Vestigial Structures

Vestigial structures, sometimes called vestigia, rudimentary structures, or remnants, are non-functional features fully developed and functioning in earlier species but serve little or no present purpose for an organism. A broader definition of the term might include structures that have a different function than their original function and are a trace of an initial state in evolution. 

Vestigial structures originate when a species experiences changes in its behavior or environment that cause a decrease in the purpose of an organ or part of the body. As the function of that structure is no longer beneficial for survival, there is a decreased chance that future species will inherit a fully functional version of the structure over generations. Examples of vestigial structures include vestigial wings in flightless birds, non-functional eyes in cave fishes that live in absolute darkness, and pelvic bones of whales who evolved from four-legged land mammals.  

Because vestigial structures are homologous or similar, to fully functioning structures inherited by related species, they provide strong evidence of common ancestry and can help biologists trace the evolutionary origin of species.

There are several examples of vestigial structures in humans. For instance, the appendix is a hollow-ended tube attached to the large intestine's cecum that does not directly participate in digestion. Some theories suggest it might shelter gut flora or play a part in the lymphatic and immune systems, though its exact role is unclear. In human ancestors and other plant-based mammals, the appendix might have harbored symbiotic bacteria that helped to digest the cellulose from plant cell walls when the diet was predominantly herbivorous. 

Similarly, the third molars, or wisdom teeth, were previously used to help grind and chew down plant tissue. As the diet evolved, humans used fire to cook and soften food, wisdom teeth became unnecessary, and a smaller jaw with fewer teeth was favored over time.

Next, the coccyx or tailbone is a vestige of a tail. When human ancestors learned to walk upright, the tail became useless for maintaining balance and mobility, and it slowly disappeared. Nowadays, the coccyx serves some secondary functions, such as being a site of attachment for muscles and ligaments of the pelvic floor, which could explain why it has not degraded further.

Another example of a vestigial structure in the human body is the plica semilunaris, a small fold of tissue on the inside corner of the eye that helps maintain tear drainage and flushing of debris. It is the vestigial remnant of the nictitating membrane, or third eyelid, present in other animals like fish, amphibians, reptiles, and birds.

Finally, humans also have vestigial behaviors and reflexes. For example, the formation of goosebumps under stress is a vestigial reflex that serves no current purpose. However, in human ancestors, goosebumps raised body hair, making them appear larger and scare off predators more easily. Cats, dogs, and bears are examples of animals that use this reflex today. Goosebumps also keep animals warm by trapping an extra layer of air between body hair. Of note, the act of goose bump formation when cold is not vestigial in humans; however, it is vestigial to form them under stress.

Specific vestigial structures in humans are of medical importance, especially in appendicitis. Appendicitis is the most common surgical emergency of the abdomen and occurs when the appendix becomes inflamed. It typically presents with right lower quadrant abdominal pain, nausea, vomiting, and decreased appetite. If inflammation persists, the appendix may rupture, causing widespread, painful inflammation of the inner lining of the abdominal wall and sepsis. The treatment of appendicitis is an appendectomy, which is the surgical removal of the appendix with antibiotics. 

Vestigial structures are features of an organism that are considered to have lost much or all of their original function through evolution. These structures can provide insight into the environment and behaviors of a species' ancestors and how they have evolved through time. Examples of vestigial structures in the human anatomy include the appendix, the coccyx, and wisdom teeth.