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...

I'd distinguish verbal inner monologue from consciousness. Many people think on the daily without narrating things to themselves in sentences, without feeling brutish or animalistic. I can plan complex human actions including theory of mind considerations without narrating it as a monologue. It's still very conscious.

Also that reddit post seems like a fake story made up by a teenager as a sort of scifi. It's just too on the nose. Yeah he lived life without understanding that his friends still exist while out of sight... I guess the OP just learned about some developmental stages of toddlers or object permanence etc and wrote a story like this as fanfic.