I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. ~ Charles Darwin
The fascination with Neanderthal just goes on and on.
Given what a lousy rap he had for years, I suppose it's appropriate that we now pay some attention to getting the picture right. We've now definitely established that Neanderthals walked as upright as you and I, were a successful species for hundreds of thousands of years, and probably had brains that made them potentially as smart as we think we are. We don't know, though, why they changed so little in that long period of existence, to the point that they used similar tools from beginning to end, never created cave drawings or other art, and may not have spoken a formal language.
We now know that Neanderthal and our immediate ancestors (anthropologically speaking) Cro-Magnon co-existed in several places quite possibly for 10,000-40,000 years. We have absolutely no idea how the two species got along, whether we lived and let live or massacred them at every opportunity. And, of course, did the two species interbreed?
That last has nothing to do with prurient interests. If there was interbreeding, then the Neanderthals might not have been wiped out by Cro-Magnon or simply died out on their own. The two species might have merged, at least to some extent.
So far, DNA analysis has not revealed any traces of Neanderthal genetic inheritance in modern humans. That isn't a definitive “no” to the interbreeding question because there's a lot more DNA to investigate, but so far it doesn't look good. In fact, scientists might have thrown in the towel on the subject by now were it not for some very suggestive discoveries. (No, I'm not going to make any cheap jokes about football defensive lineman, tempting as that may be.)
The most recent data comes from an analysis of bones discovered in 1952 which appears to clearly show a mix of modern human and Neanderthal characteristics. It's not the only such skeleton, and it falls into that tantalizing overlap era by being 28,000 years old. To date, though, no one has been able to successfully extract DNA from such a skeleton, which could potentially put the issue to rest once and for all.
The possibility of interbreeding is certainly imaginable, but the question is whether such a mating could produce any offspring. If it could, would the offspring be sterile, like so many hybrid species are? We have hints, but nothing definitive.
In general, I think a lot of people, including scientists, would like to think that there was successful interbreeding between the species. It would be nicer to think that the older species merged into ours rather than being killed off or wandering off into the Stone Age sunset to die in some lonely place. In fact, it's interesting that scientific attitudes have changed to some extent regarding whether modern humans were running woolly mammoths, saber-toothed cats, and woolly rhinos into extinction.
Up to a few years ago, it seemed like every time an animal species and humans existed at the same time and place, it was assumed that the brainy hunters must have wiped out any species that no longer existed. The biggest problem with this idea is that it ignored the biggest extinction factor, climate. It also ignored that fact that, world-wide, the human population was a few million. There were almost certainly a lot more woolly mammoths on the planet than there were humans, and humans had other things to hunt for as well, including things that weren't as likely to fatally step on them. This is not to say that humans didn't hunt mammoths; they almost certainly did. But they also probably scavenged dead ones.
It's quite possible that humans killed the last mammoth, but the mammoths were on the way out due to major changes in the ecosystem that had nothing to do with human activities.
Having freed ourselves of killing of the mammoths, it appears that we'd like to be declared “not guilty” in the matter of Neanderthal's passing. Much as I hate to disappoint everyone, but evidence of interbreeding won't prove anything. The history of human warfare is filled with tribes and civilizations being destroyed with survivors being taken as captives, generally to be made into slaves. The slaves interbred with other slaves and with the conquerors. That is not a peacefully “merging” of two groups.
I don't think we'll ever conclusively know the answer to this one, but my money is on a declining Neanderthal population that was wiped out by a growing Cro-Magnon people. I am certainly willing to maintain an open mind on the subject, but it's difficult to imagine that a behavior pattern that's manifested itself throughout the historical period and shows itself even in the prehistoric evidence, where sites have been found showing signs of conflict with bodies having spear and/or arrow points embedded in them.
I guess one might be able to argue for coexistence if one could find a site that clearly showed simultaneous habitation by Neanderthals and modern humans. There have been sites located that were inhabited by both species, but one could have followed the other (or killed off the other and moved in). The problem is that both groups were not yet producing the kinds of artifacts (like pottery) that could help pin down whether they were trading and getting along with each other.
Someday a site may turn up that will clear us of the onus of having started our career by killing off the competition.