Earthworks shed light on secret life of the Amazon

Scientists have uncovered a mysterious system of ancient earthworks in the Amazon basin that have been hidden until now by rainforest, and are shedding new light on how ancient tribes may have lived there.

The 2,000-year-old works consist of a series of 450 ditched enclosures in Acre state, in the west of Brazil, and which only came to light due to deforestation.

The structures, known as geoglyphs, cover an area of around 13,000 square kilometres, but archaeologists are still baffled about their purpose. They appear to have no defensive function. Some scientists believe they could have been used as ritual gathering places.

Whatever their original function, they provide new evidence for how indigenous people lived in the Amazon before European people arrived in the region.

They also challenge the notion that the rainforest was untouched by humans until recent times.

“The fact that these sites lay hidden for centuries beneath mature rainforest really challenges the idea that Amazonian forests are ‘pristine ecosystems’,” says Jennifer Watling, a researcher at the Museum of Archaeology and Ethnography, University of São Paulo.

“We immediately wanted to know whether the region was already forested when the geoglyphs were built, and to what extent people impacted the landscape to build these earthworks.”

Watling’s team used took soil samples from a series of pits to analyse “phytoliths”, a type of microscopic plant fossil made of silica and charcoal. This helped them to reconstruct 6,000 years of vegetation and fire history around two geoglyph sites and led a startling discovery. 

People living there appear to have concentrated on planting tree species that were useful for food or other purposes, such as palms, to create a kind of ‘prehistoric supermarket’ of useful forest products.

The biodiversity of some of Acre’s remaining forests may be a direct result of these practices.

“Despite the huge number and density of geoglyph sites in the region, we can be certain that Acre’s forests were never cleared as extensively, or for as long, as they have been in recent years,” Watling told reporters.

“Our evidence that Amazonian forests have been managed by indigenous peoples long before European contact should not be cited as justification for the destructive, unsustainable land-use practiced today.

“It should instead serve to highlight the ingenuity of past subsistence regimes that did not lead to forest degradation, and the importance of indigenous knowledge for finding more sustainable land-use alternatives”.

The full article will be released in the Proceedings of the National Academy of Sciences of the USA.