In common with many people, I find whales to be the most fascinating of mammals, but had always wondered how and why they came to evolve. Here is their story...
The oldest known whale is Himalayecetus subathuensis, dating back to around 53 million years ago. As its name suggests, remains have been found in the Himalayas, in the Simla Hills of India, an area which was submerged in the Tethys sea during the Tertiary period. According to research, the animal may have been amphibious, spending much of its time on land. An earlier animal, Ambulocetus resembling the modern otter, had teeth and its pelvis was still fused to its spine, but at about 10 feet long, it was beginning to show the adaptations later associated with cetaceans.
The creature Pakicetus had pointed teeth and some adaptation to the inner ear for marine living, but still had four limbs, and no blowhole. Rodhocetus, another Ecocene creature, was becoming more like the whales we know today, with much shorter limbs (its femur only about 8 inches (20cm) long. Its snout was longer and toothed, and its pelvis was not fused to the spine. Dorudon atrox continued the trend, with flattened flippers, and greatly bout 20 feet (6 m) long. Its arms were small, flattened flippers. The rear limbs were vestigal, being about 4 inches (10 cm) long.
The baleen whales evolved during the late Oligocene period, the earliest known being Aetiocetus, which while it still had teeth, also had a loose jaw hinge like the later baleen whales, enabling them to eat in quantity.
These, the major milestones in whale evolution, still do not explain why the animals reverted to sea living. The suggestion has been made that food was at one time more plentiful in the sea, and that the rapid adaptation enabled them to find a niche in the marine environment. Whatever the truth, our fascination with whales continues.
http://www.enchantedlearning.com/subjects/whales/allabout/Evol.shtml
http://news.bbc.co.uk/hi/english/sci/tech/newsid_1553000/1553008.stm