Did prehistoric climate change help make us human?

For years, Rick Potts has excavated ancient stone tools in southern Kenya, at a site called Olorgesailie, seeking to piece together a picture of what life was like for early humans who lived there. But he and his colleagues were puzzled by what seemed to be a technological leap forward.

Dr. Potts, the director of the Smithsonian National Museum of Natural History’s Human Origins Program, said that the site yielded hundreds of thousands of years’ worth of stone hand axes, suggesting its residents were prolific toolmakers. But those tools seemed to disappear 500,000 years ago, and the next 180,000 years of the