Strong evidence indicates that whales and other cetaceans (dolphins and porpoises) are descended from land mammals. This is due to their vestigial hind limbs and anatomical similarities with fossils of certain prehistoric land mammals. These land mammals began spending more time in the water until they adapted to the marine environment and became the cetaceans.

One of the earliest ancestors of cetaceans is Pakicetus. He was a wolf-sized mammal that spent most of his time on land. His fossils were found in Pakistan. He appeared shortly after the death of the dinosaurs around 50 million years ago. At the time the region was hot and dry and there was a scarcity of land animals to hunt. Pakicetus found an abundance of food in the sea, and few predators, because many of the predators that once ruled the seas went extinct at the end of the Mesozoic Era. This would be the start of the evolution of whales.

A few million years later Ambulocetus appears in the fossil record. Its name means "walking whale," because Ambulocetus already showed characteristics of cetaceans. He is believed to have been an ambush predator (much like today's crocodiles) and was well adapted for swimming, moving his tail and hind legs in a vertical motion as modern whales do. His hind legs were elongated, his tail was flatter, and his body was more streamlined. He probably waddled awkwardly on land, much like a seal.

The ancestors of whales, such as Ambulocetus, would become further adapted to living in the ocean. Their tails changed shape to further improve their swimming, eventually developing flukes, and their legs shrank to become fins. An example of further adaptation to the water is Rodhocetus, who appears a few million years later. By his fossils he must have been a good swimmer but would have been handicapped on land due to his shrunken pelvis and hind limbs. Both Dorudon and Basilosauruswere the first true whales, and had vestigial hind limbs that were no longer used for swimming. Eventually these disappeared altogether. Even today, we can still observe tiny vestiges of hind limbs in the skeletons of modern whales.

Modern whales evolved only within the span of 10-15 million years. To some people this seems incredibly fast. But remember that adapting to life at sea does not require the evolution of brand-new features, only the modification of old ones.

Why did mammals return to the ocean in the first place? The most likely explanation is that the extinction of the marine reptiles that used to rule the seas left an open niche which marine mammals could easily occupy. There was plenty of food in the ocean, and it would take only a few modifications to reach it.