If you go deep into the Amazon, in the Brazilian state of Acre, and you look in the right spot, you’ll eventually find lots of geoglyphs: shapes dug into the dirt that are recognizable from high above.
Archaeologists have known about these geoglyphs, some of which are estimated to be up to 2,000 years old, for decades. But this week, a group of scientists shared a remarkable new finding about them: the ground beneath the Amazon’s geoglyphs, and the forests that surround them, had likely been altered by humans a couple of millennia before that.
The discovery could upend our understanding of how the rainforest developed, whether it did so largely on its own—as some have argued—or whether humans (perhaps inadvertently) assisted, by clearing large swaths of forest for their own ends.
“There’s been a very big debate circling for decades now about how pristine or man-made the Amazonian forests are,” Jenny Watling, a co-author of a study published Monday in Proceedings of the National Academy of Sciences, told Live Science.
At two geoglyph sites, known as Fazenda Colorada and Jaco Sá, Watling and her colleagues found 4,000-year-old charcoal samples after digging five-foot holes. And while that might not be particularly unusual—forests naturally burn and catch fire regularly—the researchers said that the charcoal’s age coincided with the time humans first moved to the area.
Why would humans of that era burn down the forest? One possibility is that they were trying to encourage the growth of palm trees, which are among the first to grow after a forest burns. Palm trees have long been sources of both food and sturdy building materials for humans.
It’s probably not a coincidence, Watling told Live Science, that when humans left these sites some 650 years ago, palm trees again declined. The forest, in a way, was returning to its natural state.