David Attenborough sets out to find the earliest animals that ever lived, creatures so old and so strange that for decades scientists argued over whether they were animals at all. The trail starts in the Ediacara Hills of Australia, where soft-bodied fossils over 500 million years old were mistaken for ripple marks before anyone recognized them as life. Attenborough travels to Newfoundland, Namibia, and Morocco, handling fossils in the field and using computer models to reconstruct how these boneless, mostly immobile organisms might have fed and moved. The film tracks the shift from that quiet Ediacaran world into the Cambrian explosion, when hard shells, jointed limbs, and eyes appear suddenly in the fossil record, along with early predators like Anomalocaris. Paleontologists working the sites explain what the rock layers show and what they only guess at, and Attenborough's narration stays close to the evidence rather than the spectacle. It ends with the trilobites, the first animals to see, and the arms race in vision and predation that followed.