Final answer:
Neandertal autapomorphies suggest they were not direct ancestors of modern humans but rather a divergent sister group, as indicated by their unique traits and the genetic evidence confirming a separation between these species several hundred thousand years ago.
Step-by-step explanation:
Neandertal autapomorphies, which are unique traits of the Neanderthals, are used to argue against their being ancestral to modern humans due to the distinct morphological and genetic differences that suggest a divergence rather than a direct ancestral link. The DNA evidence shows that Neanderthals and Denisovans were closely related, but both had noticeable differences from modern Homo sapiens. This is further supported by the fact that the Denisovan divergence from Neanderthals is estimated to be around 700,000 years ago, and modern humans diverged from Neanderthals approximately 370,000 years ago. Furthermore, it is believed that Homo heidelbergensis is the common ancestor of Neanderthals, Denisovans, and perhaps modern humans, affirming that Neanderthals, with their distinctive traits and genetic make-up, were more of a sister group to Homo sapiens rather than direct ancestors.