Occasionally a science article becomes popular in the general populace for a week or two. Usually this occurs because it has pretty pictures or makes sensational claims. In May of 2006 the latter sort of story appeared which raised the possibility of human and chimpanzee ancestors continuing to mate after speciation had occurred1. Most people unwillingly conjured up images of modern humans and chimpanzees mating, which is a shame, because the notion reveals a hole in our common conception of the notion of a species, and provides an important clue to the full story of how humans came to be humans.
The chimpanzee/human mating story goes like this: isolated populations of what would later become chimpanzees and humans evolved into separate species, then creatures from those species interbred, and the resulting hybrid species again separated and offspring from the two different lines eventually evolved into chimpanzees and humans (among other creatures). The most compelling argument for this theory is the fact that the chromosome of ours that diverged from chimpanzees most recently is the X chromosome -- and since X is most important for reproduction we would need a compatible X chromosome more recently in order to interbreed. If this theory is correct (and I think the evidence is strong that it is) then most of our ancestors would have diverged from chimpanzees at an earlier date, with a few chimpanzee grandparents sneaking in every ten or hundred thousand years. The timeline is so vast that it is hard to imagine pre-humans not breeding with pre-chimpanzees occasionally, as long as it was still biologically possible.
We should prepare ourselves for more and more of these sorts of stories: evolutionary convergence as well as the divergence traditionally associated with Darwinian speciation. The reason that this is not already a common view is the “species” problem. A species has traditionally been defined as a group of organisms that can interbreed. This rough definition does not adequately address anomalies like this hybridization process, which is found fairly frequently in the animal kingdom: polar-grizzly bears, lion-tigers, bison-cattle, etc. This is because a species is not a real thing that exists, like a person or a rock or a computer is, a species is merely an arbitrary label that people use to describe certain groups of organisms. As such it has only scholastic meaning. Humans and chimpanzees did not separate and interbreed. A group of creatures interbred in Africa for a few million years in distinct groups that occasionally intermingled. Eventually they stopped intermingling and the descendents of one group became creatures we call humans and descendents of another group became chimpanzees (there were probably other groups too, judging by all of the primitive hominid fossils we’ve found in Africa, but they have no modern survivors).
This pattern of divergence and convergence lends credence to an alternative explanation of human evolution currently challenging the “Out of Africa” hypothesis (Not Out of Africa2) -- regional continuity. The current theory holds that humans completed most of their evolution into the modern homo sapiens in Africa, then migrated out of the continent and diversified slightly (white skin in Europe, hairlessness in Asia, etc.) where they eventually settled. Alan Thorne’s new theory holds that creatures we now consider pre-humans, such as homo erectus, ought to be considered members of the human species -- humans that left Africa much earlier and evolved independently into modern humans in the various regions in which they settled. Migratory patterns and occasional meetings between these diverging tribes could have brought them into contact with each other every couple of hundred or thousand years and highly advantageous new mutations (such as a slightly larger brain size) could then be passed throughout the species all over the globe. In essence we evolved all over the globe together, almost simultaneously in geological time. And Thorne has powerful evidence on his side: a 60,000 year old modern human found in Australia, much earlier than it should have been there under the older theory.
New evidence from another place fits in with both “Out of Africa” and “Regional Continuity”, but it increases the possibility that Thorne is right. A study analyzing regional variations of the human genome, studying Europeans, Africans, and Asians, found a wide variety between regions for genes controlling phenotype variations that are especially helpful in differing environments (for example, adult lactose tolerance in Europeans or malaria resistance in Africans)3. These genetic divergences are only about 10,000 years old, implying a common genetic pool before that (this also coincides with the explosion of the human population following the last ice age). This indicates if humans did complete most of their evolution in Africa and then expand outward, settle and evolve independently only these traits, they must have done so very recently. The problem is, Thorne has a 60,000 year old skeleton that argues otherwise.
Human ancestors probably left Africa a lot earlier than we currently think they did, and they probably looked a lot less like modern humans than we think they did. However, they retained the ability to interbreed, and the evidence of human/chimpanzee hybridization, along with the known tendency of hominids (early and modern) to migrate more frequently than other animals means that our ancestors evolved into us globally in sync with each other.