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Ancient African Megadroughts May Have Driven
the Evolution of Humans and Fish, Researchers Say

RELEASED: Oct. 9, 2007

Dr. Kristina Beuning
Dr. Kristina Beuning

EAU CLAIRE — New research that documents extreme megadroughts thousands of years ago in Africa may provide an ecological explanation for humans' migration out of Africa during the early Late Pleistocene, said a University of Wisconsin-Eau Claire biologist whose research will be published in the Proceedings of the National Academy of Sciences this week.

From 135,000 to 90,000 years ago tropical Africa had megadroughts that were more extreme and widespread than previously known for that region, said Dr. Kristina Beuning, associate professor of biology at UW-Eau Claire. These new findings, as deduced from the study of sediment cores from Lake Malawi, indicate that Africa was an arid scrubland during the early Late Pleistocene, she said.

"At times, the ecosystem surrounding the lake probably resembled the high-elevation semi-deserts of South Africa today with insufficient ground vegetation to even carry fire," Beuning said.

The findings provide new insights into humans' migration out of Africa and the evolution of fish in Africa's Great Lakes, said Beuning, who is part of an international research team that studied the ecological consequences of the megadraughts and the impact they had on early-modern human origins.

"Our results suggest a narrow chronological window from 85,000 to 70,000 years ago in which climate was hospitable to early human populations up the entire Nile corridor, and represents the likely time of human population expansion out of equatorial Africa and the African continent," Beuning said.

Two articles by Lake Malawi Drilling Project members will be published in the Oct. 16 edition of the Proceedings of the National Academy of Sciences. "Ecological consequences of early Late Pleistocene megadroughts in tropical Africa" will be released online Oct. 9. "East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins," is already online at the Proceedings of the National Academy of Sciences.

The National Science Foundation, the International Continental Drilling Program and the Smithsonian Institution funded the research.

Researchers have worked for years to learn more about ancient Africa's climate and ecology by coring Africa's deepest lakes, Beuning said. They studied sediments cored from the bottom of Lake Malawi, an African rift lake that is 2,316 feet deep, and comparing the findings with similar records from Lakes Tanganyika and Bosumtwi, she said.

"Lake Malawi, one of the deepest lakes in the world, acts as a rain gauge," said lead scientist Andrew 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. Archaeological evidence shows relatively few signs of human occupation in tropical Africa during the megadrought period."

The scientists used radiocarbon and other dating techniques to establish the age of regions of the lake core, Cohen said, noting they took samples at 300-year-intervals.

The cores contain a record of things that fell or died in the lake, such as plankton, aquatic invertebrates, charcoal from fires and pollen from surrounding vegetation, Beuning said. Scientists used those materials to determine what the vegetation and the lake conditions were like at a particular point in time, she said.

Samples from the megadrought times had little pollen or charcoal, suggesting sparse vegetation with little to burn, Beuning said. They also found in the cores species of invertebrates and plankton that only live in shallow, algae-rich waters, she said, adding that Malawi now is a deep, clearwater lake, she said.

The African Great Lakes are known for the biological diversity of their fish species. A dramatic increase in the number of species was thought to have happened after a dry spell about 25,000 to 15,000 years ago, Beuning said. The new research suggests the rise in species diversity happened after the megadroughts, she said. By 70,000 years ago the lake had risen to its current level and it had become a freshwater lake as it is today, the scientists said.

The findings provide an ecological explanation for the Out-of-Africa hypothesis that suggests all humans descended from just a few people living in Africa sometime between 150,000 and 70,000 years ago, Cohen said.

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

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, Cohen said. As the population rebounded, people left Africa, he said.

Other researchers have documented droughts in individual regions of Africa at that time, Cohen said. But no one realized those droughts were part of a bigger picture, he said.

Cohen's co-authors on "Ecological consequences of early Late Pleistocene megadroughts in tropical Africa" are Beuning and Sarah Ivory of UW-Eau Claire; Jeffery Stone of The University of Arizona and the University of Nebraska at Lincoln; David Dettman and Peter Reinthal of The University of Arizona; Lisa Park of the University of Akron in Ohio; Christopher Scholz of Syracuse University in New York; Thomas Johnson and Erik Brown of the University of Minnesota, Duluth; John King of the University of Rhode Island at Narragansett; and Michael Talbot of the University of Bergen in Norway.

Cohen's co-authors on "East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins," are Beuning; Christopher Scholz and Robert Lyons of Syracuse University in New York.; Thomas Johnson, Erik Brown and Isla Castaneda of the University of Minnesota, Duluth; Jonathan Overpeck and Timothy Shanahan of The University of Arizona; John King and Clifford Heil of the University of Rhode Island at Narragansett; John Peck of the University of Akron, Ohio; Michael Talbot of the University of Bergen in Norway; Leonard Kalindekafe of the Malawi Geological Survey Department in Zomba; Philip Amoako of the Geological Survey Department of Ghana in Accra; Steven Forman, Jeanette Gomez and James Pierson of the University of Illinois at Chicago; and Lanny McHargue of the Scottish Universities Environmental Research Centre in East Kilbride.

-30-

JB

EDITORS NOTE:

Photos: Downloadable photos are available online

Researcher contact information:

Kristina Beuning, 715-379-7284, beuninkr@uwec.edu

Andy Cohen, 520-621-4691, cohen@email.arizona.edu

Jeffery Stone, 402-840-5294, jstone@unlserve.unl.edu

Related Web sites: Lake Malawi Drilling Project

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