Vestigial structures take many forms: organs, behaviors, and even biochemical processes. Their defining characteristic is they are structures that a species has retained, but no longer serve their original purpose.
In "On the Origin of Species," Darwin calls vestigial organs "vestiges of bygone eras." The vestigial structures derived from a time when our primate ancestors needed them, due to their environment. When the environmental circumstances changed, our ancestors' bodies evolved and also changed. The vestigial structures that were left behind act as physical evidence of what we once were.
The appendix is probably the most well-known vestigial organ. It is a small organ that doesn't serve a function. For some herbivores in the animal kingdom, it is involved in the digestion of cellulose. Thus, scientists believe it helped our primate ancestors digest the cellulose in our once plant-heavy diet. Research now points to the human appendix as being a reserve of beneficial bacteria.
The pilomotor reflex is the reflex that produces goose bumps. A person gets goose bumps when they are cold or experiencing a very strong emotion, such as fear or arousal. The reflex is considered vestigial in humans, although it still functions effectively in the animal kingdom. For instance, when a porcupine is feels threatened, it raises its quills. If an animal is covered in fur, when cold, the reflex pulls their hair erect and creates another layer of insulation.
The coccyx, or tailbone, is a vestige of a time when our ancestors had tails. It is the small bone at the base of our spines that currently acts as an anchor to which all of our muscles stick to. However, since that was not its original function, it is considered a vestigial structure.
Wisdom teeth come from a time when our jaws were big enough to devour large animals. Our jaws have shrunk because our diets have changed, and they can no longer accommodate wisdom teeth. Wisdom teeth often cause pain when they attempt to grow in and need to be taken out by a professional.
Other examples of vestigial structures include tonsils, sinuses, and the palmar grasp reflex.
