the Near East. It seemed to take about fifty thousand years spent in the Near East for Homo sapiens to acclimatize, both physically and organizationally, to these extreme conditions. But there was another way out of the Near East that did not involve adaptation to life on the tundra and an unrelenting diet of bison and reindeer. That exit was along the coasts of Arabia, the Persian Gulf and Pakistan, south of the great mountain ranges of central Asia, into India and south-east Asia. This route was much warmer, and much more like conditions in Africa, than the freezing northern route. It could have been used straightaway, without the long interlude adapting to the cold of the higher latitudes. Did people travel this southern route by sea thousands of years before their distant relatives eventually moved into Europe and northern Asia? Unfortunately there is no inland archaeology to support the idea of an ancient movement of people along this southern route, and thanks to sea-level rises coastal sites are now under water. But recently hand axes and flakes of the volcanic glass obsidian were found on a raised fossil beach at the edge of the Red Sea. Although no human skeletons were recovered from the site, which means we cannot be sure that the occupants were anatomically related to Homo sapiens, this is direct proof of human occupation of coastal sites at a very early date.
Whoever first discovered Australia certainly knew how to travel by boat. Even when sea levels were at their lowest, it was still necessary to make a journey of at least fifty kilometres over open sea to reach Australia. But how long ago did they arrive? Like the early American sites, the dating of very early archaeological finds in Australia has been controversial. However, judging by a recently dated burial site in south-east Australia, Homo sapiens was already there at least sixty thousand years ago. Even if these dates are only approximately accurate, they mean that modern humans reached Australia thousands of years before the colonization of Europe and northern Asia had even started.
If the archaeology is inconclusive, what can the genetics tell us? For understandable reasons, native Australians are very wary about participating in genetic tests, particularly those orchestrated by their former oppressors. The outcome i
s that only very few mitochondrial sequences are known from native Australians. Those that have been published show only the remotest connection with the four clans from northern Asia that settled America. This rules out the possibility that the same hunters that crossed Asia north of the Himalayas and went on to colonize America also turned south and were the first to reach Australia. That much we can be sure of, and it does suggest that there might have been an earlier movement of people from the Near East across southern Asia. Sadly, we currently know so little about the mitochondrial genetics of native Australians that we are not in a position to be more specific about their genetic connections to people from other parts of southern Asia. From the few sequences that have been made public we can see that Australia probably holds several as yet unidentified clans. These are the signs of a very early arrival, leaving plenty of time for mutations to accumulate. They are also the signs of a relatively small population held constant over thousands of years. This fits well with what we know about the arid and hostile conditions that have persisted over this vast continent, which would have kept population growth to a minimum.
I am quite sure that genetics will be able to tell us a lot about how and when the first Australians arrived. I am equally sure that this history belongs to native Australians and not native Europeans like myself. It is their history, not mine. I, for one, would love them to share it with us.
