Charles Darwin's theory of evolution rests on evidence that keeps accumulating long after his death, and this film walks through three discoveries that scientists treat as decisive tests of it. Radiometric dating on 3.5-billion-year-old fossils from Australia anchors the timeline for when life first appears in the rock record. The film builds out a 4.5-billion-year picture of life on Earth, laid out like a skyscraper with single-celled organisms in the basement and complex animals near the top floors, to show how gradual and layered the fossil record actually is. The centerpiece is Tiktaalik, the fish-like animal with wrist bones and a mobile neck found in the Canadian Arctic, presented as the clearest physical link between water-dwelling and land-dwelling vertebrates. Paleontologists and geologists explain how each find was dated, classified, and slotted into the evolutionary tree, treating the fossils as evidence to be tested rather than settled dogma. It is a straightforward case for the science, built one specimen at a time.