Mammal lineages have regularly evolved to become aquatic, sub-aquatic and pure land animals. Some have switched back and forth. The human species itself is thought to have shed its fur as it became a subaquatic hunter and gatherer, spending much of the day foraging in the shallows of East Africa.

Elephants, too, were once aquatic animals; their skin is testament to this. Fossils found on both sides of the Pacific indicate they were actually deepwater creatures who swam the entire ocean.