Wet Weather Nudged Human Africa Exodus

Oct. 1, 2007 — The view through a new window into the climatic changes in northeastern Africa suggests that it was a wetter climate that encouraged humans to migrate out of Africa between 130,000 and 100,000 years ago.

The key to the discovery comes from a cutting-edge technique which uses dating of cave formations, called speleothems, to glean information about past wet and dry periods.

The speleothems in this study were 11 stalactites, stalagmites and flowstones collected from five caves found along the central and southern Negev Desert.

The Negev is the bottleneck through which all humans had to travel to expand into Eurasia. If it had always been as arid as it is today, humans could never have walked out of Africa, since the Sinai-Negev desert land bridge between the continents is now bereft of water and food.

"All possible migration routes leading from Africa...converge in the arid to hyper-arid Negev, Sinai, and southern Jordan Deserts, making it a key region for understanding climatic constraints on early modern human dispersal," reports Israeli geologist Anton Vaks and his colleagues in a paper on their discovery in the September issue of the journal Geology.

Uranium-thorium radioisotope dating of 33 samples from the speleothems show that most of their material — and therefore most of the rain — fell between 140,000 and 110,000 years ago.

During those 30 millennia there appear to have been relatively frequent wet periods, which made the deserts of the northeastern Sahara, Sinai, and the Negev more hospitable for early humans.