Odd shapes hidden in dense Amazon rainforest reveal sprawling ancient civilization
Newly found earthworks suggest about 3 million people might have lived in just about 3% of the forest’s entire area
By Julia Vaz
The first hint that sprawling ancient populations once occupied the Amazon rainforest in Brazil was found through the most mundane of happenstances. More than 20 years ago, geographer Alceu Ranzi, now at the University of Acre, sat on the right side of a plane flying over the ocean of green. Looking down, he saw a perfect circle inscribed on the ground, an unnatural shape produced only, he felt certain, by humans. He was right.
Decades of subsequent research by Ranzi and colleagues have slowly revealed remnants of peoples that occupied even the forest’s most remote areas. Traces like those Ranzi saw from the plane are what archaeologists call earthworks—a type of architecture carved into the landscape. Now, using a laser-based form of radar known as lidar, researchers suggest ancient Amazonians were widespread in territories previously considered uninhabited.
Ranzi’s and colleagues’ latest research, published today in Nature, details how they uncovered nearly 400 previously undocumented earthworks in a small patch of the southwestern region of the rainforest. Extrapolating the results to similarly dense zones, there may be more than 20,000 additional earthworks in that area just waiting to be discovered, the team says. Those numbers far exceed some previous estimates, which predicted about 10,000 earthworks for the whole of Amazônia—the Portuguese word for the Amazon rainforest.
“That old idea of a pristine Amazônia falls apart,” Ranzi says.
Based on calculations of the workforce necessary to create and maintain the earthworks, the researchers estimate that up to 3 million people could have lived in just about 3% of the forest’s entire area. “If we go back 20 years, there was no record of pre-Columbian occupation in the Amazônia forest,” says Vinicius Peripato, a geographer with the Brazilian National Institute for Space Research who has conducted similar studies in the Amazon but was not involved with the new work. “Those numbers are growing rapidly, showing that humans did in fact occupy Amazônia.”
The area surveyed by the researchers cuts through Acre and Amazonas, two northern Brazilian states that border Venezuela, Colombia, Peru, and Bolivia. Although scientists already knew there were earthworks in the region, the new study delineated the western and northern borders of the geoglyph-building society, which researchers had previously named Aquiry. Far away from any major rivers, which historically sustained thriving populations, the scientists did not expect the Aquiry to reach such high numbers.
Lidar, the remote-sensing technology used by the team, shoots pulses of infrared at a landscape, measuring the time it takes light to bounce back to calculate distances. The result is a highly precise 3D map of the ground below, allowing researchers to search for the ditches and raised protrusions of earthworks, even those hidden beneath tightly packed tree canopies. Before the technology, scientists were only able to identify such sites in deforested areas.
Lidar “is revolutionary” for archaeological research in the Amazon, says Eduardo Neves, an archeologist at the University of São Paulo who was not part of the study. He is the leader of Amazônia Revelada, or “Amazônia Revealed,” a new research project in Brazil that seeks to use lidar to identify earthworks in areas of the forest threatened by deforestation. Under the country’s constitution, archaeological sites must be federally protected from damage or erasure.
The team identified most of the earthworks in the new study as geoglyphs, ditches made in the ground that form geometric shapes including squares and circles, as well as more complex shapes such as pentagrams and octagons. Some are even more sophisticated, combining, for example, circles inside a perfect square.
During early surveys of these geoglyphs, Ranzi says, archaeologists encountered traces of coal that allowed them to radiocarbon date the earthworks to about 2500 years ago. However, they found very few traces of human activity, suggesting these places may have primarily been used for ceremonial purposes.
According to anthropologist Francisco Apuriña, a member of the local Indigenous tribe who worked on the study, the Apuriña tribe in Acre recognizes the geoglyphs as a type of kymyrury. The word is used to describe spaces that function as “home to spirits,” he says.
“At the same time that Pythagoras was in ancient Greece doing his theorem, we had people here in Amazônia doing geometry,” Ranzi says.
Indigenous tribes have expressed trepidation over archaeological studies using lidar, however. They worry the images captured through the technology and made public by research could reveal the precise location of their territories or expose sites they consider sacred. Martti Pärssinen, an archaeologist at the University of Helsinki and lead author of the study, says he and colleagues removed all images of earthworks captured within demarcated Indigenous territories before making their data publicly available.
Apuriña says Indigenous tribes must constantly negotiate how much to make public. “Certain things don’t need to be disclosed,” he says. “But, other things need to be showed, responsibly, so they can be used as arguments to protect and recognize Indigenous territories.”
“Lidar reveals asymmetries in knowledge production,” Neves says. “But Brazilian archaeology today has the awareness to work with local communities. It allows us to employ these technologies in ways that are less colonialist.”
To Ranzi, those efforts also include rescuing ancient Indigenous knowledge that still sustains the region. It’s clear the Aquiry population modified the forest, he says. The presence of the Brazil nut tree around the earthworks points to its domestication and cultivation by the community.
Those findings add to evidence, Pärssinen says, that the current composition of the Amazon was shaped by human populations, the knowledge of which could impact climate modeling efforts.
In the past 40 years, Amazônia has lost more than 13% of its vegetation cover. Scientists say the region might not be able to sustain itself any longer if that figure passes a 20% threshold. To both Ranzi and Neves, investing in the local production of scientific knowledge could help boost periphery communities in Brazil encroached by a growing agriculture industry.
“This civilization is not an El Dorado because of gold,” Ranzi says. “But it’s an El Dorado for science.”
doi: 10.1126/science.z5mwylx
