450 Massive Geometric Earthworks in the Amazon Suggest Ancient Civilizations

Archaeologists studying ancient human activities in the Amazon face numerous mysteries. With most evidence buried under the world's most biodiverse ecosystem, uncovering clues about these ancient peoples remains a significant challenge.

A recent study in the Proceedings of the National Academy of Sciences reveals that ancient earthworks, or geoglyphs, discovered over the last 25 years due to deforestation in the Brazilian rainforest, indicate that the Amazon was managed sustainably for thousands of years before modern industrial practices. Interestingly, these ancient methods may have been more effective than our current approaches.

The 450 geoglyphs span approximately 5,000 square miles (13,000 square kilometers) in Acre state, located in the western Brazilian Amazon. These massive earthworks, featuring trenches up to 12 yards (11 meters) wide and 13 feet (4 meters) deep, come in various shapes, including circles, squares, and ovals. Their discovery began in the 1980s during rainforest clearing for cattle ranching, with many more identified using Google Earth as satellite imagery became accessible.

Archaeologists have long been puzzled by their purpose: Why would a civilization need a perfectly octagonal moat in the heart of the rainforest? With few human artifacts nearby, they likely weren’t used as village sites or military trenches. However, the authors of this new study were more intrigued by the extensive land management required to clear space for their construction 2,000 years ago.

"Contrary to popular belief, the Amazon rainforests have not remained untouched by humans," explains coauthor Dr. Jennifer Watling, an archaeologist at the Museum of Archaeology and Ethnography at the University of São Paulo, Brazil. "For over 25 years, archaeologists have highlighted human presence and influence in Amazonia. Yet, many ecologists and biologists still focus on natural factors like soil and climate, often overlooking human impact."

Watling and her team propose that the geoglyphs served as occasional public gathering spots for ceremonies and rituals. Their primary focus, however, was to determine the effort ancient peoples invested in constructing these structures. To investigate, they reconstructed a 6,000-year land use history at two sites by analyzing soil samples.

By collecting soil from inside and outside the geoglyphs, they studied microscopic silica plant fossils, known as phytoliths, to identify the dominant vegetation over time. They also examined charcoal remains to understand the frequency and scale of forest clearing through burning, and conducted carbon-stable isotope analyses to evaluate changes in vegetation density across different periods.

The study revealed that 4,000 years ago, the region was predominantly bamboo forest. Humans began burning small areas to create space for agriculture and geoglyph construction. They selectively removed underbrush and non-useful trees, making room for palms and other plants that provided food and materials.

In summary, unlike modern agroforesters who have cleared approximately 20 percent of the world's largest rainforest, ancient Amazonians demonstrated remarkable skill in sustainable land management.

"Today's deforestation in this region is extensive and permanent," Watling explains. "Our findings show that in the past, deforestation was limited in scale and temporary. The focus was on forest management to enhance the availability of valuable resources like trees."

Why are we continuing to destroy rainforests, and is there a better way to manage this process? Reflecting on these questions and learning from historical practices could provide valuable insights.