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“Megadroughts” may have driven human evolution

Oct. 8, 2007
Courtesy University of Arizona 
and World Science staff

From 135,000 to 90,000 years ago, now-lush trop­i­cal Af­ri­ca suf­fered “megadroughts” more ex­treme and wide­spread than any pre­vi­ously known for the re­gion, new stud­ies sug­gest. The find­ing of­fers in­sights in­to hu­mans’ ev­o­lu­tion in and migra­tion out of Af­ri­ca, the re­search­ers say.

A­long­side Lake Ma­la­wi to­day. (Im­age cour­te­sy Lake Ma­la­wi Drill­ing Proj­ect)


Tropical Af­ri­ca’s Lake Ma­la­wi, one of the world’s deep­est, “acts as a rain gauge,” said An­drew S. Co­hen of the Un­ivers­ity of Ar­i­zo­na in Tuc­son, Ariz., one of the sci­en­t­ists.

At the time of the great droughts, “the lake lev­el dropped at least 600 me­ters [1,968 feet]—an ex­tra­or­di­nary amount of wa­ter lost,” he went on. “This tells us that it was much dri­er” in that per­iod.

Ar­chae­o­log­i­cal ev­i­dence, he added, “shows rel­a­tively few signs of hu­man oc­cupa­t­ion in trop­i­cal Af­ri­ca” at the time. 

Al­though the dis­cov­er­ies sug­gest an ex­plana­t­ion for early hu­man migra­t­ions from Af­ri­ca, Co­hen said, the on­set of drought it­self is­n’t what seems to have driv­en peo­ple out. Rath­er, the find­ings point to a pic­ture in which droughts cor­re­spond­ed with a po­pu­la­t­ion crash, and peo­ple left the con­ti­n­ent only as their num­bers lat­er re­cov­ered.

A the­o­ry pop­u­lar among sci­en­tists, called the Out-of-Af­ri­ca hy­poth­e­sis, sug­gests all hu­mans de­scend from just a few peo­ple liv­ing in Af­ri­ca some­time be­tween 150,000 and 70,000 years ago. Now “we’ve got an ex­plana­t­ion for why that might have oc­curred,” Co­hen said.

Oth­er re­search­ers have doc­u­mented droughts in var­i­ous parts of Af­ri­ca then, “but no one had put it to­geth­er that those droughts were part of a big­ger pic­ture,” Co­hen said. Trop­i­cal Af­ri­ca 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, he added; as the popula­t­ion re­bounded, peo­ple left Af­ri­ca.

The find­ings are sched­uled for pub­lica­t­ion in the Oct. 16 edi­tion of the re­search jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences.

Co­hen and col­leagues have been stu­dy­ing an­cient Af­ri­can cli­mate and ecol­o­gy us­ing a tech­nique known as cor­ing. This in­volves ex­tracting a ver­ti­cal col­umn of earth from the ground that con­tains sam­ples of dif­fer­ent lay­ers from dif­fer­ent time pe­ri­ods. Thus, each lay­er can be an­a­lyzed. 

The re­search­ers have been cor­ing some of Af­ri­ca’s deep­est lakes and hit on the megadrought find­ings based on sed­i­ments from the bot­tom of Lake Ma­la­wi, now 2,316 feet (706 me­ters) deep. They com­pared those find­ings with si­m­i­lar rec­ords from Lakes Tan­gan­yika and Bo­sum­twi, two others in tro­pi­cal Af­rica. Such lake cores con­tain a rec­ord of the things that fell in or died in the lake—plank­ton, in­ver­te­brates, pol­len, or char­coal from fires on land. Sci­en­tists an­a­lyze the ma­te­ri­als to learn what the vegeta­t­ion and lake con­di­tions were like at var­i­ous times. 

Some cores were as much as as 1247 feet (380 me­ters) long, rep­re­sent­ing hun­dreds of thou­sands of years of his­to­ry, they said. Cor­ing Lake Ma­la­wi was a spe­cial chal­lenge be­cause it’s land­locked. Re­search­ers had to ship the nec­es­sary equip­ment over­land, then rent a barge and out­fit it as a sci­en­tif­ic drill­ing ves­sel. It re­quired a type of GPS po­si­tion­ing sys­tem that would steady the boat through wind and waves—other­wise, they said, the costly drill­ing equip­ment might snap.

Sam­ples from the mega­drought times had lit­tle pol­len or char­coal, sug­gesting sparse vegeta­t­ion in the sur­round­ing ar­ea with lit­tle to burn, re­search­ers said. “The ar­ea around Lake Ma­la­wi, which to­day is heavily forested and has rain­fall lev­els com­pa­ra­ble to the south­east­ern U.S., at that time would have looked like Tuc­son,” Co­hen re­marked. An­oth­er sign of drought in the cores: spe­cies of in­ver­te­brates and plank­ton that only live in shal­low, algae-rich wa­ters, he said. “Dur­ing the mega­drought, Lake Ma­la­wi was algae-filled and pea-soup green.”


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From 135,000 to 90,000 years ago, now-lush tropical Africa suffered “megadroughts” more extreme and widespread than any previously known for the region, new research suggests. The finding offers insights into humans’ evolution and migration out of Africa, the scientists say. Africa’s Lake Malawi, one of the world’s deepest, “acts as a rain gauge,” said Andrew S. Cohen of The University of Arizona in Tucson, Ariz., one of the researchers involved. At the time of the megadrought, he added, “the lake level dropped at least 600 meters (1,968 feet)—an extraordinary amount of water lost... This tells us that it was much drier” in that era. Archaeological evidence, he added, “shows relatively few signs of human occupation in tropical Africa” at the time. Although the discoveries suggest an explanation for early human migrations from Africa, Cohen said, the onset of drought itself isn’t what seems to have driven people out. Rather, the findings point to a picture in which droughts corresponded with a population crash; humans seem to have left Africa only as populations later recovered. A theory popular among scientists, called the Out-of-Africa hypothesis, suggests all humans descend from just a few people living in Africa sometime between 150,000 and 70,000 years ago. “We’ve got an explanation for why that might have occurred,” Cohen said. Other researchers have documented droughts in various parts of Africa then, “but no one had put it together that those droughts were part of a bigger picture,” Cohen said. Tropical Africa 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, he added; as the population rebounded, people left Africa. The findings are scheduled for publication in the Oct. 16 edition of the research journal Proceedings of the National Academy of Sciences. Cohen and colleagues have been studying ancient African climate and ecology using a technique known as coring. This involves extracting a vertical column of earth from the ground that contains samples of different layers of earth from different time periods. Thus, each layer can be analyzed. The researchers have been coring some of Africa’s deepest lakes and hit on the megadrought findings based on sediments cored from the bottom of Lake Malawi, an African lake currently 2,316 feet (706 meters) deep. They compared those findings with similar records from Lakes Tanganyika and Bosumtwi. Such lake cores contain a record of the things that fell in or died in the lake—plankton, invertebrates, pollen, charcoal from fires on land. Scientists analyze the materials to learn what the vegetation and the lake conditions were like at various times. Some of the cores were as much as as 1247 feet (380 meters) long, representing hundreds of thousands of years of African history, they said. Coring Lake Malawi was a special challenge because it’s landlocked. Researchers had to shipped the necessary equipment overland, then rent a barge and outfit it as a scientific drilling vessel. It required a type of GPS positioning system that would steady the boat through wind and waves—otherwise, they said, the costly drilling equipment might snap. Samples from the megadrought times had little pollen or charcoal, suggesting sparse vegetation in the surrounding area with little to burn, researchers said. “The area around Lake Malawi, which today is heavily forested and has rainfall levels comparable to the southeastern U.S., at that time would have looked like Tucson,” Cohen remarked. Another sign of drought in the cores were species of invertebrates and plankton that only live in shallow, algae-rich waters, he said. “During the megadrought, Lake Malawi was algae-filled and pea-soup green.”