Marine Biology

How Whales Evolved



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Whales are among the most specialised of all mammals. Their evolutionary adaptations to a wholly aquatic life have led to a body structure that has made them almost physically unrecognisable as mammals at all, and many people hold a popular misconception they are fish.

The evolution of whales, dolphins and porpoises (collectively known as Cetaceans) has been of great debate among evolutionary scientists over the years, but one thing that is agreed on is that they evolved from land-living mammals. Cetaceans retail a vestigal trace of their former hind limbs, hidden internally, and retain mammalian features such as warm-bloodedness, milk-production and hair, although the latter is limited to very tiny amounts. They also need to breathe air, returning to the surface of the water at regular intervals to so.

Their terrestrial origins are also given away by the fact their spines move up-and-down, which is a characteristic of running mammals, rather than the side-to-side motion of swimming fish,

The true ancestors of Cetaceans have long been a mystery, partly due to huge gaps in the fossil record. It was long thought that whales are the direct descendents of a strange group of prehistoric mammals called Mesonychids, which were predatory carnivores that resembled wolves, but had hooved feet. This conclusion was made due to the similarity between Mesonychids' triangular teeth and those of Cetaceans, as well as their skull-structure and other morphological traits.

However, recent DNA analysis shows a very different picture, and suggests whales and dolphins are closely related to modern day Artiodactyla, the group of even-toed hoofed mammals that includes cows, sheep, camels and pigs. More specifically, the whale's closest living relative is the hippopotamus, and hippos and ruminants are in fact more closely related to whales than they are to pigs. While whales are not closely related to modern hippos, they sharer a closer living ancestor with them than any other living mammals.

The animals thought to be the earliest known Cetacean are a group of carnivorous mammals called Pakicetans, which lived during the Eocene. These land-swelling animals have peculiar structures to their ears, shared only by Cetaceans, and similar teeth to fossil whales. They barely resembled whales, however, with the appearance of small, hooved dogs, with thick tails.

Indohyus was a deer-like prehistoric creature discovered in Kashmir and lived about 48 million years ago. It also shared the same peculiar ear structures as the Pakicetans, but shows some adaptions to an aquatic lifestyle such as a thick coating to its bones that reduce buoyency. It is thought that perhaps this adaptation comes from a survival technique similar to modern Mouse Deer, in which the animal hides underwater to escape from predators, and shows the begins of how whales' ancestors may have originally taken to the water.

Several animals bridge the gap between this land-living acestors and modern whales. Ambulocetus was a huge, three-metre long creature that resembled a mammalian crocodile, and lived a similar lifestyle. Protocetids resembled modern-day whales, but still had a limited capacity to move about on land. Basilosaurus was a huge, wholly-marine creature that strongly resembled today's whales, but had a small brain, and small hind limbs which it is thought were used as graspers during mating.

As well as those features already mentioned, whales have also undertaken many other changes to become the masters of the seas that they are today. One is the location of their nostrils, which have fused into a single blowhole on the tops of their heads, allowing them to take breaths from the water's surface with more convenience. Pakicetans' nostrils were at the end of their snouts, but intermediate forms such as a creature named Rhodcetus saw the nostrils drifting towards the top of the head.

Echolocation is another feature that has evolved in toothed whales. The whales emit a series of high frequency clicks, focused in an air sac in the whale's cranium and modulated through a fatty "melon" organ in the head. The echos are channeled to the whale's inner ear through their jawbone, and an accurate picture of the world can be assembled from these echos. The earliest known Cetacean with echolocation capabilities was a creature called Squalodon, which lived 33-14 million years ago.

It's astounding to think that whales could have evolved from creatures that looked so unlike modern species. But it's perhaps even more astounding that the same evolutionary path has been taken on at least two more occasions. Ichthyosaurs were prehistoric marine reptiles that resembled whales, that evolved from land-living lizard-like animals in the Triassic period. Modern sirenians (manatees and dugongs) are superficially whale-like mammals that evolved from primitive hooved animals, and have elephants as their closest living relatives.

But it could be said that this adaption to a wholly aquatic life has never been as successful as it has been in the Cetaceans. Evolution is a beautiful process, and nowhere can this be seen more majestically than in the whales.

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