Slate Star Codex reviewed Jaynes' book as well, which is how I first heard about it: https://slatestarcodex.com/2020/06/01/book-review-origin-of-...

To your point:

> Isn't this a testable hypothesis? If consciousness spread virally and isn't a natural outgrowth of the physical structure of the brain, then presumably isolated indigenous tribes would still have pre-consciousness mental processes, and that should be detectable somehow.

SSC's article points to a Reddit thread that suggests even folks in the mainstream of culture may undergo a "consciousness transition" as adults: https://www.reddit.com/r/self/comments/3yrw2i/i_never_though...