Researchers have long speculated that ancient climate changes have had a powerful influence on human evolution, spurring our ancestors to walk upright, to migrate out of Africa, and to adapt to varied habitats. Now, scientists have found a concrete example: evidence of intense droughts in tropical Africa right when many anatomically modern humans were probably dying. The new findings provide an ecological explanation for the Out-of-Africa hypothesis that all modern humans are descended from a small number of ancestors who survived a population bottleneck in Africa between 150,000 and 70,000 years ago and gave rise to offspring who migrated to Asia and Europe.
The researchers discovered the ancient droughts by drilling cores of sediments from the bottom of Lake Malawi, an ancient lake in the southern African tropics. They trucked in an ocean-drilling rig, then rented a barge and used it as a stage for lowering drilling equipment 592 meters to the lake bottom where they bored another 592 meters into the sediment. As they analyzed the core, they used radiocarbon and other dating methods to date segments of the core, which spanned about 1 million years. They find not only that the lake lost about 95% of its water between 135,000 and 75,000 years ago but also that materials that had settled on the lake floor during that time also revealed that the lake was surrounded by extremely arid conditions. Species of plankton and crustaceans were those that lived in shallow, alkaline waters, rather than deep clear-water species in the lake today. There also was little pollen or charcoal, suggesting that there was too little vegetation to burn or shed pollen--and that the area around the lake would have resembled the highlands of the Karoo National Park in South Africa or Tucson, Arizona.
Starting 70,000 years ago, the climate turned wetter. From the core of Lake Malawi and records from lakes Tanganyika in East Africa and Bosumtwi in West Africa, the researchers documented a major rise in water levels. That climate change could have allowed surviving humans to migrate along the Nile River valley or other routes out of Africa, perhaps giving them a route through the dry lands and driving migrations into Eurasia, where they underwent a population explosion, says Andrew Cohen, a paleolimnologist at the University of Arizona in Tucson, who led one of two studies published online this week in the Proceedings of the National Academy of Sciences.
"This is really a first--they have shown that enormous things have happened [to the climate] in the past that were totally unexpected 10 years ago," says paleoanthropologist Andrew Hill of Yale University, who adds that earlier models viewed the east African environment as changing gradually over millions of years. "This shows things were changing fairly rapidly--huge droughts were happening where hominids were living."