A study of African sediment cores suggests that ancient climate change stimulated the expansion, migration and, ultimately, evolution of early humans. Writing in this week's Proceedings of the National Academy of Science, the researchers say that 75,000 to 135,000 years ago, a series of "megadroughts" dried up many of Africa's lakes and other water sources. But just 5,000 years after those droughts, the climate swung wildly, becoming much wetter. That change to a wetter world (which, with global warming, we may again be entering) was more favorable to early humans. It fostered their migration to various parts of Africa and eventually to other parts of the world. The theory that a changing climate helped human development is supported by a 2005 report that periods of great, rapid climate change were accompanied by increases in human brain size and complexity. Scientists theorize that the stress of adapting to a rapidly changing climate, with its altered food and water sources, forced humans to become more adaptable and find new ways to reap benefits from whatever resources were immediately available.