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.
Heck, I wouldn’t be surprised if people didn’t stick bluetooth devices in their ear just to have a voice constantly keeping them on track “You’re going to the store ... you need aspirin and deodorant ... aspirin ... and deodorant ... aspirin AND deodorant ... stop looking at the magazine — you forgot the deodorant ...”