Since Charles Darwin and Alfred Russell Wallace formulated, in 1858, their theory of evolution by natural selection, one of the most discussed aspects has been the origin of cetaceans. Its very high level of adaptation to the aquatic environment makes analyzes using comparative anatomy truly complicated.
Charles Darwin on the evolution of whales
The first hypotheses about the evolution of whales were provided by Darwin himself, who speculated in The Origin of Species how natural selection could give rise to a whale-like creature from an animal such as a whale. bear.
In North America, the black bear was seen by Hearne swimming for hours with its mouth open, thus catching, like a whale, insects in the water. Even in such an extreme case as this, if the supply of insects were constant and there were no better adapted competitors in the region, I find no difficulty in a race of bears becoming, by natural selection, more and more aquatic in structure. and habits, with larger and larger mouths, until such a monstrous creature as a whale is produced.
Darwin was heavily criticized for this paragraph, with many incorrectly assuming that the naturalist placed bears as the ancestors of whales. To counteract this trend, in his sixth edition of the book, in 1876, he decided to deal with a fossil that had not been given enough attention: Basilosaurus .
Comparative Anatomy Scientific Discussions
To strengthen his arguments he enlisted the help of Thomas Henry Huxley , probably one of the most brilliant biologists of his time. The one nicknamed Darwin’s bulldog did not have many doubts: Basilosaurus represented the type of animal that linked whales with their terrestrial ancestors —one of the biologist’s predictions, which would later be confirmed—.
But the clues Basilosaurus gave were confusing. It was clearly a carnivorous animal, and its morphology suggested that whales came from some type of terrestrial carnivore . If that hypothesis was true, they must indeed be related to bears, or perhaps to seals. However, other experts, such as the anatomist William H. Flower , presented beavers and otters as the best candidates, due to their form of propulsion.
When bears or seals swim they propel themselves with their hind legs, but Basilosaurus and modern whales do so with their tails.
It was Flower himself who, some time later, thought of other candidates to be the ‘evolutionary cousins’ of the cetaceans: the ungulates . Oddly enough, the skull of Basilosaurus had more similarities to the skulls of ancient pig-like ungulates than to bears or seals.
To solve the puzzle it was necessary to find fossils of animals in the amphibian phase; that is, mammalian ancestors of the whales, which were already beginning to enter the water, but were still living on land.
Pakicetus, the intermediate fossil
We had to wait until 1981, when paleontologists Phillip Gingerich and Donald Russell described a fossil in Pakistan that would solve the puzzle. It was called Pakicetus, it lived about 53 million years ago, and it is the kind of intermediate fossil that was needed. Although at first it was misunderstood.
Pakicetus was a clearly terrestrial animal, of a size and appearance similar to a wolf, but with a particular feature that only cetaceans have: they have a very thick, dense and highly mineralized internal part of the auditory bulb in their skull, a feature that they do not share with any other mammal. Pakicetus was thus a terrestrial cetacean .
Since the discovery of Pakicetus , many other fossils have added to the list of intermediate forms between totally terrestrial mammals and cetaceans totally adapted to aquatic life: Rhodocetus , Maiacetus , Ambulocetus…
The skull of these animals seemed to link cetaceans to a group of prehistoric mammals called mesonychians , which were thought to be associated with carnivores. The problem seemed solved, until phylogenies based on genetics began to be carried out.
Genetics, correcting the puzzle
When genetics entered the field of play, it gave such strange results that they did not seem to fit what the fossils showed. The cetaceans seemed to locate themselves not only near the artiodactyls, but within them ; a group that includes deer, pigs, or giraffes. Hippopotamuses and cetaceans were a group of their own, and they had more in common with each other than hippos had with deer or pigs.
This new scheme, which ruled out carnivores as ancestral animals of the cetacean group, did not seem to square with the fossil discoveries. But as more fossils of early cetaceans were discovered, and especially when leg bones were found, it became clear that there was a key bone that fit the genetic explanation. This is the talus , an ankle bone that in artiodactyls has a characteristic double-pulley shape. This bone was also present, in the same appearance, in primitive whales.
The problem was solved. And as we now know, cetaceans are artiodactyls , descended from a group that acquired amphibious habits, which split off from the hippopotamus lineage, and that they are their closest living evolutionary relatives.
References:
Flower, W. H. et al. 1866. Recent Memoirs on the Cetacea. Ray Society.
Gingerich, P. et al. 1981. Pakicetus inachus, A New Archaeocete (Mammalia, Cetacea) from the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan). Contributions from the Museum of Paleontology, 25.
Spaulding, M. et al. 2009. Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution. PLoS ONE, 4(9), e7062. DOI: 10.1371/journal.pone.0007062
Thewissen, J. G. M. et al. 2001. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature, 413(6853), 277-281. DOI: 10.1038/35095005
Ursing, B. M. et al. 1998. Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade. Proceedings of the Royal Society of London. Series B: Biological Sciences, 265(1412), 2251-2255. DOI: 10.1098/rspb.1998.0567
