Land animals, including mammals, originally evolved from creatures that  lived in the sea.

 

Dolphins, rather surprisingly, are mammals that evolved from animals that used to live on the land; in other words they went back to live in the sea.

 

Scientists believe that dolphins evolved from a hoofed, land-living mammal called 'Mesonyx', and returned to live in the seas some fifty million years ago. They may have looked like a large dog originally, but have looked dolphin-shaped for millions of years. The nearest living relative of the dolphin on land may be the hippo. 

 

Dolphins have existed in their current form for some 10 to 15 million years. Modern humans have only been knocking around for the last 100,000 years or so, and so we are the new kids on the block.

 

Dolphins are a ‘cetecean’, which basically means they are mammals of the whale family.

 

Dolphins are also 'odontocetes', which means toothed whales. Though closely related to whales, dolphins belong to the delphinidae family. See dolphin species.

 

Shaping up to a life on the ocean wave: Dolphins have adapted to their life in the water in other ways, including:

 

• becoming more hydrodynamic, so that they can swim more easily through the water, 

• replacing external ears by pinholes; and 

• swapping legs with pectoral flippers. Hind legs have disappeared altogether but dolphins have developed tail flukes (that’s a tail to you and me, as well as a dorsal fin on their back)