I found this interesting pdf of a discussion involving Jaynes (and Dennett) and it makes clear what he believed, which was that the change was mostly cultural, and that uncontacted tribes might be bicameral, but there were none left. ( I'm not sure this is true - anyone reading this have an anthropologist handy ? )
Also contains a very odd fact (?) about children.
EDIT: Oops, didn't notice it was on Jaynes' own website. So presumably quite a lot more stuff there.
Also contains a very odd fact (?) about children.
Are you talking about the bit about imaginary friends on page 5?
http://www.rifters.com/crawl/?p=791