No sex please
The difference in interpretation is crucial to debate between the multiregionalists and supporters of the out-of-Africa theory, because a species is defined as a group that is reproductively separate. By lumping all H. erectus and H. sapiens together, multiregionalists reinforce their claim that modern humans gradually emerged by a process of parallel evolution and interbreeding between populations of hominids spread throughout Africa, Asia and Europe. And by splitting Homo specimens into erectus and sapiens, out-of-Africa proponents slam the door on interbreeding.
Recent studies have only added fuel to the debate. For example, evidence from the three Neanderthals whose DNA has been analysed so far indicates that genetically there is more in common between us and them than there is between members of individual chimpanzee social groups. Few palaeoanthropologists now doubt that early modern humans were biologically capable of interbreeding with other hominids. But the other half of the species definition is whether two groups would in fact interbreed, or whether they would consider themselves too different from one another and remain aloof. "You can't tell that for fossils. You can't tell that in many cases for living animals unless you have an army of graduate students to go out and watch them," says Delson. "For example, chimps and gorillas live in the same forest and they don't interbreed, although, if you take a small female gorilla skull and a large chimp skull, they're very hard to tell apart."