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.
The researchers analyzed the cores for elements like hydrogen that leave distinctive signatures in sediment. These geochemical markers correspond with past precipitation levels, which influence weathering. They also examined ratios of aluminum and potassium, which indicate weathering intensity, because potassium is a highly mobile element whereas aluminum is one of the most immobile. As expected, the weathering patterns closely followed precipitation levels—that is, until about 3000 years ago. At that point, Bayon says, the pattern became completely different. The sediment appeared to have undergone intense chemical weathering, which the climate alone could not explain. So the team began suspecting another factor was responsible.
As it turns out, around this time Bantu farmers—an African ethnic group—had begun a large-scale expansion across Central Africa and settled in the rainforest. Linguistic studies and archeological evidence, such as stone tools and iron artifacts, support this event. Perhaps most importantly, archeologists have shown that the Bantu brought agriculture to the region, growing crops such as pearl millet and yams. But in order for pearl millet to grow, seasonality, or distinct wet and dry seasons, is necessary. In other words, climate shifts toward more pronounced seasonality paved the way for agriculture. To cultivate crops, the Bantu had to cut down stretches of forest, exposing the soil to weathering. Such intensive land use can lead to dramatically higher rates of chemical alteration, the researchers say, which would explain the sudden shift in weathering patterns 3000 years ago.
"Climate did play an important role in the arrival of agriculture," Bayon says. "But what we show is that the impact of those people developing and introducing agriculture probably had a quite significant impact on soil erosion."
"This is a very compelling study," 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. Even when compared with the traces natural climate change leaves, he says, "the human footprint on the environment can be very large."
"This article presents a very elegant study showing that early Bantu farmers did indeed have an impact on the environment," adds Terry Brncic, a paleoecologist previously based at the University of Oxford in the United Kingdom and now the coordinator of the Central Africa Forest Project for the World Resources Institute in the Democratic Republic of the Congo.. "I've been waiting for years for this kind of evidence."
But Katharina Neumann, an archeobotanist at Goethe University in Germany, disagrees with the researchers' conclusions. "The forest was not replaced by savanna and grassland, it was replaced by secondary forest formations, which is quite a different thing," she says. Secondary forests grow when original trees are cut down and are replaced by new trees. Even under today's intensive slash-and-burn land use, Neumann says, the forests regenerate very quickly, leaving no time for erosion. Instead of shifting into a new ecological zone, as the authors suggest, the forest never disappeared in the first place, she says. Neumann says she and her team's excavations of forest sites corresponding to the same time period found no evidence that the Bantu helped speed the forest's decline. "I think the whole assumption on which this paper is based is wrong."
There's "no question" the results are controversial, deMenocal says. The researchers put together "a really solid example" of the large impact human activities can have upon the environment, he adds, but it will take more studies to determine how much people were to blame for Central Africa's lost rainforests.