Darwinism doesn’t absolutely require its adherents to be racist, but it has repeatedly tended that way.
Slavery was common in the ancient world, but it was a question of vanquishing your enemies, not of different races that were superior and inferior. “Race” was mostly a scientific theory of the nineteent century, and it fit right in with evolution.
It is common for Darwinists to distinguish themselves from Social Darwinists. But Social Darwinists certainly never saw it that way.
In the nineteenth century, “primitive” didn’t just mean culturally inferior. It also meant racially inferior.
Darwinists still automatically tend to think in those terms. If someone expresses religious beliefs, it means that he is backward, undeveloped, crude, not as far evolved up the tree of life as the superior Darwinists.
I question this. If by the term "race" you are referring to the realization by one group noticing, and capitalizing on, those different from themselves in language, physical features,etc. than "race" has been a factor for just about forever, and found in every place inhabited by man.
As to "race" being a scientific theory, I'm not sure what you mean. Does it mean human curiosity about why humans come in the endless variety they do? Or why some societies ("race") seemed eternally becalmed in, for example, the hunter-gather stage of development? Is it the fear that in doing so it would cause/encourage one group to consider itself superior to the other(s)? Again, that is hardly a 19th century development.