From this really interesting interview with philosopher Elizabeth Anderson:
The idea that human beings are fundamentally equals from a moral point of view is ancient. I suspect it can be traced all the way back to the origins of monotheism, in the idea that we are all equally creatures of God, all made in God’s image, all in principle equally eligible for salvation. However for most of history most monotheistic churches have promised equality only in the next life; in this one a thousand reasons were invented to uphold various forms of social, political, and religious hierarchy.
I also recommend Anderson’s paper “What is the point of equality?” (PDF), though it’s a bit philosophically dense.