Who were the Beni in Charles mann's book, 1491?

Actually, the better question is "Where was Beni?"  It is a flatland area between Brazil and Bolivia, and it serves as part of the thesis of the book 1491,where Mann writes that the native groups who lived in the Americas did not live in a Garden of Eden; rather, they exerted change on their environments by cutting trees, selectively burning the savanna, and trapping fish.  Also, all of this happened thousands of years ago and may even pre-date the Bering Land bridge that attempted to explain how all native groups started from a band of Asiatic nomads.  Mann attempts to explain the mounds of Beni not as happenstance or religious but as elevated platforms that allowed the people of the area to grow trees and crops.  He also states that there was a commerce that went from the region to other parts of Central and South America.  This new work in native anthropology is a growing field in history that attempts to modify it from its European-centric narrative.