On Tuesday night’s episode of Finding Your Roots, the Henry Louis Gates Jr.-hosted PBS celebrity genealogy show, the musician and producer Questlove (Amir Thompson) will get a chance to uncover his family’s earliest days in the United States. But, while the show always relies on surprising reveals about its subjects’ backgrounds, it turns out that Questlove’s family has a distinction that sheds light not just on those individuals but on a larger story from American history.