"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," said researcher Clayton Magill, from the Penn State University.
"These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years," Magill said in a statement.
According to co-researcher Katherine Freeman, the current leading hypothesis suggests that evolutionary changes among humans during the period the team investigated were related to a long, steady environmental change or even one big change in climate.
"There is a view this time in Africa was the 'Great Drying,' when the environment slowly dried out over three million years. But our data show that it was not a grand progression towards dry; the environment was highly variable," she said.
According to Magill, many anthropologists believe that variability of experience can trigger cognitive development.
"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.
"Our data are consistent with these hypotheses. We show that the environment changed dramatically over a short time, and this variability coincides with an important period in our human evolution when the genus Homo was first established and when there was first evidence of tool use," said Magill.
The researchers examined lake sediments from Olduvai Gorge in northern Tanzania. They looked at bio-markers - fossil molecules from ancient organisms - from the waxy coating on plant leaves.
"We looked at leaf waxes because they're tough, they survive well in the sediment," said Freeman.
The team used gas chromatography and mass spectrometry to determine the relative abundances of different leaf waxes and the abundance of carbon isotopes for them.
The results showed that the environment transitioned rapidly back and forth between a closed woodland and an open grassland.
"The orbit of the Earth around the sun slowly changes with time. We find complementary forcing mechanisms: one is the way Earth orbits, and the other is variation in ocean temperatures surrounding Africa," Freeman said.
"The research points to the importance of water in an arid landscape like Africa," said Magill.
