"Neanderthal culture developed at a slow rate. We may speculate on the disappearance of Neanderthal man, and we can note that his successors, for example, Cro Magnon man, who inhabited some of the old Neanderthal sites in the Dordogne (Boule and Vallois, 1957), had the skeletal structurethat is typical of Man's speech mechanism. Neanderthal man's disappearance may have been a consequence of his linguistic -- hence intellectual -- deficiencies with respect to his spaiens competitors. Inshort, we can conclude tht Man is human because he can say so."
http://web.haskins.yale.edu/sr/SR021/SR021_09.pdfPhilip Liberman & Edmund S. Crelin, On the speech of Neanderthal man