October 11, 2007
Drought Pushed Ancient African Migration
A University of Arizona researcher suspects that a monster drought is behind our ancestors' massive bug-out from Africa.
Lake Malawi, the third-largest lake in Africa and ninth-largest in the world, dropped nearly 2,000 feet during a mega-drought roughly 100,000 years ago, according to the UA's Andrew S. Cohen, the lead scientist on a report in the latest edition of the Proceedings of the National Academy of Sciences.
The research was funded by the National Science Foundation, the International Continental Drilling Program and the Smithsonian Institution.
Cohen says the now lush, forested tropical area -- with rainfall like that of the Southeastern United States -- probably looked something like Tucson during the drought. And beautiful, deep and remarkably clear Lake Malawi would have been a salty, pea soup pond by comparison.
The massive droughts, thought to have occurred between 135,000 and 90,000 years ago would overlap the Out-of-Africa theory that said modern humans descend from a relatively small number of people living in Africa between 150,000 and 70,000 years ago.
Cohen theorizes that the drought and resulting desertification likely caused a crash in population. And the rebound of the lake coincides with archaeological work that shows an increase in population and people leaving the continent.
Members of the Malawi Drilling Project used a ship to drop a deep sea-style rig 1,942 feet to the lake bottom and extract core samples 415 feet beneath the lake bottom. The core samples give a picture of conditions there over hundreds of thousands of years, Cohen said.
Carbon and other sample analysis -- taken at 300-year intervals -- provides a record of things that fell into or died in the lake. Besides remains of fish, plankton and invertebrates there was pollen from plants surrounding the lake and charcoal from fires.
In the case of fish bones, scales and teeth, chemical analysis of the fish parts in the core samples could tell whether the sample point was beneath shallow or deep water when the sample drifted down to the bottom.
"You are what you eat," said David Dettman, a member of the science team and the director of the UA's Environmental Isotope Laboratory.
As such, the ratio of carbon and nitrogen isotopes differs between fish that lived in shallow and deep water, Dettman said. (An isotope is a different form of an element, having a different number of neutrons.)
"It tells you about the food resources of these fish," says Dettman. "By the chemistry in the bones you can tell if they're fish feeding in the rocky shallows near the shore" or in the deep open water.
During some periods, the core sample taken from what is now the deepest part of the lake shows the lake was much shallower.
Like an "in box," the oldest deposits in the core sample were at the bottom of the stack -- the farthest from the present lake bottom.
Cohen says it's completely different from drilling for oil.
Oil drillers, Cohen says, want to make a hole. The drilling scientists want the stuff that was in the hole (and truly hope they don't hit oil or gas).
"We have to get the mud back intact," says Cohen. "The upper 100 meters is full of water, squishy."
Cohen is already working on another Malawi drilling project, one that he hopes will be cheaper and easier. Coring on dry land, in former lake beds that have been elevated by geologic faults, should give them the same kind of climate and environmental records -- but without the expense and difficulties of drilling in a 2,000-foot deep lake.
"We hope to get the same quality record," Cohen says, "but be able to drive a drill rig up to the (rock) outcropping."
But Lake Malawi itself is interesting for other reasons, too. It's a freshwater equivalent to the Galapagos Islands: an isolated place where scientists can study dramatic evolutionary changes in the lake's fish the way Darwin and later scientists studied the Galapagos' finch species.
Only four of Lake Malawi's 500 to 1,000 fish species are found anywhere else, says Peter Reinthal, a UA adjunct associate professor of ecology and evolutionary biology and curator of fish.
He compared them to Darwin's Galapagos finches, highly specialized fish that fill niches in the giant lake's food chain.
Some have made downright weird adaptations.
Many of Lake Malawi's fish are mouth brooders, incubating eggs in their mouth, even sheltering the hatchlings after birth to keep them out of danger. Nice adaptation, but it doesn't end there.
Other fish have adapted to take advantage of that trait.
"One (predator fish) has a head like a battering ram," Reinthal says. "They'll swim directly at a female with a mouthful of young and try to disgorge them."
Others, Reinthal said, feed on the scales of other fish, or scrape them off and mimic them.
--Contact reporter Dan Sorenson at 573-4185 or [email protected]