What were whales like 40 million years ago?

Indohyus

Pakicetus

Ambulocetus

Rhodocetus

Protocetus

Blue whale (Balenoptera musculus)

But how do we know?

Evolution of Cetaceans Name comes from “Cetus” the seamonster in Greek mythology Evolved from a predator on land 40 million years ago Special adaptations: Lack of external hind limbs Streamlined shape and tail Fluid filled jaw for hearing Echolocation (in toothed cetaceans only) Baleen for feeding (in baleen cetaceans only)

How do we know they were once land mammals? 1. Skeletal structure They have the same bones in their flippers as we have in our hand Their tails move in a horizontal motion like land mammals’ tail bones

Gray Whale skeleton

Evolution of Echolocation

Blowhole evolution

2. Vestigial structures Hip and leg bones (Structures that once served a purpose but no longer do) Hip and leg bones

Vestigial hind limbs on a living bottlenose dolphin

3. Embryological relationships - vestigial structures that occur during development of the fetus Human fetus 

Spotted dolphin fetus

4. DNA Similarities Cetaceans show much greater similarities in their DNA to land mammals (e.g. the hippo) than to fish or other aquatic animals