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: Mesonychids, Hippos or Artiodactyls:
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 hippopotami (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.
Hippos are artiodactyls too, but the 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 the earliest known proto-whales; they lived in the early Eocene, around 52 million years ago. They looked rather like dogs with long, thick tails. It is not known exactly how they lived, but they may have roamed the seashore or hunted in rivers. What links the Pakicetids to whales is the structure of their ears, which contain an adaptation to underwater hearing that is possessed only by whales. It seems that the Pakicetids were amphibious carnivores which, thanks to their remarkable ears, were able to hear better in the water than other aquatic mammals. This adaptation began the journey that would lead to the whales.
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.
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, hence 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, while Dorudontids were dolphin-sized, 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. They also had small brains; this suggests they were solitary and didn't have the complex social structure of modern whales. Most remarkable of all, Basilosaurus retained two tiny, useless hind limbs, a small reminder of the lives of their ancestors.