One of the difficulties with most theories of human evolution is that they assume that many of the alleged early hominids were members of different species, and that each of these species gradually gave way to another over millions of years on the way to producing modern man.
The problem is that the concept of a species is so nebulous as to be almost useless in these speculations. The textbook definition of species is a reproductively isolated population. In other words, if members of two disparate populations are unable, for whatever reason, to produce fertile offspring they are said to be reproductively isolated and thus belong to different species.
This definition entails that organisms isolated by geographical, physical, behavioral, or even temporal factors would be different species even if they were genetically compatible. Two individuals would be reproductively isolated if they were separated from each other by either distance or time (as are members of different generations of humans), or if they were physically incompatible, or if behavioral idiosyncrasies prevented synchronization of mating cycles.
This being the case it's a little absurd to think that just because two different hominids were physically different and/or lived at different times that they were therefore different species, but that's the assumption that's almost always made in the study of human evolution. Now, however, comes a story in the New York Times that bodes ill for this assumption. The report explains that indeed two different lines of hominids actually did interbreed and, though the Times never draws this conclusion, were thus conspecific:
Neanderthals mated with some modern humans after all and left their imprint in the human genome, a team of biologists has reported in the first detailed analysis of the Neanderthal genetic sequence.
The biologists, led by Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, have been slowly reconstructing the genome of Neanderthals, the stocky hunters that dominated Europe until 30,000 years ago, by extracting the fragments of DNA that still exist in their fossil bones. Just last year, when the biologists first announced that they had decoded the Neanderthal genome, they reported no significant evidence of interbreeding.
Scientists say they have recovered 60 percent of the genome so far and hope to complete it. By comparing that genome with those of various present day humans, the team concluded that about 1 percent to 4 percent of the genome of non-Africans today is derived from Neanderthals.
It's interesting that the implications of this discovery are not mentioned in the article which insists on referring to "human-Neanderthal" interbreeding as if Neanderthals were not humans. The fact is that if such unions produced viable offspring then Neanderthals were human, and if that's the case then there wouldn't seem to be much warrant for treating Homo habilis and Homo erectus as different species from H. sapiens.RLC