this doesn't seem to be a variation of "wild rice" such as some communities in the area still gather, or similar to the "wild rice" that the Indians of Minnesota still harvest every year (Zizania palustris) but the Oryza species.
if it was Oryza, then one wonders if it was native or introduced there. the map lists other species of rice.
Increasingly large Oryza sp. husk phytoliths recovered from mid-Holocene levels of a shell mound in southwestern Amazonia (Fig. 1) dating to ca. 4,000 cal. yr BP show the progressive selection of larger wild rice seeds by its pre-Columbian residents, who were already engaged in the cultivation of maize (Zea mays) and squash (Cucurbita sp.)19.
alas, being Nature, I can't get the full article, but then I probably don't know enough to interpret it anyway.
this article in Science points out that the Amazon area was not "pristine and untouched" as some ecology types insist: it was actually well developed and many trees and plants were domesticated and cultivated by locals in Pre Colombian times.