In 1923, an Indian American man named Bhagat Singh Thind argued before the U.S. Supreme Court that he was a white man and was therefore eligible to become a naturalized citizen. He based his claim on the fact that he was a member of India's highest caste and identified as an Aryan and therefore white. His claims were supported by the so-called Indo-European language theory, a controversial idea at the time that says nearly half the world's population speak a language that originated in one place. Theories about who lived in that place inspired a racist ideology that contended that the original speakers of the language were a white supreme race that colonized Europe and Asia thousands of years ago. This was used by many to define whiteness and eventually led to one of the most horrific events in history. On this episode of Throughline, we unpack the myths around this powerful idea and explore the politics and promise of the mother tongue.