All shared a group identity, and in that way, shared a consciousness.

But the food that we ate that enabled us to operate in this was died out, probably killed by the climactic changes that came with end of the last Ice age, as we entered the Holocene period.

Neccesity being the Mother of Invention, we began to invent ways to survive and to communicate without the old cast systm in place. In the same way that money only works as long as we all believe in it, the concept of giving and receiving orders and expecting them to be followed is what has kept the cast system in place despite the disappearance of the human equivalent of Royal Jelly.