The spanish explorer Francisco de Orellana encountered advanced societies along the Amazonas river. He's the guy that came up with that name. What's left from these civilizations are Terra Preta deposits (extremely fertile black earth of very likely human origin) and petroglyphs that can be detected using LIDAR. Also, small tribes of natives with a noble class, which is quite unexpected for hunter-gatherer societies, but makes sense for more complex societies that suffered civilizational breakdown and had to change lifestyle. Sort of like the northern Dundedain :-)