In 2001 a team of paleoanthropologists announces the discovery of a 3.5-million-year-old skull, among the oldest human ancestor fossils ever found, and the film follows what that find means for the story of how one ape species ended up dominating the planet. Researchers walk through the excavation sites, the painstaking work of piecing together fragments of bone, and the arguments over where this skull fits on the human family tree. Interviews with the scientists behind the dig sit alongside reconstructions of early hominid life, tracing the physical and behavioral changes, upright walking, tool use, bigger brains, that separated our ancestors from other primates. The film treats the fossil record as a series of open questions rather than a settled timeline, showing how a single skull can upend existing theories about when and where key traits appeared. It closes on the larger puzzle the discovery reopens: what actually made this one lineage of ape different enough to spread across the world.