In the often impenetrable lushness of the Amazon rainforest, it's easy to see why many believed it was untouched wilderness before Europeans explored the area. But more discoveries show that indigenous people altered the land long before then.

Most recently, Brazilian and U.K. researchers have discovered more than 450 massive geoglyphs that had been concealed for centuries by trees. The earthworks only came to light due to deforestation in the Brazilian rainforest. They cover an area of roughly 5,000 square miles.

The geoglyphs are striking in their size and number. Some of the trenches are as large as 36 feet wide and 13 feet deep, reports Live Science.

Reconstructing history

Hortigranjeira Amazon rainforest geoglyph The earthworks were scattered over an area of about 5,000 square miles. (Photo: Edison Caetano)

"The geoglyphs are earthworks consisting of an external bank and inner ditch, enclosing a typically circle- or square-shaped area," lead researcher Jennifer Watling tells MNN. Watling is a post-doctoral researcher at the Museum of Archaeology and Ethnography, University of São Paulo.

"Their forms are often geometric and they can measure hundreds of meters in diameter. We think their original functions were public gathering places where ceremonial and ritual activities were carried out. They seem to have been used sporadically, as archaeologists find very sparse cultural remains during excavation of these sites."

The researchers were able to reconstruct 6,000 years of vegetation history in the area of two of the earthworks sites, says Watling.

"We found that these societies profoundly altered the composition of the rainforest over thousands of years by encouraging species that were useful to them, such as palms. We also found tantalizing evidence to suggest that the structure of some of the region's remaining forests, once thought to be 'pristine,' owes itself to these ancient land-use practices."

Who changed the rainforest and when?

Chinésio Amazon rainforest geoglyph The function of the earthworks is likely a public gathering place, but researchers don't know for sure. (Photo: Edison Caetano)

At those two geoglyph sites, researchers found evidence that the forest had already been altered by humans long before people had built the earthworks, Watling says.

"At these locations, palms made up a large component of the vegetation, though in some other locations we tested, we didn't find so many palms," she says. "We deduce that the region's vegetation probably consisted of a mosaic of forest types and small-scale, temporary clearings where people planted crops such as maize and squash."

Although researchers aren't certain what the purpose of the geoglyphs was, Watling says she was most surprised by the time that passed between the modifications that were done in the forest.

"In two locations where geoglyphs were built roughly 2,000 years ago, we could see that people were already altering the forest in a detectable way almost 2,000 years before that," she says. "Many researchers have long proposed the antiquity of cumulative human impacts in Amazonia, but to finally prove it with your our data was something special."

The early edition of the study has been released in the journal The Proceedings of the National Academy of Sciences.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.