About 3000 years ago, Central Africa was a landscape in transition. Lush evergreen forests were gradually giving way to savannas and grasslands as regional climate change pushed the formerly humid weather patterns toward drier, slightly warmer conditions. But climate was not the only factor at play. According to a new study, an influx of humans into the region at this time may have helped drive some of the original rainforests into oblivion.

The paper's results, published online today in Science, came as a surprise to the researchers. "To be honest, at the beginning we were not at all aware of this human issue," says lead author Germain Bayon, a geochemist at the French Research Institute for Exploration of the Sea in Plouzané.

He and his colleagues originally set out to investigate the relationship between precipitation and chemical weathering, or the breakdown of soils and rocks. They analyzed marine sediment cores collected near the mouth of the Congo River, where thousands of years of runoff have accumulated. Because rocks are composed of different minerals, Bayon explains, those materials that are more susceptible to weathering will more readily erode away, eventually washing into the ocean and forming layers of clay on the bottom. By analyzing clay's composition, scientists can reconstruct the intensity of past weathering and infer environmental conditions....