- [Narrator] All whales and dolphins have this unexpected feature of multiple stomach chambers.

(gentle music) It's a trait they inherited from their ancient relatives that walked on land.

- [Michael] Just like cows have multiple stomachs to digest their planned matter, whales have multiple stomachs to digest what they're eating, which is completely different fishes and squid, but it's still coming from the same structure as a terrestrial mammal.

- [Narrator] This anatomy is more evidence that whales are related to hoofed mammals, (dramatic music) and this terrestrial heritage can even be revealed in their genes.

(gentle music) In his lab, Michael uses modern whales, DNA, to map their past.

(gentle music) It's opening up a vast new world of information about the origins of whales.

- So if you think about different marine mammals, such as a manatee or a seal sea line, they all swim and live in the ocean and have similar adaptations to whales and dolphins.

But we can look at the DNA to see whether whales and dolphins are closely related to those other groups or whether they're related to another species entirely.

- [Narrator] Scientists wanted to identify the whale's closest living relative, so they compared whale DNA with a range of other animals.

(gentle music) - They came up with a really surprising finding, and the finding was that the whale's closest relative using DNA was the hippo.

(intense music) - [Narrator] Whales and hippos both descended from a common hoofed ancestor that lived about 5 million years before Pakasedas.

- Some of the earliest whales like Pakasedas may have lived like hippos.

(cheerful music) Also, hippos get birthed underwater.

They nurse underwater, their skin is also very thick.

So it's interesting to think that maybe the common ancestor whales and dolphins had these particular features.

(gentle music) - [Narrator] But life in the ocean is very different from life on the riverbank.

Over time, whale's ancestors adapted to this new environment.

(gentle music) Scientists compared the DNA of hippos and whales to find out how.

- Genome of whales.

We see that whales still have a lot of the genes from when they used to live on land.

So they still have genes involved in smelling, sweat glands, color vision, producing saliva.

But these genes are inactivated and they gradually degrade.

But I think this is incredibly powerful evidence that shows that whales come from land ancestors, that they still have these genes in their genomes, even though they're inactivate.

(gentle music)