Vestigial structures are anatomical structures of organisms in a species which are considered to have lost much or all of their original function through evolution. These structures are typically in a degenerate, atrophied, or rudimentary condition or form. Vestigial structures are often referred to as vestigial organs, though not all of them are actually organs

Although the structures most commonly referred to as “vestigial” tend to be largely or entirely functionless, a vestigial structure need not necessarily be without use or function for the organism. Vestigial structures have lost their original main purpose, but they may retain lesser functionalities, or develop entirely new ones. Thus, a “vestigial wing” need only be useless for flight to be vestigial; it may still serve some other purpose than that of a wing.

The coccyx, or tailbone, is the remnant of a lost tail. All mammals have a tail at one point in their development; in humans, it is present for a short time during embryonic development. The tailbone, located at the end of the spine, has lost its original function in assisting balance and mobility, though it still serves some secondary functions, such as being an attachment point for muscles, which explains why it has not degraded further. In rare cases it can persist after birth and must be surgically removed.

Humans also bear some vestigial behaviors and reflexes. For example, the formation of goose bumps in humans under stress is a vestigial reflex; its purpose in human evolutionary ancestors was to raise the body’s hair, making the ancestor appear larger and scaring off predators. Raising the hair is also used to trap an extra layer of air, keeping an animal warm. This reflex formation of goosebumps when cold is not vestigial in humans, but the reflex to form them under stress is.

There are also vestigial molecular structures in humans, which are no longer in use but may indicate common ancestry with other species. One example of this is L-gulono-gamma-lactone oxidase, a gene, found functional in most other mammals, which produces a Vitamin C-catalyzing enzyme. In humans, an earlier mutation may have caused it to become disabled (unable to produce the enzyme), and it now remains in the human genome only as a vestigial genetic sequence.

Dolphin with Legs
Creationists dispute such evolutionary theories: dolphin with legs?
Tracing evolutionary development: mitosearch