There would seem to be a problem with that idea in that you'd figure any common ancestor of both modern man and the neanderthal would be further removed from us than the neanderthal. If we can't be descended from neanderthals because the genetic gap is too great, which is what we hear, then how could we be descended from something even further removed?
I'm not espousing it, just repeating it.
I don't buy the development of modern man as represented, anyway. We popped up about 35,000 years ago, and eradicated entirely Neanderthal - who had been here over 100,000 - in 5000 years.
Why?
A silly objection! Hint to the un-hintable: You're talking about this kind of thing.
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Both lineages have diverged from a starting point removed in time from the last examples of either. Some of the differences between humans and neanderthals arose along the human line. Some arose along the neanderthal line. You might as well be protesting you can't have common ancestors with your sixth cousin because you don't seem all that related.
The fossil record is increasingly showing us this history. H.s. idaltu would be one example of a recent addition. There's nothing unthinkable about it and the evidence for it is better now than it ever was. The only question, and it's still a little bit open but closing fast, is whether humans and neanderthals had completely speciated by the time neanderthals went extinct. It's looking more and more as though this is the case.