Slavery probably IS in the natural order of things, along with rape, abuse of women and children, child abuse and buggery. That is not to say that they are civilized behavior. We can figure out a lot about the "natural order" of humans 10,00 years ago by observing our fellow primates: gorillas, orangutans, chimps binabos and baboons.
Slavery is not conducive to a high-tech, industrial civilization, as we can see from the development of this country in the early years. I do not believe that a high tech industrial civilization is part of any 'natural order'. Natural order dictates that women begin to reproduce in their mid or even early teens. It also dictates that high status men should have as many wives as they can support, and that low status men have none. Mohammad's view of civilization is closer to a 'natural order' than pretty much any other society on this planet.
Without industry in the developed world, Saudi Arabia, Kuwait, the UAE and other oil countries would still be poor and primitive, since no one would need their oil.
Both the Baptist and the Methodist Church split over slavery into Northern and Southern factions. Each, of course, claimed God was on their side. God, as usual, remained silent on the issue. The Baptists have still not rejoined, and probably never will.