A series of rapid environmental changes in East Africa roughly 2 million years ago may be responsible for driving human evolution, according to researchers at Penn State and Rutgers University. "The landscape early humans were inhabiting transitioned rapidly back and forth between a closed woodland（林地） and an open grassland about five to six times during a period of 200,000 years," said Clayton Magill, graduate student in geosciences at Penn State. "These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years."

According to Katherine Freeman, professor of geosciences, Penn State, 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 3 million years," she said. "But our data show that it was not a grand progression towards dry; the environment was highly variable."

According to Magill, many anthropologists（人类学家） 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（运动，移动） 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."

The researchers -- including Gail Ashley, professor of earth and planetary sciences, Rutgers University -- examined lake sediments from Olduvai Gorge in northern Tanzania. They removed the organic matter that had either washed or was blown into the lake from the surrounding vegetation, microbes and other organisms 2 million years ago from the sediments. In particular, they looked at biomarkers -- fossil molecules from ancient organisms -- from the waxy（柔软的，蜡色的） coating on plant leaves.

"We looked at leaf waxes because they're tough, they survive well in the sediment," said Freeman.