When the dinosaurs became extinct about 65 million years ago, the mammals got a chance to flourish and take the dinosaurs' place. The mammals roamed the land and diversified greatly. Among the newly emerging groups were primitive hoofed animals (ungulates). These animals were called Condylarthra. They are considered to be the common ancestors of the modern day whales (Cetacea) and Artiodactyla, which include pigs, hippopotami, cows etc. From the Condylarthra, the Mesonychidae developed.

These mesonychids were large-bodied animals. Some were carnivorous, some herbivorous and others omnivorous. They lived in or near river estuaries and lagoons. In the Paleocene period, about 50 million years ago, these creatures adapted for the capture of fast-swimming fish, a food resource that was more or less untapped after the disappearance of marine dinosaurs like the plesiosaurs and ichtyosaurs.

They took to the seas and became the oldest known, now extinct, group of whales, the Archaeoceti. The archaeocetes still had teeth like land mammals, with canines, incisors and molars. They had dense ear bones, space for fat deposits around the bones and air sacs to isolate the ear from the bone. These are typical whale characteristics. Their nostrils were located on top of their snout. Their body was elongated with a long tail and a short neck. The hind limbs were reduced in size and the front limbs were paddle-shaped. Their tail could move up and down.

The oldest members of the archaeocetes, Pakicetus species, have been found in Pakistan. Some slightly more recent archaeocete remains have been found in Europe, the US and Australia. One of the best known fossils was originally mistaken for a reptile and was named Basilosaurus. It is now recognized as being a member of the Archaeoceti.

In the Oligocene period, 38-25 million years ago, the archaeocetes were replaced by primitive groups of Odontoceti (toothed whales) and Mysticeti (baleen whales). The largest toothed whales, the sperm whales, probably separated from the baleen whales later than the other toothed whale groups. DNA and anatomical studies have shown, that the sperm whales are closer related to the modern baleen whales than to any of the modern toothed whale species. Nowadays, there are about 80 whale species.

The ancestors of modern whales originally lived close to the water's edge. Over time they ventured into the water more and more and discovered a rich source of food, for which there was virtually no
competition. Over time, they adapted to an increasingly aquatic life style: their bodies became more streamlined and better insulated and they adapted to the kind of food they were eating. Some specialized
on feeding on large fishes. For this they needed sharp teeth so their teeth became pointed. These developed into the toothed whales.

Other specialized towards a more grazing life style, living off the abundant small creatures of the sea. To effectively collect these small animals, they had no use for teeth but needed a sieve-like
apparatus instead. They lost their teeth and developed baleen: horny structures with fine hairs that could filter small creatures out of the water. These developed into the modern baleen whales. For a while there was another group of whales, which also had teeth. This group, the Archeoceti, became extinct some 25 million years ago.

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---Adaptations

The similarities between the skeletons of land mammals and whales are quite clear. The hind limbs have all but disappeared. There is only a small remnant of the pelvis, which is not attached to the spine or any other bone. In males, this small pelvic bone serves as an anchor for the muscles of the penis. Occasionally a whale is found in which the hind limbs are somewhat more developed are even visible externally.This is very rare, though.

The front limbs in all whales, the pectoral fins or flippers, basically contain all the bones that can be found in the front limbs of land mammals. Some bones are shortened, others are slightly modified, but the bones of 5 digits are still clearly recognisable. Externally, no digits are visible. The flippers are smooth, flattened paddle-shaped fins. The bones in the peduncle (the tail stem) are extensions of the tail vertebrae in land mammals.

Land mammals have some form of hairy fur, which serves among others as insulation. Whales don't have fur. In water, fur needs a lot of maintenance in order to maintain its insulating properties. Furry marine mammals like sea otters spend a considerable amount of time preening. Also, the fur would cause a drag when swimming. Losing the fur gave whales a perfectly streamlined body. For insulation, they have a thick layer of blubber (fat) under their skin, which helps to maintain the body's streamline, serves as energy storage and protects the whales from heat loss. In whales, the remnants of the fur can still be seen. Young dolphins have small whiskers and in older animals, the hair follicles can still be seen on the snout. Some dolphin species (e.g. river dolphins) have whiskers throughout their life.

The streamline of whales is also enhanced by the disappearance of the auricles, the external ears. All that is left is a tiny hole behind the eye. Because the density of the auricles would be close to that of the surrounding water, they would not have much use collecting sounds. Most of the sounds would pass right through.

Because whales breath air, but spend a lot of their time under water, they have developed an efficient system to get enough oxygen. When they breath, they exchange almost the entire air volume in the lungs. When breathing normally, humans exchange little more than 12% of the lung volume. Also, whales have the capacity to store much more oxygen in the muscles than land mammals. The nostrils have in whales moved to the top the their head, which makes breathing while swimming a lot easier. This breathing opening is called the blowhole. Baleen whales have a double blowhole, toothed whales have a single blowhole.

Most whales are excellent divers. When they breath, most of the oxygen is transported from the lungs to the blood and to the muscles. The muscles are rich in a special oxygen-binding protein, called myoglobin. It is related to haemoglobin, the protein that binds oxygen in the red blood cells. When they dive, the blood flow, and hence the oxygen flow, is limited to some vital organs, like the heart and the brain. Also, the heart start beating at a considerably lower rate. This is called bradycardia.

In most mammals (also in humans) the heart slows somewhat when the head is submerged, but in whales and also in seals and sea lions, this slowing is very pronounced. A whale's body has to be able to withstand enormous pressures when diving. For every 10 m they go down, the pressure increases by 1 bar. Whales can dive and surface rapidly without any risk of the bends or nitrogen narcosis, which are potential problems for human divers. The main reason is that whales and dolphins do not breathe at all when under water, so the amount of nitrogen in the body is minimal. Human divers use compressed air and constantly absorb extra nitrogen when submerged.

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