The other humans, and how they relate to you #3: Africa and enigmas

So we've looked at the three human species we know to have been alive when the great migration out of Africa occurred around 50-60,000 B.C. Have we overlooked anyone? Were there other human populations still around? It seems pretty likely.

Just how "fossil" are non-sub-Saharan African humans? The minimum archaic DNA in Europeans and Han Chinese, for instance, seems to be around 1-4%--the equivalent, on the upper end, of a great-great grandparent. Melanesians, who have Neanderthal as well as Denisovan ancestors, range slightly higher: 4-9%. The existence of yet another population of archaic humans might be also be inferred from genetic analysis of sub-Saharan Africans themselves, who average around 13% archaic DNA even when they lack European, Asian, or Melanesian ancestry. In short, while there is evidence that Africans encountered and bred with other human species outside of Africa, they also bred with archaic humans inside of Africa as well.

But wait...if modern humans came from Africa, what the heck were non-modern humans doing in Africa? The simple answer is that modern humans originated within a certain part of Africa--either in the south or the east. Africa is a big continent, big enough to allow multiple types of humans to coexist for long periods of time without much interaction (remember, this was back in caveman times when you couldn't just hop in a car or plane and suddenly be in a different part of the continent a few hours later). When modern African humans and archaic African humans finally did interact, they apparently interacted like human populations did elsewhere in the world--sex, sex, and more sex. The fact that 13% of their DNA is still hanging around in modern Africans just means that the mixing process probably had a longer time to occur in Africa than elsewhere.

But who were these archaic guys? Nobody is really sure.

A guess might be Homo rhodesiensis:

This is a species whose validity as a taxon is somewhat controversial. Based primarily on a fossil known as the Kabwe Skull (discovered in Kabwe, Zambia) and a few other fossils around 125-400,000 years old, H. rhodesiensis has been variously classified as Homo erectus, Homo sapiens (albeit a very old version), Homo heidelbergensis, and as its own species. Whatever his true nature, we can say a few things about these remains with confidence: they looked distinctively different from modern humans--for starters, they had the largest brow ridges known of any member of the human line, and secondarily, they were extremely robust, more so than even Neandethal (now that's robust!). In fact, some researchers have called H. rhodesiensis the "African Neanderthal" because of these features, although he was probably only distantly related to the Neanderthals proper.

Part of what makes H. rhodesiensis so confusing from a taxonomic standpoint is his apparent close relationship with modern H. sapiens; anthropologist Tim White considers rhodesiensis to have been the immediate ancestor of Homo sapiens idaltu, an East African population that may have given rise to the very people who overtook the great journey out of Africa 50,000+ years ago. So if the anatomically-modern ancestors of today's African interbred with H. rhodesiensis, they were in a sense breeding with grandpa. Before you say ewwww, remember that this really isn't any different than an American WASP having children with someone from England, or a woman from Quebec having children with a Frenchman, but instead of hundreds of years there might be hundreds of thousands of years separating the two populations.