Jaynes claims it wasn't physical evolution but an adaptation to changing social conditions, i.e. the hardware (brain) remained the same but the software (mind) changed.
Can raise awareness of the likelihood that when right-handed people learn to read & write from other right-handed people, that a certain amount of verbal concentration can be made exclusively to the left side of the brain in some kind of a self-perpetuating pattern. The numerical predominance of right-handed people makes this into an underlying mainstream development.
Going back quite a ways but who knows how far?
OTOH those who are left-handed often do not seem to conform as uniformly in this respect, and those having a tradition of large vocabularies but without reading or writing might even have the advantage of some verbal or other skills handled by both sides of the brain simultaneously far differently than the mainstream.
Some types of thinking may or may not be accomplished in a relatively verbal way at all.
