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Also, as this survey article indicates, the anatomical differences are far more pronounced in Europe than they are elsewhere, especially the Near East. There thus may have been a local "species gap" that didn't exist everywhere at once.

I think most scientists agree that at some point you have to have been able to interbreed with something to have been descended from it. The curious total lack of any evidence of interbreeding between modern humans and neanderthals is most problematical precisely in the levant in which moderns and neanderthals are known to have existed in close proximity for long periods of time and you'd figure interbreeding should have been very common IF it was possible.

James Shreeve's 1995 Discover Magazine article on the topic is apparently now being used in university courses in paleontology. You can get to the article by going to Discover Magazine's Search Page, clicking the Archive Search Engine, and searching on "Neanderthal Peace" for the year 1995.

Actually, there have been claims of interbreeding between Neanderthals and Cro-Magnons, based on a number of "intermediate" type skeletons. But academic opinion is divided on these claims. In reality, two "species" can diverge in evolutionary terms, without losing their ability to interbreed. Case in point: the various "species" of big cats, etc., which can be interbred in captivity, but which would never interbreed in Nature. Mankind being what it is, if the two groups could have interbred, then it would have happened, at least occasionally. But the fossil record is extremely incomplete, and rare half-breed individuals would be extremely unlikely to have been preserved in the fossil records. Our current level of genetic knowledge is very rudimentary; studies showing lack of "neanderthal" genes in modern humans are very primitive, given our current level of ignorance, and in any case, lack of evidence of neanderthal genes in current populations does not prove that the two groups did not interbreed in the past; it only indicates that if they did, that the neanderthal genes passed into the cro-magnon population were few in number, and were eventually bred out of the population.

The curious total lack of any evidence of interbreeding between modern humans and neanderthals is most problematical precisely in the levant in which moderns and neanderthals are known to have existed in close proximity for long periods of time and you'd figure interbreeding should have been very common IF it was possible.

A curious statement. "The Levant" is exactly where the intergrading of human and Neanderthal features is most pronounced. Note the comments on the linked page regarding "Skhul V" specimen in particular. Then there's the Lagar Velho child from Portugal, which looks a lot like a human-neanderthal hybrid.