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. Biochemistry Biochemistry also reveals similarities between organisms of different species. For example, the metabolism of vastly different organisms is based on the same complex biochemical compounds. The protein cytochrome c, essential for aerobic respiration, is one such universal compound. The universality of cytochrome c is evidence that all aerobic organisms probably descended from a common ancestor that used this compound for respiration.