Evolution of cetaceans facts for kids

A phylogeny showing the relationships among cetacean families.

Whales, dolphins, and porpoises are amazing ocean mammals. But did you know they actually came from land animals? It's true! We can tell because:

• They need to breathe air from the surface of the water.
• The bones in their fins look a lot like the limbs of land mammals.
• Their spines move up and down, just like a running land animal, not side-to-side like a fish.

For a long time, how land animals became giant ocean creatures was a mystery. But recent discoveries in Pakistan have shown us many steps in this incredible journey from land to sea.

How Whales Evolved

Further information: Archaeoceti

Possible relationships between cetaceans and other ungulate groups.

Reconstruction of Indohyus

Scientists have found many fossils that help us understand how whales evolved. Even though hippos and whales look very different today, they might share a common ancestor. This ancestor was probably a semi-aquatic animal, meaning it lived partly in water and partly on land. It lived about 60 million years ago (mya).

Around 54 mya, this group of animals split into two branches. One branch eventually evolved into the cetaceans we know today. This journey might have started with an animal called Pakicetus, which lived about 52 mya. These early whale ancestors slowly adapted to life in the sea, eventually becoming fully aquatic.

Meet Indohyus

Some modern whales have traces of their land-dwelling ancestors. The skeleton of a Bowhead whale shows its hind limb and pelvic bone structure (circled in red). This bone structure stays inside its body its entire life: it is a vestigial structure, a leftover part from its land ancestors.

Indohyus was a small, deer-like creature that lived about 48 million years ago in Kashmir. It was part of a family called Raoellidae. Scientists believe it is the closest relative to whales among land animals.

This animal was about the size of a raccoon or a domestic cat. Even though it ate plants, it shared some features with whales. It also showed signs of being adapted to life in water. For example, its outer bones were thick and heavy. This is similar to the bones of modern animals like the hippopotamus. Heavy bones help an animal sink and stay underwater.

This suggests Indohyus might have used a survival strategy similar to the African water chevrotain. When threatened by a bird of prey, the water chevrotain dives into the water and hides beneath the surface for several minutes.

Why Whales Are So Big

Scientists from the University of Plymouth studied many animals that dive and hold their breath, from tiny insects to giant whales. They found that larger animals can hold their breath longer than smaller ones. This is because bigger animals can store more oxygen for their size. This difference was much greater for warm-blooded animals than for cold-blooded ones.

This might explain why modern whales and extinct diving animals like plesiosaurs grew to be so enormous. Being large helped them stay underwater for longer periods, which was useful for hunting or escaping predators.

Images for kids

• 

  Cladogram showing the position of Cetacea within Artiodactylamorpha

• 

  Archaeocetes (like this Basilosaurus) had different kinds of teeth

• 

  Artistic impression of two Eobalaenoptera pursued by the giant shark Otodus megalodon

• 

  Acrophyseter skull

• 

  Pakicetus attocki skeleton

See also

In Spanish: Evolución de los cetáceos para niños