The Denisovan session at the American Society of Physical Anthropology meeting was very interesting. In Science Anne Gibbons reports on the findings, Our mysterious cousins—the Denisovans—may have mated with modern humans as recently as 15,000 years ago:
The elusive Denisovans, the extinct cousins of Neanderthals, are known from only the scraps of bone they left in Siberia’s Denisova Cave in Russia and the genetic legacy they bequeathed to living people across Asia. A new study of that legacy in people from New Guinea now suggests that, far from being a single group, these mysterious humans were so diverse that their populations were as distantly related to each other as they were to Neanderthals.
The finding of two Denisovan lineages in Southeast Asia adds to results reported in Cell last year by Sharon Browning of the University of Washington in Seattle and her colleagues. They had suggested that New Guineans had a separate source of Denisovan DNA than people in East Asia, suggesting at least two mixing events.
“I’m skeptical,” added Cosimo Posth of the Max Planck Institute for the Science of Human History in Jena, Germany. He suggests the hints of a late mating could reflect an encounter of previously isolated modern populations rather than of moderns and Denisovans. In this scenario, modern humans mated with Denisovans, then the modern populations diverged, with each branch retaining a different set of Denisovan genes. The moderns then reconnected, mixing the two sets of Denisovan DNA together again.
Here is one thing that I think is important to remember: unlike Western Eurasia parts of Eastern Eurasia were better insulated from extreme climatic events. Neanderthals show strong evidence of repeated die-offs and population expansion so that in general Neanderthal relatedness is more a function of time than location (i.e., Neanderthals tended to go extinct in much of their range periodically, to be repopulated from refuges).
In contrast, the Denisovan range likely went far into Southeast Asia. It is not surprising that this is a highly structured population, with deep lineages. This is exactly what we see in Africa for the same time period. Tropical Southeast Asia is not as extensive as Africa, but it was more expansive during the Pleistocene due to lower sea levels. Hominins with low population densities occupying a huge range of territory almost certainly had developed local lineages and traditions.
As should be clear in the quotes we shouldn’t take these presented results as definitive. But they are suggestive and align well with earlier work that there were several Densiovan admixtures across Eurasia.