I think this is illuminating about what the invention of race means for the nature of social relationships. From Lawrence Hirschfeld, Race in the Making (MIT 1996):
Indeed, racial kinds differ form other intrinsic kinds in that racial kinds are *supposed* to be self-reproducing, not only in a strict biologistic sense (you don't need members of two different intrinsic kinds to produce offspring, as you do with gender) but also in a societal sense. Unlike, say, intrinsic construals of class in 17th and 18th century France or caste in South Asia, races are conceptualized as distinct subpopulations whose interrelationships are contingent, not necessary. Castes coexist in a web of mutual dependency, as do classes in their intinsic guises. Although their members may be able to reproduce endogamously (bakers begetting bakers, nobility begetting nobiity), taken together the society cannot reproduce without these various kinds' being locked together in a system of exchange and mututal responsibility. Races *may* coexist in this way, but they do not have to. Typically each race is seen as traceable to an independent population, with its own history, its own habitat, and so forth. Relatons of interdependence between races, as the discourse of slavery and its subsequent social forms often stipulate, are historical and often are dismanteable. Even when racial hierarchies have been conceived as part of the natural order of things with respect to their relative place in hierarchies of value, their continued interdependence is not seen as a necessary part of the natural order--slaves *could* have been sent back to Africa, Jews *could* have been expelled from the Iberian peninsula, and so on. " p. 198 rb