“This is a place where humans appear to have been a positive force on this landscape and its biodiversity over thousands of years.”

A rainforest in the Putumayo region of Peru has been home to relatively unaltered forest for 5,000 years. The people who have lived there found a long-term way to coexist with nature–and the evidence is in microscopic bits of silica and charcoal in the soil.

In recent years, scientists have learned that many parts of the Amazon have been cultivated by Indigenous peoples for thousands of years, and mere centuries ago were the sites of cities and farmland.

Researchers studied the soil samples from the Putumayo region of the Amazon rainforest in northeastern Peru and inventories of the plants and animals of the region. They also built relationships with the people who live there to better understand how the forest had developed over such a long period of time.

Soil samples collected in the rainforest. Image credit: Nigel Pitman, Field Museum

“It’s very hard even for experienced ecologists to tell the difference between a 2,000-year-old forest and a 200-year-old forest,” says Nigel Pitman, an ecologist at Chicago’s Field Museum

“There’s more and more research showing that many Amazonian forests we think of as wilderness are actually only 500 years old, because that’s when the people who were living there died from the pandemics brought by Europeans, and the forest has regrown.”

Dolores Piperno, a researcher at the Smithsonian Tropical Research Institute added:

“Far from implying that complex, permanent human settlements in Amazonia had no influence over the landscape in some regions, our study adds substantially more evidence indicating the bulk of the Indigenous population’s serious impact on the forested environment was concentrated in the nutrient-rich soils near rivers, and that their use of the surrounding rainforest was sustainable, causing no detectable species losses or disturbances, over millennia.”

Many plants take up silica from the soil and use it to produce microscopic mineral particles called phytoliths that provide structural support. After a plant dies, these phytoliths linger in the soil for thousands of years. Different kinds of plants produce differently-shaped phytoliths, meaning that phytoliths in the soil can be used to determine what kinds of plants lived there in the past.

Pitman and his colleagues, including Field Museum associates Juan Ernesto Guevara Andino, Marcos Ríos Paredes, and Luis A. Torres Montenegro, found that the types of trees growing in the region today have been growing there over the past 5,000 years–an indicator that unlike in other parts of the Amazon, the Putumayo wasn’t home to cities and farmland prior to European colonization.

In addition to phytoliths, the researchers also looked for microscopic bits of charcoal. The low levels of charcoal in the soil show that while the forest remained unaltered by humans for 5,000 years, people did live in the area–they just coexisted with the forest in a way that didn’t change it.

Nigel Pitman explained:

“One of the scary things for conservationists about research showing that so much of the Amazon used to be towns and cropland, is that it’s allowed people who aren’t conservationists to say, ‘If that was the case, then you conservationists are getting upset for no reason–500 years ago, half the Amazon was cut down and all grew back, it’s no big deal. We don’t have to worry so much about cutting down the Amazon, we’ve already done it and it turned out fine.” This study suggests that while people are able to coexist with wilderness without altering it, the Amazon isn’t simply a resource that can be destroyed and regrown from scratch in a matter of centuries.

Dolores Piperno added:

“To me, these findings don’t say that the Indigenous population wasn’t using the forest, just that they used it sustainably and didn’t modify its species composition very much.

“We saw no decreases in plant diversity over the time period we studied. This is a place where humans appear to have been a positive force on this landscape and its biodiversity over thousands of years.”

“It’s an important finding, and a hopeful one, because it shows that people have been living in the Amazon for thousands of years, in a way that allows them to thrive and the forest to to thrive,” says Pitman. “And since this particular forest is still being protected by Indigenous peoples, I hope this study reminds us all how important it is to support their work.”

The research was published in PNAS

Aerial view of the Putumayo region of the Amazon rainforest in Peru. Image Credit Alvaro del Campo, Field Museum

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