The ancestors of whales were four-limbed creatures living on land. Though their body structure was originally well adapted to terrestrial life, this mammal changed its structure again to lead an aquatic life. Its body shape moved to a morestream-lined form,andits front limbs evolved into fins asits tail spread horizontally: all these changes represent evolutionary shaping to fitan aquatic life. Whales spend all their time in the water. Thanks to their buoyancy function,they don't have to support their body. They can consume the abundant food available in the expansive oceans. The huge size of whales is a direct consequence of all these changes.