"So if you suppose that a final set of changes was enough to produce a sudden huge leap in effective intelligence, it does beg the question of what those changes were."

Perhaps the final cog was language. The original innovation is concepts: the ability to process thousands of entities at once by forming a class. Major efficiency boost. But chimps can form basic concepts and they didn't go foom.

Because forming a concept is not good enough - you have to be able to do something useful with it, to process it. Chimps got stuck there, but we passed abstractions through our existing concrete-only processing circuits by using a concrete proxy (a word).