From Ella Dias
Mammals have gone back into the sea at least seven times. This has given us whales, porpoises and dolphins (Cetacea), sea cows and dugongs (Sirenia), seals, sea lions and walruses (Pinnipedia), polar bears, sea otters and two extinct groups: Desmostylia and aquatic sloths (Thalassocnus). Only Cetacea and Sirenia became fully aquatic. Around 50 million years ago, the ancestors of these two groups started to adapt to feeding and hearing in the water. They could still walk on land, but as they spent more and more time in the water their bodies began to change; their front legs became flippers and they started to grow tail flukes. By the end of the Eocene about 35 million years ago, both were living in the water full time.
Answered by Laura Mears for Brain Dump in How It Works Issue 107