For a long time, it was believed that the Amazon rainforest was only sparsely populated before the arrival of Europeans. However, dense vegetation has been concealing most traces of ancient societies. Using airborne laser scanning, scientists have uncovered hundreds of previously unknown earthworks and now estimate that the population of southwestern Amazonia may have reached up to three million people around two thousand years ago.
The scale of settlement in the pre‑Columbian Amazon has long been a matter of debate. The thick forest canopy obscures the terrain, which is why archaeological sites were often discovered only in deforested areas or where features were visible from the air. This made it difficult to assess how large these societies were and how extensively they transformed their environment. The situation began to change with the use of airborne laser scanning (LiDAR), which allows researchers to “see” the ground surface beneath the tree canopy.
In a new study published in Nature, researchers conducted a detailed investigation of the Brazilian states of Acre and Amazonas, focusing on geoglyphs — large circular, square, and polygonal enclosures surrounded by ditches and earthen embankments. Some of these structures span several hectares, with ditch depths reaching up to five meters. They are likely linked to the Aqiri culture. Construction of these geoglyphs began more than 2,500 years ago, and most of the sites associated with this tradition date from approximately 600 BCE to 850 CE.
Scientists carried out airborne LiDAR surveys along routes totaling 4,430 kilometers. After processing the LiDAR data, they generated digital elevation models that revealed ditches, embankments, roads, and platforms hidden by vegetation. In total, they identified 432 earthworks, although only 36 of them had been known before.
By combining LiDAR data with satellite imagery, excavation results, and information about the distribution of known geoglyphs, the team estimated the Aqiri territory to cover approximately 183,000 square kilometers. They believe this area could contain between 24,000 and 30,000 earthworks, roughly two‑thirds of which belong to the Aqiri period. The peak of construction and use of the geoglyphs likely occurred between 100 and 300 CE.
Many of these geoglyphs were not villages or fortresses but rather public, ceremonial, and political centers where people from surrounding areas gathered periodically. Building and maintaining such complexes required a large workforce, division of labor, knowledge of geometry, and a stable organizational structure. Based on all available evidence, researchers estimate the Aqiri population at its height to have been between 1.25 and 3 million people — a smaller population simply could not have created and sustained such a vast network of sites.
This conclusion is further supported by evidence of intensive agriculture: the inhabitants cultivated maize, pumpkins, and other crops, managed soil fertility, used controlled burning, and spread useful tree species, including Brazil nuts and various palms. They carried out these activities over large areas and for extended periods. As a result, the current composition of the forests, the distribution of plant species, and soil properties may partly reflect centuries of human activity.
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