Environmental changes that occurred over two million years ago may have lead to human evolution, according to a new study.
"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 Clayton Magill, graduate student in geosciences at Penn State.
Magill added that these changes occurred very abruptly, with each change taking just few hundred or thousand years.
The research challenges the current hypothesis that humans evolved due to steady, long-lasting changes in the environment. Instead, researchers say that sudden changes may have kicked-off human evolution, improving cognitive skills.
"There is a view this time in Africa was the 'Great Drying,' when the environment slowly dried out over 3 million years. But our data show that it was not a grand progression towards dry; the environment was highly variable," said Katherine Freeman, professor of geosciences, Penn State, in a news release.
For the study, researchers analyzed lake sediments from Olduvai Gorge in northern Tanzania. They removed the organic matter from these sediments that was either blown or washed away from plants that were growing around the lake. In particular, researchers looked for biomarkers from the waxy coating on the plant leaves. These biomarkers were fossil molecules from organisms that inhabited the landscape about 2 million years ago.
Study results showed that the area changed from closed woodland to open grassland many times in a short span of time.
"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. 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," said Magill.
Researchers also used statistical and mathematical models to determine what caused this abrupt change in the environment. They found that earth's movement along with sea-surface temperatures were among the many changes that may have contributed to the change in the environment.
Another paper by the research team showed that during the same period, rainfall was greater when there were more trees and sparse when the area was a grassland.
"Together, these two papers shine light on human evolution because we now have an adaptive perspective. We understand, at least to a first approximation, what kinds of conditions were prevalent in that area and we show that changes in food and water were linked to major evolutionary changes," Magill concluded.
The study is published in the journal Proceedings of the National Academy of Sciences.