A fascinating body of research has revealed how intimately our ancestors were connected to the earth – specifically, ancient Amazonians, who intentionally created a fertile dark earth or ‘terra preta’. The study has found that despite the highly acidic and low nutritional content of the Amazonian soil for agricultural purposes, ancient humans intentionally modified the environment around former human settlements to allow their expansion.
This study published in Sciences Advanceswas led by researchers at MIT, the University of Florida, and institutions in Brazil. In the past, while conducting research in a region of the Amazon inhabited by the Kuikuro people, a team of researchers led by anthropologist Michael Heckenberger from the University of Florida, alongside Morgan Schmidt, who was a graduate student at the time, discovered that these communities utilize sophisticated agricultural methods.
Indians of the Kuikuro ethnic group at the closing ceremony of the ninth edition of the Indigenous Peoples Games (Olinda PE) (CC BY 3.0 Br)
Intentionality of Dark Earth: Inadvertent or Deliberate?
One such technique involves the establishment of centralized centers—accumulations of waste that break down over time, enriching the soil, which in turn supports crop cultivation.
The researchers essentially helped address a longstanding debate about intentionality – was this dark earth, known as ‘terra preta’, an inadvertent byproduct of certain practices, or very intentional? This study helped confirm that these practices were very much intentional.
Aerial view of the Kuikuro II village in Indigenous territory of Xingu, Brazil. (Joshua Toney/MIT News)
The researchers concentrated their fieldwork efforts within the Kuikuro Indigenous Territory, situated in the Upper Xingu River basin in the southeastern Amazon. This area encompasses not only contemporary Kuikuro villages, but also archaeological sites believed to be the former residences of Kuikuro ancestors.
Together, they conducted a soil analysis revealing the presence of dark earth deposits arranged in a radial pattern, primarily located at the center of both contemporary and historic settlements. The composition of both modern and ancient terra preta was comparable, characterized by high levels of elements like carbon, phosphorus, and other essential 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 told Technology Review.
Over time, these waste piles undergo decomposition, blending with the soil to create a dark and fertile earth, which residents subsequently utilize for crop cultivation. Additionally, the researchers noted that Kuikuro farmers distribute organic waste and ash across distant fields, contributing to the formation of dark earth in those areas as well, allowing for the cultivation of additional crops in these enriched soils.
Test pit in Ngokugu archaeological site, showing the dark color and ceramic pieces characteristic of terra preta soil. (Courtesy the researchers/MIT News)
Anthropogenic Dark Earths: Science and Timeline
The age and distribution of these soil deposits serve as a narrative of the evolution of ancient indigenous civilizations throughout the Amazon region, with the oldest layers of these dark soils dating back approximately 5,000 years, there is a notable increase in evidence of Anthropogenic Dark Earths (ADEs) being produced around 4,000 years ago, indicating heightened activity and significant cultural changes during that period.
However, it wasn’t until around 2,000 years ago that these ADEs reached their peak. This timeframe corresponds to the average age of the widespread black deposits found across the Amazon basin. During this period, communities grew larger and established extensive networks, indicative of a peak in cultural and societal development.
Upper Xingu River study area showing locations of the modern and historic Kuikuro villages and five archaeological sites. Inset map shows the location of the study area in the Amazon Basin (red star) and locations of documented archaeological sites with dark earth (black points). (Schmidt et al./Science Advances)
ADEs are not only exceptionally nutrient-rich but also serve as potent carbon sinks, containing up to 7.5 times more carbon compared to the surrounding soils! As ADEs accumulate, the carbon becomes sequestered underground, where it remains stable for extended periods, effectively locking it away and delaying its release into the atmosphere.
The mechanism behind this behavior of carbon within ADEs is not fully understood, but scientists speculate that it may be related to the presence of “black carbon,” also known as “biochar.” This crucial component is derived from organic materials subjected to high temperatures in the absence of significant oxygen, resulting in the conversion of organic matter into nearly pure carbon.
Unlike traditional charcoal production, this process generates less carbon dioxide emissions and yields a fine, crumbly black product that has been consistently identified within ADEs throughout the Amazon region.
Then – a disruption occurs around 500 years ago, i.e., with ADE production dropping off.
This links to the aftermath of Christopher Columbus’ arrival on South American soil on August 1, 1498. When he planted the red and gold flag of Spain into the ground on the Paria Peninsula in Venezuela, it symbolized the onset of a tragic era known as the “great dying.”
It is estimated that by 1600, approximately 56 million indigenous people had lost their lives across the Americas due to various factors, including violence, diseases introduced by Europeans, and forced labor. The scale of this devastation was so immense that it had a discernible impact on the Earth’s climate, contributing to a cooling effect, reports BBC.
“The ancient Amazonians put a lot of carbon in the soil, and a lot of that is still there today,” concludes co-author Samuel Goldberg, who performed the data analysis as a graduate student at MIT and is now an assistant professor at the University of Miami. “That’s exactly what we want for climate change mitigation efforts. Maybe we could adapt some of their indigenous strategies on a larger scale, to lock up carbon in soil, in ways that we now know would stay there for a long time.”
Top image: Aerial view of Amazon tributary river, San Jose do Rio Claro, Mato Grosso, where ancient manufacturing of terra preta has been found. Source: Uwe Bergwitz/Adobe Stock
By Sahir Pandey
References
Chu, J. 2023. Ancient Amazonians intentionally created fertile “dark earth”. Available at: https://news.mit.edu/2023/ancient-amazonians-intentionally-created-fertile-dark-earth-0920.
Gorvett, Z. 2024. The ‘dark earth’ revealing the Amazon’s secrets. Available at: https://www.bbc.com/future/article/20240116-the-dark-earth-revealing-the-amazons-secrets.
McKey, D., Rostain, S. (2015). Farming Technology in Amazonia. In: Selin, H. (eds) Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures. Springer, Dordrecht. Available at: https://doi.org/10.1007/978-94-007-3934-5_9893-2.
Schmidt, M.J., et al. 2023. Intentional creation of carbon-rich dark earth soils in the Amazon. Sciences Advances, 9(38). Available at: https://DOI: 10.1126/sciadv.adh8499.
