Lets assume that, at some point around the earliest findings of Terra Preta in the Amazon Basin (so, circa 500 BC), the Amazonian tribes somehow come into possession of some elements of the Eurasian crop and livestock package. We're fortunate that this puts us right in the range of when the Carthaginians were sending expeditions down the West African coast and setting up small colonies there. So, lets wave our hands, ask the more rigorous members of the forum not to look behind the curtain and say that some Carthaginians get blown off course, land at the mouth of the Amazon, and teach the locals how to take advantage of their crops.
This happens right as the locals are (as far as current estimates can tell) perfecting their technique of soil enrichment. With that super fertile soil and a robust crop package, what sort of civilization could arise on the banks of the Amazon?
I'm picturing a riverine civilization, isolated from any major neighbors/trading partners, spreading along the Amazon river, a sort of Tropical Egypt.