You might think, looking at a whale, that their closest living relative was some kind of fish. This would be a fair conclusion: they both live in water, and have many similar adaptations to aid this, after all. However, as air-breathing mammals, whales actually separated from fish in the animal family tree some 450 million years ago – any similarities between them now are likely the result of pure coincidence (or, as scientists call it, convergent evolution: when two different animals in the same environment develop the same adaptations independently). The closest living relative to the whale group, surprisingly, is actually the hippopotamus – a decidedly un-whale-like animal that split off from its submarine relatives a mere 54 million years ago. A blink of an eye, evolutionarily speaking.
Evolution began the same day that life did, somewhere around 3.7 – 4.3 billion years ago. If you picture the tree of life, it starts at this root. Every species on earth stems from here, shaped into the life forms they exist as today by a long series of gradual changes to their DNA, each one supporting the ‘survival of the fittest’. What constitutes ‘the fittest’ is what drives evolutionary change: if the fittest animal is the fastest because it needs to hunt fast prey, and faster animals get more food and therefore live longer lives with more offspring, then the ability to travel at high speed will evolve. Around 400 million years ago, the ability to breathe air became an advantage, and air-breathing animals first appeared. Just shy of 200 million years later, a small, shrew-like animal evolved the ability to produce milk to feed its young on demand, and mammals were born.
Since then, mammals have evolved to occupy almost every habitat on earth, and crucially, this includes the ocean. Whales and dolphins – collectively known as cetaceans – are mammals too, despite their aquatic environment. They breathe air, give birth to live young that are grown with a placenta inside the mother, produce milk, and even have tiny rudimentary hairs. So given that mammals started life on land, how exactly did they end up back in the ocean, the very same place their ancestors left some 400 million years ago?
The answer lies in an unsuspecting creature called Pakicetus. 50 million years ago, Pakicetus – scientifically considered the first whale species – was a land animal that inhabited modern-day Pakistan (the name Pakicetus is a combination of the prefix Paki- for Pakistan and suffix -cetus, meaning sea creature). It measured 1 – 2 meters long, or about the size of a goat, with the head of a shrew, the body and legs of a deer, and the tail of a kangaroo. It had fur, could not swim, and did not live near water, and yet the skull shape of this species was distinctly cetacean in nature – especially the morphology of the inner ear – making it a clear predecessor to modern whales.
Over the years, Pakicetus, and likely many intermediate ancestors that remain undiscovered, gravitated towards the water for reasons unknown. Found in sediment that once formed an estuary, the fossils of Ambulocetus show that the ‘next step’ in whale evolution lived aquatically, in areas where saltwater and freshwater meet. As time progressed, early whales moved further and further from land into saltwater environments (as shown by increasing levels of saltwater isotopes in their bones over millions of years of evolution), and at the same time, their nostrils moved further back along their head to make breathing underwater easier. This change would have been driven by a need for the ‘fittest’ animals to be able to remain in water for long periods of time, and as early whales subsequently found themselves with more of a need to be able to move in water, their front limbs became fins, their back limbs became non-existent, and their historically kangaroo-like tails split and became fluked.
The first recognisable whales, named Basilosauridae, appeared 41 million years ago and have been found in fossil beds all over the world. They are widely believed to be the first cetacean species to live entirely in the oceans and never on land, otherwise known as obligate cetaceans. They did still have hind limbs that joined to a pelvis, however, this pelvis was not connected to the spine in any way. Crucially, this means the hind limbs were incapable of supporting any weight on land. All Basilosauridae species also had similar teeth with distinct dental patterns (i.e. molars, canines, incisors), which is no longer the case in whales today.
34 million years ago – 16 million years after the emergence of the very first whale, and several intermediate species later – we see some of the most important evolutionary changes in the whale lineage arise. Through the acquisition of two key adaptations, echolocation and baleen, evolution introduced the feeding mechanisms that have allowed whales to flourish into what we see today. Echolocation, adopted by odontocete whales (whales with teeth, like sperm whales, killer whales, and beaked whales), allows for active hunting of hidden or evasive prey even in environments where sight is limited. Baleen, a keratinous substance that hangs in rows from the mouths of mysticete whales (including blue whales, humpback whales, and grey whales), allows for filter feeding on smaller prey that exists in the water column or on the seabed. It was at this point in history that whales had really conquered the oceans.
Finally, after millions of years of evolution, we reach the final turning point in whale evolution. 4.5 million years ago, cooling of the climate led to the formation of ice caps in the northern hemisphere, impacting the distribution of plankton globally due to the trapping of nutrients in ice and current-driven accumulations. The end result of this change was that, rather than existing widely spread throughout the oceans, plankton started to congregate seasonally in areas that were at times thousands of kilometers apart. Suddenly, size became an advantage. Larger whales, with more energy reserves and a better ability to travel further, were now the ‘fittest’ individuals, and as such survived and thrived. It is around this time that we see the sudden growth – literally – of the whale group, resulting in the ocean giants we see today. Indeed, the blue whale is not only the biggest animal on earth now, but the biggest animal ever known to have existed.
The beautiful thing about evolution is that it never ends. All species alive today are constantly working with their environments to be the ‘fittest’, and thus ensure survival. It’s clear from whale evolution that it’s possible to come a very long way in even just a few million years, which begs the question… What’s next for these incredible creatures?