Atheist Amazonian tribe.

[Esquilax]

1. The xigagai account is evidence of the supernatural. THere's a group of people, otherwise known to be sane and down-to-earth, not prone to religion, who only believe in what they've seen themselves or heard first-hand from an eyewitness, and they see and hear this spirit in daylight while it's not seen or heard by another group of observers.

Noooooooooooooo...

All that is evidence of is that the human mind is just as fallible and susceptible to hallucination whether it has a religious concept in it or not. Religion may thrive in that susceptibility and convince people of things despite their rational impulses, but the weakness itself is a universal thing.

2. These people are amazingly simple in terms of language and culture. They have very few sounds in their language, and even no words for colors. They have no interest in constructing things like boats. They have no oral history. Is this what we would expect to find considering current thought on human migration? They could be explained as ancient and isolated if found in Africa, but South America is supposedly the last place humans reached. Were there only a few sounds, no words for colors, and no oral histories at the time people reached South America? Or, did these people devolve, actually losing such things?

It's probably their relative isolation acting as an echo chamber. Like a culture wide version of chinese whispers: over time their language mutated and changed, as all languages are wont to do, only shrinking in this case, and without cross pollination from other cultures there was no outside information and linguistic forms coming in.