The Amazon rainforest was extensively modified by pre-Columbian indigenous societies. Some anthropologists have even called it a "manufactured landscape". Modern LiDAR mapping has revealed thousands of large geometric ditches, reservoirs, causeways, mounds, and earthen infrastructure.[0][1] Up to 10% of soils are highly fertile man-made terra preta soils and trees like cocoa, acai, and brazilnut show an unnatural "hyperdominance" distribution due to ancient orchards. Many plants throughout show some degree of domestication[2][3]

And some have even proposed a possible human role in the transition of parts of the Amazon from grassland to rainforest about 10k years ago[4]

https://royalsocietypublishing.org/rspb/article/282/1812/201...

You don't achieve a population of 3M people without such extensive landscape modification.

[0] https://www.zmescience.com/science/archaeology/archaeologist...

[1] https://www.sciencedirect.com/science/article/abs/pii/S02773...

[2] https://www.theguardian.com/world/2020/apr/08/humans-living-...

[3] https://www.frontiersin.org/journals/environmental-archaeolo...

[4] https://www.discovermagazine.com/10-800-years-ago-early-huma...