Neil Shubin, a paleontologist who co-discovered the fossil fish Tiktaalik in the Canadian Arctic, argues that the blueprint for the human body was assembled long before humans existed. He traces the origins of arms, legs, necks, and lungs back roughly 375 million years to a fish that pushed itself onto land, using Tiktaalik's flattened skull and stubby limb bones as the hinge point between water-dwelling and land-dwelling anatomy. The film follows Shubin through fossil sites and into the lab, where he and other scientists compare genes in mice, fish, and humans to show the same genetic switches building fins in one animal and hands in another. Reenactments and animation walk through how a one-piece fish skull split into the movable jaw and inner-ear bones humans still carry, and how the same genetic toolkit shows up in wildly different creatures. The throughline is that hiccups, hernias, and other everyday quirks of human anatomy are leftover fish and reptile engineering, not fresh design, still doing a job it was never quite built for.