What is the land ancestor of cetaceans?

How does one get from a small four-legged mammal running along the banks of a river in eastern Pakistan to the 180-tonne blue whale swimming in the Southern Ocean? It took fifty million years, and a succession of fossils now dense enough to reconstruct the journey step by step. The evolution of cetaceans is one of the finest documented cases of macroevolution in action, both rapid on a geological scale and richly attested by fossils. And the most unexpected revelation of this story, confirmed by modern genetics, is that the closest living relatives of whales are not other marine animals, but the hippopotamus.

The ear that revealed everything

The story begins in the late 1970s, in the Kuldana valley in Pakistan. The palaeontologist Philip Gingerich, of the University of Michigan, was searching for Eocene mammal fossils. In 1979 he found a fragmentary skull in sedimentary deposits 50 million years old. The skull resembled that of a medium-sized canid, with the shearing teeth of a carnivore. Nothing apparently out of the ordinary.

But when Gingerich examined the inner ear, the petrosal bone and the ectotympanic, he found something astonishing: the structure is characteristic of cetaceans. The same architecture, almost feature for feature, as that of modern dolphins. This distinctive feature, which in cetaceans serves to perceive sounds under water, is anatomically diagnostic. Gingerich had just laid hands on the oldest known member of the cetacean lineage, an animal that in all the rest of its body resembled a land mammal. He named it Pakicetus inachus in 1981.

The evolutionary march: Pakicetus, Ambulocetus, Rodhocetus, Basilosaurus

Pakicetus, then, at around 50 million years. An amphibious quadruped 1.5 to 2 metres long. Probably a bank-side predator, hunting from ambush like a crocodile, able to listen under water thanks to its specialised ear.

Somewhere around 49 million years, Ambulocetus natans appears: the “walking and swimming cetacean”. Discovered in 1994 in the same Pakistani basin, it has the outline of a furred crocodile: short but webbed limbs, a powerful tail, an elongated snout. Its very name indicates its ecological hybridity: it walks, but it also swims. It hunts from ambush in estuaries, perhaps lying in wait for land prey come to drink, like a modern crocodile. It measures about three metres.

At around 47 million years, Rodhocetus crosses a major threshold. The hind limbs are smaller, the vertebral column more flexible. Its most important character: it begins to show propulsion by vertical undulation: not lateral as in a fish, but vertical as in a modern dolphin. This is the locomotor signature that will characterise the whole cetacean lineage. Rodhocetus probably spends most of its time in the water, and comes out onto the banks only to give birth or to rest.

At around 40 million years, Basilosaurus and Dorudon are fully marine. Basilosaurus, whose first fossils had been described as early as 1834 without their nature being understood, reaches 15 to 20 metres. Its outline is strangely elongated, almost serpentine. It has lost all capacity to walk: its hind limbs are reduced to tiny vestiges, a few centimetres across, with no functional connection to the vertebral column. It is one of the most striking arguments in favour of evolution: a large fossil cetacean that retains, against all functional logic, vestigial hind limbs, solely because its ancestors had them. Dorudon, smaller (5 metres) and probably more representative of the modern lineage, lives in the same waters. It is probably the common ancestor of today’s mysticetes and odontocetes.

At around 34 million years, at the end of the Eocene, the Neoceti lineage (modern cetaceans) splits into two groups that will follow distinct ecological trajectories: the mysticetes, developing baleen and filter feeding, and the odontocetes, developing echolocation and active predation. See our detailed feature on evolution.

The hippopotamus cousin

The connection with the hippopotamus is one of the most unexpected revelations of modern palaeontology and genetics. For a long time cetaceans were held to descend from a particular group of carnivorous mammals, the Mesonychia. That hypothesis, founded on morphological similarities in the teeth, appeared solid.

But from the 1990s onward, genetic analyses began to say something else. In 1997 John Gatesy published a study based on blood proteins (Molecular Biology and Evolution): cetaceans group closely with the artiodactyls (even-toed ungulates: cattle, deer, pigs, antelopes, hippopotamuses). More precisely, they are genetically closer to hippopotamuses than the latter are to the other artiodactyls.

This conclusion was reinforced by later DNA analyses (Nikaido et al., 1999) and then by the discovery, in 2007, of a decisive fossil: Indohyus, a small mammal 48 million years old whose characters clearly place the cetaceans within the artiodactyls. Cetaceans and hippopotamids now form the clade Whippomorpha, which is thought to have separated from the other artiodactyls about 55 million years ago.

Why return to the water?

The leading hypothesis: ecological opportunity. At the end of the Cretaceous, 66 million years ago, the mass extinction had removed the large marine predators (mosasaurs, plesiosaurs, ichthyosaurs, ammonites). For the ten million years that followed, those niches remained largely vacant. The semi-aquatic ancestors of the cetaceans, already adapted to coastal settings, were able to colonise those open waters gradually, without major competition.

The speed of the transition, about ten to fifteen million years between the terrestrial Pakicetus and the fully marine Basilosaurus, is regarded as one of the fastest examples of macroevolution ever documented. The whole anatomical machinery was restructured: respiratory system, locomotion, thermoregulation, reproduction, sensory perception. To an anatomist, the modern cetacean is still a land mammal, but profoundly remodelled.

Sources

• Thewissen, J.G.M. (1998). The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea. Plenum Press.
• Gingerich, P.D. et al. (2001). “Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan”. Science, 293.
• Gatesy, J. (1997). “More DNA support for a Cetacea/Hippopotamidae clade”. Molecular Biology and Evolution, 14.
• Thewissen, J.G.M., Cooper, L.N., Clementz, M.T., Bajpai, S. & Tiwari, B.N. (2007). “Whales originated from aquatic artiodactyls in the Eocene epoch of India”. Nature, 450, 1190-1194.
• Nikaido, M., Rooney, A.P. & Okada, N. (1999). “Phylogenetic relationships among cetartiodactyls based on insertions of short and long interspersed elements”. PNAS, 96.

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