I don’t remember whether I actually read Jaynes’ book back in the day, or just read some discussions about it. It didn’t strike me as worth much thought.
Of course, you could certainly say that we have bicameral minds—left side of the brain, and right side, one primarily intellectual and the other primarily emotional, and so forth. (Although there, again, I believe that one should distinguish the brain from the mind. The mind uses the brain, but is not the same thing.)
Still, I have always found it difficult to believe that “evolution” could have changed humanity in such a short time, merely a few thousand years. That always struck me as a delusion of the 19th-century mind, which developed the idea of “progress.” It seems to me that the Greeks and Romans—or the Chinese—were much the same as we are, and just as smart.
Why so much science and technology in the modern world, then? That’s another story, in which—contrary to the usual modernist view—Christianity and its view of the world played a major part. But it’s preposterous to think that people a few thousand years ago were differently screwed together in their heads.
Jaynes attributes the cognitive change and “the breakdown of the bicameral mind” to a variety of social changes, but largely due to the invention of writing wherein current events and such can be documented and no longer reliant on changeable folklore and unreliable human memory, and our literacy lives in just the one lobe.
This is similar to the aftermath of the Black Plague — prior to, the local council of elders (semi-literate at best) decided disputes based on their recollection of past practice and otherwise undocumented events and agreements. In the Plague the elders had mostly died, and written recordkeeping became all-important for settling property disputes, not least regarding the estates of those who had died by the bushel during the Plague itself.
As we enter a new era of quasi-literacy — more people can read than ever before, but don’t read anything but ads, chosen headlines, and text messages — we may see another re-emergence of the bicameral mind.