The pollen in this time period came mostly from grasses and a few drought-resistant species of trees. After about 2,000 years ago, more and more tree pollen appears in the samples, including fewer drought-resistant species and more evergreens, the researchers report today (July 7) in the journal Proceedings of the National Academy of Sciences. Charcoal levels also went down, indicating a less-fire-prone landscape. These changes were largely driven by an increase in precipitation, Carson said.
The earthworks predate this shift, which reveals that the diggers of these ditches created them before the forest moved in around them. They continued to live in the area as it became forested, probably keeping clear regions around their structures, Carson said.
"It kind of makes sense," he said. "It's easier to stomp on a sapling than it is to cut down a big Amazonian tree with a stone ax." (Gallery: Biodiversity of the Amazon (Photos))
The discovery that the human activity came before the forest answers some questions, like how Amazonian people could have built in the rainforest with no more than stone tools (they didn't have to), how many people would have been necessary to construct the structures (fewer than if clear-cutting had been required), and how the population survived (by growing maize).
The study also has wider implications for the modern day, Carson said. The question of how to preserve the Amazonian rainforest is difficult to answer; some people say humans need to get out, and others believe people and the forest can coexist. Ancient history could provide a guide, as well as a greater understanding of how the forest has recovered from earlier perturbations. (The Amazon also drives climate as well as responds to it, thanks to its ability to take up carbon from the atmosphere.)
The new study suggests that the modern forest is a coproduction between humans and nature, Carson said. Natural cycles drove the rainforest to sprout, but humans stayed on-site for 1,500 years afterward, he said.
"It's very likely, in fact, that people had some kind of effect on the composition of the forest," Carson said. "People might favor edible species, growing in orchards and things like that, altered the soils, changing the soil chemistry and composition, which can have a longer-lasting legacy effect."
Those long-range changes are next for Carson and his colleagues to investigate. "This kind of study has only just started in Amazonia," Carson said.
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