The Amazon may contain thousands of undiscovered ancient structures

There are probably more than 10,000 undiscovered pre-Columbian archaeological sites hidden in the Amazon, researchers have concluded after surveying a fraction of the sprawling rainforest.

The study adds to growing evidence suggesting that the region isn’t a pristine tropical forest, but has been significantly altered by Indigenous societies that have inhabited it for more than 12,000 years.

Luiz Eduardo Oliveira e Cruz de Aragão at the National Institute for Space Research in Brazil and his colleagues surveyed 5315 square kilometres of the Amazon using lidar, a technology that uses reflected laser light to create a 3D representation of a landscape.

By beaming pulses of light into the rainforest, usually from planes or drones, lidar records slight variations in topography and has uncovered numerous archaeological sites in recent years.

The team discovered 24 previously unknown earthworks in the areas it surveyed, which are thought to be the remnants of civilisations that lived between 1500 and 500 years ago.

The discoveries include a fortified village in the southern Amazon, a region known to have been densely populated due to the high concentration of earthworks that were connected by ancient roads.

Defensive and ceremonial sites in the southwestern Amazon were also brought to light, along with permanent settlements and ceremonial sites with large stone structures arranged in circular clusters in the northern Amazon.

The survey covered only 0.08 per cent of the Amazon’s 6.7 million square kilometres. Aragão and his colleagues used a computer model to predict how many other sites could remain hidden under the forest canopy, based on the concentration of earthworks in the new data and 937 earthworks previously discovered. They estimate that between 10,272 and 23,648 earthworks may lay undiscovered.

The model analysed the typical characteristics of known earthworks, including the local temperature, rainfall, soil clay content and distance from the nearest river, to predict where others are likely to be. “These are the characteristics needed for building the structures, but also surviving in these regions,” says Aragão.

Most of the predicted structures are in the southwestern Amazon, many in the Brazilian state of Acre.

Emerging evidence indicates that the Indigenous societies that occupied the Amazon for more than 12,000 years were larger than previously thought, at one point numbering as many as 5 million people. It is unclear why the jungle cities disappeared centuries ago.

In common with previous studies, Aragão and his colleagues also found high concentrations of domesticated plants that yield nuts or fruits close to archaeological sites, suggesting these lost societies significantly altered the composition of the rainforest. This might mean predictions about how the rainforest will adapt to climate change could be incorrect, as it isn’t as pristine as previously thought, the researchers say.

“There has always been this bias in Western thought that the Amazon was like a Garden of Eden, a primordial society that was inimical to human society,” says Michael Heckenberger at the University of Florida, who wasn’t involved in the study. “We are now seeing there was a significant degree of human intervention and variation just 500 years ago.”

The growing evidence that there were massive societies in the region before the arrival of Europeans could help protect the Amazon, says Heckenberger. Around 17 per cent of the rainforest has been cleared and some researchers believe it has already reached a tipping point where it no longer generates enough rainfall to support itself.

“If we’ve now demonstrated that even more of the Amazon forest is actually an artefact of cultural influence, the implication of that is that it’s the heritage of living Indigenous peoples and must be protected as these are their ancestors,” says Heckenberger.