Vestigial structures are anatomical features inherited from our ancestors that have largely or entirely lost their original function. These biological remnants demonstrate that the human form is a collection of adaptations and discarded parts from a long evolutionary lineage.
What Makes a Structure Vestigial
A structure is considered vestigial when it has been reduced in complexity and size compared to a functional counterpart found in ancestral or related species. The defining characteristic is the loss of the structure’s major ancestral role over the course of evolution. While some may retain a minor or new function, they no longer perform the significant task for which they first evolved.
This concept differs from an atavistic trait, which is the rare, sporadic reappearance of an ancestral feature absent for many generations. A vestigial structure, like the coccyx, is present in nearly every member of the species. An atavism, such as an individual born with a true tail, is an anomaly. The persistence of vestigial features indicates that the evolutionary pressure to completely eliminate them was weak or nonexistent.
Prominent Skeletal and Internal Examples
The human body retains several significant internal and skeletal structures pointing toward different ancestral dietary and locomotor needs. One recognized internal example is the vermiform appendix, a narrow, worm-like pouch attached to the large intestine.
In herbivorous ancestors, this structure was a much larger cecum, a chamber used to house bacteria that helped digest tough plant matter like cellulose. Today, the appendix no longer plays a significant role in digestion, but it contains lymphatic tissue, suggesting a minor function in the immune system. Some research also indicates it may act as a safe house, storing beneficial gut bacteria that can repopulate the digestive tract after a severe illness.
The coccyx, or tailbone, is a skeletal remnant composed of three to five fused vertebrae at the base of the spine. This structure is all that remains of the bony tail our distant primate ancestors used for balance, locomotion, and communication. Although we no longer have an external tail, the coccyx remains an important anchoring point for several pelvic floor muscles and ligaments. It also helps support the body’s weight when a person is sitting down, distributing pressure evenly.
Wisdom teeth, or third molars, offer a clear example of evolutionary change related to diet. Ancestral hominids consumed a coarse diet of uncooked vegetation and raw meat, which caused significant wear and tear on their teeth. The large, powerful jaws of these ancestors easily accommodated these molars, which served as replacements for front teeth lost early in life. With the advent of cooking and softer foods, human jaws decreased in size, leading to the common modern issue of impaction, where the molars have no room to properly erupt.
Muscular and Sensory Remnants
Other vestigial features are less obvious, consisting of small muscles or minor sensory structures. The arrector pili muscles are tiny bands of smooth muscle attached to hair follicles. When these muscles contract, they cause the phenomenon known as goosebumps, which makes body hair stand on end.
For furry mammals, this reaction, called piloerection, served two major purposes: trapping a layer of air for insulation against cold, and making the animal appear larger as a threat display. Since humans lack dense body fur, the insulating or intimidating effects of goosebumps are now negligible.
The auricular muscles are another group of tiny muscles surrounding the external ear. In many mammals, these muscles are actively used to swivel the ears, helping to precisely locate the source of a sound in a complex environment. While most humans cannot voluntarily move their ears, the muscles still exist as a remnant, and some studies suggest they unconsciously activate during intense listening situations.
The plica semilunaris is a small, pink fold of tissue located at the inner corner of the human eye. This fold is considered the vestige of the nictitating membrane, often called a “third eyelid,” which is fully functional in many animals like birds and reptiles. The membrane once swept horizontally across the eye to quickly clear debris and maintain moisture without obstructing vision. In humans, the plica semilunaris has been reduced to a simple fold, and its protective function has been replaced by the increased efficiency of blinking.