Whales and other cetaceans (such as dolphins) evolved from land-dwelling mammalian ancestors. Cetaceans are spectacularly well adapted to life in the oceans, so well that they were considered fishes until the last couple of centuries. Among the adaptations that were necessary in the evolution of whales are:

- Front flippers and a tail fluke. Pinnipeds such as seals and walruses crawl onto land for mating and childbirth, but cetaceans spend their entire lives in water. An animal that spends all of its time in the water does not need to walk, and hind legs are only a hindrance. Flippers provide a large surface area with which the front limbs can propel the cetacean through water, and the horizontal fluke on the tail also allows propulsion. Cetaceans move their tails up and down in typical mammalian fashion, unlike the side-to-side motion of the tails of fishes.

- Nostrils on the top of the head. Cetaceans, being mammals, need to breathe air. It is much more convenient to emerge from the ocean for breathing if the nostrils are on the top of the head.

- Change in eating. While many cetaceans (such as killer whales and dolphins) have teeth and pursue prey, some of the largest whales live off of plankton, which they strain from the water with baleen or whalebone, which they have instead of teeth.

- Ability to hold its breath. When diving deeply for long periods of time, whales must take as much oxygen with them as possible. Whenever they emerge, they can exchange up to 90 percent of the air in their lungs with the atmosphere, compared to the typical human breath that exchanges only 50 percent. Their muscles contain a great deal of myoglobin, a protein similar to hemoglobin that releases oxygen into muscle tissue.

- Changes in ear structure. The structure of the ear that is best for hearing in air is quite different from that which is best for hearing underwater.

- Changes in birth. Most mammals are born headfirst, to allow the newborn to begin breathing as soon as possible. This would, however, cause a baby whale to drown. Baby whales are born tailfirst.

All of these evolutionary transitions had to occur quickly, as whales that resemble modern ones were in existence by the Eocene epoch, the second epoch of the Tertiary period, following the Cretaceous extinction. The extinction of the large aquatic reptiles sufficiently reduced competition for this style of life, which allowed whales to evolve rapidly. Another possible reason for the rapid evolution of whales is that, once they stopped coming onto land during any stage of their life cycles, they were freed from the structural constraints imposed by gravity. . .

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