This allowed Denisovans to change many genes in the gene pool of this archaic human population which created a human population that diverged from our own. There’s a lot of good evidence that this interbreeding occurred several times, but a big part of that evidence came from a genetic analysis of archaic people . While a more extensive analysis may not come soon, recent work does show that Neanderthals and Denisovans may have interbred during this entire period, but the reasons why they did is not known. One possibility is that the ancient Denisovans interbred with the populations of the Middle East , but more recent archaeological and genetic work has brought that conclusion into question. Other researchers believe this interbreeding may have been triggered by some kind of ecological event like drought or famine, but it seems that we don’t yet know the full story. Some scientists have proposed a possible explanation for this mixing: Neanderthals and Denisovans may have exchanged genes with each other before then, but it’s not yet clear how.
The question of the time of the interbreeding is important because there’s evidence that the interbreeding in the past was not quite as quick as is commonly believed. For example, a study of more than 140 previously identified Denisovan and Neanderthal DNA found that between 400,000 years ago and 150,000 years ago, two separate ancient populations, those of Denisova Cave in Russia and that of Siberia, mixed before the Denisovan interbreeding that occurred around 500,000 years ago. In fact, when the Denisovan and Neanderthal DNA is compared to modern humans, its genetic structure is similar to that of modern day Western Europeans. Further analysis of those same Denisovan and Neanderthal DNA found that the Neanderthals in the area of Siberia were not quite as far to the east as previously supposed, and that as far as the Denisovans were concerned, they were not related at all. A different study, published this year in Nature Genetics , found that the interbreeding between Neanderthals and modern humans had occurred a few thousand years prior to the Denisovans coming over to Siberia, and that the interbreeding between the Neanderthals and Neanderthals was more of an intermixing of different species. However, it was previously thought that this interbreeding happened via exchange from other archaic hominins, which means we don’t yet fully understand why it has remained elusive for so long.
The study may also further change the way we look at the Denisovans. The researchers argue that this new DNA evidence shows that the Denisovans and Neanderthals may not be a discrete subgroup with identical ancestry. Rather, it seems that a number of different species, some that had been on the landscape for hundreds if not thousands, of years may have interbred together while Denisovans were still on the planet.
As we continue to learn about our genetic history and the species that were around when it was in fact ours, there are still a few other questions that we should be asking. One of the questions that is much easier to answer is why, exactly, a population like the Denisovans, a species with a well-known extinct population, would move to Siberia when they were in the same geographic region as these modern humans who had become extinct. We could, however, learn more about how this population came to inhabit our planet by exploring a different area, such as Siberia. A number of previous studies did try to answer those questions, but again a lack of scientific understanding around what was going on in that region was a crucial weakness in many of these studies. As a further development for the Siberian research, scientists believe that in the 1930s a second population of hominins, which was distinct from the Neanderthals (and other archaic hominins), might have invaded from Africa. If this is the case, there may have been a period of increased genetic isolation and migration (something that has also been shown in Africa). And unlike the Neanderthals and Denisovans, the second group was much older than either of the latter and would have also been far from a geographical area known for its extreme climates.
The question of a possible ancient migration remains but it will be interesting to see how these questions affect the way we look at the Neanderthal and Denisovan human species today. The more we learn about the ancient history of our species, the more insight we will have into our own past and it may even help us think more creatively about the future of humanity.