Vestigial structure

A vestigial structure is a part of the body that a species has kept from its ancestors, but in a shrunken or simplified form that no longer does its original job, or does it only weakly. Familiar examples are the tiny hip and leg bones buried in the bodies of whales and some snakes, the small wings of flightless birds such as the kiwi and emu, the eyes of blind cave fish, and the dewclaws of dogs, pigs and deer, which do not reach the ground.

These leftovers make sense in the light of evolution. When a lineage takes up a new way of life, natural selection may stop maintaining a structure that is no longer useful. Mutations that shrink it are no longer weeded out, and may even be favored if the part is costly. Because a vestigial part matches a fully working structure in related species, it is a kind of homology and counts as strong evidence of common ancestry.

Vestigial does not have to mean useless. The human tailbone, or coccyx, is what remains of a tail but still anchors muscles, and the appendix, often listed as vestigial, may act as a reservoir for helpful gut bacteria. What makes a structure vestigial is the loss of its ancestral function, not the absence of any role.