Deforestation and climate change may sound like familiar concerns to the modern ear. But a team of French scientists is arguing that even 3,000 years ago, humans may have played a role in transforming the Central African rain forest into the savannas we see today.
As Bantu farmers expanded south and east into the rain forest in search of fertile agricultural land, they may have created savanna "corridors" that cut into the forest and helped turn that lush landscape into drier grassland, according to a study published online this week in the journal Science.
Study leader Germain Bayon, a geochemist at the French Research Institute for Exploitation of the Sea, said he and his colleagues weren't even thinking about humans when they began studying sediment samples from the mouth of the Congo River. All they were trying to do was match rainfall rates over thousands of years with the degree of chemical weathering in minerals to see how the rocky material breaks down over time.
Soil can be broken down by a number of natural causes, a primary one being rain. As it falls to Earth, the atmospheric carbon dioxide trapped in the droplets reacts with the minerals in the exposed rock and soil. Potassium gets washed away quickly, while aluminum sticks around much longer. Thus, the more chemical weathering there is, the more abundant aluminum will be in relation to potassium.
Looking back 20,000 years, these ratios seemed in sync with the rainfall levels, with amounts of rain and evidence of chemical weathering rising or falling together.
But that tidy relationship fell apart about 3,000 years ago. Though the climate had been growing drier for some time, the ratio of aluminum to potassium in the soil sediment rose significantly — and there was no plausible natural cause for the sudden change.
"When we first took a look at the results, we were puzzled," Bayon said.
Then members of his research team learned of the Bantu migrations that began around that time, and an explanation started to fall into place.
The Bantu people were coming into rain forested areas and using tools made with the latest iron-smelting technology to chop down trees and clear the area to plant crops such as yams and pearl millet, according to the study. Several archaeological sites and the presence of iron artifacts back up this theory. With fewer trees to prevent erosion, the soil would have been much more susceptible to chemical weathering even in the absence of heavy rains.
This human activity, Bayon and his team argue, could account for the strange increase in chemical weathering rates.
"He makes a really convincing case," said William Ruddiman, a geologist and climate scientist at the University of Virginia who was not involved in the study. "I don't see an alternative."
But another outside scientist, archaeobotanist Katharina Neumann of Goethe University in Germany, said the evidence doesn't add up. Pollen records have shown that a rain forest does not immediately segue into a savanna when it's chopped down; rather, a secondary forest pops up — so humans may not have made as much of a dent in the forest as the authors assert.
And in any case, the area had been growing progressively drier long before Bantu farmers arrived, she added. Blaming humans for such a drastic change in the environment has an appealing "political dimension," but it oversimplifies the matter, Neumann said.
"I was a strong defender of this theory that everything was shaped by humans and humans destroy the environment and so on" in the past, she said. "But the archaeological and paleological data from other areas — not just the rain forest — show that the impact of humans was much lower than we thought."