Denisovans co-existed with early humans and Neanderthals.
A single finger bone found in this Siberian cave led to an amazing discovery. Early humans and Neanderthals co-existed with another humanoid species called Denisovans. And many present-day humans carry genes that prove our ancestors had children with Denisovans, too.
The new species is named after the cave where the 30,000 year-old finger bone was found. Researchers had been searching for Neanderthal bones in the area, and were surprised to discover what they initially thought was a fossil from an early human’s little finger. To find out more, they shipped the bone off to the Max Planck Institute in Germany, where evolutionary biologist Svante Pääbo had already sequenced several Neanderthal genomes. Pääbo’s tests gave a shocking result: The genome sequence they got from the bone showed that it was neither human nor Neanderthal.
And yet it was undeniably a human relative, who had clearly lived among humans and Neanderthals thousands of years ago in the caves of Siberia. After careful analysis, a team of genomics experts figured out where the Denisovans fit into the puzzle of human ancestry. Most likely they are descended from a common ancestor shared with Neanderthals. When early humans left Africa about 300 or 400 thousand years ago, they spread out across Europe and Asia. Those who went west to Europe became the heavy-browed, squat Neanderthals. And those who went East became Denisovans.
Genomics expert Richard Edward Green worked on analyzing the Denisovan DNA. In an email to io9, he explained:
The genome of the Denisovans is more diverged from modern humans than any two humans are from each other. It’s almost exactly as diverged as the Neanderthal genome was. That’s one of the reasons that we think the Denisovans and the Neanderthals are descendants of a single migration event into Eurasia.
Another wave of early human migration spilled from Africa about 70 to 80 thousand years ago. These travelers encountered both Neanderthals and Denisovans, eventually settling down and forming families with them. As a result, many Europeans have Neanderthal DNA; and, as the researchers report today in Nature, some Melanesian people from Papua have Denisovan DNA.
The picture of early human life that emerges is a lot messier than what we believed even just twenty years ago, when many anthropologists believed humans diverged from Neanderthals and the two species never interbred again. Now it seems that humans had many cousin species – at least two that we know of – and that we separated from them only to rejoin them later, forming families and creating lineages that persist to the present day.
So what makes the Denisovans and Neanderthals separate species from early humans anyway, given that all three groups co-habitated and had children together? Green said:
Answering that question – How much DNA divergence is necessary to call something a new species? – is a very difficult one. We know there was admixture between early modern humans and a population related to the Denisovans. We can see this in the genomes of individuals from Papua New Guinea, as described in the paper. Thus, from this perspective they were similar enough to successfully mate with our ancestors. The sad, frustrating truth, though is that there is no simple answer to how much divergence must be present to call something a different species or sub-species or variety or whatever.
Regardless of whether the Denisovans were another species, or just distant cousins, they are proof that humans have not always been alone among the primates. Within the last 50 thousand years, we shared the planet with other intelligent hominids who weren’t quite human.
If we want to know what humanity might look like 50 thousand years from now, after we’ve colonized space and spent millennia evolving in dramatically different environments, we should look back to the Denisovans’ humble cave in Siberia. There, three very different types of human beings met after a long time apart. And formed a community together.