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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.”