[allmost]

I had assumed that the abundance of terre preta made the concept virtually irrefutable. Somebody made all of that soil.

[SunkenCiv]

Also, satellite survey of the Amazon landscape has uncovered large artificial structures in clusters here and there.

Rainforest Researchers Hit Paydirt (Farming 11K Years Ago in South America)
Smoldered-Earth Policy: Created By Ancient Amazonian Natives, Fertile, Dark Soils
Putting the carbon back: Black is the new green
Farming That Improves the Environment