Homologous structures, such as the fins of whales and the hands of monkeys, demonstrate that while a species may use structures for different purposes, the species shared a common ancestor. By definition, homologous structures refer to those that were derived from ancestrally similar structures. The natural world is full of examples of homologous structures, which the theory of natural selection predicts should be the case.
Originally, scientists grouped animals based on their overall physical similarity. This means that bats were considered to be close relatives of insects and birds, as all of the animals fly. However, as modern DNA techniques have verified, bats are not related to insects and birds, but instead, they are mammals that are related to rodents. Their wings evolved from the same bones that the wings of birds evolved from — the hand and arm bones that their ancestors possessed.
The fact that the arms of raccoons, the wings of birds and the fins of whales are all similar in design demonstrates that they all evolved from a common ancestor. When viewed in an X-ray, all three structures share striking similarities, despite the different ways the three structures evolved. Sometimes, homologous structures do not bear obvious resemblances to each other. For example, the quadrate bones of snakes are long, thin and reside in the animals’ jaws. By contrast, the quadrate bones have evolved into small bones of the middle ear for mammals.