From 135,000 years 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," says lead scientist Andrew Cohen of the University of Arizona in Tucson. "The lake level dropped at least 1,968 feet – an extraordinary amount of water lost from the lake. This tells us that it was much drier at that time."

He adds, "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 hypothesis that suggests all humans descended from just a few people living in Africa sometime between 150,000 years 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," says Cohen, a 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 says. "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 says.

Extracting Cores from Lake Malawi

Cohen and his colleagues have been working for years to learn more about ancient Africa's climate and ecology by coring Africa's deepest lakes.

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 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," Cohen says. Extracting cores from Lake Malawi required the kind of rig used in ocean-going drilling expeditions. Those expeditions just sail a drill-equipped ship to the desired site.

However, the Lake Malawi Drilling Project's target was land-locked.

The international research team collected the equipment necessary, shipped it overland, rented a barge and outfitted it to become a scientific drilling vessel. They equipped the ship, M/V Viphya, with the type of GPS positioning system needed to hold the large ship steady under windy and wavy conditions. The drilling equipment was lowered 1,942 feet to the lake bottom and bored into the lake's sediment another 1,247 feet. If the ship didn't hold its position over the drilling site, the expensive drilling equipment might snap.

The work was successful – the team extracted a series of cores, some as much as 1,247 feet long, representing hundreds of thousands of years of African history.

Such lake cores contain a high-resolution record of the things that fell in or died in the lake – plankton, aquatic invertebrates, charcoal from fires on land, pollen from the surrounding vegetation. Scientists analyze those materials to figure out what the vegetation and the lake conditions were like at a particular point in time.

Although the team has used the lake cores to peer back in time 150,000 years, there's still much more to do: The Lake Malawi core represents as much as 1.5 million years of tropical Africa's past.

Look for the full report on this project in the December print issue of National Driller.