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Amazon: Archaeological wonder or alien crop circles?

New discoveries suggest the Amazon was home to civilizations that rivaled classical Greece.

Several decades ago, most scientists believed the soil here was too poor and its climate too extreme to support anything more than small groups of hunters and gatherers or small-scale farmers.

That began to slowly change in the 1970s and 1980s, as accumulating evidence suggested that elaborate civilizations, maybe numbering in the millions, had existed along the banks of the Amazon’s largest rivers. Then, in the 1990s, scientists began documenting sites along the southern edge of the Amazon, suggesting that area may have held ancient societies as sophisticated as those of the ancient Western world.

“In many respects they were more complex than classical Greece, or medieval Europe,” said anthropologist Michael Heckenberger, who has spent nearly two decades working near the headwaters of Brazil’s Xingu river, several hundred miles east of Acre.

His team of researchers has described communities housing at least 50,000 people, living in what he’s called garden cities — made up of small and medium residential areas built around central plazas and interconnected by elaborate road systems, aligned with the cardinal directions.

“We also found large defensive earthworks around settlements and indications of pretty complex systems of forest and wetland management,” Heckenberger said.

Heckenberger’s work was featured prominently in the best-selling book “The Lost City of Z.” The non-fiction work recounts the disappearance of world-famous British explorer Percy Fawcett, who vanished into the Amazon in 1925 while hunting for a lost civilization. At the end of the book, author David Grann concludes Heckenberger may have come as close as anyone ever will to uncovering Fawcett’s mythical city of gold.

And he’s not the only researcher revealing a real-life El Dorado in Amazonia.

In Bolivia, a team lead by archaeologist Clark Erickson has found evidence of extensive ancient land management: a landscape marked by causeways, canals, raised fields, ring-shaped ditches and massive earthworks for trapping fish.

Geographically, the earthworks in Acre are roughly contiguous with these two sites, and the archaeological remains have some characteristics in common with those in the other two areas, scientists say.

“Now we know that they were in Acre, they were in Bolivia, they were in the Xingu. Basically all along the southern borderlands of the Amazon we find another group of complex societies,” Heckenberger said. “It clearly represents a much denser population than previously assumed.”

And the research has only just begun. Ranzi’s team is working with the Brazilian government to use a new technology that can map the ground beneath forest has not yet been chopped down.

“Amazon still is an unknown quantity,” said Jaco Cesar Piccoli, an anthropologist and head of the department of social sciences at the Federal University of Acre. “It's likely we’re going to find many more remains of human occupation.”

But he says, even as scientists broaden their search, some ruins are already being destroyed. Roads have been built across many geoglyphs, and cattle graze on and around many more.

“Very little has been done, up until now, to preserve these sites,” Piccoli said. “And there’s still so much left to discover.”