Out of Africa: Why?

It is becoming the consensus of scientists that study human evolution that human beings arose out of a small group of humans living in Africa 150,000 to 75,000 years ago. This "Out of Africa" theory is supported both by genetic evidence (human gene variation is greater in Africa than in other regions of the world) and by some recent evidence based on skeletons.

One of the unanaswered questions has been: why did humans leave Africa in the first place? A group of University of Arizona scholars may have an answer--we left Africa after a population boom caused by an end to a long drought in Africa:

From 135,000 to 90,000 years ago tropical Africa had megadroughts more extreme and widespread than any previously known for that region, according to new research.

Learning that now-lush tropical Africa was an arid scrubland during the early Late Pleistocene provides new insights into humans' migration out of Africa and the evolution of fishes in Africa's Great Lakes.

"Lake Malawi, one of the deepest lakes in the world, acts as a rain gauge," said lead scientist Andrew S. Cohen of The University of Arizona in Tucson. "The lake level dropped at least 600 meters (1,968 feet) -- an extraordinary amount of water lost from the lake. This tells us that it was much drier at that time."

He added, "Archaeological evidence shows relatively few signs of human occupation in tropical Africa during the megadrought period."

The new finding provides an ecological explanation for the Out-of-Africa theory that suggests all humans descended from just a few people living in Africa sometime between 150,000 and 70,000 years ago.

"We've got an explanation for why that might have occurred -- tropical Africa was extraordinarily dry about 100,000 years ago," said Cohen, a UA professor of geosciences. "Maybe human populations just crashed."

Other researchers have documented droughts in individual regions of Africa at that time, such as the Kalahari desert expanding north and the Sahel expanding south, he said. "But no one had put it together that those droughts were part of a bigger picture."

Tropical Africa's climate became wetter by 70,000 years ago, a time for which there is evidence of more people in the region and of people moving north. As the population rebounded, people left Africa, Cohen said.

Read it all here.