With more advanced DNA analysis techniques available to researchers, our understanding of how humans spread across the globe seems to get more and more complicated. Not only did modern humans leave Africa to find a world populated with Neanderthals and the more mysterious Denisovans, but it seems that there may have been another as yet unidentified human added to the mix.
This conclusion is based on a recent whole genome analysis of DNA taken from people living on the Andaman Islands in Southeast Asia, as well as that of other Melanesians. When compared with those of people living in India, they found whole sections of DNA that did not match any currently known human species, with it neither derived from Neanderthals, Denisovans, or us. This, they argue in a study published in Nature Genetics, shows how the ancestors of people who now live in the Pacific may have been mating with an as yet unknown hominin.
The humans in question could feasibly be a whole new species, as the history of the human settlement in this region is notoriously complex. But it could also be explained by another species called Homo erectus, who we know to have been present in Southeast Asia and who may have overlapped with modern humans. Unfortunately, the fossil evidence of H. erectus have not been preserved significantly enough to allow for genetic analysis. So while it could be that the unnknown DNA is from H. erectus, there is currently no way to test it.
To the best of our knowledge, we know that a small band of modern humans, those that would eventually give rise to you and me, left the African continent at least 60,000 years ago. But they were not the first human species to make it outside of the natal homeland. From Homo heidelbergensis to Homo erectus, various ancient humans dispersed from Africa over the preceding hundreds of thousands of years before we even existed as a species. These spread out across Eurasia, and eventually gave rise to other species, such as Homo neanderthalensis (the Neanderthals) and Homo floresiensis (the "hobbits").
So far from being pioneers, when we eventually entered the scene, we were fairly late to the party. We now know that as we began to colonize Europe, we started interacting with the Neanderthals who were already present, even mating with them. But recently, the picture was made even more complex, as researchers discovered there may have been a third human species interacting with our ancestors, all based on a single fragment of bone and two teeth discovered in a cave in Siberia.
It is now widely accepted that these mysterious humans, called the Denisovans, were also mating with modern humans and Neanderthals, and that their descendants spread throughout Southeast Asia, as modern genetics show that people living there today have a much higher proportion of Denisovan DNA than any other population in the rest of the world.
But it now seems that it doesn’t even stop there, as the new research suggests that the picture from this region is far more complex and convoluted than anyone could have guessed. With little chance that well-preserved organic remains from this period survived due to the tropical environment, we may never know the true picture of our expansion out of Africa.