VANCOUVER — Scientists may now be able to explain one of the key events that shaped human history: why East Africa got drier starting around 2 million years ago, with forests giving way to grasslands on which Homo could further evolve. Ocean temperature changes, especially the arrival of a strong warm/cool difference along the equator in the Indian Ocean, could have triggered the change.
“Those gradients are responsible for shifting rainfall towards or away from East Africa,” said Peter deMenocal, a paleoclimatologist at the Lamont-Doherty Earth Observatory in Palisades, N.Y. He presented new details about his idea on February 17 at the annual meeting of the American Association for the Advancement of Science.
Researchers infer that East Africa, where humans first evolved, started shifting toward grasslands by looking at data on the proportion of fossils of grazing animals, which peaked around 1.5 million years ago. Around this time Homo began to develop new tools, diversify into new species and make its first tentative forays out of Africa.
But despite a raft of theories, scientists haven’t been able to explain what triggered the drying responsible for the shift to grasslands. The ocean gradient idea might do the trick, 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. More rain occurs where the ocean is warmest, because water can more readily evaporate and fall back as rain.
deMenocal and his colleagues looked at deep-sea sediment cores, representing the last several million years, drilled across the Indian, Pacific and Atlantic oceans. Most strikingly, a site in the eastern Indian Ocean and another in the western Indian Ocean show temperatures tracking similarly until around 2 million years ago, when the eastern site warmed and the western site cooled. If so, then the cool waters off East Africa would have dried out that part of the continent, deMenocal said.
Next the scientists ran computer simulations that erased the east/west temperature difference and showed what might happen if temperatures were the same along the equator across the Indian Ocean. That change shut down a type of atmospheric circulation, making East Africa wetter than it is today.
Taken together, deMenocal said, “I think this is pretty solid evidence for a transition to more open conditions [in East Africa] at this time.” What kicked off the change in ocean temperatures, though, remains a mystery.
Other, more recent climate changes may have also shaped the course of human evolution, said Andrew Weaver, a climate scientist at the University of Victoria in Canada. At the conference he reported new simulations looking at climate changes that happened about 105,000 years ago, around the time modern humans, Homo sapiens, were moving out of Africa.
When the northern ice sheets dumped great icebergs into the ocean, freshening the water there, rain belts in Africa also shifted, Weaver reported. In this case, the change may have dried out much of northern Africa, compelling H. sapiens to leave its birth continent.