Woah, is this common knowledge? I feel silly for not realizing that myself.

Among archaeologists, I believe yes; I think when talking to the general public, though, they emphasize what we do know rather than what we don't.

If you want something really fun...

It's becoming likely that around the time of the European contact with the Americas, there were large societies with high population densities living throughout what is now the Amazon rainforest. As parts of the jungle are cleared large geoglyphs are being discovered, along with apparent raised causeways between settlements in a number of Western basins. Furthermore, there appears to be artificial soil called Terra Preta all over along the rivers, full of charcoal and pottery fragments that collectively ameliorate the problems of low nutrient retention that hamstrings agriculture in rainforest conditions. There are vague reports of high population densities in the amazon basin that were not believed for centuries by a conquistador named Francisco de Orellana that may have been at least somewhat more accurate than they have been given credit for. What appears to have happened is that after the various European diseases (smallpox, hepatitis, etc) killed off most of the population, almost all material evidence for these people other than geoglyphs, soil engineering, and pottery shards would have rotted away. In the deep amazon, there was no stone. All building and tool materials would have been biodegradable in the very wet, warm, full-of-living-things climate.