- [Narrator] In 1978, Philip went to Pakistan to search for prehistoric horses.

Instead, his team unearthed the remains of a mysterious creature.

The team named the strange animal pakicetus.

It's about 50 million years old.

(gentle music) When Philip took a closer look, he spotted something unexpected in the creature's ear.

- So when you look at this covering bone, covering the ear, it's very dense, it's thickened, it has a sloping surface on this side, and in modern mammals, those are only found in whales.

And why?

To enable them to hear in water.

- [Narrator] This ear bone, unique to whales and dolphins, helps them locate the direction of sounds underwater.

It's proof of pakicetus' pedigree.

- This bone was the key to understanding that pakicetus is a whale.

Well, that made it the oldest fossil whale anybody ever found.

- [Narrator] It was groundbreaking, and as they discovered more pakicetus fossils, they realized something else: this whale could walk.

(gentle music) - Pakicetus is an animal a little bigger than a wolf, probably built approximately like a wolf.

It has teeth like a carnivorous mammal.

- [Narrator] But unlike a wolf that has claws on the ends of its toes, pakicetus had tiny hooves.

(gentle music) Pakicetus was a carnivore that hunted on land, but its anatomy suggests it had adapted to living in water.

Its long snout, full of sharp teeth, also allowed it to probe shallow river beds for prey.

Its eyes were squeezed onto the top of its head so it could keep watch while swimming.

And some scientists think markings on its foot bones are evidence it had webbing between its toes, allowing it to hunt underwater.

- And of course, it didn't take long until they moved into the water more permanently.

- [Narrator] Pakicetus marks the beginning of an eventful journey from land animals to today's gigantic whales.

(gentle music)