Healthy soil is the hidden ingredient

Humans can figure out a lot given enough time. While all the hype for us is finance, management, machines, electronics and software etc it is not unthinkable a previous civilization went all in on soil. Terra Preta seems to be quite sophisticated.

https://en.m.wikipedia.org/wiki/Terra_preta

South America was pretty advanced. The oldest evidence we have of widespread metallurgy comes from the tip of South America around approximately 5000 BC. Which predates metallurgy in Eurasia by thousands of years. Copper smelting was particularly important

https://en.wikipedia.org/wiki/Metallurgy_in_pre-Columbian_Am...

On the ecological side, some anthropologists argued that humans actually played a major role in transitioning Amazonia from mostly grasslands to the rainforest it is today around 10,000 years ago.

The distribution of many plant species is inexplicable without looking at human settlement patterns. So much so that other anthropologists have called the Amazon a "manufactured landscape".

https://sci-hub.ru/https://link.springer.com/chapter/10.1007...

you can also say the Saharan desert played a major role on turning what Amazon is...

now, wow, calling it a grassland before humans 10,000 years ago is to smoke too much pot before reading/making papers. 5,000,000 AD then yes, maybe... /s but Terra Preta and other indigenous interferences is not even 10% of Amazon territory. various other animals are responsible for spreading diversity be it by shitting seed or just moking stuff around to make nests or impress some partner. the rainforest are also there because mountains changing courses of rivers.

[0] "Geology and geodiversity of the Amazon: Three billion years of history" https://www.theamazonwewant.org/wp-content/uploads/2022/05/C...

[1] the grassland hypothesis (somewhere in the text) and other curiosities about its biodiversity https://www.science.org/content/article/feature-how-amazon-b...

This is common knowledge. Even the Wikipedia page states:

> There is evidence that there have been significant changes in the Amazon rainforest vegetation over the last 21,000 years through the last glacial maximum (LGM) and subsequent deglaciation.

https://en.wikipedia.org/wiki/Amazon_rainforest

Also, there's this:

https://www.discovermagazine.com/planet-earth/10-800-years-a...

the Wiki citation doesn't even have a source, nor is calling that indigenous people made it

your last link is about Llanos de Moxos, which isn't in Amazon. you don't seen to understand even basic geography... even if Llanos was 100% man-made (and isn't) and it was part of the Amazon (and not a region that borders it) it would be the equivalent of 2.6% of the whole Amazon area. concluding such a thing because 3% of an area that benefited (soil quality wise) from billions of years of geologic events and was partly modified by humans is ignorant but again, Llanos isn't even Amazon

it was common knowledge among middle age that Earth was flat. doesn't seem an argument to me

There were elephants there that humans hunted to extinction, elephants typically keep forests down and create grasslands. So it seems likely it happened, and that humans was the cause (by killing the elephants).

Edit: So it is likely that the change happened and had nothing to do with the soil change.