The ancestors of sea mammals once lived on land. About 50 million years ago, they began to return to the sea for food and their bodies adapted to life in the water. The whale's front legs became flippers, its back legs disappeared and its nostrils became a blow-hole on top of its head. Sea mammals can hold their breath for a long time. A fin-back whale is able to stay underwater for 40 minutes; a bottle-nosed dolphin for two hours.