The widespread disappearance of rainforest in central Africa 3,000 years ago may have been caused by human activity.
A team of French researchers believes that as Bantu famers spread out across the region, they created corridors of savanna that eventually spread to create the grasslands of today.
The team, from Ifremer, came across the evidence by accident, following the analysis of marine sediment cores collected in 1998 by the research vessel L'Atalante.
"The results of the analysis of the core clearly show that the intensity of erosion chemical in the Congo Basin has fluctuated along with regional variations rainfall over the past 40,000 years," the authors write.
"However, from a certain period, about 3,000 years ago, while the rainfall in Africa experienced a marked decrease, the geochemical tracers indicate a sudden acceleration of the intensity of chemical weathering of soils, reaching levels not seen in the last 40,000 years, and completely decoupled from the natural climate signal."
What happened, they suggest, is that as Bantu people moved into the Congo basin from what's now Cameroon and Nigeria 4,000 years ago, they cleared large areas of land for farming and smelting iron. This affected the rate at which minerals weathered into the water supply, producing the unexpected results.
"This study suggests that the imprint of man in his environment was already very pronounced at this time," the authors say.