Whales are mammals, like you, and their ancestors once lived on land. So how did they come to live in the sea?

This astounding transition came shortly after the rise of modern mammal groups, around 55 million years ago, during a hot period in the Earth’s history. Dinosaurs and large marine reptiles had disappeared some millions of years previously.

One group of hoofed mammals spent more and more time in the water, living on the abundant food there. Eventually they left the land altogether – to become whales.

Pakicetus – the first whale

Pakicetus attocki lived on the margins of a large shallow ocean, the Tethys Sea, around 50 million years ago. Chemical fingerprints from some of these wolf-sized meat-eaters show that they ate fish.

Ear bones from Pakicetus show a feature that is unique to whales, placing it as the earliest known ancestor to modern whales. Although Pakicetus had a land animal’s body, its head has the distinctive shape of a whale’s.

Pakicetus gives another clue to relationships with present-day animals. Its ankle bone is similar to that of hoofed, even-toed mammals, such as pigs, cows, and hippopotamuses. Genetic research confirms that hoofed mammals and whales are related.

Ambulocetus – Sea invaders

Ambulocetus natans means ‘walking whale that swims’, referring to its lifestyle both in water and on land. It probably swam by paddling with its legs, and dived by tucking in its forelimbs and giving powerful kicks with its hindlimbs.

Ambulocetus lived in near-shore environments such as estuaries. It probably came ashore to breed.

It seems that Ambulocetus heard sound through its lower jaw bone. Sound passed from the jaw through soft tissues leading to the ear. This small adaptation foreshadows the remarkable sound-receiving system used by modern toothed whales.

Kutchicetus – Conquering the sea

Kutchicetus minimus, with its small otter-like skeleton, lived between 43 and 46 million years ago. Like other early whales, Kutchicetus lived in tropical seas. Its fossils are found in sediment that formed in shallow seas sheltered by barrier islands.

How did Kutchicetus swim? Its hind legs are smaller than those of earlier whales and probably had little to do with propulsion. Perhaps the long tail helped, although there is no evidence of tail flukes as seen in living whales.

Kutchicetus probably spent more time diving than Pakicetus. Hair was no longer needed. Blubber gave insulation and helped with body streamlining, which in turn aided swimming.

Dorudon – Leaving the land for good

This model skeleton and skull represent a group of whales called basilosaurids. These whales were fully aquatic and lived between 34 and 40 million years ago. From a distance, living basilosaurids probably looked very much like modern species.

The nostrils, or blowhole, have moved towards the top of the head. The structure of the ear bones suggests that basilosaurids could hear well under water. Forelimbs have become paddle-like flippers, while the hindlimbs are rudimentary. The pelvis has detached from the spinal column, freeing up the lower spine to power greater tail movement. Squared-off vertebrae at the tip of the tail would have supported flukes.

This kind of whale came to inhabit all the oceans of the world.

Echolocation and toothed whales

Echolocation is the feature that distinguishes modern toothed whales from ancient toothed whales. This remarkable ability allows these whales to produce sound, beam it out through the water, and use the signal of the returning echo to navigate.

The noses of toothed whales are no longer used for smelling – they have become echolocation devices. Toothed whales and dolphins don’t have to rely on sight for navigation and hunting.

The skull of a small toothed dolphin found in New Zealand shows evidence of an ability to echolocate. Although extinct, Waipatia maerewhenua may be a distant ancestor of a present-day river dolphin.

The rise of baleen whales

Recently discovered fossils fill important gaps in the record for whales. These show the early ancestors of ‘modern’ baleen whales.

Teeth and baleen

For a brief time in whale history, the earliest mysticete (baleen) whales had both teeth and baleen in their jaws, as with Llanocetus.

Soon after, the eomysticete whales – or ‘dawn baleen whales’ – appeared, with baleen only. This is a defining characteristic of all the later baleen whales, whether living or extinct. The group includes the largest animal that has ever lived – the blue whale.

Baleen only

This fossil skull of an eomysticetid whale was collected by Dr Ewan Fordyce from New Zealand rocks that formed about 26 million years ago. Broad flat upper jaw bones reveal that this whale possessed baleen only. The teeth seen in all earlier stages of whale evolution had gone.