Enlarge / Whales and their kin evolved from land-dwelling mammals, a transition that entailed major physiological and morphological changes—which geneticists have begun to parse. (credit: Hayes Baxley/National Geographic for Disney+) Around 400 million years ago, the ancestor of all four-limbed creatures took its first steps onto dry land. Fast-forward about 350 million years, and a descendant of these early landlubbers did an about-face: It waded back into the water.