Unveiling the Enigma of Dark Earth: Ancient Amazonians Deliberately Cultivated Fertile “Dark Earth”

Aerial Image of the Kuikuro II village in the Território Indígena do Xingu. Credit: Joshua Toney

Recent research indicates that indigenous practices accumulated substantial carbon-rich soil over centuries, reshaping our understanding of the Amazon’s ancient civilizations.

The Amazon basin, renowned for its vast and luxuriant tropical forests, might lead one to presume that its land is inherently fertile. Contrary to this assumption, the soils beneath the forest, particularly in the hilly terrains, are surprisingly barren. Much of the Amazon’s soil is acidic and nutrient-deficient, rendering agriculture a formidable challenge.

Nevertheless, archaeologists have unearthed enigmatic pockets of dark and fertile soil known as “dark earth” across numerous Amazonian sites. These deposits, found in proximity to human settlements spanning hundreds to thousands of years, have sparked debates regarding whether they were intentionally crafted or the accidental byproduct of ancient societies.

Unraveling the Origins of Dark Earth

A recent study conducted by researchers from MIT, the University of Florida, and Brazil endeavors to resolve the longstanding debate surrounding the genesis of dark earth. The research amalgamates findings from soil analyses, ethnographic observations, and discussions with contemporary Indigenous communities, elucidating that ancient Amazonians purposefully created dark earth to enhance soil quality, thereby supporting complex and populous societies.

Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric and Planetary Sciences at MIT, affirms, “If you want to establish thriving settlements, a fertile foundation is indispensable. Yet, the Amazon’s soil is intrinsically nutrient-poor and devoid of essential conditions for most crops to flourish. Our contention is that humans played a pivotal role in shaping dark earth, deliberately altering the ancient environment to foster human habitation.”

Remarkably, dark earth harbors substantial carbon reserves. Through generations of soil cultivation, which involved enriching it with food remnants, charcoal, and waste, carbon-rich matter accumulated, remaining sequestered for centuries. In crafting dark earth, ancient Amazonians inadvertently fashioned a potent carbon-absorbing soil.

Samuel Goldberg, co-author of the study and now an assistant professor at the University of Miami, underscores this point, stating, “Ancient Amazonians introduced a substantial amount of carbon into the soil, and a significant portion remains preserved today. This aligns precisely with our contemporary aspirations for mitigating climate change. Perhaps we can scale up some of their indigenous methods to trap carbon in the soil, with the knowledge that it would endure for extended periods.”

The team’s findings were published in the journal Science Advances on September 20. Collaborators include former MIT postdoc and lead author Morgan Schmidt, anthropologist Michael Heckenberger from the University of Florida, and researchers from multiple institutions in Brazil.

Comprehending the Contemporary Perspective

In their recent investigation, the team synthesized data collected by Schmidt, Heckenberger, and others while collaborating with Indigenous communities in the Amazon since the early 2000s. Their fieldwork concentrated on the Kuikuro Indigenous Territory in the Upper Xingu River basin in the southeastern Amazon. This region houses modern Kuikuro villages and archaeological sites associated with the Kuikuro’s ancestors. Schmidt, a former graduate student at the University of Florida, was struck by the dark soil surrounding certain archaeological sites during multiple visits to the region.

Schmidt and his colleagues began documenting the modern Kuikuro’s soil management practices. These practices include creating “middens,” akin to compost heaps, where waste and food scraps accumulate in specific village locations. Over time, these waste piles break down and merge with the soil, forming dark and fertile earth that residents use for crop cultivation. The researchers also observed Kuikuro farmers dispersing organic waste and ash in more distant fields, producing additional dark earth suitable for crop cultivation.

Schmidt explains, “We observed their deliberate actions to modify the soil and enhance its properties, such as applying ash to the ground or distributing charcoal around the base of trees.”

Additionally, interviews with villagers provided insights into the Kuikuro’s beliefs and practices related to dark earth. In these discussions, villagers referred to dark earth as “eegepe” and elucidated their daily routines in creating and nurturing this fertile soil to maximize its agricultural potential.

These observations and interviews collectively affirmed that contemporary Indigenous communities intentionally generate dark earth through soil improvement practices. The question remained: did the dark earth discovered in nearby archaeological sites result from analogous intentional endeavors?

Building a Link Through Soil

In pursuit of this connection, Schmidt joined MIT as a postdoctoral researcher. Collaborating with Perron and Goldberg, the team meticulously analyzed soils from both modern and ancient sites in the Upper Xingu region. They uncovered similarities in the spatial distribution of dark earth: these deposits radiated from the center of both modern and ancient settlements, forming a wheel-like pattern. Moreover, the composition of modern and ancient dark earth exhibited striking resemblances, characterized by enrichment in elements such as carbon, phosphorus, and other essential nutrients.

Schmidt elucidates, “These elements are vital for humans, animals, and plants. They counteract aluminum toxicity in soil, a persistent issue in the Amazon, enhancing soil suitability for plant growth.”

Goldberg adds, “The pivotal link between the past and present is the soil itself. The correspondence we observed across time periods implies that the practices we witness today, and inquire about, were similarly occurring in the past.”

In essence, the team has convincingly demonstrated that ancient Amazonians purposefully manipulated the soil, possibly employing methods akin to those seen in contemporary times, to facilitate agricultural production for burgeoning communities.

Taking a step further, the team quantified the carbon content in ancient dark earth. Combining measurements of soil samples with maps indicating dark earth locations across various ancient settlements, their estimates unveiled that each ancient village contained several thousand tons of carbon sequestered in the soil over centuries due to Indigenous human activities.

As articulated in their paper, “Inspired by the enduring fertility of ancient dark earth, modern sustainable agriculture and climate change mitigation initiatives can draw inspiration from traditional methods still practiced by Indigenous Amazonians.”

Reference: “Intentional creation of carbon-rich dark earth soils in the Amazon” by Morgan J. Schmidt, Samuel L. Goldberg, Michael Heckenberger, Carlos Fausto, Bruna Franchetto, Jennifer Watling, Helena Lima, Bruno Moraes, Wetherbee B. Dorshow, Joshua Toney, Yamalui Kuikuro, Kumessi Waura, Huke Kuikuro, Taku Wate Kuikuro, Takumã Kuikuro, Yahila Kuikuro, Afukaka Kuikuro, Wenceslau Teixeira, Bruna Rocha, Vinicius Honorato, Hugo Tavares, Marcos Magalhães, Carlos Augusto Barbosa, João Aires da Fonseca, Kelton Mendes, Luís Reynaldo Ferracciú Alleoni, Carlos Eduardo Pellegrino Cerri, Manuel Arroyo-Kalin, Eduardo Neves and J. Taylor Perron, 20 September 2023, Science Advances.
DOI: 10.1126/sciadv.adh8499

This MIT research received support from MIT’s Abdul Latif Jameel Water and Food Systems Lab and the Department of the Air Force Artificial Intelligence Accelerator. Field research was funded by grants to the University of Florida from the National Science Foundation, the Wenner-Gren Foundation, and the William Talbott Hillman Foundation, with sponsorship in Brazil from the Museu Goeldi and Museu Nacional.

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More about Amazonian Dark Earth

• Science Advances Journal
• MIT’s Abdul Latif Jameel Water and Food Systems Lab
• University of Florida
• National Science Foundation
• Wenner-Gren Foundation
• William Talbott Hillman Foundation
• Museu Goeldi
• Museu Nacional