"Today we think of the Amazon as a pristine rainforest, sparsely populated by indigenous communities. But in records from the 1500s, Spanish colonizers describe a densely populated region, crisscrossed by canals and sunken roads and dotted with bustling, fortified towns. Unfortunately, fortification couldn’t protect the towns’ inhabitants against European diseases, which devastated South American indigenous populations and left the fortified villages abandoned." https://arstechnica.com/science/2018/03/network-of-fortified...
"Some 5 million people may have lived in the Amazon region in AD 1500, divided between dense coastal settlements, such as that at Marajó, and inland dwellers. By 1900, the population had fallen to 1 million and by the early 1980s it was less than 200,000."