Flexi Says: The evolution of whales is a good example of how fossils can help us understand evolution. Starting in the late 1970s, a growing number of fossils have allowed scientists to piece together the story of whale evolution. The fossils represent ancient, whale-like animals. They show that an ancient population of land mammals made their way back to the sea more than 50 million years ago. The population may have found a source of food or shelter in the shallow ocean. Over time, helpful traits for water life were passed on more frequently. This population became the ancestor of modern whales. Various intermediate species arose, and eventually some of these lost legs and gained new adaptations to life in the water, such as fins instead of forearms. Comparing body parts of different species may reveal evidence for evolution. For example, all mammals have front limbs that look quite different and are used for different purposes. Bats use their front limbs to fly, whales use them to swim, and cats use them to run and climb. However, the front limbs of all three animals—as well as humans—have the same basic underlying bone structure.