More than 10,000 undiscovered pre-Columbian archaeological earthworks — and likely as many as 23,000 — rest undiscovered throughout the Amazon basin, hidden beneath the dense jungle foliage that blankets the region, according to a new study published in the 6 October issue of Science.
The findings, which were derived from airborne laser mapping of a roughly 2,000-square-mile area of the Amazon, analysis of previously documented archaeological sites, and predictive spatial modeling, address questions about the influence of pre-Columbian societies on Amazonian forests and provide insights that could help guide modern conservation strategies and bolster support for land rights for today's Indigenous groups.
"The massive extent of archaeological sites and widespread human-modified forests across Amazonia is critically important for establishing an accurate understanding of interactions between human societies, Amazonian forests, and Earth's climate," write the study's authors.
The Amazon basin — a vast and seemingly impenetrable swath of tropical forest stretching some 2.6 million square miles across northern South America — is one of the most biodiverse regions on planet Earth. It's estimated that roughly 1 in 10 known species in the world live in the Amazon rainforest.
For centuries, Western scientists considered these forests to be "pristine" landscapes, relatively unsullied by human hands.
Archaeologists now understand that Indigenous societies have called the Amazon basin home for more than 12,000 years, settling in large groups that built diverse earthwork structures and molded domesticated forest landscapes that have had long-lasting effects on modern forest composition.
Over the past several decades, a growing number of pre-Columbian earthwork structures, including ditches and wells, pathways, and other geoglyphs that form geometric patterns and shapes, have been revealed in deforested areas throughout the Amazon. These sites, which mostly date between 1,500 and 500 years before the present, have been linked to various social, agricultural, ceremonial and defensive activities. Evidence for widespread forest management, tree domestication, and the cultivation of enriched, fertile soils surrounding these earthen constructions further underscores that the Amazon has long been guided by intentional human activity.
However, the extent of human settlement and landscape transformation across the Amazon is poorly understood. The region is enormous and remote, with many areas nearly impossible to access and potential archaeological sites obscured by dense tangles of rainforest vegetation.
As such, there has never been a comprehensive survey of pre-Columbian sites across the Amazon basin.
Seeing the Forest Without the Trees
Airborne LIDAR (light detection and ranging), a remote sensing technique that uses laser pulses, can penetrate dense foliage and measure small changes in topography on the ground surface, producing highly detailed, three-dimensional images of what lies beneath the forest canopy.
This technology has been revolutionary in revealing the size and scale of many ancient settlements and archaeological features in heavily forested sites throughout Asia, Mesoamerica and South America, including the recent highly detailed mapping of two monumental pre-Columbian settlements in the Bolivian Amazon.
"We carried out the most extensive search for earthworks beneath the forest canopy across Amazonia," said Carolina Levis, study co-author and researcher at the Federal University of Santa Catarina in Brazil.
Levis and her colleagues searched roughly 2000 square miles of LIDAR survey data and discovered 24 previously unreported human-made earthworks across the Amazon basin.
"The new sites included ringed structures that might have been ceremonial or defensive, fortified villages, mountaintop settlements, road networks, and even megalithic structures akin to Stonehenge," said Levis.
However, the LIDAR survey area represents a minuscule fraction — just 0.08 percent — of the entire Amazon. The number and diversity of new sites revealed suggests that these large-scale archaeological sites are much more widespread than currently thought, explained Levis.
To better understand where and how many pre-Columbian sites might occur throughout the entirety of the Amazon, the researchers combined data from their survey with information on previously identified sites with a predictive spatial distribution model.
The model's results indicate that between 10,272 and 23,648 large-scale pre-Columbian sites remain to be discovered, particularly in the southwestern region of the Amazon, suggesting that currently documented sites account for a tiny fraction of what might be hidden beneath the forest canopy.
Cultivating Sustainable Forest Gardens for Millennia
"These new predictive sites co-occurred with domesticated tree species, including fruit and nut trees," said Levis. "Indigenous people have been developing the land in a way that they didn't destroy the forest but managed it so that the forest keeps providing essential services, such as soil quality and food provision."
According to the authors, high concentrations of 53 domesticated tree species, including cacao, Brazil nut, breadnut and rubber trees, were found near earthwork sites, suggesting that active pre-Columbian forest management practices have long shaped the ecology and composition of forests across Amazonia.
A deeper understanding of Indigenous groups' role in the Amazon's natural and cultural heritage, in both the past and the present, could also support their claims for land rights.
"Indigenous-modified forests throughout the Amazon are of critical importance in building a new understanding of this ancient interaction between Indigenous peoples and the forest itself," said Levis. "This new understanding opens our minds to further investigate the contributions of Indigenous people on shaping Amazonian biodiversity and its resilience to past and current changes."