Indian Ocean Set Stage for Humans: Temperature Shifts Dried out East Africa 2 Million Years Ago
Witze, Alexandra, Science News
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 the genus Homo further evolved. 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.
Researchers infer that East Africa started shifting toward grasslands by looking at 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. …