Some estimates say over 100 million people lived in complex cities in the Amazon.
Then they got hit by European diseases carried in by explorers.
This is part of a much larger story. Amongst other things, the "rumours" that have circulated for centuries about complex kingdoms of gold that lived in the deepest regions of the Amazon were obviously true. Hundreds of European explorers vanished in the Amazon, many of them probably killed by people who feared them for good reason - it is likely they were carrying diseases that wiped out these entire societies. These people were afraid of multiculture and did not appreciate the improvementation that Europeans were importing.