VANCOUVER, British Columbia, Feb. 21 (UPI) -- U.S. scientists say they now know why East Africa got drier starting 2 million years ago, creating grasslands on which early humans could further evolve.
Peter deMenocal, a paleoclimatologist at the Lamont-Doherty Earth Observatory in New York, says ocean temperature changes, including the arrival of a strong warm/cool difference along the equator in the Indian Ocean, could have triggered the change, ScienceNews.org reported.
"Those gradients are responsible for shifting rainfall towards or away from East Africa," deMenocal said at the annual meeting of the American Association for the Advancement of Science in Vancouver, British Columbia.
Around the time East Africa started shifting toward grasslands, early man began to develop new tools, diversify into new species and make the first migrations out of Africa, researchers said.
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The ocean gradient could have been the trigger for forests giving way to grasslands, deMenocal said.
"At first blush it doesn't seem intuitively obvious, but what controls rainfall in the tropics is where the warm water is," he said, noting more rain occurs where the ocean is warmest because water can more readily evaporate and fall back as rain.
Deep-sea sediment cores show a site in the eastern Indian Ocean and another in the western Indian Ocean had similar temperatures until around 2 million years ago when the eastern site warmed and the western site cooled, deMenocal said.
If so, then the cool waters off East Africa would have dried out that part of the continent, he said.
"I think this is pretty solid evidence for a transition to more open conditions [in East Africa] at this time."
