The direct evolution of whales is as ancient and mysterious as the seas in which they roam. Yet there are, in fact, a number of vital clues providing an answer to the most important question about them. How did these large-brained mammals- air-breathing, warm-blooded and producing live young- come to live entirely in water?
First of all, paleontologists discovered the 52 million-year-old skull of a mammal from the Eocene period in Pakistan and named it Pakicetus.
Its importance lay in the fact that it was found to have features of both land-dwelling mammals, called Creodonts, and the earliest archaic whales, Archaeocetes. Its most relevant characteristic was a water adapted inner ear; an aid for directional hearing underwater. And so, although Pakicetus was a land mammal it was also the first transitional creature between this and a fully aquatic mammal.
Another discovery, slightly more recent and also of primary importance as a source of whale biology, came to us from fossils found in rocks in Northern India and Pakistan. Dating from the Eocene period of 49 million years ago, it was named Ambulocetus. One of these fossils is a near-complete skeleton called Ambulocetus Natans.
This skeleton suggests that it was an amphibious animal. It had forelimbs with fingers and small hooves, and the hind feet were clearly adapted for swimming.
An analysis of the type of movement the creature made indicated that it moved slowly on land and could swim, otter-like, by pushing back with its hind feet and undulating its tail. Because of its slowness on land it was probably an ambush hunter, attacking prey in or near shallow water; like modern crocodiles.
It was similar in size to a sea-lion but its pelvis was still fused to the back-bone; like land mammals but unlike whales.
In a third discovery, partial skeletons of mammals dating from the mid-Eocene period were uncovered in rocks in Indo-Pakistan, Africa, Europe and North America.
These were, as such, the oldest whales to disperse across the oceans. They were, too, the first whale group to develop tail flukes and were named Protocetus.
Rhodocetus- a sub-species of Protocetus- showed evidence of an escalating marine lifestyle. Its shorter neck vertebrae meant it had a less flexible but more stable neck. This is a swimming adaptation seen in aquatic animals such as sea-cows and, in more radical form, in modern whales. The ear region had become even more specialized for underwater use and the whale-like body had a pelvis no longer fused to the back bone. It had a long snout with many teeth and was an agile, quick. aquatic hunter; preying on small animals.
The 2 final species significant to the evolution of the modern whale were named Basilosaurus and Doruntodids and lived 35-41 million years ago.
Basilosaurus was enormous (about 60 feet long) with a snake-like body and tail flukes. It had a tiny head with a pointed, toothed snout. Doruntodids were similar in size to dolphins.
Both of these species had complete hind limbs that included a mobile knee and several toes. However, these extremities were tiny: so small that they were certainly not important for swimming. These were clearly creatures that were fully adapted to living in the seas, propelled by sturdy flippers and long, flexible bodies. Yet they still retained small, weak hind legs -a legacy from their evolutionary past- even though they could not walk on land.
Although none of these species are necessarily direct ancestors of modern whales, the important thing is that each fossil whale shares progressively new whale-like features, unique to the heritage of modern whales.
Put simply: each of these species and sub-species are merely branches of the modern whale's family tree.
Microsoft Encarta 2006