I think that Jaynes' hypothesis does not hold up the scrutiny. We have too many records of regular folks from antiquity that are counterexamples.
Somewhat relatedly, as late as the 13th century, the late Averroes held a substantial philosophical following that claimed that there was only one mind (the agent intellect, or the active intellect), and we all shared it. Its purpose was to explain the congruence of our thoughts, meanings, concepts, etc.