I'm currently reading the book [1] and while I haven't finished it yet, the thrust (if I'm understanding it correctly) seems to be that it was the increasing complexity of civilization (as populations rose in an area) that lead the brain to reform connections to be less bicameral in nature.

There are evolutionary changes in humans that are known to us. For instance, people of European descent can still digest milk into adulthood; the father you get from Europe, the greater that lactose intolerance grows. So it could also be for bicameralism.

I too, am skeptical about how this came across the world, but he does mention in passing that some of the Pre-Columbian cultures of North America could have still been bicameral as late as the early 1500s. It's really an interesting read.

[1] I found a copy in a used book store and the price was right.

I see it not as a hardware change but a software change. That doesn’t prove Jaynes’ theory, of course. There is another book called Metaprogramming and the Human Biocomputer.

Jaynes’ theory is interesting to me because it reminds me of disassociative disorders.

I don't think you can evaluate if that's true by reading books by people in the past. There are a lot of different states of mind which are consistent with the same output. I mean, just think about how many people with wildly different states of mind (schizophrenic, autism) today who can write books which are indistinguishable from books by "normal" people.

I think you can assume most people write characters that think somewhat like they do.

And is it true that schizophrenic and autistic authors do write the same?

There are a lot of people who don't think verbally, who nevertheless manage (because they have to, because that's basically the only channel we have) to share their ideas using language (in an indistinguishable way from people who do think with words). Some people think with sounds, some are primarily visual thinkers, some are informed by their emotions. I don't think it's infeasible that there are other ways of thinking (or other states of consciousness), which nevertheless have the capacity to produce the same type of output, in terms of writing, as what we with our consciousness produce today.

Are you saying how our brains work never evolved and we think the same as primates?

That seems really far fetched.

It's not so far fetched to say that mammal brains are all pretty similar and have a similar "consciousness" that "lower" mammals may be mediated/obscured by inability to use complex language or some other constraints

The ability to use language is a core part of jayne’s theory though. That would be part of the evolution process.