Whales are thought to have evolved from land animals similar to large otters. as evidence of this, whales have useless leg bones (structure c) that float inside their bodies. these leg bones are structures.

These useless leg bones are vestigial structures. It was believed that the only vertebrates existing were water creatures. Over time, they developed hips and legs as they eventually walked out of the water which later evolved to land creatures. Some of these are mammals, and they later moved back to the water. Biologists believe this to have occurred 50 million years ago.