Back then African people were basically considered evolved apes but definitely not humans, at least not the same concept used for Europeans. They were even kept in zoos in Europe[1]. The USA also extensively used eugenics[2] to try to purify the race, which was a huge inspiration for the Nazis and their "final solution". Social Darwinism[3] at its best.

[1]https://en.wikipedia.org/wiki/Human_zoo

[2]https://en.wikipedia.org/wiki/Eugenics_in_the_United_States

[3]https://en.wikipedia.org/wiki/Social_Darwinism