Present-day humans may not be the first to have affected world climate, researchers say, citing possible changes in Africa triggered by farmers 3,000 years ago.

French geochemist Germain Bayon, writing in the journal Science, says early farmers in Central Africa may have contributed to the disappearance of rainforests that were "abruptly" replaced by savannas, broad grasslands dotted with shrubs and trees.

Bayon and his colleagues studied weathering of sediment samples drawn from the mouth of the Congo River. Because deforestation would intensify weathering, the samples are a record of the climate for the past 40,000 years.

Between 20,000 and 3,500 years ago the samples showed weathering consistent with patterns of rainfall in the region, but around 3,000 years ago, "there was a complete decoupling" between rainfall and the rate of weathering, Bayon said.

The findings show "climate could not be the only factor in explaining deforestation," he said.

Bayon suggests Bantu-speaking peoples from what is now the border area between Nigeria and Cameroon moved into the Congo Basin around 4,000 years ago and had "a significant impact on the rainforest" as they cleared land for farming and iron-smelting.

That could have had a major impact on the environment, Bayon said, since it's now understood that agriculture contributes to carbon emissions and that trees help trap that carbon and keep it from the atmosphere, and during the Bantu's stay in Central Africa there was more agriculture and fewer trees.

Bayon said the study shows how the combination of culture and climate can affect the environment.

"Humans can have a huge impact on natural processes," he said.