Modern Humans Interbred with Two Groups of Denisovans

There is now more evidence that were were “one big happy family.”  Here is a short recap:

Beginning around 1.8 million years ago, a hominin form called Homo erectus left Africa for points east, eventually settling in Indonesia and China. These early humans were characterized by having heads roughly ¾ the size of modern humans with very large brow ridges and with their widest point just above the ears. They were also the first humans to conquer fire and perfect hunting.

Then, at some point, between 300 and 500 thousand years ago, a population group migrated from North Africa into Europe and, eventually into East Asia. The European branch became the Neandertals, between 200 and 250 thousand years ago, and the East Asian group eventually became the Denisovans. The Denisovans then spread east and south, eventually mixing with other populations, some of which were the precursors of the Melanesians and native Australians. The bulk of the Neandertals hunkered down in Europe and tried to outlast the bitter cold of not one but two glaciations.  Despite this, while often pilloried in cultural literature as being half-witted brutes, Neandertals were a very complex society, with advanced weaponry and hunting behavior, grave goods, habitation structures and who practiced ritual behavior. Some populations of Neandertals eventually  expanded their range into Western Asia and steppic Russia and interbred with the Denisovans.  Unfortunately, as a culture, we know next to nothing about the Denisovans. 

Roughly 100 thousand years after this, there was yet another wave of migration, between 100 and 60 thousand years ago, of early modern humans from North Africa, who moved north and East mixing with both the Neandertals in Europe and, perhaps, Western Asia and the Denisovans in East Asia.

And now we learn that the modern humans arriving from Africa interbred with not one but two groups of Denisovans.  From Gizmodo:

We know so little about the Denisovans that they don’t even have a formal scientific name, though scientists are considering Homo sp. Altai or Homo sapiens ssp. Denisova. Indeed, as these names suggest, Denisovans were a branch of humans, having diverged from Neanderthals some 200,000 years ago. We know this because the Altai fossil yielded a near-complete genome, which scientists have been poring over since it was first sequenced in 2010.

But in addition to the Neanderthal ancestry, genetic anthropologists also learned that Denisovan DNA lives on in modern humans, especially among Oceanians and East and South Asians. This means anatomically modern humans, or Homo sapiens, must’ve interbred with a population of Denisovans. But as new research published today in the science journal Cell points out, our ancestors mated with Denisovans on at least two different historical occasions. So the traces of Denisovan DNA embedded in the genomes of some people living today originated from at least two distinct Denisovan populations.

Given Palaeolithic population densities, this is not surprising.The research seems to indicate that there were early modern human/Denisovan mixes in both Asia and Oceania.  What this means is that, once the Denisovans and Neandertals split, the Denisovans migrated east and northeast (as humans will do) and established population centers in these areas.  When the modern humans came (Huh.  I wonder what is over that hill?  Oh look, humans...sort of.) it made sense to intermingle with them.  We already know that Neandertals and modern humans could, and did, mix.  It is, absent any knowledge to the contrary, reasonable to assume that the Denisovans looked mostly modern human. 

Interestingly, the research seems to indicate that the rate of interbreeding of Neandertals to early moderns was much more limited than with moderns and Denisovans.  This is at variance with other studies (and fossil material) which seems to indicate more sustained contact.  It would be nice if we could find a bit more fossil evidence to get a handle on what at least one Denisovan looked like.