MAMMAL ORIGINS | A Journey from Land to Sea:
The Evolution of Ireland's Whales
Last month's post ended with a bombshell, including the implication that Ireland's cattle may be more closely related to the island's dogs than they are to its horses. However, this month we can begin with an even more mind-blowing revelation: that Ireland's cattle are even closer kin to the whales (which include the dolphins) that inhabit Ireland's seas than they are to the horses or dogs that they often share Ireland's fields with! This revelation bears re-stating: that Ireland's cattle are more closely related to Fungi the dolphin than they are to their fellow four-legged, grass-munching terrestrial neighbours the horses.
This all seems incredibly strange at first glance – how could a four-legged terrestrial mammal like a cow be more closely related to an aquatic mammal like a whale, which has flippers and a quite fish-like form, than it is to other four-legged terrestrial mammals like horses?
Well, as with the true relationship between cows, horses, and dogs, the relationship between cows and whales becomes less and less shocking the more we delve into their evolutionary history, revealing the path the whales have taken over tens of millions of years which has resulted in their modern, specialised form.
Just like the ancestors of most other modern mammal orders, the ancestors of the whales which inhabited the Earth around 65 million years ago, after the demise of the non-avian dinosaurs, would have been small, shrew-like forms dwelling in ancient forests, scampering through the leaf litter on the forest floor or zipping up the trunks of trees on the hunt for insects. This fact, that the whales evolved from terrestrial ancestors, was known as far back as Darwin's day in the nineteenth century, but for many decades there was a giant gap in this evolutionary story as the only known fossils of whales were of forms which had already assumed more or less their modern shape.
This, then, was something of an abominable mystery: just how had a tiny terrestrial 'shrew' evolved into an 'extraterrestrial' aquatic giant, the largest of which, the blue whale, can reach around 180 tonnes in weight and 30 metres or more in length?
After a long interlude filled with speculation and debate, solid answers to this evolutionary puzzle began to accumulate in the twentieth century, when a steady stream of fossil discoveries, mainly from the 1970s onwards, were bolstered by a growing body of DNA research built up in subsequent decades to provide an extremely clear picture of the evolution of the whales.
Numerous genetic studies confirmed something which had been suggested by a study of whale blood by immunologists in the 1950s – that the whales were descended from artiodactyls. These studies even revealed which of the artiodactyls was the whales' closest living relative, which, as it turns out, is the hippo. This means that the whales and the hippos are more closely related to each other than the hippos are to their fellow terrestrial artiodactyls.
This revelation, that the whales emerged from a line within the artidactyls, has led to some experts merging these two mammal orders, the whales (Cetacea) and the artiodactyls (Artiodactyla), into one larger order called Cetartiodactyla.
According to the genetic evidence, the ancestors of the whales and the hippos diverged around 55 million years ago when they would have looked quite different to their specialised modern descendants. Some indication of what the early ancestors of the whales may have looked like is provided by fossils of an artiodactyl family called the raoellids, which are the whales' closest extinct relatives, which also split from their line around 55 million years ago.
The raoellids are typified by a form called Indohyus, which lived around 47 million years ago in the area of India/Pakistan which was then bordered by the ancient Tethys Sea which stretched from China to France. In many ways, Indohyus resembled the early artiodactyl Diacodexis (see last month's post), being about the size of a cat and sporting a long tail and slender legs which terminated in little hooves. However, this little creature also displayed another feature which revealed a more aquatic lifestyle: its limb bones were very thick – an adaptation which allowed it to stay submerged at times in water.
These thick limb bones were likely present in the common ancestor shared between the whales and the raoellids as they are also present in one of the earliest whales, Pakicetus, which lived around 50 million years ago. From our modern perspective, Pakicetus is a fascinating creature, as it bears a number of features which bridge the gulf between the terrestrial artiodactyl form and that of modern whales.
Pakicetus is thought to have spent a great deal of its time swimming, paddling, or walking in freshwater streams on the hunt for tasty fish, and while its thick limb bones facilitated this movement in water they would have made sustained running on land more tiring. Pakicetus, then, already shows that the early whales were becoming more committed to a life in the water by 50 million years ago, and this conclusion is bolstered greatly by the fact that its inner ear bears features particular to whales, having become modified to hear in water as well as on land.
However, despite these aquatic adaptations, in other aspects of its form Pakicetus was very much a terrestrial creature, looking quite like some kind of primitive wolf, and it is a feature of this still-quite-terrestrial form which betrays its affinities with the artiodactyls. Artiodactyls have a very distinctive ankle, and it is now known that members of the whale group to which Pakicetus belongs, the pakicetids, also possessed this ankle, which places these ancient whales firmly with the even-toed ungulates.
As the early whales continued to exploit the abundant resources of the water, it set off an evolutionary explosion which saw a variety of novel forms appear, including the bizarre Ambulocetus natans (the 'walking-swimming' whale) which lived around 49 million years ago. This whale has been called the 'mammalian crocodile' as it is known to have had a mainly terrestrial diet, and this, allied with the fact that it had eyes positioned high up on its head like a crocodile, has led to suggestions that it lived in a similar way, lying patiently underwater at the mouths of rivers for passing prey before launching its powerful, three-quarter-tonne frame and massive befanged jaws at its unsuspecting victims and dragging them to a quick death.
Over the next 10 million years, the whales became ever more aquatically adapted as they moved out further into the oceans, as seen in forms like Rodhocetus (cover image). Their necks became shorter and their heads longer which allowed them to cut through the water more efficiently, while their spines became longer and more flexible and their tails more powerful, greatly enhancing their swimming abilities. Their eyes slipped down to the sides of their heads as they became more and more accustomed to diving into the oceans' depths, while their ears became buried under a layer of fat for better underwater hearing and their nostrils began to migrate to the tops of their heads to become the blowhole(s).
By around 40 million years ago, then, the whales had assumed more or less their modern form and had become such powerful swimmers that they broke free of their ancestral waters to populate the oceans of the world. One of the most famous forms from this time, Basilosaurus, was a 16-metre-long giant and is known to have inhabited the waters surrounding Ireland.
Although in most respects this oceanic titan resembled its modern relatives, it yet displayed a feature which betrayed the whales' terrestrial origins – it had tiny hind legs. These were now absolutely useless for walking on land, and these whales lived their entire lives in the sea. They may not have been wholly redundant appendages, though, and it has been suggested that they may have been used to help these whales orient their extremely long bodies when mating.
It was from some relative of Basilosaurus that the first ancestors of Ireland's modern whales emerged around 35 million years ago, and of all the whales which inhabit the world's oceans today, almost a third either live in or pass through Irish waters.
The most iconic of the whales in Ireland's seas must be the blue whale, which inspires wonder not only by its massive dimensions but also by the fact that it somehow manages to be elusive and enigmatic, as well as enormous, at the same time. However, the whales in Ireland's waters that are most familiar and celebrated must be the dolphins, not least due to the activities of their member Fungi who has entertained tourists in the waters around the Dingle peninsula for years.
This affable dolphin crests the waves very close to fields which hold many of its distant artiodactyl cousins, the cattle, their bodies separated by a short distance but their forms by an ocean of time, each now committed to life in a different domain. And yet their genes and the bones of their ancestors speak of an ancient and enduring bond; of an artiodactyl heritage which stretches back tens of millions of years, to a time when whales still walked the Earth.