Cetaceans evolved from land mammals about 50 million years ago, making the transition to water through a variety of adaptations. A modern cetacean is so streamlined for swimming that it looks like a fish, despite its terrestrial origins. Its head is connected smoothly to its body, with no neck or shoulders showing. Nothing sticks out of a cetacean's body other than fins and a tail. External ears have been replaced by holes leading to inner ears. Male reproductive organs and female teats are housed behind slits. Cetaceans have body hair during development, but lose it as adults, even though they live in extremely cold water. Instead of insulating hair, they rely on a suit of body fat (blubber) just under their skin. Cetaceans include the largest mammal ever to have lived on earth (the blue whale), thanks to the support that water provides. Air, less dense than water, could never support a land animal the size of a whale.

All cetaceans are aquatic, and they swim by pumping their tail up and down, using paddle-like flippers for steering. The tail fins (flukes) are flexible, with no bones, and attached to the body with cartilage. Huge back muscles power the tail, making them powerful swimmers. Cetaceans are also good divers, some able to stay underwater for hours and reach depths of hundreds of meters. As air breathers, they must hold their breath to dive, yet their lungs are small and compress to an even smaller size during a dive. The stored air is pushed into their windpipe, where a thickened lining keeps it out of contact with the rest of the body. Sealing off the air prevents a buildup of nitrogen (from air) in the bloodstream, which protects cetaceans from the "bends" that human divers are in danger of when they submerge. During diving, oxygen reserves stored in special molecules (myoglobin) supply oxygen to cetaceans' bodies.