Long before the arrival of Europeans in the Americas, people living in western Brazil carved hundreds of huge geometric shapes into the ground. These mysterious earthworks, like the one shown above near the Rio Branco in the Brazilian state of Acre, are called geoglyphs, and they are one of the few clues that large-scale, complex societies may have once occupied the Amazon rainforest. Composed of ditches precisely laid out in circles and squares, the monuments stretch up to 300 meters across. Scientists only noticed them after the forest had been cut away, implying that the builders may have also deforested the region. To find out what impact these geoglyphs had on the environment, a team of scientists extracted long cores of the soil from two geoglyph sites in western Brazil and dated the material to reconstruct the vegetation that covered the landscape at different times. The scientists discovered a peak in charcoal remnants between about 2300 and 1400 years ago, suggesting that the geoglyph builders burned clearings into the forest to carve the forms into the earth during that time. But the clearings didn’t exist for long. When a patch of the Amazon is deforested, grasses quickly move in to colonize the open space. Around the time of geoglyph construction, however, hardly any of the plant remains came from grasses, hinting that the clearings were quickly reabsorbed by the forest. Instead, the cores were dominated by bamboo and palm, and ratios of carbon isotopes in the soil were consistent with tall trees, the team reports online today in the Proceedings of the National Academy of Sciences. That suggests that the geoglyph builders created only small, temporary clearings to build their monuments; in fact, it’s likely the geoglyphs were not even consistently visible, the researchers report. Therefore, they conclude, the ecological impact of the geoglyphs does not come close to the clear-cutting and burning endangering the Amazon today.