From Scientific American: 
Shifts in climate that occurred in Africa between three million and one million years ago may have played a pivotal role in the speciation and dispersal of early humans, scientists say. Conventional wisdom holds that our hominid forebears evolved under increasingly arid conditions in East Africa. But the results of a new study suggest that this drying trend may have been interspersed with episodes of humidity, forcing humans and other mammals to adapt to their fast-changing environs.
More here.