Otters, water voles and sea lions are just a few examples of mammals that spend a large amount of time in water. Some can swim under water for long periods. But cetaceans are a unique class of mammal. They are the only mammals to never completely leave the water. This has left an evolutionary puzzle: did a fish evolve to breathe air, or did a land animal evolve to live in the sea?
Some characteristics of whales are consistent with them having evolved from a land-living creature. Their bone structure is suited to land-based movement. Occasionally, more contemporary whale skeleton finds have been reported to show evidence of vestigial limbs – small stumps which could be the evolutionary left-over of legs in the species millions of years ago.
The remains of an ancient baleen whale, from 27 million years ago, were discovered in Washington State, USA, by Lawrence Barnes and colleagues. The pelvis was found to have a deep socket of the type to hold a thigh bone. Barnes, of the Natural History Museum of Los Angeles, estimates that the mammal could have had small legs of about one and a half feet long.
There has been much speculation as to what kind of land creatures could be a precursor to whales. One idea was that whales are related to an extinct hoofed mammal, the teeth of which were similar to whale teeth. However, more recent theories about the land-based ancestors of whales would put the whale closer to hippopotami.
The discovery of complete fossilised remains of a creature named Pakicetus has changed the course of whale evolution theories somewhat. First discovered by palaeontologists in 1983, this creature lived about 56 million years ago, around the area now known as Pakistan. At that time, the region was on the shores of the ancient Tethys Sea which separated Africa from Asia. The Pakicetus, about the size of a wolf, was an aquatic land mammal. While it had a land animal’s body in many ways, there are some important distinctions. The Pakicetus skull shape is more like that of a whale, and the teeth of Pakicetus and of whales are very similar. Fossils show its ear shape and position to be of a type unique to whales. This puts the Pakicetus in the Cetacean class, the same class as whales, dolphins and porpoises, even though it lived on land.
It is suggested that the carnivorous Pakicetus found it easier to hunt for food in water, where there was less competition from other mammals. It eventually evolved to stay in water completely, and to no longer have legs, because it had no more need, in survival terms, to venture on to land. The Pakicetus ankle bone links it to artiodactyls, even-toed hoofed animals such as pigs, cows, giraffes – and hippopotami. That is how it is thought that the hippopotamus is in fact the closest living relative to the whale. This is not to say that whales directly evolved from hippos, just that they share some evolutionary links.