'Megadroughts' Once Left Tropical Africa Desert-Like

Megadroughts that left lush parts of tropical Africa looking more like Arizona might have caused early human populations to crash, with the survivors eventually migrating out to other regions of the globe, according to new research from the University of Arizona (UA).

A UA research team reached that conclusion after studying cores representing tens of thousands of years of history from the bottom of Lake Malawi, one of Africa's Great Lakes.

"The area around Lake Malawi, which today is heavily forested and has rainfall levels comparable to the southeastern U.S., at that time would have looked like Tucson," said UA geoscientist Andrew S. Cohen, who conducted the research with Marshall Pardey and Doug Schnurrenberger. "During the megadrought, Lake Malawi was algae-filled and pea-soup green."

Multiple megadroughts are believed to have occurred from 135,000 to 90,000 years ago, the team writes in this week's online edition of the Proceedings of the National Academy of Sciences. During that period, core samples from Lake Malawi show few signs of pollen or charcoal, which indicates sparse vegetation and little to burn on land. The research results also indicate the great lake's water levels dropped dramatically during that time.

"Lake Malawi, one of the deepest lakes in the world, acts as a rain gauge," Cohen said. "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 that archeological evidence shows few signs of humans living in that part of Africa during that period.

The evidence of megadroughts, not only around Lake Malawi but in the Kalahari and the Sahel, at that time suggests that the changing climate might have been a major factor in human evolution.

"Maybe human populations just crashed," Cohen said.

As the climate started growing wetter again, more signs of human populations are found in the area. By 70,000 years ago, there is ample evidence not only for growing populations in the region, but for a growing number of people migrating north to other parts of Africa and beyond.

According to a leading theory for human development, known as the Out-of-Africa hypothesis, all humans on Earth are descended from just a few people who lived in Africa between 150,000 and 70,000 years ago.

University of Arizona, "Newfound Ancient African Megadroughts May Have Driven Evolution of Humans and Fishes." URL: (http://uanews.org/node/16277)