Scientists have discovered that mysterious mounds in the southwest corner of the Amazon River basin were once the site of ancient urban settlements. Using remote sensing technology capable of mapping topography from the air, they discovered that beginning approximately 1,500 years ago, ancient Amazonians built and lived in densely populated urban areas.
The cities featured 22-meter-high earthen pyramids, and the city was surrounded by kilometers-long causeways. Archaeologist Heiko Prümers of the German Archaeological Institute, a member of the team, called the complexity of these settlements "astonishing."
A Shift in Thinking
Humans have inhabited the Amazon Plain—a watershed system roughly the size of the continental United States—for approximately 10,000 years. Researchers previously believed that before the arrival of Europeans in the 16th century, all Amazonians lived in small, nomadic bands with little impact on their surroundings. Although early European visitors described a landscape dotted with villages and towns, later explorers failed to find them.
By the 20th century, archaeologists were still unsure of these rumors and believed that the Amazon's poor soils could not support large-scale agriculture, hindering the development of tropical civilizations similar to those in Central America and Southeast Asia.
However, in the 21st century, this view began to shift. Some researchers suggest that the unusual abundance of domesticated plants in the Amazon, combined with rare patches of fertile soil that may have been created by humans, may indicate that ancient Amazonians did shape their environment.
This hypothesis is supported by a 2018 study: Archaeologists have discovered and reported hundreds of large, geometrically shaped mounds due to deforestation in the southern Amazon rainforest.
These structures suggest that organized ancient societies could thrive in a single location for many years, but direct evidence of settlements remains lacking. In 1999, Prümers began studying a group of mounds located just outside the dense rainforest in the Bolivian Amazon River basin. There, numerous tree-covered mounds rise from a lowland area that is flooded during the rainy season. Earlier excavations have shown evidence of human habitation in these "forest islands," including remains of the enigmatic Casarabe culture, which emerged around 500 AD.
During one excavation, Prümers and his colleagues realized they had uncovered a structure that resembled a wall, indicating a permanent settlement. The team also discovered tombs, platforms, and other signs of a complex society. However, the dense vegetation made it difficult to investigate the site using traditional methods.
Unsolved Mysteries
The reasons for the abandonment of these settlements nine hundred years after their establishment remain a mystery. Radiocarbon dating indicates that the Casarabe culture disappeared around 1400.
Prümers notes that lidar images reveal the presence of reservoirs within the settlements, potentially indicating that the climate was not consistently humid and that a single environmental shift could have forced human migration. However, consistent pollen records indicate that maize was cultivated uninterruptedly for thousands of years, suggesting sustainable agricultural practices.
Eduardo Neves, an archaeologist at the University of São Paulo in Brazil, says that, at best, the discovery of long-lost Amazonian societies "changes the general view of archaeology in the Amazon." He says that while logging and agricultural activities in the Amazon are almost certainly destroying important, yet undiscovered, archaeological sites, the growing interest in Amazonian archaeology may lead to the preservation of vulnerable areas.
The findings also refute the view that indigenous peoples in the Amazon were merely passive inhabitants before the arrival of Europeans. "The people who lived there permanently changed the landscape."
