Some 3,000 years ago, the Bolivian Amazon didn’t look like a jungle at all—it was more like an African savannah. A new study reveals how the ancient landscape took shape, and sheds new light on the mysterious ditches the early Amazonians built there for purposes unknown. The key to the finding is pollen: As LiveScience reports, British researchers studied sediment cores from two lakes near the ditches and found that the ancient Amazon was neither deforested or virgin, but just very dry. The ancient pollen found in the cores hailed from grasses and drought-resistant trees; then about 2,000 years ago something shifted—pollen from fewer drought-resistant species and more evergreens showed up, suggesting more precipitation.
How this ties back to the ditches, which are square, straight, and circular in shape; measure up to 16 feet deep; and have been found throughout the Bolivian and Brazilian Amazon: There had long been a debate over the toll the area's early people exacted on the forest. As LiveScience explains, some believed they "conducted major slash-and-burn operations"; other researchers thought they were gentle. As the lead researcher now puts it, they were "neither." The ditches came before that 2,000-year-old shift, indicating the people made their ditches—whether for defense, drainage or ceremonial purposes—before the forest took shape ... which explains why they were able to build there with only stone tools. LiveScience has some photos of the earthworks. Unfortunately, the deforestation that revealed them has increased. (Read more scientific study stories.)