How the whale took a spectacular evolutionary path

Image detail from the cover of the book The Walking Whales by Hans Thewissen, published in 2014. Illustration by Jacqueline Dillard. 

In eight million years, land-dwelling hoofed mammals evolved into fully marine whales. An evolutionary path that was partly mapped by palaeobiologist Hans Thewissen, discoverer of the ‘walking whale’ Ambulocetus.

‘We have probably found even more fossil transitional forms for whale evolution than for horse evolution. Horses took around fifty million years to go from a small forest animal like Eohippus with four toes on the forefoot to adapting to life on grasslands and transforming into larger, faster animals with taller molars, longer legs, and just a single toe. In that respect, whales underwent a far more spectacular evolutionary trajectory. In just eight million years, a small, herbivorous, deer-like hoofed mammal such as Indohyus gave rise to the enormous, carnivorous, fully marine Basilosaurus, which received their name because they were mistakenly thought to be sea reptiles. All the major adaptations in hearing, locomotion, and physiology for life at sea were already present in whales forty million years ago, and subsequently diversified into the modern array of dolphins and other toothed and baleen whales. That is extraordinarily profound, difficult to imagine, yet it happened and has been documented’, says palaeobiologist and whale expert Hans Thewissen, professor of anatomy and neurobiology at Northeast Ohio Medical University, in impeccable Dutch during a Teams session.

He left the Netherlands forty years ago (see the box ‘Accidental whale expert’) but enjoys the chance to practise his mother tongue and talk about his passion. ‘To convey the drama of whale evolution, I still use the analogy in lectures of a technician given the impossible task of converting a Batmobile into a submarine. Because almost everything that works well on land is doomed to fail in water. The transition from land to water means you have to move differently, use your senses differently, and completely reorganise your reproduction and osmoregulation. And all of this whilst every transitional form along the way also had to function effectively in its environment’, says Thewissen.

He published an accessible review article on the subject (Evo Edu Outreach, 2009) and described his adventurous searches for whale fossils and his research on transitional forms such as Pakicetus, Ambulocetus, Kutchicetus, Remingtonocetus, Protocetus, Basilosaurus and recent whale species in his book The Walking Whales (2014). A title that refers to his most spectacular find in Pakistan: a near-complete skeleton of a whale some 48 million years old that possessed legs with which it could genuinely walk, and which Thewissen named Ambulocetus natans (Science, 1994). A species that the scientific world and the media embraced as a ‘missing link’ and that creationists denounced as ‘impossible’.

‘We did have the wind in our sails, because molecular data from DNA were increasingly pointing to whales being related to hoofed mammals. So with Pakicetus and Ambulocetus, the skeletal puzzle pieces fell into place as well’, observes Thewissen.

Slow
In the early evolution of whales, there was a simplification of the dental elements, changes to the ear ossicles (still the defining characteristic for identifying whales), elongation of the snout, high-set and large eyes in the skull, and an increase in the number of vertebrae and ribs. ‘With Ambulocetus, I picture an alligator-like animal that was probably too slow to pursue prey on land or in water, but instead lay in ambush in shallow water just below the surface, waiting to seize them. They had a narrow throat and so could certainly not swallow prey whole, as some modern toothed whales can today. They therefore chewed their food to swallow it in smaller portions, and you can see this in the wear patterns on the molars’, says Thewissen.

The evolutionary changes in locomotion are equally spectacular and far-reaching – from four-legged paddling to dorsoventral propulsion with a horizontal tail fluke. The forelimbs evolved into flippers, and in modern whales the hindlimbs have largely disappeared entirely from the outside: although they remain visible during embryological development, and pelvic bones and a few hindlimb bones are still present inside the body. ‘These are not rudimentary but actually essential for reproduction, because the muscle attachment to the penis is anchored to them’, says Thewissen.

A large part of whale evolution has been reconstructed thanks to fossil finds from Pakistan and India, the ‘whale valley’ (Wadi Al-Hitan) in Egypt, North America (Mississippi and Alabama), and South America (Chile and Peru; see the box ‘Extinct giant whale’).

Baleen
Thewissen now increasingly combines his work with anatomical, embryological, and genetic research on modern toothed and baleen whales, and regularly undertakes fieldwork in Alaska. This has led, among other things, to the discovery that baleen formation in the bowhead whale (Balaena mysticetus) is driven by the same fibroblast growth factor (FGF-30) that controls tooth formation in toothed whales (Journal of Anatomy, 2017). This may explain why, 34 million years ago, some whales very rapidly lost their teeth and replaced them with baleen, enabling efficient foraging on krill and other forms of plankton. Even for this transition, fossil intermediate forms have been found (Current Biology, 2017).

Another interesting line of research is the discovery that belugas or white whales (Delphinapterus leucas) off the coast of Alaska have growth lines of dentine in their teeth that can be used to estimate age and, in females, even indicate when they were pregnant (PLOS ONE, 2018). ‘We are now working to extract additional information from fossil whale teeth using the same approach’, says Thewissen. ‘There are still many major unanswered questions about whales. One of them concerns the strict way in which the mammalian developmental blueprint is regulated – for example in dental formula, numbers of vertebrae, and phalanges. Marine mammals appear to be an exception to this rule and show considerable internal variation, whilst externally they look very similar to one another. They are all streamlined, hairless, lack a neck and hindlimbs, and always have a horizontal tail fluke. Apparently they managed to circumvent the strict genetic rules, yet paradoxically have all arrived at the same external morphology. That cries out for answers, for which we need to study both whale fossils and modern whales.’

Extinct giant whale
The blue whale is currently – at a body weight of 100 to 190 tonnes – the largest living animal on Earth. Yet some 39 million years ago an even larger whale was probably swimming the seas: Perucetus colossus, with vertebrae of nearly 100 kilograms and ribs of around 1.4 metres, estimated to have had a total weight of up to 340 tonnes (Nature, 2023). The enormous fossils – thirteen vertebrae, four ribs and a hip bone – were excavated from the Paracas Formation (Middle Eocene) in the Ica Desert of southern Peru. An international research team analysed the internal structure of the bones, scanned them for volume calculations and used skeletons of modern whales to estimate how heavy Perucetus must have been. The estimated 20-metre-long skeleton alone weighs between 5 and 8 tonnes, two to three times as much as the blue whale skeleton hanging in the Natural History Museum in London. The enormous weight of the skeletal elements is attributable to two adaptations of Perucetus: extra bone deposition on the outer surface (pachyostosis) and infilling of cavities with bone (osteosclerosis). Such exceptionally heavy bones may have given Perucetus the ability to dive and forage in shallow water – a phenomenon that also occurs in manatees and other whale species, but Perucetus possesses extraordinarily heavy bones. It has the heaviest skeletal mass relative to total body weight of all mammals, reptiles and birds (amniotes). Because no skull or teeth of the whale have been found, it is not possible to reconstruct the feeding habits of Perucetus, but this relatively slow-swimming whale was almost certainly a representative of the Eocene basilosaurs that attained gigantic proportions early on. ‘Discoveries of such extreme body forms offer an opportunity to reconsider our understanding of animal evolution’, Thewissen argues in a simultaneously published analysis in Nature. ‘We are barely aware of how remarkable the form and function of whales can be.’