UA scientist: Drought recovery 100,000 years ago spurred African exodus

Recovery from a "megadrought" 100,000 years ago precipitated the exodus of ancient humans from tropical Africa to Europe and Asia, according to a UA researcher.
Analysis of soil samples extracted from below the bottom of the 2,000-foot-deep Lake Malawi in Africa shows the area believed to the cradle of anatomically modern humans depicts an area hit hard by drought, said Andy Cohen, University of Arizona geology professor.
Cohen is lead researcher and lead author of "Ecological consequences of the early Late Pleistocene megadroughts in tropical Africa," which was released Monday.
Cohen's findings offer an ecological explanation for the "out of Africa" hypothesis that suggests that all modern humans descended from a core group of ancient people living in Africa between 150,000 and 70,000 years ago.
The lake-bottom core samples show the area became rejuvenated between 90,000 and 70,000 years ago, with lakes rising to current levels following the severe drought.
"That period of time gives the ideal window when people might have gotten out of Africa," he said. "It was getting wetter and the Nile was flowing fully providing a north/south corridor.
"This might provide a mechanism for explaining the timing," he said. "When conditions got better we saw an expansion into a lot of other areas. Population growth allowed people to expand."
The spring 2005 field work was filled with challenges.
Lake Malawi, bordered by Malawi, Mozambique and Tanzania, is landlocked, so a ship specially equipped for ocean-floor drilling could not be used.
Cohen's group leased a 160-foot, 800-ton barge and converted it into a drilling platform using equipment and staffing from Seacore, a English drilling company.
"Technologically and logistically it was very tough," he said. Vessel issues forced expenses and delays that cut 45 planned drilling days to 23, he said.
Despite the challenges, the rig was able to bore into the lake floor sediment more than 380 meters - 1,250 feet - and secure 3-inch diameter core samples for analysis.
Cohen spent more than a year analyzing the top 80 meters, or 265 feet, representing the past 150,000 years, of the 380 meter long core samples prior to writing the report that was released Monday.
Research continues, and the bottom end of the core samples may chart history from as long ago as 750,000 years.
"We're just scratching the surface of what these core samples have to tell us," Cohen said.