RAPID climate change in Africa two million years ago may have driven human evolution, researchers say.
The early landscape shifted between woodland to grassland half a dozen times over 200,000 years, meaning man had to adapt to survive.
Experts from Penn State University in the US say that this may have set the tone for the rapid evolution which then took place.
Writing in the Proceedings of the National Academy of Sciences, Clayton Magill said: “The landscape early humans were inhabiting transitioned rapidly back and forth. These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years.”
Early man’s diet would have to have adapt as a result. “Changes in food availability, food type or the way you get food can trigger evolutionary mechanisms to deal with those changes,” he said. “The result can be increased brain size.”
The findings appear to contradict previous theories which suggest evolutionary changes were gradual, and in response to either long and steady climate change or one drastic change.
Professor Katherine Freeman said: “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.”
This rapid change could have triggered development of the brain, said Magill. He 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.
“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.”
The research was carried out on lake sediments in the Olduvai Gorge in northern Tanzania.
Gas chromatography and mass spectrometry was used to determine the relative abundances of different leaf waxes and the abundance of carbon isotopes for different leaf waxes.
This data enabled the team to reconstruct the types of vegetation present at very specific time intervals.
The results showed that the environment transitioned rapidly back and forth between a closed woodland and an open grassland.
Prof Freeman said a number of factors may have caused this, including changes in the Earth’s movement and changes in sea-surface temperatures.
She said: “The orbit of the Earth around the sun slowly changes with time.
“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.”
There was also a correlation between changes in the environment and sea-surface temperature in the tropics.
Prof Freeman said: “We find complementary forcing mechanisms: one is the way Earth orbits, and the other is variation in ocean temperatures surrounding Africa.”
Magill added: “The research points to the importance of water in an arid landscape like Africa.
“The plants are so intimately tied to the water that if you have water shortages, they usually lead to food insecurity.
“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.”