Slavery is much older than the period that the term "scientific racism" refers to. Older slave holders had no trouble justifying slavery.
Interestingly, the many slave rebellions of antiquity (and after) by and large did not seek to abolish slavery, only to switch who's a slave, and who's a slave holder.
> Romans (or whatever) did not treat their slaves as sub-humans
I mean, that's pushing it. Obviously, their concept of slavery wasn't really based on ethnicity, and manumission was generally more common and socially acceptable than it was in American slavery. But the treatment of slaves in Ancient Rome was certainly inhuman by modern standards.
The Romans also had mechanisms to, in practice, suppress the political power of descendants of slaves; in practice almost all freedmen went into one of the urban tribes (the very wealthy could buy into a rural tribe), making their votes largely irrelevant, for instance. It's reasonable to view New World slavery as a special evil, but Roman slavery was Pretty Bad, too.
> The Romans also had mechanisms to, in practice, suppress the political power of descendants of slaves; in practice almost all freedmen went into one of the urban tribes (the very wealthy could buy into a rural tribe), making their votes largely irrelevant, for instance. It's reasonable to view New World slavery as a special evil, but Roman slavery was Pretty Bad, too.
Those mechanisms sound a lot like present day gerrymandering[1][2] coupled with redlining[3][2].
Interestingly, the many slave rebellions of antiquity (and after) by and large did not seek to abolish slavery, only to switch who's a slave, and who's a slave holder.