The Neandertal EnigmaFrayer's own reading of the record reveals a number of overlooked traits that clearly and specifically link the Neandertals to the Cro-Magnons. One such trait is the shape of the opening of the nerve canal in the lower jaw, a spot where dentists often give a pain-blocking injection. In many Neandertal, the upper portion of the opening is covered by a broad bony ridge, a curious feature also carried by a significant number of Cro-Magnons. But none of the alleged 'ancestors of us all' fossils from Africa have it, and it is extremely rare in modern people outside Europe." [pp 126-127]
by James Shreeve
Assimilation
One skeleton that has led some researchers to claim that it shared Neanderthal and Cro-magnon features has been found at Lagar Velho in Portugal; it is uncertain whether this is in fact a hybrid of the two species, or simply an extreme individual of one or the other. This may suggest the two species may have interbred. The child skeleton does seem to be more robust than what we would expect for modern humans. However, most researchers think that it represents extreme variation within modern humans. Moreover, the skeleton is dated to about 24,000 years BP. Until recently, this implied that a hybrid population survived in the region for thousands of years. However, a Neanderthal population in Gibraltar dated to about the same time has recently been found. The dating evidence for this claim is debated, though. Claims for Neanderthal sites that were advanced in the past have in the end all been revised to pre-30 kyr*. It has also been speculated that these hybrid individuals could have been sterile.
* kyr = kiloyear.
http://www.talkorigins.org/faqs/homs/lagarvelho.html
http://www.guardian.co.uk/science/story/0,,1871842,00.html
http://johnhawks.net/weblog/reviews/neandertals/gorhams_28000_date_2006.html