Though the limbs look strikingly different on the outside and though they vary in function, they are very similar in skeletal structure. More significantly, they are derived from the same structures in the embryo. Structures that are embryologically similar, but have different functions, are called homologous structures. Though these animals look different, a comparison of homologous structures indicates that they are quite similar. This suggests that these animals evolved from a common ancestor.
Some organisms have structures or organs that seem to serve no useful function.
For example, humans have a tailbone at the end of the spine that is of no apparent use. Some snakes have tiny pelvic bones and limb bones, and some cave-dwelling salamanders have eyes even though members of the species are completely blind. Such seemingly functionless parts are called vestigial organs or structures. Vestigial organs are often homologous to organs that are useful in other species. The vestigial tailbone in humans is homologous to the functional tail of other primates.