Climate research gives clues to human expansion out of tropical Africa (10/10/2007)

New research that involves a University of Nebraska-Lincoln scientist has shed light on an important, but previously little-understood period in Africa's climate history that has implications for understanding human evolution and the expansion of Homo sapiens out of tropical Africa.

In a paper published this week in the online version of the Proceedings of the National Academy of Sciences, a team headed by Andrew Cohen of the University Arizona and including UNL's Jeffery Stone as the second author, reported findings from sediment cores recovered from one of the world's deepest lakes, Lake Malawi in East Africa's Great Rift Valley. Cohen, Stone and colleagues reported finding evidence of two extended periods of extreme aridity between 135,000 and 70,000 years ago, an important time in human pre-history.

"Prior to this research, there was not a really good terrestrial record of climate that stretched back through the period of human development and migration from the tropical region of Africa," said Stone, an adjunct faculty member in the UNL Department of Geosciences who also has a research appointment at Arizona.

"Most of the previous records basically stretch back to the last glacial maximum, maybe 20,000 years. They show some really dry conditions and it's assumed that that had a huge impact on human populations in Africa, but nobody really had a sense of what was going on before that."

The scientists studied a variety of fossils and other sediments that settled to the lake bottom over the millennia and used them as proxies to interpret the climate at various times during the last 140,000 years. For example, diatoms, Stone's specialty, are one-celled organisms having a silica skeleton that fossilizes readily. Different species of diatoms flourish or fail during different climatic conditions, so their relative abundance or absence provides a good indication of contemporary climate conditions.

What the diatoms and other proxies indicate is a Lake Malawi basin between 135,000 and 70,000 years ago that looked a lot different from the lush conditions found there today. Modern Lake Malawi has a surface area of 29,500 square kilometers (more than 11,000 square miles) and reaches a depth of 706 meters (more than 2,300 feet). Annual rainfall in its watershed varies from 800 to 2,400 millimeters a year (31-93 inches; for comparison, Lincoln averages 27.8 inches of rainfall per year).

But records from the sediment cores reveal two periods of megadrought, from 135,000 to 127,000 and 115,000 to 95,000 years ago, when the level of Lake Malawi fell 550 to 600 meters (1,800-1,970 feet) below present-day levels. The surrounding watershed was a semidesert that received less than 400 millimeters (16 inches) of rain per year, creating much drier conditions than occurred during the last glacial maximum, 35,000 to 15,000 years ago, when Lake Malawi's level fell by only 30 to 200 meters.

There is little archaeological evidence of human habitation in tropical Africa during the megadroughts, a period that coincides with the earliest evidence of humans outside the region -- about 125,000 years ago in North Africa and the Middle East.

The research by Stone and colleagues, however, indicates that tropical Africa's climate became wetter after 70,000 years ago and reached conditions comparable to today by about 60,000 years ago. That period coincided with increased evidence of human habitation in the area, and closely coincided with increased aridity in other parts of the continent.

Cohen said the new finding provides an ecological explanation for the "Out-of-Africa" hypothesis that suggests that all humans descended from just a few people living in Africa sometime between 150,000 and 70,000 years ago. He said it's possible that the human population crashed during the megadroughts, but rebounded when the climate became more hospitable. The growing human population eventually expanded down the Nile valley and dispersed around the globe.

The PNAS paper concluded that this timing is "consistent with the idea that the earlier (approximately 125,000 years ago) documented occurrence of modern humans in North Africa and the Levant represents ultimately unsuccessful 'excursions' out of Africa."

The article is scheduled for publication in the Oct. 16 print edition of PNAS. The National Science Foundation, the International Continental Drilling Program and the Smithsonian Institution funded the research.

Note: This story has been adapted from a news release issued by the University of Nebraska-Lincoln

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