Our understanding of The Great Human Diaspora continues to become increasingly complex..!!
First, it was presumed that Homo neaderthalensis, a subline splitting off from Homo sapiens following migration of early humans Out of Africa, did not interbreed with modern humans, and then subsequently died out ~28,000 years ago.
Then, Neaderthal DNA was found in genomes of northern Europeans, indicating that admixture (interbreeding) had occurred after all––within the past 47,000 to 65,000 years.
Then it was discovered that Neaderthal DNA was also found in the genomes of some Africans, indicating that some ‘admixed’ northern Europeans that had interbred with Neaderthals had back-migrated to Africa.
It was also established that a tribe living in Denisova Cave (southern Siberia, within the border of present-day Russia) and were named Homo denisoviensis, represented an earlier migration out of Africa and eastward across southern Asian––and that this migration was independent of the migration of modern humans northward through the Caucasus Mountains leading to the major geographical subset we call Caucasian.
The present report [ref below] shows analysis of genomes of a Neanderthal and a Denisovan from the Altai Mountains in Siberia together with sequences of chromosome 21 of two Neanderthals from Spain and Croatia. They discovered that a population that had diverged early from other modern humans in Africa contributed genetically to the ancestors of Neanderthals from the Altai Mountains––roughly 100,000 years ago. In contrast, they did not detect such a genetic contribution in the Denisovan or the two European Neanderthals. It was therefore concluded that, in addition to later interbreeding events, ancestors of Neanderthals from the Altai Mountains and early modern humans met and interbred, possibly in the Near East, many thousands of years earlier than had been previously thought.
Nature 25 Feb 2o16; 530: 429–433