The occupational divisions in ancient times weren't rigid enough to be true castes, with the exception of a few untouchable-like groups. When was the last time a Potter was killed by a lynch mob for marrying a Marshall? (I deliberately chose those occupational surnames because they exist in both Chinese and English.)

The US segregation system did not differ much from SA's apartheid, though the origins were somewhat distinct. In both cases there was an earlier period where blacks and whites mixed to some extent, then social conventions changed and forbade it. Interracial marriage may be increasing in the US but it is still very much frowned upon, and not even talking about blacks, when was the last time you saw an Oriental man married to a white American?

Until fairly recently the French had cagots, and the Japanese still have burakumin. But their untouchables could not be physically distinguished from anyone else, unlike blacks in the US.