Experts believe the Neanderthal genome sequence will be of extraordinary importance in understanding human evolutionary history since the two species split apart some 600,000 years ago.Keep in mind that only the "strong" (absolutist) version of the Out-of-Africa hypothesis that is in trouble. The weak version (that most of our genes are descended from people whose ancestors were in Africa 100,000 or so years ago) looks pretty reliable.
So far, the team has identified only about 100 genes â€” surprisingly few â€” that modern humans have evolved since the split. The nature of the genes in humans that differ from those of Neanderthals is of particular interest since they bear on what it means to be human, or at least not Neanderthal. Some of the genes seem to be involved in cognitive function and others in bone structure.
â€?Seven years ago I really thought that it would remain impossible in my lifetime to sequence the whole Neanderthal genome,â€? Dr. Paabo said in a news conference. But the Leipzig teamâ€™s second conclusion, that there was probably interbreeding between Neanderthals and modern humans before Europeans and Asians split, is being greeted with reserve by some archaeologists.
A degree of interbreeding between modern humans and Neanderthals in Europe would not be greatly surprising given that the two species overlapped there for some 15,000 years, from 44,000 years ago when modern humans first entered Europe to 30,000 years ago when the last Neanderthals fell extinct. Archaeologists have been debating for years whether the fossil record shows evidence of individuals with mixed features.
But the new analysis, which is based solely on genetics and elaborate statistical calculations, is more difficult to match with the archaeological record. The Leipzig team asserts the interbreeding they detect did not occur in Europe but in the Middle East and at a much earlier period, some 100,000 to 60,000 years ago, before the modern human populations of Europe and East Asia had split apart. There is much less archaeological evidence for an overlap between modern humans and Neanderthals at this time and place.Â ...
Dr. Paabo said that episode of human-Neanderthal breeding implied by Dr. Reichâ€™s statistics most plausibly occurred â€?in the Middle East where the first modern humans appear before 100,000 years ago and there were Neanderthals until 60,000 years ago.â€? According to Dr. Klein, people in Africa expanded their range and reached just Israel during a warm period some 120,000 years ago. They retreated during a cold period some 80,000 years ago and were replaced by Neanderthals. It is not clear whether or not they overlapped with Neanderthals, Dr. Klein said.
These humans, in any case, were not fully modern and they did not expand from Africa, an episode that occurred some 30,000 years later. If there was any interbreeding, the flow of genes should have been both ways, Dr. Klein said, but Dr. Paaboâ€™s group sees evidence only for gene flow from Neanderthals to modern humans.
The Leipzig groupâ€™s interbreeding theory would undercut the present belief that all human populations today draw from the same gene pool that existed a mere 50,000 years ago. â€?What we falsify here is the strong Out-of-Africa hypothesis that everyone comes from the same population,â€? Dr. Paabo said.
In his and Dr. Reichâ€™s view, Neanderthals interbred only with non-Africans, the people who left Africa, which would mean that non-Africans drew from a second gene pool not available to Africans. Dr. Reich said that the known percentage difference in DNA units between African and non-African genomes was not changed by his proposal that some of the non-African DNA is from Neanderthals.