A paleoclimatic hypothesis is presented to account for the evolution and eventual replacement of Homo sapiens neanderthalensis. Neandertal populations in the European Late Pleistocene were largely isolated by geographic barriers. Populations of modern Homo sapiens replaced Neandertals at 34000 years ago, near the end of the relatively cold oxygen isotope stage 3. These population were pushed into Europe by conditions brought on by increasing aridity affecting North Africa and southwestern Asia, and their dispersal was facilitated by lowered sea level.