How did whales evolve from land animals?

Whales descend from hoofed land mammals. Molecular and anatomical studies point to the hippopotamus as their closest living relative: the whale and hippo lines split from a shared ancestor about 55.5 million years ago. The first whales were not sea monsters but small, four-legged animals that lived in and around water.

Pakicetus, from the family thought to be the earliest known whales, was a wolf-like mammal about one to two metres long that lived in and around water and ate fish and other animals. Scientists call it a whale mainly because of the shape of its ear bone, since a skeleton built for running and jumping looks nothing like a whale.

Then the fossils start to look like a slow-motion film. Ambulocetus, whose name means walking whale, hunted like a crocodile, waiting near the surface to ambush large mammals, and swam something like a modern river otter. Later whales spent their whole lives in water, and by roughly 41 million years ago the basilosaurids were fully ocean-going, with flipper-shaped front limbs and only tiny hind limbs that played no part in swimming.

The story is a good reminder of how evolution works: no animal ever decided to become a whale. Each fossil is just a hoofed mammal that spent a bit more of its life in the water than the one before it.