I don't really think that's the case, the evolving. I think it's more of a matter of people always striving to be accepted as equal human beings with full protection of the law of that status, and the society and rulers blocking that, reserving equal status only for some groups (i.e. ruling men,) and slowly starting to change that when circumstances and activism push against the prevailing view. I would not term that as an evolution because it validates the ruling body as having the superior right to evolve while everybody else in lower, lesser human status sits around and waits for them to decide that hey, maybe slavery is not a good thing to do. But I do think human populations do always strive for true egalitarianism and the world is in balance more equal than it was even fifty years ago. But that does not seem able to come without a great deal of unnecessary pain and death to it. The world has become less violent, but equality is a harder sell because it requires sharing being legal humans in the society.
But if you brought today's notion of egalitarianism up as a social goal in, say, pre-Elizabethan England, I don't think it would match the realities that operated at that time. Being egalitarian back then meant something different than it does now. So I think it's the "slow striving" I object to -- I think the general tendency is to always strive toward the most egalitarian result, but 'egalitarian' as defined with respect to the time you're looking at. As a concept, egalitarianism evolves slowly, and we always strive towards it.