The two hemispheres started to communicate (better) so wouldn't feel each other as strangers anymore. Or at least that was my understanding, in very simple words and without asking any AI :)

The phenomenon of auditory hallucinations often attributed to schizophrenics in modern day, was, far from being an aberrant condition, in fact the normal every day condition of ancient bicameral man. There was no concept of "I" or "free will;" the gods merely issued their auditory commands, and man obeyed, not having any choice in the matter. This was simply every day life for the bicameral man, who lived in a community of other "like-minded" beings that also experienced similar auditory hallucinations, often commands or admonishments, from the same god-king ruling the civilization.

Neurologically Jaynes locates these voices, and even defines gods to be those particular neurological phenomena located in the right hemisphere of the brain, that communicated its preverbal judgements to "man," located in the left hemisphere, which interpreted such judgments as speech:

    The gods were in no sense "figments of the imagination" of anyone. They *were* man's volition. They occupied his nervous system,
    probably his right hemisphere, and from stores of admonitory and preceptive experience, transmuted this experience into articulated
    speech which then "told" the man what to do.

Several factors were attributed to its decline: the development of written language localizing a disembodied voice that was once omnipresent into a stele or rock carving; the emergence of trade and the contact of other societies, governed by different god-kings, leading to proto-theories of mind meant to explain the differing behaviour of the rival civilization; selective pressures against the viability of such "bicameral theocracies;" the development of free will. Reasons abound, but what followed was the collapse of bicameral mind and the disappearance of the voices, substantiated by various observations of ancient cultural artifacts: hypnotic induction of trance at the Oracle of Delphi, the practices of divination and omen reading, the production of artistic works in Mesopotamia depicting empty thrones and absent gods, or the crying out to the gods for their assistance and return, as in The Babylonian Theodicy, or the Psalms. All this in order to "re-awaken" the voices of gods that had once dispensed wisdom and now fell dormant with the emergence of modern, self-conscious, free-willed, individualistic man.

people drank a lot more alcohol back in the day.