The DenisovansCory Panshin on December 25, 2010
A study that came out this week adds tantalizing new details to the story of how modern humans reached eastern Asia, and I’ve found it particularly intriguing in light of the non-orthodox scenario of human expansion that I’ve been developing in recent entries.
It appears that the sequencing of DNA extracted from a finger-bone found in a cave in Siberia has demonstrated the existence of a previously unknown species of archaic humans. Researchers have dubbed them the Denisovans, after the cave where the bone was discovered, and describe them as the Far Eastern cousins of the Neanderthals, with whom they shared a common ancestor 400,000 years ago.
This study has a number of fascinating implications. For one thing, it overturns the old Eurocentric assumption that eastern Asia was inhabited solely by the relatively primitive and small-brained Homo erectus until the arrival of modern humans.
But the really extraordinary conclusion it reaches is that the first modern humans to arrive in Southeast Asia must have interbred with Denisovans in the same way that modern humans in the Middle East interbred with Neanderthals. The researchers determined that about 5% of the DNA of present-day Melanesians — who inhabit New Guinea and the smaller islands nearby — is of Denisovan origin.
The news coverage I have seen of this discovery has attempted to fit it into the conventional short chronology for human expansion outside of Africa, which implies that modern humans could not have encountered the Denisovans any earlier than 50,000 years ago. But I believe that in the long run, it will turn out to be far more compatible with the idea I’ve been presenting here — that a first wave of modern humans left Africa as early as 150,000 years ago, crossing from Ethiopia to Southern Arabia and then moving along the coast to Southeast Asia.
For example, human-Denisovan interbreeding might help explain the enigmatic fossils, some of them more than 100,000 years old, which have been been found in the Philippines and southern China and are variously described as “not a perfect match with any known group of humans” or as displaying “a mixture of modern and archaic features.”
But it’s the big picture that I find really exciting. A couple of months ago, I suggested that during the interglacial period between 125,000 and 75,000 years ago, there were two non-African centers of human population — one in Southeast Asia founded by those initial migrants and the other in the Middle East, established once warmer and moister conditions made it possible for modern humans to move into North Africa and Israel.
It now appears that both of these groups also encountered and interbred with the previous inhabitants — the Middle Easterners with Neanderthals and the Southeast Asians with the Denisovans.
The two communities would have been largely separate — though possibly overlapping in India — but the separation would have come to an end when the most recent ice age forced many of the Middle Easterners to seek greener pastures to the south and east. By no later than 60,000 years ago, this second wave of migrants reached Southeast Asia and began moving down the western coast of what is now Indonesia but which thanks to lower ocean levels was then the subcontinent of Sunda.
At the same time, some of the original settlers appear to have moved from southern China down the eastern coast of Sunda and into New Guinea. The two lines of migration must have eventually converged, since both genetically and linguistically, the peoples of Australia and New Guinea have distinct affinities. But it appears that when they did, the Melanesians of New Guinea included more of the older strain — a strain which had a significant Denisovan component.
On balance, I believe this two-migration scenario accommodates the new findings far more efficiently than the single-migration theory. For example, the British professor who discussed the study with the BBC had to resort to explaining the presence of Denisovan genes among Melanesians — but not in the sample from northern China — by speculating that there had been just a single brief “interbreeding event” involving one isolated group of migrants.
Quite frankly, given the nature of human sexual behavior, I find that idea wildly implausible. It seems a lot more likely that the first modern humans in the region interbred with the Denisovans whenever and wherever they could, giving rise to a hybrid population. It was only the overwhelming number of later arrivals that led to the present situation where a significant Denisovan component is found in just a few genetic backwaters.
If that was the case, however, I would expect that a smaller percentage of Denisovan DNA would also be present among Indonesians and Southeast Asians, as well as Filipinos, aboriginal Taiwanese, and the Ainu. It would be absent only among the peoples of northern China and Mongolia, whose genetic roots lie primarily in the ancient Middle East.
It will be extremely interesting to see the results of future genetic studies in this area.
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