Researchers using airborne laser scanning across 4,430 kilometers of flight in Southwest Amazonia mapped 432 precolonial earthworks, 396 of which were previously unknown. Nature published the findings, which revise estimates for human settlement and population density in the region between 100 AD and 300 AD.
Airborne light detection and ranging (LiDAR) surveys conducted in 2024 covered 4,497 square kilometers across the Brazilian states of Acre and Amazonas. The canopy-penetrating lasers identified five times more earthworks in deforested areas than previous satellite imagery had detected along the same flight lines. In one test block, LiDAR uncovered 156 earthworks compared to 30 visible via satellite images.
LiDAR mapping and population estimates
Extrapolating the survey data across the 183,000-square-kilometer region of the Aquiry civilization yields an estimated total of 23,760 to nearly 30,000 earthworks. The Aquiry civilization constructed ditched geometric enclosures between 600 BC and 850 AD. Ditch structures measured between 9 and 13 meters wide and up to 5 meters deep, encircling sites ranging from 0.35 to 14 hectares, with the largest covering nearly 50 hectares.
Geoglyph construction peaked between 100 AD and 300 AD, when 25% to 60% of the Aquiry sites were simultaneously in use. Based on workforce calculations requiring 300 people to construct and maintain an average earthwork, the authors estimated the regional population during this period at 1.25 million to 3 million people. When accounting for forest canopy areas where laser penetration was reduced by 11.5% to 41.9%, the estimated population ranges from 1.6 million to 3.8 million people.
Later village structures
Later mound villages without ditches appeared in the same area after 1000 AD. Analysis of roundish and rectangular mound settlements showed an average area of 1.28 hectares containing 11.75 elliptical mounds. These settlements averaged 235 people per hectare, or approximately 300 inhabitants per mound village.
Survey flights along two 800-kilometer lines between the Purus and Juruá river interfluves detected no earthworks in the upland area, supporting hypotheses of uninhabited ancient buffer zones between regional population centers.
