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The cetaceans (whales, dolphins and porpoises) are descendants of land-living mammals, and remnants of their terrestrial origins can be found in the fact that they must breathe air from the surface; in the bones of their fins, which look like huge, jointed hands; and in the vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish. The question of how land animals evolved into ocean-going behemoths has been a mystery for a long time, owing to gaps in the fossil record. However, recent discoveries in Pakistan have managed to solve many of these mysteries, and it is now possible to see several stages in the transition of the cetaceans from land to sea.
Earliest ancestors
Before the recent discoveries in Pakistan, one popular theory of cetacean evolution was that whales were related to the mesonychids, an extinct order of carnivorous ungulates (hoofed animals), which looked rather like wolves with hooves. These animals possessed unusual triangular teeth that are similar to those of whales. For this reason, scientists had long believed that whales evolved from a form of mesonychid.
However, DNA analysis generated an alternative hypothesis. Whale DNA is more similar to that of the hippopotamids than to any other living animal. Therefore, a debate arose as to whether hippopotamuses (hippos) or mesonychids were the closest relatives of the whales.
The recent discovery of Pakicetus, the earliest proto-whale (see below) has helped to settle the debate. The skeletons of Pakicetus demonstrate that whales did not derive directly from mesonychids. Instead, they are a form of artiodactyl (another type of ungulate) that began to take to the water after the artiodactyl family split from the mesonychids. In other words, the proto-whales were early artiodactyls that retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have since lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, today's artiodactyls and perissodactyls having switched to a plant diet later in their evolution. Whales, due to the readier availability of animal prey and their need for higher caloric content to live as marine endotherms, naturally retained their carnivorous diet, as did mesonychids, who were however out-competed by better-adapted animals like the Carnivora later on (mesonychids became specialized carnivores when the overall availability of large animal prey was still low; thus their adaptation was likely at a disadvantage when new forms had filled the gaps left by the dinosaurs).
There is evidence that Hippos are the closest relatives of the whale. Not only are hippos of artiodactyl origin, but also they share similar protein and amino acids, brain structure, and chambered digestive tracts with whales. Furthermore, there is molecular evidence suggesting that hippos and whales are more closely related than the hippos are to any other living species, including pigs. Yet, a new discovery suggests that the origins of whales and hippos are not directly related. The reason for the physical and genetic similarities between them is that hippos split off from the main Artiodactyl line shortly after the proto-whales did, and thus, like whales, hippos retain some characteristics of early Artiodactyls. Both hippos and whales are Artiodactyls that became adapted to life in the water, but they did so separately and evolved in quite different directions.
Pakicetids: the earliest Cetaceans?
The pakicetids are hoofed-mammals that are sometimes classified as the earliest whales. They lived in the early Eocene, around 52 million years ago. They looked rather like dogs with hoofed feet and long, thick tails. They have been linked to whales by their ears: the structure of the auditory bulla is formed from the ectotympanic bone only, a feature shared by modern whales. It was initially thought that the ears of Pakicetus were adapted for underwater hearing, but, as would be expected from the anatomy of the rest of this creature, the ears of Pakicetus are specialized for hearing on land, and if Pakicetus is related to the ancestors of whales, underwater hearing must have been a later adaptation. According to Thewissen, the teeth of Pakicetus also resemble the teeth of fossil whales, which is another link to more modern whalesWhale Origins.
Ambulocetids and remingtonocetids
The most remarkable of the recent discoveries in Pakistan has been Ambulocetus, which looked like a three-metre long mammalian crocodile. Ambulocetus was clearly amphibious, as its back legs are better adapted for swimming than for walking on land, and it probably swam by undulating its back vertically, as otters, seals and whales do. It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting prey.
A smaller cousin of Ambulocetus was the remingtonocetid family, which had longer snouts than Ambulocetus', and were slightly better adapted for underwater life. It has been speculated that they lived like modern sea otters, hunting for fish in the shallows.
Protocetids
The protocetids, which include Rodhocetus and Artiocetus, are another recent discovery. They lived around 45 million years ago. Their principal adaptation was flukes (horizontal bars) on their tails, which enable faster swimming; however, they retained substantial hind legs. They lived in shallow seas, and may have had a similar lifestyle to seals, or even dolphins; it is not known whether they ever came onto the land.
Basilosaurids and dorudontids: fully marine cetaceans
Basilosaurus (discovered in 1840 and initially mistaken for a lizard, whence its name) and Dorudon lived around 38 million years ago, and were fully recognisable whales which lived entirely in the ocean. Basilosaurus was a monstrous creature, up to 18m long; dorudontids were within the range of modern cetacean size, about 5m long. Although they look very much like modern whales, basilosaurids and dorudontids lacked the 'melon organ' that allows their descendants to sing and use ultrasound as effectively as modern whales. They had small brains; this suggests they were solitary and did not have the complex social structure of modern whales. Basilosaurus had two tiny but well-formed hind legs which were probably used as claspers when mating; they are a small reminder of the lives of their ancestors.
Modern Whales
Today, the whale hind parts are internal and reduced, and they serve as anchor for the muscles of the genitalia. Occasionally, the genes that code for longer extremities cause a modern whale to have a sprouted leg or two (known as atavism). Whereas early whales such as the Pakicetus had the nasal openings at the end of the snout, later whales such as the Rodhocetus began to drift the openings toward the top of its skull. Today's modern whales have blowhole that allows it to break to the surface, inhale, and submerge with convenience. (This is known as the Nasal Drift Theory.) The ears began to move inward as well, and, in case of Basilosaurus, the middle ears received vibrations from the lower jaw. Today's modern toothed whales use the 'melon organ', a pad of fat, for echolocation.
References
See also
External links
"Evolution of cetaceans"