What Is a Vestigial Structure?

Vestigial structures are anatomical structures that either serve no physiological function or have a function that has been greatly reduced in the course of the organism's evolution. According to About.com, humans have several structures that can be considered vestigial, whether or not they still serve some function.

Some vestigial structures serve no identifiable purpose. One such structure is the coccyx, or tailbone. This bony prominence, About.com explains, is the vestigial remnant of distant human ancestors that had fully formed tails. In the human body, the tailbone serves no obvious purpose, but the evolutionary cost of reworking human embryological development is greater than the energetic cost to the organism of growing the coccyx. The plica luminaris, which is a small flap located at the corner of the human eye, is the evolutionary remnant of a nictitating membrane and is also entirely without apparent function.

Some vestigial structures do have a function, but that function is either greatly reduced or different from what the homologous structure did in ancestral bodies. Human fingernails are vestigial claws, for example, and the human appendix, which may have once been useful in digesting leaves, according to About.com, may still play a role in maintaining the balance of gut bacteria.