A sophisticated civilisation once flourished in the Amazon basin

In northern BoliviaLIDAR, on the edge of the Amazon rainforest, lies a savannah called the Llanos de Moxos. This is the stamping-ground of Umberto Lombardo, of the Autonomous University of Barcelona. Archaeologists once thought the Amazon basin’s soil would have been too poor to have sustained a large human population before Europeans arrived. Dr Lombardo is head of a team proving them wrong.The idea that the pre-Columbian Amazon was pristine has taken a nose-dive in recent years. Scientists have found several lines of evidence suggesting habitation, including once-populated sites and tree species that seem to have been transplanted. But just how big the civilisation was, and how many people it may have supported, has remained unclear.It is clearer now. In a paper published this week in Dr Lombardo and his colleagues provide evidence of large-scale hydrological engineering and maize-planting, bolstering the case that a good number of people lived in the area in the centuries before Columbus, during the period when the Inca ruled the Andes, and the Maya and the Aztecs Mesoamerica. The Llanos de Moxos is mostly flat, and is flooded for between three and six months a year. But some hillocks rise above the water level. These are where trees grow, and where people dwelt—building with earth, for lack of stone.Remnants of some of their earth-built structures can still be seen. One area in particular, the 4,500km Monumental Mound Region, is home to hundreds of mounds, some more than 20 metres tall and spanning 20 hectares, linked by causeways that run for kilometres. These were constructed by people known to modern scholarship as the Casarabe culture, who flourished for roughly 1,000 years. The extent of the Casarabe’s earthworks suggests there were a lot of them. That raises the question of how they fed themselves.Using satellite images and —an optical equivalent of radar that can peel away vegetation to reveal the topography beneath—Dr Lombardo’s group has identified a system of canals and ponds near the mounds. They suggest the canals drained water from the savannah into the ponds during the rainy season, keeping parts of it dry enough to be farmed, and that this water was then used for irrigation during the dry season. That arrangement would have allowed year-round farming. They also searched local sediments for pollen and phytoliths—microscopic silica structures that form in many plant tissues. The pollen and the phytoliths suggested that the Casarabe grew maize, to the exclusion of almost anything else.How numerous the Casarabe were remains unclear. Though the earthworks are extensive, they could have been built gradually, over the centuries, by a population that was never particularly big. Estimating how much maize was produced—and the numbers this could have supported—will need further fieldwork. It will also require identifying the varieties of maize grown, for these would not have been as productive as modern cultivars.Recent attempts to estimate the pre-Columbian population of the entire Amazon basin have gone as high as 8m-10m, but these are just educated guesses, according to Eduardo Neves of the University of São Paulo, in Brazil, a co-author of the paper. As for the fate of the Casarabe people, many suspect they went the way of other indigenous populations of the Americas, both South and North: ravaged by Old World diseases, especially smallpox, even before direct contact between victims and incomers. But that, too, is largely speculation. In the case of the Monumental Mound Region, radiocarbon dating suggests people stopped living on at least some of the mounds around 1400, almost a century before Columbus’s landfall. But others remained, and were still cultivating maize as late as 1550.The Casarabe themselves remain mysterious. Stone axe heads, and jewellery made of copper and lapis lazuli, suggest commerce with the Andes and what is today Brazil. Diverse ways of burying the dead—some more opulent than others—imply a social hierarchy. The most colossal of the mounds might have carried specific religious significance, or been associated with political power.In any case, the thread between past and present inhabitants of the Monumental Mound Region was broken. Most of the savannah is now owned by ranchers. The Sirionós—the indigenous people who live there today—have no connection to the Casarabe culture. Locals are aware of the remains, though; some have built their homes on top of ancient mounds. Others continue to use the Casarabe causeways and canals. Many report turning up ceramics and bones while farming. And they are, says Dr Lombardo, curious to know more about the history of their lands.