New research from Penn State and Rutgers University has reshaped the idea of what drove human evolution 2 million years ago, pointing the finger at a series of rapid environmental changes in East Africa rather than one single environmental change.
“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. “These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years.”
Previous hypotheses revolved around a long steady environmental change or even one big change.
“There is a view this time in Africa was the ‘Great Drying,’ when the environment slowly dried out over 3 million years,” said Katherine Freeman, professor of geosciences, Penn State. “But our data show that it was not a grand progression towards dry; the environment was highly variable.”
“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,” Magill 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.”
Magill and Freeman, along with Gail Ashley, professor of earth and planetary sciences at Rutgers University, examined lake sediments from Olduvai Gorge in northern Tanzania. By studying leaf waxes removed from the Gorge they were able to determine the levels of vegetation in the region at very specific time intervals.
Subsequently they were able to find that the environment transitioned rapidly back and forth between closed woodland and an open grassland over the period in question.
The researchers then turned to statistical and mathematical models to correlate the changes they saw in the environment with other factors taking place at the same time, including changes in the Earth’s movement and changes in the temperatures of the oceans.
“The orbit of the Earth around the sun slowly changes with time,” said Freeman. “These changes were tied to the local climate at Olduvai Gorge through changes in the monsoon system in Africa. Slight changes in the amount of sunshine changed the intensity of atmospheric circulation and the supply of water. The rain patterns that drive the plant patterns follow this monsoon circulation. We found a correlation between changes in the environment and planetary movement.”
The team also found a correlation between changes in the environment and sea-surface temperature in the tropics.
“We find complementary forcing mechanisms: one is the way Earth orbits, and the other is variation in ocean temperatures surrounding Africa,” Freeman added.