Scientists at Penn State and Rutgers University said their study challenges a long-held belief humans evolved in response to a long, consistent altering of the climate.
"The landscape early humans were inhabiting transitioned rapidly back and forth between a closed woodland and an open grassland about five to six times during a period of 200,000 years," Penn State geoscience graduate student Clayton Magill said.
"These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years," he said in a Penn State release.
This is in contrast to the current leading hypothesis of a single change in the environment that brought on drier conditions, researchers said.
"There is a view this time in Africa was the 'Great Drying,' when the environment slowly dried out over 3 million years," Penn State geoscience Professor Katherine Freeman said. "But our data show that it was not a grand progression towards dry; the environment was highly variable."
The reaction of early humans to the variations could have triggered cognitive development, Magill said.
"Early humans went from having trees available to having only grasses available in just 10 to 100 generations, and their diets would have had to change in response," he said. "Changes in food availability, food type, or the way you get food can trigger evolutionary mechanisms to deal with those changes. The result can be increased brain size and cognition, changes in locomotion and even social changes -- how you interact with others in a group."