It was the first European visitors to the Amazon region, such as Spain's Francisco Orellan, who reported seeing large numbers of people living in villages, tending farms, and traveling using roads; their reports were dismissed as too fanciful to be true, however, because subsequent explorers found the results of European conquest and prevalent diseases. It was thought that the population of the entire Amazon basin was 1 million. Recent estimates, based on newfound evidence, put that total population at 10 million. The 1 million figure should be assigned to just the southern part of the Amazon basin, the archaeologists said.

The Amazon River and its tributaries, collectively known as the Amazon basin, include the current countries of Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, Suriname and Venezuela. The area is dominated by the Amazon Rainforest, the largest in the world.

Explorations that did occur focused on the low-lying river plains. Because of the belief that the higher areas were relatively unpopulated, archaeologists did not examine them in a systematic fashion.

Deforestation of the high-lying area, known as terra firme, revealed some of the evidence that encouraged archaeologists to investigate further. One prime target of the investigations was the Tapajós river basin, hundreds of miles upstream from the river mouth, in what is now the Brazilian state of Mato Grosso. What they found, primarily by studying satellite images but also by investigating in person, was 81 sites that dated to before the arrival of Christopher Columbus to North America in 1492. The team has already found one settlement that dates to the early 1400s.

The 81 settlements are not entirely concentrated; as well, earthworks are a common element found in multiple settlements, but enough variety has been identified on the satellite images that archaeologists have reported a relatively widespread pattern of settlement.

Results of the study, led by Jonas Gregorio de Souza and José Iriarte of the University of Exeter, appeared in the journal Nature Communications.