Throughout human history, people have treated groups of people unfairly based on race, gender, and/or social class. Two examples of this occurred during the colonial period. First, Europeans and white Americans treated the Native Americans reprehensibly. They destroyed their cultures (Aztec, Inca, etc.), forced them to convert to Catholicism (in Spanish colonies), and stole their lands. In the United States, for instance, Native Americans were forced to either conform to the conventions of white American society or abandon their homes and move to reservations in the West.
Second, Europeans and white Americans enslaved Africans and forced them to work on plantations in the New World. These slaves--who grew labor-intensive crops such as sugar, tobacco, and cotton--reaped huge profits for their masters but were often treated even worse than animals.
In both cases, the oppressive group justified their actions by claiming that the group they were treating unfairly was subhuman.