The ethical abyss always has been that slaves where treated as objects that had the same legal status as other ''things''. But this changed especially during the late roman republic when first vital signs of antique ''humanism'' forced society to guarantee to slaves the right of having property too. Legal regulation of treating slaves changed dramatically and there where more and more families born out of that status that gained citizenship.

To the second part of Your question if I get it right: we lost the path. I do not see any deeper impact of contemporary philosophy or reflection nowadays to be honest.