[plorg]

It's never been 100% clear to me what the accepted timeline of human inhabitation of the American continents was, but it seems clearly possible (or likely?) that humans and other megafauna existed close together. Given that fact, it's entirely possible that the plant (and the others mentioned) were cultivated by human, and one megafauna was replaced by another.

[empath75]

I’m pretty sure the consensus is that humans wiped them out.

[barberousse]

The fate of one of Daenerys' dragons in Game of Thrones comes to mind. A real good look at ecology and evolution really reveals how fragile everything really is.