The story of cetacean evolution

Whales and dolphins are mammals, just like humans. Even though they look very different from us, we actually have quite a lot in common! Cetaceans didn’t always live in the sea – in fact, up until about 55 million years ago, they were small, terrestrial omnivores, with the appearance of a cross between a racoon and a deer.

While cetaceans went on to evolve into fully aquatic predators, most of their relatives have remained on land, preferring to munch on vegetation – you might not have guessed it, but the closest land-dwelling relative of modern whales and dolphins is actually the hippopotamus! Cetaceans and hippos belong to the same order as deer, sheep, cows, pigs, and camels: the artiodactyls, or even-toed ungulates.

Going back to the sea

Meet Pakicetus – the first cetacean. Pakicetus had a semi-aquatic lifestyle, living mostly on land but hunting for fish and other small animals in the rivers and streams of what is now northern Pakistan. Fossils of Pakicetus have been found in river deposits but not in marine deposits, suggesting that they only inhabited freshwater environments.

Pakicetus was fairly small – only about one metre long. Their overall body plan much more closely resembled terrestrial artiodactyls than modern whales. They are the only cetacean to have four fully functioning legs, and were probably not particularly good swimmers!

Despite this, Pakicetus shares enough traits with modern whales and dolphins to be considered a cetacean. The journey back to the sea started here, just over 50 million years ago.

By 49 million years ago, cetaceans were fully aquatic – this is a startlingly rapid change, in evolutionary terms. Ambulocetus still had four legs but they were much shorter, broader, and generally much better suited to paddling and swimming than walking on land.

Over the next 10 million years, cetaceans slowly lost their hind limbs and developed powerful tails for swimming, and eventually stopped emerging from the water altogether. The Basilosaurids, a family of large cetaceans which lived about 30 to 40 million years ago, are among the earliest known obligate water-dwellers, and their general appearance was similar to that of modern whales and dolphins. The largest known Basilosaurid, Perucetus colossus, could grow up to 20 metres long and might even have weighed more than a blue whale!

How do we know?

Much of our understanding of cetacean evolution comes from fossil evidence, but modern cetaceans still carry around some remnants of their ancestors’ terrestrial lifestyle, if you know where to look!

Some of the best evidence of cetaceans’ land-dwelling origins can be found in their skeleton.

The front flippers, or pectoral fins, are derived from the limbs of their land-dwelling ancestors, and still retain the same bone structure. If you compare the anatomy of a whale’s pectoral fin with our own wrists and hands, the wings of a bat, the lower leg and paw of dogs, you’ll see that they are remarkably similar! The pectoral fins are the only fins which have a skeletal structure – the dorsal and tail fins, which are not derived from any ancestral limb, are both constructed of cartilage rather than bone and therefore are not part of the skeleton.

As the hind limbs receded and disappeared altogether, the pelvis has been reduced to a few small bones which are no longer connected to the spine at all. Without back legs to power, it may seem strange that cetaceans still have pelvic bones at all.

The pelvis of whales and dolphins is often referred to as vestigial – just like our tail bone – but this is actually not quite true, for it does still serve an important function. The pelvic bones act as an anchor for certain muscles which control the reproductive organs: even though whales and dolphins no longer need their pelvis for walking, they still need it to control these muscles. Rather than being a remnant of evolution, the cetacean pelvis is in fact continuing to evolve to this day!

Cetaceans’ digestive system points to their herbivorous ancestry – just like their close relatives, the ruminants, whales and dolphins have multi-chambered stomachs! Most cetaceans have four stomachs, but some beaked whales have many more, with up to eleven subdivisions in one of the stomach chambers.

In addition to this, the gut microbiome of baleen whales is broadly similar to that of terrestrial ruminants. Chitin, one of the main components of crustacean exoskeletons, has a very similar chemical structure to cellulose, which is found in plants. The bacterial communities require to break down and ferment these two polymers are almost the same, so the digestive system of baleen whales adapted very easily to their plankton prey.

Divergence and specialisation

Cetaceans split into two lineages about 25 million years ago, which went on to form the groups that modern cetaceans are divided into – the toothed whales, or odontocetes, and the baleen whales, or mysticetes.

Although cetaceans share a general body plan, they are not all identical. Across the ninety-four currently recognised species, we can see some degree of specialisation to each particular ecological niche. Arctic endemics, for example, have lost their dorsal fin as a method of reducing body heat loss. Beaked whales have a suite of physiological adaptations which allow them to dive to extraordinary depths in search of their prey.

River dolphins have long, narrow beaks highly flexible necks with unfused vertebrae, allowing them to twist around and easily snatch fish in the flooded forests where they hunt. Rorqual whales have slender, streamlined bodies and expanding ventral pleats along their throat which aid in their preferred hunting technique – lunge feeding.

Cetaceans may be a relatively small and recently-emerged taxon, but they have adapted so superbly to their watery home that they have since expanded into almost every marine habitat on the planet, as well as several freshwater systems. There is no doubt that cetaceans occupy the top tiers of the food chain wherever they are found, from the deep sea to the open ocean to shallow coastal waters. Their return from land to sea has been so successful, their divergence and specialisation so rapid, that cetaceans have outcompeted a great number of other large predators – their ancestors may even have contributed to the extinction of Megalodon, 3.5 million years ago!

From small and deer-like to the largest animals on Earth: cetaceans’ evolutionary history is a whale of a success story!