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 External ears have been replaced by holes leading to inner ears.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.