Washington, Oct 9 : Tropical Africa had megadroughts more extreme and widespread than any previously known region from 135,000 to 90,000 years ago, a new study has revealed.

The scientists discovered the ancient megadroughts by studying sediments cored from the bottom of Lake Malawi, an African rift lake that is currently 2,316 feet (706 meters) deep, and comparing those findings with similar records from Lakes Tanganyika and Bosumtwi.

“What's unique about the Malawi, Tanganyika and Bosumtwi cores is that they're continuous records. We can see what happened in one place over a long period of time,” said lead scientist Andrew S. Cohen of The University of Arizona in Tucson.

The researchers used radiocarbon and other dating techniques to establish the age of regions of the Malawi cores. Then they took samples at 300-year-intervals.

Samples from the megadrought times revealed they had little pollen or charcoal, suggesting sparse vegetation with little to burn.

“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 Cohen.

Cohen said one indicator of drought present in the cores were species of invertebrates and plankton that only live in shallow, turbid, algae-rich waters – a situation very different from the deep, clearwater lake that Malawi is now.

“During the megadrought, Lake Malawi was algae-filled and pea-soup green, much like modern-day Lake Turkana. Lake Turkana is known as the Jade Sea,” said Cohen.

He said the discovery that the now-lush tropical Africa was an arid scrubland during the early Late Pleistocene provided 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. 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 said.

“Archaeological evidence shows relatively few signs of human occupation in tropical Africa during the megadrought period,” he added.

Cohen said the new finding provided an ecological explanation for the Out-of-Africa hypothesis, which suggested that 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. Maybe human populations just crashed,” said Cohen, a UA professor of geosciences.

Prof. Cohen said 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, said Prof. Cohen, adding that 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.

“But no one had put it together that those droughts were part of a bigger picture,” he said.

The two researches, “Ecological consequences of early Late Pleistocene megadroughts in tropical Africa,” and “East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins,” appear online in this week’s Proceedings of the National Academy of Sciences (PNAS).

Both articles are scheduled for publication in the Oct. 16, 2007, print edition of the PNAS. (ANI)