This is a pretty controversial viewpoint. See link to the article below. Makes sense to me.

Volcanoes wiped out the Neanderthals some 40,000 years ago, suggest archeologists, setting the stage for modern humans in Europe.

Stumpy but strong, the Neanderthals disappear from the European fossil record by about 30,000 years ago, replaced about that time by modern-looking humans. In the upcoming October Current Anthropology journal, researchers led by Liubov Golovanova of Russia's ANO Laboratory of Prehistory in St. Petersburg report that volcanic dust deposits in a cave in the Caucasus point to an ecological catastrophe wiping out our Neanderthal cousins, not warfare or competition for food.

According to the study, ash, pollen and tools left in Mezmaiskaya Cave suggests that "volcanic eruptions had an unusually sudden and devastating effect on the ecology and forced the fast and extreme climate deterioration ('volcanic winter') of the Northern Hemisphere." They point to evidence for a massive eruption around 40,000 years in southern Italy as well as a smaller eruption a bit earlier in the Caucasus.

Read more here:

http://content.usatoday.com/communities/sciencefair/post/2010/09/volcanoes-neanderthals-extinct/1