Archaeologists have discovered the ruins of a vast network of settlements hidden beneath the undergrowth of the Bolivian Amazon, in what has been described as the clearest example yet of the complex societies that thrived in a region once held to be pristine wilderness.
The system of monumental centres, towns and villages spans hundreds, if not thousands, of square kilometres of the Llanos de Mojos region, a tropical savannah in the Amazonian basin.
Mysterious mounds were first noted in the region by archaeologists more than 100 years ago.
Since then, excavations have unearthed evidence of the Casarabe culture, which developed in the area from AD500 to 1400.
Remote sensing had revealed the possible presence of hundreds of settlements. But the difficulties of working in the tropics – and a thick cover of vegetation – obscured the true extent and pattern of the sites.
In 2019, the archaeologist Heiko Prümers and his team began flying over the region by helicopter, mapping the land beneath them with a laser. They were then able to digitally strip away the vegetation, revealing the topography of the ground underneath.
In a paper published in Nature, they have now documented a range of settlement sites in detail for the first time – and discovered numerous previously unknown ones.
Within the largest sites, they found monumental platforms and pyramids, some 20 metres (65ft) high. Smaller settlements surrounded the larger ones, linked by causeways running for kilometres. Canals and reservoirs show how the Casarabe shaped the land for agriculture and aquaculture.
The authors describe it as a new form of urbanism in Amazonia.
Other complex societies have been found in the Venezuelan and Brazilian Amazon. “But this, in my opinion, is the clearest example of low-density urbanism in the Amazonia,” said Michael Heckenberger, an archaeologist who works in the Brazilian Amazon and did not participate in the project.
“It is like an index fossil of what full-blown Amazonian urbanism might have looked like,” he added. “They really nailed not what caused these urban societies to appear, not what caused them to collapse – but what they were like at their peak.”
For most of the 20th century, it was held that the Amazon was unsuitable for large permanent settlements. Some still resist the idea of urban societies in the Amazon.
“There is a very entrenched position that the Amazon is supposed to be about nature, and that the human footprint is very, very slight, almost nonexistent,” said Heckenberger.
Umberto Lombardo, another archaeologist who works in the Llanos de Mojos but was not involved in this project, said he saw the findings as definitive. “I think that old debate is settled. Now the discussion is the extent to which people changed the Amazon.”
Much of what was assumed to be untouched wilderness might in fact have been shaped by the activities of cultures like the Casarabe.
“Very little of that landscape was not directly influenced, if not constructed or managed, by pre-Columbian societies,” said Heckenberger. “These were not natural forests – they were a mix of the natural features of the tropical environment and cultural patterns of management.”
Given the scale and complexity of Casarabe culture revealed by these findings, it prompts the question of why the archaeological record appears to stop at about AD1400.
“They lived there for 1,000 years, then they disappeared,” said Lombardo. “And we don’t really know why. But it seems they disappeared before the arrival of Columbus.”
More broadly, says Heckenberger, the Amazon is the last significant world region to reveal the archaeological secrets of its “deep past”.
In some cases, archaeologists have only become aware of settlement sites because of deforestation. “It’s one of these tragic ironies,” said Heckenberger. “We know a lot more about the archaeology of these areas because of remote sensing of formerly forested areas.”
“But the fact remains that the vast majority of the Amazon is terra incognita.”