As cetaceans (the mammal group that includes whales, dolphins, porpoises) began moving from land to water about 55 million years ago, they evolved special adaptations including streamlined bodies, blowholes for nostrils, and loss of external hind limbs. Forelimbs became flippers. Note the similarity between the bones of flippers and the human hand.

A hoofed land mammal and close whale relative that lived about 45 million years ago, Andrewsarchus mongoliensis is known from a single skull discovered in 1923 during a Museum expedition to Inner Mongolia.