Porpoises do belong in their own family and are quite different to a dolphin. The final family in the suborder Odontoceti is the Platanistidae family, including five species of river dolphins. They are generally regarded as separate species though. The water-dwelling cetaceans that we know today were once land based animals. Scientists are unsure of the exact reason why the ancestors of whales, dolphins and porpoises took to the sea millions of years ago.
Fossils show that their bodies adapted gradually to an aquatic way of life.
The transition from walking to swimming was brought about by changes to the shape of the body. As water is a denser medium than air and therefore harder to move about in, a streamlined torpedo-shaped body was required to slip easily through the water. Paddle-like blades are more efficient for power and propulsion in the water than the limbs of a land based mammal. Progressively the forelimbs were modified into blades, known as pectoral fins , for steering and balance. The hind limbs disappeared altogether, the only evidence of these in today's cetaceans being in the skeleton beneath the blubber and muscle in the abdominal region.