And it is beginning to appear that it was FREQUENT and not rare!!
Archaic admixture with modern humans is believed to have take place via interbreeding between modern humans
and archaic humans such as Neanderthals, Denisovans, and possibly others. And I would certainly add Homo erectus.
Neanderthal DNA accounts for between 1-4% of the Eurasian genome in modern humans. However, this admixture is uncommon or nonexistent in the genomes of Sub-Saharan Africans. In Oceanians, Southeast Asians we find Denisovan DNA admixture. In Melanesian people we find 4-6% Denisovan DNA in their genomes.
African populations have a genetic contribution from a now extinct archaic African hominin in their genomes. Fossils, especially ancient human ancestor fossils, are rare because certain conditions must be present for fossils to form. Most species that die simply rot but every now and then the conditions are right and fossils form. In African environments there is a rapid decay of fossils and this makes it very difficult to compare modern human admixture to archaic African hominins such as H. ergaster and H. habilis et al. Further, about 2% genetic material found in some Sub-Saharan African populations entered the human genome about 35-40 kya. This was added by interbreeding with archaic human species (ancient human ancestors). Evidence for this suggests that this interbreeding mostly took place in CENTRAL Africa and NOT Eastern Africa as we would expect! In terms of geologic time 35-40 kya is NOT a long time.
Most people know that modern humans interbred with Neanderthals in Europe and Asia to some extent but many don’t realize that our species also interbred with more archaic hominins. New research is beginning to show the actual extent to which our ancestors interbred.
Research conducted by Michael Hammer of the University of Arizona has provided EVIDENCE that our species not only interbred with Neanderthals in Eurasia but also with several other ancient human ancestors not only in Eurasia but across Africa. And, further, while some are claiming this was a “rare interaction” the EVIDENCE suggests it was NOT! In fact, the evidence suggests such interbreeding was OFTEN!! “It [interbreeding] happened relatively extensively and regularly,” according to Professor Hammer.
Skulls have always been good for providing evidence of interbreeding. In several fossil skulls we can see features of one or more hominin species. In some we can see features of modern humans and Neanderthals. In others we can see features of H. erectus and Homo sapiens. Not only were these different hominin species interbreeding but they were also producing hybrid offspring that were functional enough to be integrated into the larger ancient society, clan, or tribe. Some of the hybrids were able to have offspring of their own, who in turn had their own offspring, etc right up to our modern times.
Due to the lack environmental conditions necessary for fossil formation we haven’t been able to do the same kind of genetic research with African populations as we have with Eurasian populations. DNA doesn’t survive long in tropical climates and this is one reason why we find even FEWER fossils in places like Indonesia, Southeast Asia, and (I think) Central and South America.
We find mixtures of early modern human features and other hominin species features in H. sapiens so we know interbreeding took place. In some of the fossil specimens found we can also see Erectine features such as a low forehead, remains of a saggital crest or occipital bun at the back of the skull, et al. To me these features suggest that H. sapiens interbred not only with Neanderthals but also H. erectus. As we find more fossil remains of “extinct” human ancestors I’m pretty sure we are gong to see just how common interbreeding between ancient human ancestors really was!
As things stand now we know there have been several interbreeding events via the study of ancient DNA. These events or episodes include interbreeding between:
-Early modern humans & Neanderthals (both Eastern and Western)
-Early modern humans & the Denisovans
-Neanderthals & Denisovans
-and then…..interbreeding between the mysterious Denisovans and some as yet unidentified “ghost” population of hominins!
Once we though human evolution was like a ladder in which one ancient human species gave way to a newer human species but as research and discoveries have continued we have come to realize that the NORM is that more than one human species has coexisted on the planet MOST of the time if not ALL of the time. We have also come to realize that human evolution is not like a ladder at all but it is more like a twisted vine or the numerous branches of a winding stream. Now we are beginning to discover that evidence is mounting for the interbreeding between these coexisting ancient human species and we are finding this evidence via DNA analysis and fossil analysis.
Once we thought it was all so simple but we are quickly discovering that the “human story” is much more complex than we thought. And, we are also discovering that these MANY episodes of human species interbreeding produced viable HYBRID offspring capable of breeding themselves and producing viable offspring. Some people conceive of this phenomena as different “species” of humans but, actually, it is more like different BREEDS of humans….like dog breeds or cat breeds (no offense intended). When we consider dogs we know we can breed a Chow with a Collie with no problem and that resulting hybrid can reproduce its own offspring perfectly well and viable. So, really, we are not looking at different “species” of humans in “our story” because what we are actually looking at is different BREEDS of humans and instead of calling them collies, chows, shepherds, etc we call them H. habilis, H. erectus, Neanderthals, Denisovans, Cro-mags, Homo sapiens, etc. And, in at least one case we are calling one of these “breeds” of humans a “ghost species.” Why? Because at this point we really don’t know what species the Denisovans were interbreeding with other than the Neanderthals and modern humans. But, I do think that as research continues we will uncover that mystery too.