Since the discovery, archaeological study of the earthworks and other evidence has challenged the notion that the rain forests of the Amazon were untouched by human hands before the arrival of European explorers in the 15th century. And while the true purpose of the geoglyphs remains unknown, a study published on Monday in The Proceedings of the National Academy of Sciences offers new insight into the lives of the ancient people who lived in the Amazon. Thousands of years before the earthworks were built, humans were managing the forests, using what appear to be sustainable agricultural practices.

“Our study was looking at the environmental impact that the geoglyph builders had on the landscape,” said Jennifer Watling, an archaeologist at the University of São Paulo, Brazil, who conducted the research while a student at the University of Exeter in Britain. “A lot of people have the idea that the Amazon forests are pristine forests, never touched by humans, and that’s obviously not the case.”