Whale Cetaceans

Both cetaceans and artiodactyl are now classified under the super-order Cetartiodactyla which includes both whales and hippopotamuses. Whales are the hippopotamus's closest living relatives.
Evolution
All cetaceans, including whales, dolphins and porpoises, are descendants of land-living mammals of the Artiodactyl order (even-toed ungulates). Both descended from a common ancestor, the Indohyus (an extinct semi-aquatic deer-like ungulate) from which they split around 54 million years ago. Primitive whales probably first took to the sea about 50 million years ago and became fully aquatic about 5-10 million years later.