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.
Similarly rainy periods in the northern and southern parts of Saharan-Arabian desert at the same time would have eliminated the desert between Africa and the Levant — the area in Southwest Asia bounded by the Mediterranean Sea in the west, the Arabian Desert in the south, and Mesopotamia to the east.
"The humid period in the Negev Desert between 140,000 and 110,000 years ago was preceded and followed by essentially unbroken arid conditions; thus creating a climatic window for early modern human migration to the Levant," the researchers explain.
"This is a taste of what can be gleaned from speleothems all over the world," said cave researcher Penelope Boston of New Mexico Tech in Socorro.
Already the same sort of work is underway in the American Southwest to make sense of mysterious settlement patterns of early Native Americans. This kind of research may not only help clarify the early history of humans, but also of other animals, she said.
"The potential for this is huge," said Boston. "I think it's hot."