What Is a Vestigial Structure? A Leftover That Lost Its Original Job

A vestigial structure is one that has been reduced from a more developed form in an ancestor and no longer performs the function it originally evolved for. The definition concerns loss of the original function rather than absence of any function whatever, which is the point most commonly misunderstood, and a structure can be vestigial while doing something useful, since a reduced part may be co-opted for a new and smaller job. The evidence that a structure is vestigial comes from comparison, showing that relatives have a fuller version doing a clear job, from development, showing the structure forming and then reducing, and increasingly from genetics, where the genes for building the full structure are frequently present in broken form and can be identified as such.

The clearest cases come from several groups and include some that are easy to check:

• •Hind limb and pelvic bones in whales and in some snakes, isolated within the body wall
• •Wings in flightless birds, retained at reduced size and used for balance, display or swimming
• •Eyes in cave-dwelling fish and salamanders, sometimes forming and then degenerating during development
• •The muscles that move the human ear, present and functional in relatives and barely usable in most people
• •Wisdom teeth, reduced and frequently failing to erupt, in a jaw shortened relative to ancestors
• •The tail bone, formed from fused vertebrae of a tail that is present and then reabsorbed in the embryo

The human appendix is the standard example and the standard counterexample, and the honest account includes both. It is much reduced compared with the large fermenting caecum of herbivorous relatives, where that structure houses the microbes that break down plant material, and in humans it plays no such digestive role, which makes it vestigial in the strict sense of having lost its original function. It is not functionless, since it contains substantial lymphoid tissue, appears to serve as a refuge for gut bacteria that can recolonise the intestine after a severe disturbance, and has been shown in comparative work to have evolved and been retained repeatedly across mammal groups in a way that suggests it is maintained by selection. Both statements are true together, and the argument about it is largely a dispute about the definition rather than about the anatomy.

Calling something vestigial when it is not is a real error and the history of the idea includes a large instance of it. Lists compiled in the late nineteenth and early twentieth centuries named dozens or by some counts over a hundred human structures as functionless leftovers, including several endocrine glands, parts of the immune system and structures whose roles were simply not yet known. Nearly all of those have since been shown to do something, and the episode is cited both as a caution about arguing from ignorance and, misleadingly, as evidence against the concept itself. The correct lesson is narrower, since the failure was in claiming no function rather than in identifying reduction from an ancestral form, and the structures that are genuinely vestigial in the strict sense remain so regardless of what else they may have been recruited to do.

The question of why a useless structure is not simply eliminated has several answers. Selection removes a structure only if maintaining it costs something, and a small reduced structure may cost so little that there is no pressure either way, leaving it to drift. Development is interconnected, so a structure may be produced as a by-product of a process needed for something else and cannot be removed without disturbing that. Reduction takes time, and a lineage may be caught partway through. A structure may retain a secondary function that keeps it, which is the appendix case. And genetic evidence shows the machinery frequently breaks before the structure fully disappears, with broken versions of genes for building teeth, for making vitamin C and for detecting smells scattered across genomes where the corresponding function was lost.

The term means a structure reduced from an ancestral form that no longer does its original job, which is compatible with doing something useful now. Whale pelvic bones, flightless bird wings, cave fish eyes and human ear muscles are standard cases. The appendix has lost its fermenting role and retains an immune and bacterial refuge role, which is why the argument about it is about definitions.