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.
A Thinking Reed 
Well, I wouldn’t say “the origins of monotheism,” but Genesis 1 and the subsequent mythological history from Genesis 2 forward do certainly wrap the older narratives of inequality in the assertion of a fundamental radical equality that is God’s original order, but which became tainted by sin. Paul’s statement, “There is neither Judean nor Hellene, neither slave nor free, not male and female; for all of you are one in Christ Jesus,” can easily be a version of Christ and Adam, in this case as the recapitulation of lost original equality.
And Barth’s analysis of Genesis 1 in CD III.1 certainly goes after the point that the duality of male and female in humanity is a description of one kind, one being that is two in its kind, and then makes the analogy to God’s distinguished-yet-singular being. We are not species of male and female; der Mensch simply is male and female. And it is der Mensch, not “man,” who is at the center of creation and given singular, signal importance for demonstrating divine intent toward the whole.
I saw this the other day, an old post at The Guardian … http://www.guardian.co.uk/commentisfree/belief/2011/sep/07/political-bible-equality
Maybe the idea of equality is older than even monotheism – if it’s true, as some studies have seemed to show, that we and animals too have a built in preference for fairness, than it seems like equality would be assumed?