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Ancient Amazonians created “dark earth”

The rich soil sustained complex societies and also sequestered large amounts of carbon, a new study suggests.

January 4, 2024

The Amazon is known for its lush forests, but much of its soil is acidic and low in nutrients—with the exception of patches of “dark earth” found in and around human settlements dating back hundreds to thousands of years. It has been a matter of debate whether this rich soil was created on purpose, but a study led by researchers at MIT, the University of Florida, and institutions in Brazil suggests that it was.

“If you want to have large settlements, you need a nutritional base,” says EAPS professor Taylor Perron. “We argue here that people … intentionally modified the ancient environment to make it a better place for human populations.”

An aerial photo shows the layout of a Kuikuro village in Brazil.

Working some years ago in a part of the Amazon populated by the Kuikuro people, researchers including University of Florida anthropologist Michael Heckenberger and Morgan Schmidt, then a Florida grad student, learned that these communities practice agricultural techniques such as creating centrally located middens—piles of waste that gradually decompose and enrich the soil, which is then used for crops. 

Schmidt, the study’s lead author, joined Perron’s group as a postdoc in 2018 and collaborated on a soil analysis that found deposits of dark earth in a radial pattern, concentrating mostly in the center of both modern and ancient settlements. Modern and ancient dark earth was also similar in composition, enriched with elements such as carbon, phosphorus, and other nutrients.

“These are all the elements that are in humans, animals, and plants, and they’re the ones that reduce the aluminum toxicity in soil, which is a notorious problem in the Amazon,” Schmidt says.

Besides the confirmation that the dark earth is the product of age-old human ingenuity, the research found that the enriched soil in each settlement has sequestered thousands of tons of carbon for hundreds of years, suggesting that some of these practices could be adapted as tools to fight climate change. 

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