What Killed the Neanderthals? And from Discover comes this short article describing a novel theory about the demise of the Neanderthals:
The detective story began at Cambridge University seven years ago, when Tjeerd van Andel and a team of paleoclimatologists started combing through environmental and archaeological data to try to solve an old mystery: Why did Neanderthals vanish from Europe 28,000 years ago? Researchers had assumed they died out because they weren’t as smart or as good at manipulating tools as modern humans. Van Andel came up with a different explanation: bad weather. The animals Neanderthals hunted—mostly bison and giant deer—died off from extreme climate change.
Between 60,000 and 20,000 years ago, Europe became drier and experienced rapid phases of warming and cooling. “If you have very quick changes, that wipes out the trees,” Van Andel says. With less vegetation to eat, large herd animals could not survive. Suddenly, the Neanderthals' hunting method—running after prey animals and stabbing them with a spear—no longer worked. The smaller game taking over the continent simply outran them.
Incidentally, that's pronounced Neander-tal not Neander-thal.