Return to the Sea
Marine mammals—cetaceans (whales, dolphins, porpoises), pinnipeds (seals, sea lions, walruses), sirenians (manatees, dugongs), sea otters, and polar bears—evolved from land-dwelling ancestors that returned to the ocean over millions of years. Cetaceans descended from hoofed terrestrial mammals; the fossil record shows a gradual transition from Pakicetus (a wolf-like terrestrial animal, 50 million years ago) through Ambulocetus (a walking whale) to modern fully aquatic forms. These animals retain mammalian characteristics: they breathe air, are warm-blooded, nurse their young with milk, and have hair (reduced to a few sensory bristles in most cetaceans). The evolutionary pressures of aquatic life drove convergent body shapes—the streamlined fusiform form of dolphins parallels that of sharks and ichthyosaurs despite their very different ancestry.