Every spring, rains and snowmelt swamp vast grasslands that stretch between the Andes Mountains and Amazon rainforest in northern Bolivia, but thousands of tree-covered mounds stand solid, several feet above the flooded grasses.
     "They are like islands in a sea of savannah," says Umberto Lombardo of the University of Bern, Switzerland. In 2006, Lombardo first stepped onto a forest island in this Llanos de Moxos region, puzzling over how such features could form naturally. One theory suggested that over the past few centuries, ranchers had carved away rainforest to create pastures, leaving scattered groves of about 300 trees each - but that didn't explain why the trees grew on higher ground.
     It turns out the forest islands were made by people, and are much older than suspected: roughly 10,800 years ago, humans cultivated crops in the Llanos de Moxos - confirming that Amazonia was one of the first places on Earth where people domesticated wild species.
     In 2012, Lomnardo and José M. Capriles, study co-author and archaeologist at Penn State, launched excavations, which confirmed that three mounds were made by ancient people, based on burned clay, food scraps and human burials found at the sites.
     For the new study, Lombardo mapped 6,643 forest islands in the region. The researchers probed some and found archaeological debris similar to the fully excavated sites in 64 out of 82 of the tested mounds between 2,300 and 10,850 years old. From that ratio, they estimated humans erected at least 4,700 of the 6,643 forest islands mapped.
     In the probes, the team also identified microscopic plant remains from the oldest-known squash in Amazonia and the oldest-known crops of the tuber cassava (also known as manioc or yuca) in the world, as well as nearly 7,000-year-old corn - a plant domesticated about 2,000 years earlier in Mexico. It seems people passed seeds from one community to the next, spanning over 2,000 miles from Central to South America. The new data also suggests an 8,000-year gap between the start of garden-scale cultivation and full-blown agriculture with canals, fields and dependence on domesticated species.
     
Edited from Discover Magazine (19 December 2020)