The Mark of Cain comes from a story in the Bible. Cain killed his brother, Abel, so God cursed him by sending him away from his family. Cain feared that someone would kill him because of his crime, so God put a mark on him to warn others not to kill Cain.
Much later, white slave owners and white supremacists imagined that the Mark of Cain must have been dark skin. (They appeared to be confusing two Bible stories, as the Curse of Ham bears a tenuous relationship to skin color, while the story of Cain bears none.) They assumed that God cursed dark-skinned people by making them inferior to whit...