There's an interesting article on human evolution at MSNBC. The piece starts out by claiming that Neanderthals were a separate species from modern Homo sapiens, but everything in the article suggests that Neanderthals and H. sapiens interbred and produced feertile offspring. Since the ability to produce fertile offspring is the distinguishing characteristic of a species the finding that Neanderthals and H. sapiens were interfertile would mean that they're really members of the same species.
Indeed, none of the alleged predecessors of modern humans are actually known to have been unable to breed successfully with H. sapiens. It's simply assumed that sexual unions between them would have been sterile because it's assumed that they were different species. And it's assumed that they were different species because it's assumed that H. sapiens is the culmination of a long evolutionary history with lots of ancestral species leading up to it. But, for all we really know, creatures like H. habilis and H. erectus could have been conspecific with H. sapiens. We simply don't know whether they were capable of producing fertile offspring or not.
Of course, if it were ever found that these members of genus Homo were all the same species it would throw modern human evolution models into a state of complete chaos, so I suppose we have to assume that they really were all distinct species.
RLC
