Thousands of Ancient Structures Are Still Hidden Deep In the Amazon, Study Says

Scientists have estimated that anywhere from 10,000 and 24,000 archaeological sites made by pre-Columbian Indigenous peoples lie hidden beneath the forest canopies of the Amazon Basin, according to a new study. 

In other words, ancient earthworks that are already known in the Amazon represent only a small fraction of the predicted total, hinting that 91 to 96 percent of Pre-Columbian Amazonian sites remain undiscovered. The results shed new light on the complex and fascinating past of the Amazon’s Indigenous peoples, and bolster the case for Indigenous territorial rights in the region today.

Indigenous peoples have inhabited Amazonia, a vast rainforest biome that extends across 2.6 million square miles, for more than 12,000 years. Over the millennia, these societies constructed expansive earthworks such as roads, settlements, and ceremonial architecture; they also substantially modified local ecosystems with forest management practices, including the domestication of plants that are useful to humans.

Now, scientists led by Vinicius Peripato, a researcher at Brazil’s National Institute of Space Research, report the discovery of 24 previously unknown earthworks—including fortified villages, plaza towns, and ceremonial rings of megaliths—that were detected across Amazonia with a remote-sensing technique called LiDAR. 

Peripato and his colleagues used this sample of newly discovered earthworks, which cover only 0.08 percent of Amazonia, to run sophisticated models of the distribution and abundance of large-scale archaeological sites across this massive basin. The team’s approach revealed that “between 10,272 and 23,648 sites remain to be discovered and that most will be found in the southwest” Amazon, a discovery that “opens opportunities for better understanding the magnitude of ancient human influence on Amazonia and its current state,” according to a new study published in Science.

“During the pre-Columbian era, Amazonia was home to dense and complex societies throughout its vast forested area,” Peripato and his colleagues said in the study. “These ancient Indigenous societies had profound knowledge of earthmoving, riverine dynamics, soil enrichment, and plant and animal ecology, which allowed them to create domesticated landscapes that were more productive for humans.” 

“With earthmoving techniques, Indigenous peoples created a wide variety of earthworks (i.e. ring ditches, geoglyphs, ponds, and wells), mostly between 1500 and 500 years before present, with social, ceremonial, and defensive functions,” the team added. “Around these earthworks, they also managed hundreds of tree species, some of which show evidence of domestication, and effected long-lasting changes in forest composition. The scale and intensity of that landscape transformation remain unknown, in part because there has never been a comprehensive inventory of pre-Columbian sites across the basin.”

Given the scarcity of information about these ancient societies, Peripato and his colleagues set out to constrain the rough number of Amazonian earthworks that remain unknown. These sites are often located in remote and inaccessible regions and can be difficult to spot from the air through dense forest foliage.

LiDAR, which stands for Light Detection and Ranging, is a technique that involves shooting lasers at the ground from airborne devices and mapping out landscapes by measuring the amount of time it takes for the reflected pulses to bounce back up into the sky. This approach has completely revolutionized our view of archeological sites around the world by revealing human-made (or anthropogenic) structures that have eluded other methods of detection.

Peripato and his colleagues were able to spot two dozen previously unknown earthworks in an LiDAR survey that covered sites across Amazonia. To get a better sense of the total amount of undiscovered Amazonian sites, the team then combined the newfound sites with a comprehensive catalog of known archaeological sites “to model areas likely to harbor as yet undetected earthworks hidden beneath remote forest landscapes,” according to the study.

“This predictive model indicated that earthworks are likely concentrated in southwestern Amazonia and corroborated previous studies that found this region to be a hotspot of earth-building societies,” the team said, noting that the Brazilian state of Acre may hold particularly high concentrations of undiscovered earthworks. 

“Indeed, southwestern Amazonia contains the earliest plant cultivation and domestication, the oldest anthropogenic soils, low-density urbanism, and now a much higher density of earthworks,” the researchers added. “The underlying spatial data distribution may offer valuable information about pre-Columbian practices before European contact.”

The new study also linked the presence of earthworks to an abundance of certain plants that were domesticated by these ancient societies for use as food, shelter, tools, and even disaster mitigation. Plants that were preferentially grown near earthworks include Brazil nut trees, breadnut trees, rubber trees, and cacao trees.

“The massive extent of archaeological sites and widespread human-modified forests across Amazonia is critically important for establishing an accurate understanding of interactions between human societies, Amazonian forests, and Earth’s climate,” the team said. “Considering the widespread extent of locations modified by pre-Columbian management and cultivation practices, Amazonia can be viewed as an ancient social-ecological system, with long-term responses to climate change, more similar to old secondary forests than pristine climax ecosystems.”

For this reason, Peripato and his colleagues emphasized that these past Indigenous societies offer many lessons for global challenges we all face in the present, such as climate change and the struggle for Indigenous rights. Across thousands of years, these ancient societies were able to survive in this key region without leaving it vulnerable to deforestation, fires, and other anthropogenic pressures that now threaten its ecosystems. In this way, they offer an example of successful conservation practices and Indigenous ancestral heritage that is incredibly relevant in our times.

“These archaeological legacies can play a role in present-day debates around Indigenous territorial rights,” Peripato’s team said. “They serve as tangible proof of an ancestor’s occupation, way of life, and their relationship with the forest. Today, Indigenous peoples struggle to recognize their right to land originally inhabited by their ancestors, along with the protection of their territories, languages, cultures, and heritages. In addition to protecting the native peoples that remain, the institution of Indigenous lands also collaborates with forest conservation in times of debates on climate change and the search for solutions that minimize impacts on the climate and promote carbon neutrality.”

“Ironically, modern-day deforestation is removing the very evidence of pre-Columbian land-use strategies that were able to transform the landscape without causing large-scale deforestation,” the researchers concluded. “Amazonian forests clearly merit protection not only for their ecological and environmental value but also for their high archaeological, social, and biocultural value, which can teach modern society how to sustainably manage its natural resources.”