What is the evolutionary history of cetaceans?

An extraordinary evolutionary history

Cetaceans have one of the most incredible evolutionary histories known to date1 .

Cetaceans are marine placental mammals, now perfectly adapted to life in the ocean (see also article on hydration of cetaceans in seawater). This group includes a large number of species, including whales, dolphins, orcas, belugas, sperm whales, etc. They are ungulates belonging to the order artiodactyls. These mammals distribute their weight evenly over two (or an even number) of their five fingers/toes, with the other fingers/toes either present, vestigial or absent. This group is in contrast to perissodactyls, which distribute their weight on a single (or odd number of) fingers/toes. This means that they belong to the same large family as cows, camels, giraffes and even hippopotamuses! It also means that all these different animals have at least one common ancestor2–4 .

Cetaceans have had a unique evolutionary history: their distant ancestors were small land animals that gradually evolved, adapting to a new, exclusively aquatic environment3–5 . The earliest ancestor groups of cetaceans are called archaeocetes. The first archaeocetes, the pakicetes, were adapted to move and feed both in water and on land4,6,7 . Before becoming exclusively marine animals, archaeocetes inhabited shallow waters, probably returning to land to reproduce5 . This transition from the terrestrial environment to the ocean took place during the Eocene, between 55 and 37 million years ago, and is well documented in the fossil record. This is one of the most rapid evolutions known to date, with a complete transition from a terrestrial to a marine environment in just 8 to 12 million years8,9 . The discovery of valuable fossils, including a pregnant female and her nearly full-term foetus dating from the middle Eocene (between 49 and 37 million years ago), has even allowed us to learn about the ecology of the earliest cetaceans10 . We now know that they gave birth head first, a typical birth pattern for land mammals, but different from modern whales, whose calves are born tail first. The calf was already well developed at birth, an essential factor for the survival of a newborn at the interface between land and sea10 . 

Fossil records from the last 40 million years, after cetaceans became exclusively aquatic creatures and after the extinction of the first whales, show us some fascinating adaptations. These adaptations include extreme gigantism, echolocation, the appearance of baleen plates for feeding by filtering water, and even the recolonisation of certain freshwater environments11 .

Bone structure and physiology: changes on all levels

Mitogenomic studies have revealed an explosive evolutionary radiation between 30 and 35 million years ago12. This led to major morphological changes in the skeleton13. For example, the structure of the skull14,15, the spine and pelvis2,4–6,10,16–23, and the structure of the limbs20,24 changed significantly. The bone structure itself also changed, adapting to life in the open sea and enabling them, among other things, to dive to great depths²⁵⁻²⁷. Moreover, if we look at the bone structures inside the fins of cetaceans (i.e. their front limbs), we find a structure similar to that of their distant terrestrial cousins10,20,24. Some fossils, such as that of Ambulocetus, show good adaptation to both terrestrial and aquatic environments24 .

The change in environment has also caused changes in the physiology of these animals, enabling them to live in an exclusively marine environment. This includes, among other things, changes in the structure of their myoglobin to enable them to dive to great depths28, changes in lipid assimilation29, hormonal and renal changes (e.g. article hydration of cetaceans in seawater), loss of taste, and changes in the structure of their skin31.

Some studies also highlight a parallel between the evolutionary history of cetaceans and that of other lineages of terrestrial tetrapods that returned to life in ocean ecosystems since the end of the Permian period more than 250 million years ago. These lineages (e.g. ichthyosaurs, plesiosaurs, mosasaurs, penguins and sea turtles) have similar morphologies and evolutionary convergences, such as forelimbs that have become flippers32.

Towards two large groups: Mysticeti and Odontoceti, two ecological niches, two ways of feeding.

The skull of cetaceans is one of the parts of their body that has undergone the most significant and rapid evolutionary adaptation33. The nostrils have migrated from the tip of the snout to the top of the skull to become blowholes, a major adaptation to facilitate breathing at sea. Two key innovations also appeared during their evolution: the presence of baleen and echolocation, adaptations that also involved changes in skull morphology34,35. Cetaceans are today divided into two large groups: Mysticetes (baleen whales) and Odontocetes (toothed whales), which diverged around 30 million years ago12,36,37.

In addition to their teeth, odontocetes also have the ability to echolocate38. This evolutionary adaptation is thought to have appeared very early on after the two groups of cetaceans separated: during the Oligocene, approximately 28 million years ago39,40. Even the oldest odontocetes taxa have a skull morphology compatible with the emission of high-frequency sound signals and the ability to hear them. They were therefore capable of echolocation and already possessed a functional sonar system40,41. Most odontocetes also have the ability to dive to great depths, where their sonar allows them to navigate and hunt effectively37. Noise pollution caused by human activities may also affect this second group's use of their bio-sonar42.

Baleens are long keratin structures attached to the palate, which Mysticetes whales use to feed by filtering large quantities of water43. This morphological trait appears to have emerged in three major phases linked to significant changes in the global climate: the first between 39 and 28 million years ago, the second around 23 million years ago, and the third around 3 million years ago44. We now know that the ancestors of baleen whales had teeth and that the transition from teeth to baleen represents a major change in the morphology and ecology of these cetaceans36. This transition was accompanied by significant changes in cranial ontogenesis (development of the foetal skull in-utero) and did not occur all at once45,46. Indeed, it has been shown that baleen whales develop teeth on their upper and lower jaws during their growth in-utero and that these teeth never emerge, being completely reabsorbed by the body before the calf is born36,45,47–49. The disappearance of small Mysticetes (and the favouring of large ones) can be explained by the need to migrate between the poles to feed and the equator to reproduce. This hypothesis is supported by evidence of migration among Mysticetes since at least the end of the Pliocene44,50. Mysticetes may even have used the Mediterranean as a breeding zone in old times50 !

Some cetaceans also feed using a suction mechanism. This is quite common among Odontocetes and known to occur in one species of Mysticete: the grey whale. This behaviour appears to be associated with a particular skull structure, which has also been found in some fossil species51.

Body and brain size: gigantism is the key word

Phylogenetic studies have shown that the different evolutionary lines of neocetes (the first fully aquatic cetaceans) very early on spread out into different ecological niches depending on their size, and that changes in cetacean size are closely linked to their feeding strategy52 .

Cetaceans are among the largest and heaviest mammals that have ever lived on Earth. Recent studies show that their gigantic size can be explained by the selection of certain specific genes linked to increased body size but also involved in the management of growth hormone and a hormone similar to insulin (involved in the body's storage of energy in the form of fat)53,54. These genes were selected differently between small and large cetaceans54. In addition, cetacean-specific amino acid changes may also have played a key role in the evolution of their body size after the divergence between cetaceans and their terrestrial cousins54. Other marine mammals such as pinnipeds (i.e. the large order of seals and sea lions) show similar trends towards gigantism32,55.

But why were these genes favoured?

Being large and, more importantly, covered in fat, is a significant advantage when it comes to living in icy waters for much of the year, as well as building up reserves for long migrations. The body loses a considerable amount of heat when submerged. Beyond the sometimes extreme temperatures, water cools the body much faster than air. With aquatic life comes a new challenge: conserving energy.

Physiologically, the smaller the organism and the greater its surface area (skin) in contact with its environment, the more energy it requires and therefore the higher its metabolism must be to compensate for heat loss. Even though large whales need more energy (= food) than smaller whales in absolute terms, they need less food in proportion to their mass thanks to a slower metabolism. Being large, round and fat is therefore an evolutionary advantage37. Furthermore, thanks to the density of seawater, the constraints on the body due to gravity are reduced. Under these conditions, weighing several tonnes becomes possible.

Cetaceans also have large brains relative to their overall size: they have a high encephalization quotient. Researchers have studied the evolution of cetacean size and brain size over time and have found significant differences in encephalization quotient between the two existing suborders, Mysticeti and Odontoceti. According to their findings, the mass of cetaceans and their brains has increased significantly during their evolution, but the encephalisation quotient has tended to decrease. Mysticeti appear to have lower encephalisation quotients than Odontoceti due to a change in the ratio of brain mass to body mass over time. This change is explained by a rapid increase in body mass in Mysticeti, while Odontoceti appear to have undergone a period of reduction in body mass41.

Bibliography

1. Gatesy, J. et al. A phylogenetic blueprint for a modern whale. Molecular Phylogenetics and Evolution 66, 479–506 (2013).

2. Gingerich, P. D., Haq, M. U., Zalmout, I. S., Khan, I. H. & Malkani, M. S. Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from Pakistan. Science 293, 2239–2242 (2001).

3. Thewissen, J. G. M., Cooper, L. N., Clementz, M. T., Bajpai, S. & Tiwari, B. N. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450, 1190–1194 (2007).

4. Thewissen, J. G. M., Williams, E. M., Roe, L. J. & Hussain, S. T. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature 413, 277–281 (2001).

5. Thewissen, J. G. M., Cooper, L. N., George, J. C. & Bajpai, S. From Land to Water: the Origin of Whales, Dolphins, and Porpoises. Evo Edu Outreach 2, 272–288 (2009).

6. Madar, S. I. THE POSTCRANIAL SKELETON OF EARLY EOCENE PAKICETID CETACEANS. Journal of Paleontology 81, 176–200 (2007).

7. Clementz, M. T., Goswami, A., Gingerich, P. D. & Koch, P. L. Isotopic records from early whales and sea cows: contrasting patterns of ecological transition. Journal of Vertebrate Paleontology 26, 355–370 (2006).

8. Marx, F. G., Lambert, O. & Uhen, M. D. Cetacean Paleobiology. (Wiley, 2016). doi:10.1002/9781118561546.

9. Thewissen, J. G. M. H. The Walking Whales: From Land to Water in Eight Million Years. (University of California Press, 2014). doi:10.1525/9780520959415.

10. Gingerich, P. D. et al. New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism. PLoS ONE 4, e4366 (2009).

11. Pyenson, N. D. The Ecological Rise of Whales Chronicled by the Fossil Record. Current Biology 27, R558–R564 (2017).

12. Arnason, U., Gullberg, A. & Janke, A. Mitogenomic analyses provide new insights into cetacean origin and evolution. Gene 333, 27–34 (2004).

13. Uhen, M. D. Evolution of marine mammals: Back to the sea after 300 million years. The Anatomical Record 290, 514–522 (2007).

14. Roston, R. A. & Roth, V. L. Cetacean Skull Telescoping Brings Evolution of Cranial Sutures into Focus. The Anatomical Record 302, 1055–1073 (2019).

15. Coombs, E. J. et al. The tempo of cetacean cranial evolution. Current Biology 32, 2233-2247.e4 (2022).

16. Moran, M. M. et al. Intervertebral and Epiphyseal Fusion in the Postnatal Ontogeny of Cetaceans and Terrestrial Mammals. J Mammal Evol 22, 93–109 (2015).

17. Uhen, M. D. New material of Natchitochia jonesi and a comparison of the innominata and locomotor capabilities of Protocetidae. Marine Mammal Science 30, 1029–1066 (2014).

18. Geisler, J. H. Whale Evolution: Dispersal by Paddle or Fluke. Current Biology 29, R294–R296 (2019).

19. Lambert, O. et al. An Amphibious Whale from the Middle Eocene of Peru Reveals Early South Pacific Dispersal of Quadrupedal Cetaceans. Current Biology 29, 1352-1359.e3 (2019).

20. Vautrin, Q. et al. From limb to fin: an Eocene protocetid forelimb from Senegal sheds new light on the early locomotor evolution of cetaceans. Palaeontology 63, 51–66 (2020).

21. Thewissen, J. G. M. & Williams, E. M. The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations. Annu. Rev. Ecol. Syst. 33, 73–90 (2002).

22. Evolutionary history of cetaceans: a review. in Secondary adaptation of tetrapods to life in water: proceedings of the international meeting, Poitiers, 1996 (eds Fordyce, R. E., De Muizon, C., Mazin, J.-M. & de Buffrénil, V.) (Verl. Friedrich Pfeil, München, 2001).

23. Buchholtz, E. A. Modular evolution of the Cetacean vertebral column. Evolution and Development 9, 278–289 (2007).

24. Gavazzi, L. M., Cooper, L. N., Fish, F. E., Hussain, S. T. & Thewissen, J. G. M. Carpal Morphology and Function in the Earliest Cetaceans. Journal of Vertebrate Paleontology 40, e1833019 (2020).

25. Sun, D. et al. Accelerated evolution and diversifying selection drove the adaptation of cetacean bone microstructure. BMC Evol Biol 19, 194 (2019).

26. Gray, N., Kainec, K., Madar, S., Tomko, L. & Wolfe, S. Sink or swim? Bone density as a mechanism for buoyancy control in early cetaceans. The Anatomical Record 290, 638–653 (2007).

27. Buffrénil, V. de, Ricqlès, A. de, Ray, C. E. & work(s):, D. P. D. R. Bone Histology of the Ribs of the Archaeocetes (Mammalia: Cetacea). Journal of Vertebrate Paleontology 10, 455–466 (1990).

28. Isogai, Y. et al. Common and unique strategies of myoglobin evolution for deep-sea adaptation of diving mammals. iScience 24, 102920 (2021).

29. Endo, Y., Kamei, K. & Inoue‐Murayama, M. Genetic signatures of lipid metabolism evolution in Cetacea since the divergence from terrestrial ancestor. J of Evolutionary Biology 31, 1655–1665 (2018).

30. Zhu, K. et al. The loss of taste genes in cetaceans. BMC Evol Biol 14, 218 (2014).

31. Menon, G. K., Elias, P. M., Wakefield, J. S. & Crumrine, D. Cetacean epidermal specialization: A review. Anat Histol Embryol 51, 563–575 (2022).

32. Kelley, N. P. & Pyenson, N. D. Evolutionary innovation and ecology in marine tetrapods from the Triassic to the Anthropocene. Science 348, aaa3716 (2015).

33. Milinkovitch, M. C. Molecular phylogeny of cetaceans prompts revision of morphological transformations. Trends in Ecology & Evolution 10, 328–334 (1995).

34. Churchill, M., Geisler, J. H., Beatty, B. L. & Goswami, A. Evolution of cranial telescoping in echolocating whales (Cetacea: Odontoceti). Evolution 72, 1092–1108 (2018).

35. Churchill, M., Martinez-Caceres, M., de Muizon, C., Mnieckowski, J. & Geisler, J. H. The Origin of High-Frequency Hearing in Whales. Current Biology 26, 2144–2149 (2016).

36. Berta, A., Lanzetti, A., Ekdale, E. G. & Deméré, T. A. From Teeth to Baleen and Raptorial to Bulk Filter Feeding in Mysticete Cetaceans: The Role of Paleontological, Genetic, and Geochemical Data in Feeding Evolution and Ecology. Integr. Comp. Biol. 56, 1271–1284 (2016).

37. Goldbogen, J. A., Pyenson, N. D. & Madsen, P. T. How Whales Dive, Feast, and Fast: The Ecophysiological Drivers and Limits of Foraging in the Evolution of Cetaceans. Annu. Rev. Ecol. Evol. Syst. 54, 307–325 (2023).

38. Jensen, F. H., Johnson, M., Ladegaard, M., Wisniewska, D. M. & Madsen, P. T. Narrow Acoustic Field of View Drives Frequency Scaling in Toothed Whale Biosonar. Current Biology 28, 3878-3885.e3 (2018).

39. Geisler, J. H., Colbert, M. W. & Carew, J. L. A new fossil species supports an early origin for toothed whale echolocation. Nature 508, 383–386 (2014).

40. Park, T., Fitzgerald, E. M. G. & Evans, A. R. Ultrasonic hearing and echolocation in the earliest toothed whales. Biol. Lett. 12, 20160060 (2016).

41. Montgomery, S. H. et al. THE EVOLUTIONARY HISTORY OF CETACEAN BRAIN AND BODY SIZE: CETACEAN BRAIN EVOLUTION. Evolution 67, 3339–3353 (2013).

42. Wisniewska, D. M. et al. Ultra-High Foraging Rates of Harbor Porpoises Make Them Vulnerable to Anthropogenic Disturbance. Current Biology 26, 1441–1446 (2016).

43. Goldbogen, J. A. et al. How Baleen Whales Feed: The Biomechanics of Engulfment and Filtration. Annu. Rev. Mar. Sci. 9, 367–386 (2017).

44. Marx, F. G. & Fordyce, R. E. Baleen boom and bust: a synthesis of mysticete phylogeny, diversity and disparity. R. Soc. open sci. 2, 140434 (2015).

45. Peredo, C. M., Pyenson, N. D. & Boersma, A. T. Decoupling Tooth Loss from the Evolution of Baleen in Whales. Front. Mar. Sci. 4, (2017).

46. Fitzgerald, E. M. G. A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proc. R. Soc. B. 273, 2955–2963 (2006).

47. Ishikawa, H. & Amasaki, H. Development and Physiological Degradation of Tooth Buds and Development of Rudiment of Baleen Plate in Southern Minke Whale, Balaenoptera acutorostrata. J. Vet. Med. Sci. 57, 665–670 (1995).

48. Ishikawa, H., Amasaki, H., Dohguchi, H., Furuya, A. & Suzuki, K. Immunohistological Distributions of Fibronectin, Tenascin, Type I, III and IV Collagens, and Laminin during Tooth Development and Degeneration in Fetuses of Minke Whale, Balaenoptera acutorostrata. J. Vet. Med. Sci. 61, 227–232 (1999).

49. Davit‐Béal, T., Tucker, A. S. & Sire, J. Loss of teeth and enamel in tetrapods: fossil record, genetic data and morphological adaptations. Journal of Anatomy 214, 477–501 (2009).

50. Bianucci, G., Landini, W. & Buckeridge, J. Whale barnacles and Neogene cetacean migration routes. New Zealand Journal of Geology and Geophysics 49, 115–120 (2006).

51. Johnston, C. & Berta, A. Comparative anatomy and evolutionary history of suction feeding in cetaceans. Marine Mammal Science 27, 493–513 (2011).

52. Slater, G. J., Price, S. A., Santini, F. & Alfaro, M. E. Diversity versus disparity and the radiation of modern cetaceans. Proc. R. Soc. B. 277, 3097–3104 (2010).

53. Silva, F. A., Souza, É. M. S., Ramos, E., Freitas, L. & Nery, M. F. The molecular evolution of genes previously associated with large sizes reveals possible pathways to cetacean gigantism. Sci Rep 13, 67 (2023).

54. Sun, Y. et al. Insights into body size variation in cetaceans from the evolution of body-size-related genes. BMC Evol Biol 19, 157 (2019).

55. Churchill, M., Clementz, M. T. & Kohno, N. Cope’s rule and the evolution of body size in Pinnipedimorpha (Mammalia: Carnivora): COPE’S RULE IN PINNIPEDS. Evolution 69, 201–215 (2015). 

February 2026

Legal notices / Mentions légales