A recent study earlier this month by the Max PLanck Institute for the Science of Human History reports that Neanderthals received a genetic contribution from Africa by hominins who were closely related to modern humans more than 220 kya. The study obtained mtDNA (inherited from our mothers) from the femur bone of a prehistoric European hominin in an effort to resolve what appears to be our complicated relationship with Neanderthals. The genetic data has now given us a timeline for a possible migration out of Africa that took place after Neanderthal ancestors arrived in Europe and suggests Neanderthals were more closely related to modern humans than any other hominin species. What this means is that there may have been a migration out of Africa of hominins 220 kya.
What the study suggest is that after the Neanderthals arrived in Europe there was another migration out of Africa and those people interbred with the Neanderthals in Europe leaving their genetic marker in the Neanderthals mtDNA and these hominins appear to be closely related to modern humans. In addition, although likely this took place 220 kya it could have taken place as far back as 470 kya and may NOT have been a one time event either!
Previous research using nuclear DNA (inherited from our fathers) suggested modern humans and Neanderthals split about 765-550 kya but mtDNA suggests this split is far more recent (470-220 kya). Further, it appears that the mtDNA in Neanderthals is more similar to that found in modern humans (Homo sapiens) and this would also indicate a more recent common ancestor researchers say. In fact, the mtDNA in Neanderthals is actually more similar to modern humans than it is to their close nuclear DNA relatives the Denisovans!
In many ways modern humans had a complex relationship with the Neanderthals and this study along with others is seeking to resolve some of the mysteries surrounding the relationship. This mtDNA has not been found in Denisova remains but only in Neanderthals.
Frankly, I think there were many back and forth migrations in and out of Africa. And I think interbreeding between modern humans and Neanderthals was more frequent than we think. And, as a result H. sapiens/Neanderthal HYBRIDS were being born and those hybrids were viable and able to reproduce their own fertile offspring. Think of cats or dogs and the various BREEDS they have. The different breeds are able to mate with each other and produce fertile offspring. I think rather than various “species” of ancient human ancestors it may be more accurate to start thinking of them as different BREEDS of ancient human ancestors and this includes ourselves!