http://www.bbc.co.uk/worldservice/sci_tech/highlights/010710_neanderthal.shtml
From 2001
Aside from archaeology, another line of evidence for speech would be genetics. The announcement in August 2002 of the unique form of the FOXP2 gene in modern humans was seen as a possible line of evidence for the lack of language in human ancestors. The FOXP2 gene seems to be vital in allowing human to speech to develop much more clearly, as mutations cause problems with movements of the lips and tongue as well as selection of the correct word tense.
The human version of the gene does not seem to appear until 200,000 years ago, after the neanderthals split from the ancestors of modern humans. This suggests neanderthals may have lacked a fine-tuned speech ability. However, there is unlikely to be a single 'language gene'. Language relies on an incredibly fine-tuned interaction between brain and throat, and is likely to be dependent on several genes. Further studies may show neanderthals used different genes to perform a similar function. The debate continues.
http://www.bbc.co.uk/science/horizon/2003/learnthinkqa.shtml
"The debate continues" hmmmmm
Yes, of course the debate continues, but there are two relevant points here:
In modern humans quite clearly the gene that regulates speech and language is our unique FOXP2 gene.
Even if Neanderthals did have a language capability, then it would be based on "different genes to perform a similar function".
The topic of debate here is how closely related the Neanderthals were to the Cro-Magnons as a species, or sub-species.
Therefore, regardless whether the Neanderthals had their own unique genes to enable speech, their genes that performed the function would be different from that of Cro-Magnons.
And so, that would be yet another in the lengthy list of items above that clearly point to Neanderthals as being fully distinct species.
And note also that even if you hypothesize "different genes to perform a similar function" of speech in Neanderthals and Cro-Magnons, there are no known modern humans that evidence this alleged Neanderthal alternative to the unique human FOXP2 gene. So, that would be yet another sign that Neanderthals and Cro-Magnons did not intermix, because if they did then you would plausibly expect to find some descendents with this hypothetical Neanderthal speech-enabling alternative.