Two mysteries for the price of one: were some parts of the Amazon rainforests actually grassy plains just a few thousand years ago, and why (and how) were the ancient people of that area building massive circular earthworks? Environmental scientist John Francis Carson and his colleagues are trying to find the answers:

A series of square, straight and ringlike ditches scattered throughout the Bolivian and Brazilian Amazon were there before the rainforest existed, a new study finds.

…Since the 1980s, deforestation has revealed massive earthworks in the form of ditches up to 16 feet (5 meters) deep, and often just as wide… These human-made structures remain a mystery: They may have been used for defense, drainage, or perhaps ceremonial or religious reasons.

Carson and his colleagues wanted to explore the question of whether early Amazonians had a major impact on the forest. They focused on the Amazon of northeastern Bolivia, where they had sediment cores from two lakes nearby major earthworks sites. These sediment cores hold ancient pollen grains and charcoal from long-ago fires, and can hint at the climate and ecosystem that existed when the sediment was laid down as far back as 6,000 years ago.

An examination of the two cores — one from the large lake, Laguna Oricore, and one from the smaller lake, Laguna Granja — revealed a surprise: The very oldest sediments didn’t come from a rainforest ecosystem at all. In fact, the Bolivian Amazon before about 2,000 to 3,000 years ago looked more like the savannas of Africa than today’s jungle environment.