Vestigial Structures and Atavisms

A vestigial structure is one that has been reduced from a functional form in an ancestor. The term does not mean useless — it means reduced or repurposed relative to the ancestral condition, and Darwin's own examples included structures with retained secondary uses.

An atavism is the related but distinct case: an individual developing a structure its species does not normally have, but its ancestors did. A vestigial structure is present in every member of a species in reduced form; an atavism is an abnormality in occasional individuals.

Both bear on the same question. A structure reduced from a known function, or a structure that reappears by developmental accident, requires an ancestor that had it.

Reduced structures across animals#

The whale pelvis#

The clearest case, because the sequence is documented in fossils rather than inferred.

Modern whales have no external hind limbs. They retain reduced pelvic bones embedded in muscle, unattached to the vertebral column — an arrangement with no mechanical role in swimming, since a pelvis exists to transmit force from hind limbs to spine.

The fossil sequence shows the reduction step by step: Pakicetus with full hind limbs, Ambulocetus with functional legs, Rodhocetus with reduced ones, Basilosaurus with hind limbs a few centimetres long on a fifteen-metre animal.

Dines et al. (2014) showed the bones do have a current function — they anchor the muscles controlling the penis, and their size across species tracks the intensity of sexual selection. This is a real function and exactly the kind of secondary use the definition anticipates: the structure was retained and repurposed, not built for its present job.

Cave fish eyes#

Blind cave forms of Astyanax mexicanus begin building eyes as embryos — optic cup, lens, retina — and the structures then degenerate.

Yamamoto and Jeffery (2000) transplanted a lens from a surface fish into a cave fish embryo and recovered a substantially restored eye, with cornea, iris, and pupil. The developmental programme is intact; a single tissue's failure triggers the collapse of the rest.

An animal built without eyes would not carry a working programme for them.

Inherited constraint#

Related to vestigiality is the case where an inherited arrangement persists despite being a poor solution.

The recurrent laryngeal nerve runs from the brain to the larynx. Instead of travelling directly, it descends into the chest, loops beneath the aortic arch, and returns up the neck. In fish, the equivalent nerve serves gill arches and the route is short and direct. As necks lengthened across vertebrate evolution, the nerve stayed trapped behind the vessel.

In a giraffe the detour runs roughly five metres to reach a destination a few centimetres from the origin, and the same arrangement is reconstructed for sauropod dinosaurs, where it would have been longer still.

Atavisms#

Bejder and Hall (2002) surveyed the whale cases and the developmental basis for limb loss across vertebrates.

The specific prediction is that atavisms reproduce traits of a lineage's own ancestors and not arbitrary ones. That holds: horses grow extra toes and not wings, whales grow legs and not gills, chickens grow conical reptilian teeth and not mammalian molars.

The developmental machinery required for these reversions can be tested directly rather than assumed — see Embryology and Development for the snake limb enhancer that still drives limb growth when placed in a mouse, and Pseudogenes for the enamel genes birds still carry.

Common objections#

"Every 'vestigial' organ turned out to have a function"

A fair criticism of older claims. Robert Wiedersheim's 1893 list of 86 vestigial human structures included the thyroid, thymus, and pineal gland, all of which have known functions.

That history is real, and the corrections came from within biology.

The objection targets a definition biologists do not use. Vestigial means reduced from an ancestral function, not functionless. Discovering that whale pelvic bones anchor reproductive muscles does not restore their role in supporting hind limbs, and does not explain why a legless animal has a pelvis at all.

"These structures were designed for their current purpose"

Compatible with any observation, which is the difficulty with it.

It does not account for the cross-species pattern. If the whale pelvis were designed to anchor muscle, there is no reason for it to resemble a reduced terrestrial mammal pelvis, occupy the same position, develop from the same embryonic tissue, and appear in a fossil sequence of progressive reduction.

Pythons make the point more sharply. They have an internal femur and a pelvic girdle attached to nothing, with external spurs used in courtship. A structure designed for courtship does not require a femur.

"Loss of function is degeneration, not evolution"

Accurate as description. These are losses, and creationist authors accept that organisms degrade over time.

Both accounts predict loss; they differ on the pattern it should form. The losses are nested — whales share reduced pelvises with each other and with no other mammals — and the reduced structures resemble specific functional structures in specific other groups rather than simply being absent.

Degeneration from created kinds gives no reason for a cave fish to retain a working eye programme that runs and then fails, or for a snake to retain a limb enhancer whose decay tracks the snake family tree.

"Atavisms are birth defects, not evidence of ancestry"

Correct, and the medical and veterinary literature classifies them that way.

The classification does not reach the argument. The question is why a particular abnormality produces a coherent, well-formed ancestral structure rather than disorganised tissue.

A whale born with a femur, tibia, and tarsal bones in the correct arrangement is not what a random developmental error produces from nothing. It indicates that instructions for building a hind limb are present and were expressed.

What the evidence shows#

Vestigial structures are reduced forms of structures that remain functional in related animals. Several retain secondary functions, which the definition allows and which is expected of inherited material under new conditions.

The whale pelvis, python hind limbs, and cave fish eyes each have a documented fossil or developmental sequence showing the reduction rather than only the endpoint, and the cave fish eye can be experimentally restored.

Atavisms reproduce ancestral traits specific to each lineage and respect group boundaries, which requires that the developmental programmes persist in suppressed form.

Applied to human anatomy, see Human Vestigial Structures and Human Atavisms.