From what I’ve read the early explorers of the Amazon reported large numbers of people and vast open fields where agriculture was practiced. It was not dense impenetrable rain forest like it largely is now.
Humans had apparently terraformed the Amazon along with much of North America. This is covered in the book “1491”. The book is not only a great read but provides a lot of food for thought.
I seem to recall reading a few years ago that according to satellite data or some kind of survey the rainforest had terraced farms and fields under it. So much for it being “the lungs of the planet”.
The relatively recent reports on the early travels of Spaniards from the conquest of the Incas who then traveled down the Amazon to the Atlantic by boat have opened our eyes to a lot. Large settlements were reported along the river banks. Agriculture in the poor soild of the Amazon jungle had been converted by long term use and development to something called Terra Prieta, or Black Soil. This agriculture enabled the larger populations, although river resources like fish were also an important resource.
Although a population, subsequently disappeared, of 5 million is certainly possible, it is unlikely that it occupied more than a modest portion of the Amazon jungle. It takes a while to develop black soil, and it is certainly NOT found in large parts of the Amazon, especially locations not near major rivers. Diseases carried by the Spaniards are probably the reason that 100 years later the large settlements along the Amazon no longer existed.
Looking at the LIDAR photos you can clearly see the square and rectangular construction foundatioos. Also with careful looking one can also find much more faint similar forms which might be hundreds of years older. In North America, temples were often found to have layers on layers of newer construction over original buildings. I wonder what the existance of fain traces of earlier buildings means for continuity of occupation or rise and fall of prosperity in towns and villages.