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.

Martin H. Trauth of the University of Potsdam in Germany and his colleagues found evidence for these wet periods in the sedimentary record of East Africa's Rift Valley lakes. The sediments contain assemblages of diatoms (silica algae), which are well known to be sensitive environmental indicators and can be used to reconstruct water depth and salinity. Studying the ancient diatoms, the team identified three times during which the lakes were deep as a result of regional climate change. These episodes also correlate with significant global climatic changes.

Rapid shifts between sustained periods of humidity and aridity, the researchers propose, would have provided the stress required for species to diverge. It is perhaps no coincidence, then, that the interval between three million and one million years ago witnessed the emergence of the genus Homo, the rise of Homo erectus and the first migration of hominids out of Africa, among other paleoanthropological events of note. A paper detailing these findings was published today in the journal Science.