Not since “Roots” has an American drama taken such an ambitious, all-encompassing approach to the stain of slavery. “Origin” is not about ancestry, but the seeds of a system that dehumanizes one group so that others may dominate them — a dynamic for which Pulitzer-winning author Isabel Wilkerson found analogs in Nazi Germany and the Indian caste system. If “Origin” sounds like a lecture (of the sort the Florida school system seems determined to avoid), think again. Rather than making another documentary, à la remarkable “13th,” director Ava DuVernay personalizes Wilkerson’s research, dramatizing how a woman wounded by national tragedy (the murder of Trayvon Martin) and personal setbacks (casual racism, the loss of loved ones) connected disparate ideas to reframe the country’s most difficult conversation.