Researchers say the growth of arid grasslands at the expense of tropical forests may have prompted the first humans to split off from other primates.
For instance, a study in the journal Nature in 2004 identified adaptations for running in human fossils more than two million years old. These adaptations may have enabled early humans to chase down prey on the open plains of Africa, researchers said. (See "Humans Were Born to Run, Fossil Study Suggests.")
More recent research, published last year in Science, suggested that a period of more sudden, regional climate fluctuation played a key role in human development.
Analysis of soil layers in the Great Rift Valley showed evidence for three unusually wet periods between 2.7 and 1 million years ago.
Fossils of aquatic algae indicated the presence of extinct lakes, some more than 328 feet (100 meters) deep, which quickly formed then disappeared.
Researchers say the lakes are evidence of the type of rapid climate swings that might have driven human evolution, forcing populations to adapt and readapt to cope with fast-changing environmental conditions.
"These temporary humid periods would have imposed huge impacts on early humans, and our research provides strong evidence for theories in which early human species evolved and spread out in response to a rapidly changing environment," said Mark Maslin, a co-author of the new study and a prehistoric-climate scientist at University College London, England.
But Behrensmeyer, the ancient-biology expert, said it remains unclear how fluctuating lake levels "could have exerted selective pressure on the immediate ancestor of Homo and resulted in the emergence of [the first humans]."
There is also uncertainty over the precise time and place of humanity's origin, she adds. Estimates range from 2.6 million to 1.7 million years ago. (Read "Did Early Humans First Arise in Asia, Not Africa?")
Behrensmeyer says samples from the bottoms of some of the world's oldest lakes, in East Africa, should provide more complete data "and a stronger bridge between oceanic and continental climate records."
Unlike sediment records found on land, the beds of deep tropical lakes are thought to hold relatively undisturbed records of climate change laid down in layers over thousands of years.
Now an international team of scientists hopes to get a better picture of African and global climate history from mud drilled up from the bottom of Lake Malawi in the Great Rift Valley.
The samples, dated to as many as 1.5 million years ago, should enable researchers to make detailed comparisons with climate change records from temperate and polar regions, according to lead researcher Christopher A. Scholz, of the Department of Earth Sciences at Syracuse University in upstate New York.
The project should uncover "an archive of environmental change that occurred in concert with human evolution," Scholz said.
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