Mysterious geoglyphs can teach us about the Amazon's past—and its worrisome future

Enormous shapes etched onto the Earth

Geoglyph
Modern deforestation has revealed more than 450 of these strange shapesJenny Watling

We know that the forests of the Amazon have a long history of human interference. Scientists have been studying the region's ancient geoglyphs—large designs traced into the ground with rocks and other debris—since the 1960s, when deforestation by cattle ranchers first revealed the stunning shapes. Because the geoglyphs only appeared once forests had burned away, it follows that the ancient artists must have burned their own trees to the ground to build them.

But according to new research, this knowledge shouldn't give us hope that it will be easy to bounce back from modern deforestation—which, in the past half a century, has destroyed around 20 percent of the Amazon rain forest.

The geoglyphs have been cited as a sign that Amazonian forests aren't meant to be kept "pristine". Some forests have evolved to occasionally burn. If the Amazon rain forest has been influenced by humans for thousands of years—and grew back just fine when swaths sat unoccupied for a few centuries—then is it right to say we should leave it alone entirely?

New findings suggest that indigenous farmers cleared their surroundings on a much, much smaller scale than today's cattle ranchers. So while the area might not be adapted to total seclusion, we should still worry that our excessive meddling will kill the forest for good.

"In our records, we didn't find any ancient parallels for this kind of vegetation deletion," says Jennifer Watling, a post-doctoral researcher at the Museum of Archaeology and Ethnography, University of São Paulo. Watling led the study, published this week in Proceedings of the National Academy of Sciences, back when she was a PhD candidate at the University of Exeter.

"We didn't find any ancient parallels for this kind of vegetation deletion."

The team did find evidence of ancient controlled burns in the geoglyphs of Acre, a state in western Brazil that's currently experiencing heavy deforestation. They analyzed the charcoal in surrounding soil, as well as searching for carbon stable isotopes and microscopic plant fossils called phytoliths. This data enabled them to reconstruct 6,000 years of the area's ecological history. Unsurprisingly, they found charcoal layers heavy enough to suggest that humans cleared the area with a blaze when they first moved in about 4,000 years ago.

But these indigenous peoples didn't keep on slashing and burning all willy-nilly. Instead, according to Watling's findings, they selectively encouraged the growth of palm trees while burning down bamboo.

And when it was time to build a geoglyph—a mysterious practice that began about 2,000 years ago, the purposes of which are still unknown—the locals did clear the land with fire, but kept the burns contained to small areas around the design's intended location.

"Indigenous peoples managed their environment through mixed subsistence strategies, many of which involved concentrating useful tree species," Watling says. "The lack of evidence of large-scale deforestation in the past means that we cannot assume Acre's forests will recover from the unsustainable land-use practices of today."