Hundreds of circles in the ground of the Amazon rainforest that date back thousands of years have been discovered, but scientists don't know what purpose they served.

The Amazon rainforest has long been thought of a pristine ecosystem that was untouched by humans before the Europeans arrived, but new research shows that people manipulated the landscape centuries before their arrival.

Modern deforestation in the Amazon led to the discovery of more than 450 geometrically shaped ditches, known as geoglyphs, which occupy roughly 13,000 square kilometres in the Acre state of Brazil.

These circular geoglyphs went unnoticed for centuries, because they were covered by trees. Years of deforestation in the region made them visible from the air.

Researchers say they aren't certain what function the geopglyphs served.

Researchers recently discovered the presence of geoglyphs in deforested sections of the Amazon rainforest, which was previously believed to have been a pristine ecosystem. (Exeter University)

Archeologists didn't find enough artifacts to determine it was a village, and the design of the geoglyphs make a defensive or military purpose unlikely, the researchers said.

They theorize they could have been ritual gathering spaces

Though the purpose of the geoglyphs is of interest, it was how they were made that was a focus of the research.

"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," lead researcher Jennifer Watling said in a release.

Managed by indigenous peoples

Watling and the research team used technology to reconstruct 6,000 years of vegetation and fire history around two geoglyph sites. They found that humans had been altering the bamboo forests to create temporary clearings for the geoglyphs.

"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 said.

"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 practised 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 research was conducted by the universities of Exeter, Reading and Swansea, Sao Paolo, Belem and Acre and the published in the Proceedings of the National Academy of Sciences.