Tim Weaver has concluded that Neandertals evolved in different direction from modern human precursors because of nothing more than genetic drift. According to the article:
In their new study, Weaver and his colleagues crunched their fossil data using sophisticated mathematical models -- and calculated that Neanderthals and modern humans split about 370,000 years ago. The estimate is very close to estimates derived by other researchers who have dated the split based on clues from ancient Neanderthal and modern-day human DNA sequences.
This suggests that Neandertals were an isolated group in Europe and had no outside contact with other population groups. The article states that it is in this week's PNAS. The problem is that it isn't. I guess I will have to check next week.