We modern humans know the origins of our species, don’t we? Homo sapiens started out in one place — Africa. We then spread out across the Earth during the last 60,000 years or so, replacing archaic species such as Neanderthals and Homo erectus. And as modern humans spread out across the world in small numbers, our “racial” features began to develop in each region.
Over the past 20 years this “Out of Africa” school of thinking has risen to dominance. A wealth of evidence — by discoveries in fossils, archaeology and genetics — supports this explanation of how Homo sapiens evolved.
But now, a team of scientists has reconstructed the Neanderthal genome sequence for the first time, and the results have produced an intriguing twist to our evolutionary tale. If you are European, Asian or New Guinean, the research suggests you may have some Neanderthal blood racing through your veins — but not if you’re African. These surprising results are probably only the first of many, as the emerging science of fossil genomics builds up momentum.
So who were these Neanderthal relatives of ours? They have certainly had an image problem over the years, and brutish behaviour is often dismissed as “Neanderthal”. Yet they were actually highly evolved humans who walked as upright as we do, and whose brains were as large as ours. Fossil and DNA evidence suggest that their lineage split from ours about 400,000 years ago, and over the next 370,00 years in Europe and western Asia the Neanderthals developed their own bodily (and no doubt behavioural and social) characteristics until they disappeared about 30,000 years ago. Their demise has often been explained by losing out in competition with modern humans, although I think that very unstable climates between 30,000 and 60,000 years ago were also a big factor.
Because, like us, they buried their dead, many of their remains in caves have been saved from erosion and damage, and so we have a good idea of their bodily form: they were relatively short, wide-shouldered and hipped, and barrel-chested. Their physique looked more suited to short powerful bursts rather than endurance running, and it’s thought they were mainly ambush hunters, armed with thrusting spears. Their stocky body shape may also reflect their evolution through the Ice Ages, although they probably actually preferred temperate weather and environments.
Above the neck, the Neanderthals were particularly distinctive. Their braincases were big, but long and low, with a large browridge instead of the domed forehead of modern humans. Neanderthal faces were dominated by an enormous and projecting nose, which was accentuated by swept-back cheekbones and a receding chin.
As to their intelligence, we cannot be certain about how smart they were, but the Neanderthals were clearly capable hunters, gatherers and toolmakers. While it does not seem they were great innovators, during the final 30,000 years of their existence they started to make more advanced tools, showed increasing use of pigments and developed the production of jewellery.
These changes have been the source of much debate — were the Neanderthals becoming increasingly inventive, or were they under the influence of modern humans who had recently emerged from Africa?
In this week’s issue of the journal Science, a truly international team of more than 50 researchers have published their findings from a reconstructed Neanderthal genome of more than three billion bits of DNA coding. The full sequence is posted online, but even the descriptions of their methods, results and conclusions cover more than 200 pages. Using massive improvements in DNA recovery techniques and computing power, three small fragments of bone excavated from the Croatian cave of Vindija have provided most of the sequence. These three female Neanderthals who died around 40,000 years ago have been immortalised through their DNA.
The results still largely confirm the Out of Africa thesis, the overall distinctiveness of the Neanderthals, and a separation time from our lineage 270,000-440,000 years ago. But when the Neanderthal genome is compared with those of modern humans from different continents, the results challenge some of the orthodoxy, because the genomes of people from Europe, China and New Guinea lie slightly closer to the Neanderthal sequence than do those of individuals from Africa.
The most likely explanation is that the ancestors of people in Europe, Asia and New Guinea interbred with Neanderthals (or at least with populations that had a component of Neanderthal genes) in North Africa, Arabia or the Middle East, as they exited Africa about 60,000 years ago. This ancient human exodus involved perhaps just a few thousand people, so it would have taken the absorption of only a few Neanderthal women into a group of Homo sapiens for the genetic effect — greatly magnified as modern human numbers exploded — to be felt tens of thousands of years later.
The amount of Neanderthal genetic input is estimated to be about 2 per cent, a surprisingly high figure to me and many other adherents of Out of Africa. As one of the architects of this model, I have regarded Neanderthals as representing a separate lineage and most likely a separate species from us, although I never ruled out the possibility of interbreeding. But even if there had been interbreeding I believed it to have been small and insignificant in the bigger picture of our evolution — for example, the results of isolated instances of interbreeding could easily have been lost in the intervening millennia.
But now, the Neanderthal genome strongly suggests those genes were not lost, and that many of us outside Africa have a tangible Neanderthal heritage. What these shared genes do for us, if anything, remains to be determined, but that will certainly be a focus for the next stages of this fascinating research. With much more genetic data to come, including further studies of ancient DNA from the “Lineage X” fossil from Siberia, which has yet to be matched to an ancient human species, and of recent DNA in Oceania that hints at additional archaic interbreeding there, we may yet discover new surprises about who our distant ancestors were.
Professor Chris Stringer, Research Leader in Human Origins at the Natural History Museum. His book The Origin of Our Species will be published next year.