Hundreds of giant geometric earthworks have turned up in northern Brazil, and they are pushing archaeologists to rethink how many people once lived in the Amazon.

A new paper published July 29 in Nature says millions of people may have inhabited the southwestern Amazon rainforest about 2,000 years ago as part of the Aquiry civilization, which researchers now suspect was far larger than previously thought.

Scientists estimate between 1.25 million and 3 million people lived across more than 182,800 square kilometres of what is now northern Brazil. They say the civilization likely peaked between 100 and 300 C.E.

The estimates come from a recent aerial survey using lidar, a laser-based remote sensing technology that creates detailed, three-dimensional maps of the ground. In a 4,507-square-kilometre study area, researchers found nearly 400 previously undocumented geometric earthworks, known as geoglyphs, and used them to estimate the Aquiry’s population and reach.

The findings add to growing evidence that the Amazon was shaped by humans over centuries. “That old idea of a pristine Amazônia falls apart,” study co-author Alceu Ranzi, a geographer at the University of Acre, tells Science’s Julia Vaz.

Scientists think the Aquiry civilization first emerged around 600 B.C.E. They estimate people lived in villages with about 300 inhabitants on average, in multi-family longhouses, and grew crops including squash, maize and manioc. They also cultivated Brazil nut trees and peach palms for fruit.

Researchers think the Aquiry built the earthworks as public, ceremonial and political centres that were rarely used. They were “venues for gatherings, political decision-making and displays of chiefly generosity, ritual dancing, music, feasting, athletic competitions and ball games,” lead author Martti Pärssinen, an archaeologist at the University of Helsinki, tells NewScientist’s James Woodford.

The earthworks include circles and squares, along with octagons, petagrams and circles inside squares. Most cover about 2.8 hectares on average, but some stretch to nearly 50 hectares. Their ditches are about 12 metres wide and up to 4.9 metres deep.

Scientists think the Aquiry built them without metal tools or animals. “The scale and precision of these constructions is simply astonishing,” Pärssinen tells NewScientist.

Archaeologists have now identified about 1,700 geoglyphs in the region, but they think there may be between 23,000 and 30,000 in total. That is well above earlier estimates that suggested the entire Amazon rainforest might contain about 10,000 earthworks.

The Aquiry geoglyphs appear to have been abandoned between 850 and 950 C.E. Researchers do not know why, but Pärssinen tells Live Science’s Kristina Killgrove that the evidence points to a “relatively rapid collapse” that may be linked in some way to the decline of the Maya, Tiwanaku and Wari civilizations.

Researchers said they were surprised by the scale of the Aquiry civilization because it developed far from major rivers, which have historically supported large ancient settlements. They also note the study area covers less than 3 percent of Amazonia.

They think other parts of Amazonia may have had similar population densities, raising the possibility that tens of millions of people lived there before European colonization. “That’s something amazing and something new,” Pärssinen tells Scientific American’s Jackie Flynn Mogensen.

Michael Heckenberger, an anthropologist at the University of Florida who was not involved in the study, called the new population estimates “mind-blowing,” but also “plausible and extremely important.”

“The idea of pre-Columbian urbanism in southern Amazonia has gained momentum over the past two decades,” he tells NewScientist. “What we now need to know is what is going on around the detectable earthworks, such as occupational sites, to support the high estimated population.”

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