What an interesting historical interpretation!!
I would have said slavery was historically universal once any culture reached the social status of an agriculture based economy. That would have been the necessary prerequisite toward a surplus economy that could lead to trade and an advance into a more technological age. Those society's that never did make these transitions still have various forms of slavery to this day. (certain areas of Africa and SW Asia and a few other scattered spots)
Slavery as an institution isn't about rights or equality. It is simply much cheaper in a modern society to use machines to do drudge work, people, even slaves are high maintenance. The idea that "All people are created equal." Is a political truth, not a human one. as such it can only be enforced by the power of a government.
just a thought
Mad Lews