About 3000 years ago, Central Africa was a landscape in transition. Lush evergreen forests were gradually giving way to savannahs and grasslands, as the regional climate shifted toward drier, slightly warmer conditions. But according to a new study, climate was not the only factor at play - an influx of humans into the region at this time may have helped push some of the original rain-forests into extinction.
     The paper's results 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 Plouzane.
     He and his colleagues originally set out to investigate the relationship between rainfall and the weathering of soils and rocks. They analysed marine sediment cores collected near the mouth of the Congo River, where thousands of years of runoff have accumulated. By analysing clay's composition, scientists can reconstruct the intensity of past weathering and infer environmental conditions.
     As expected, the weathering patterns closely followed rainfall levels - that is, until about 3000 years ago, when the pattern became completely different. The sediment appeared to have undergone intense chemical weathering, which the climate alone could not explain.
     As it turns out, around this time Bantu farmers had begun a large-scale expansion across Central Africa and settled in the rainforest. Linguistic studies and archeological evidence support this event. The Bantu brought agriculture, growing crops such as pearl millet and yams. Climate shifts toward more pronounced seasonality made this possible, but to cultivate crops the Bantu had to cut down stretches of forest, exposing the soil to weathering. Such intensive land use, the researchers say, which would explain the sudden shift in weathering patterns 3000 years ago.
     There's "no question" the results are controversial, says  Peter deMenocal, a marine geologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, who was not involved in the work, adding that it will take more studies to determine how much people were to blame for Central Africa's lost rain-forests.

Edited from Science (9 february 2012)