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The Amazon rainforest is one of the worlds greatest natural resources, with its incredible vegetation continuously recycling carbon dioxide into oxygen. Having been referred to as the Lungs of our Planet, about 20% of Earths oxygen is produced here.

The impenetrable lushness of this revered rainforest lends to both its ominous reputation and the misconception that it was an untouched wilderness prior to Europeanexploration.As discoveries continue to show, indigenous people altered the land far before then.

Now, researchers have found that theAmazonian rainforest was transformed over two thousand years ago by ancient people who constructed hundreds of massive, mysterious earthworks.

Brazilian and U.K. researchers found the more than 450 massive geoglyphs that had been hiddenfor centuries by trees by way of deforestation in the Brazilian rainforest. The earthworks span an area of about 5,000 square miles.

It is unknown what function the sitesserved. Archaeologists say it is unlikely they were villages, since they recovered very few artifacts during excavation, and the layout doesnt appear defensive, either. Experts believe the sites were used on occasion, potentially as ritual gathering places.

The research, carried out by Jennifer Watling, a post-doctoral researcher at the Museum of Archaeology and Ethnography, University of So Paulo, is yet another challenge to the overwhelming assumption that the rainforest ecosystem was untouched by humans.

The fact that these sites lay hidden for centuries beneath mature rainforest really challenges the idea that Amazonian forests are pristine ecosystems, statedDr. Watling.We immediately wanted to know whether the region was already forested when the geoglyphs were built, and to what extent people impacted the landscape to build these earthworks.

The researchers used state-of-the-art methods to reconstruct 6,000 years of vegetation and fire history around twogeoglyph sites. They discovered that humans heavily changed bamboo forests for millennia, and small temporary clearings were created to construct thegeoglyphs.

Perhaps one of the most useful pieces of information the experts discoveredwas that, rather than burning large tracts of forest to farm or constructgeoglyphs, people altered their environment by way of focusingon economically valuable tree species like palms, which created a sort of prehistoric supermarket of useful forest products.

The team also found evidence suggesting that the biodiversity of some of the remaining forestsin Acre state in the western Brazilian Amazon, where the ditched enclosures were found, may have a strong history of the ancient agroforestry practices.

Despite the huge number and density of geoglyph sites in the region, we can be certain that Acres forests were never cleared as extensively, or for as long, as they have been in recent years, said Dr. Watling.Our evidence that Amazonian forests have been managed by indigenous peoples long before European Contact should not be cited as justification for the destructive, unsustainable land-use practiced today. It should instead serve to highlight the ingenuity of past subsistence regimes that did not lead to forest degradation, and the importance of indigenous knowledge for finding more sustainable land-use alternatives.

The team extracted soil samples from a series of pits dug both within and outside of the geoglyphs to conduct the study. They then used the soils to analyzephytoliths, which are a type of microscopic plant fossil made of silica, in order to recreate ancient vegetation. Charcoal analysis revealed the amount of ancient forest burning, and carbon stable isotopes revealed just how open the vegetation had been in the past.

The early edition of the study has now been published in the journalThe Proceedings of the National Academy of Sciences.

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Mysterious Ancient Stonehenge-Like Circles Found in Amazon Rainforest - Collective Evolution