Human Vestigial Structures

The general case — what "vestigial" means, why reduced structures count as evidence, and the objections the term attracts — is set out in Vestigial Structures and Atavisms. This article applies it to human anatomy.

The definition matters before the list. Vestigial means reduced or repurposed relative to an ancestral function, not useless. Several of the structures below have demonstrated present-day roles, and that is compatible with the claim rather than a refutation of it.

The human cases#

The coccyx and the embryonic tail#

Every human embryo develops a tail. It reaches its maximum extent around the fifth week with ten to twelve vertebrae, and is then reduced by programmed cell death, leaving the coccyx.

The coccyx is not functionless — it anchors the pelvic floor musculature and the gluteus maximus. It is vestigial in the technical sense: a reduced remnant of a structure that supports a tail in other primates, formed by the same developmental process, truncated.

When the reduction fails, the result is a true tail, which is treated in Human Atavisms.

The appendix#

The case where the science has genuinely moved, and the older textbook treatment was too strong.

The appendix contains gut-associated lymphoid tissue and appears to act as a reservoir of gut flora allowing recolonisation after severe diarrhoeal illness. Bollinger and Parker's work at Duke made this case in 2007, and it is now widely accepted.

It remains vestigial in the technical sense. In animals that ferment cellulose — rabbits, horses, koalas — the caecum is large and the appendix is a functional extension of it. The human structure is the reduced remnant of that organ with a secondary role retained.

Its removal has no significant long-term effect in populations with clean water, and untreated appendicitis is fatal, which is difficult to read as optimal design.

Structures with measurable remnant activity#

The ear muscles are the most interesting case, because the vestigial system can be detected rather than merely inferred. The auriculares are fully innervated by the facial nerve, and electromyographic work has recorded them responding to sounds from the side — the residue of a pinna-orienting reflex that still operates in cats and horses, firing muscles too weak to move the human ear.

The vomeronasal organ points the same way from the genetic side. The structure forms in the human embryo but lacks the neural connections needed to function, and TRPC2, the ion channel gene required for signalling in the organ, is a pseudogene in humans and other catarrhine primates, disabled by mutations shared across the group. The anatomy and the genome record the same loss.

Inherited constraints in the human body#

Some human features are not reduced structures but consequences of an inherited developmental route.

The recurrent laryngeal nerve descends from the brain into the chest, loops under the aortic arch, and returns to the larynx a few centimetres from where it started. The general case traces the arrangement to the gill arch supply in fish, where the route is direct.

The inguinal canal. Human testes develop near the kidneys and descend through the abdominal wall, leaving a weakness that makes inguinal hernia common in men and rare in women. The developmental route makes sense in ancestral terms and is a structural liability in its own right.

The vertebral column carries a lumbar curve superimposed on a spine whose basic architecture is shared with quadrupeds, and lower back pain is correspondingly common.

Common objections#

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

The strongest form of the objection, and its historical premise is correct. Wiedersheim's 1893 list of 86 vestigial human structures included the thyroid, thymus, and pineal gland. Tonsils were removed routinely on the assumption they were useless.

That history is real, and the corrections came from biologists rather than from critics of the field.

The objection answers a claim the term does not make. Vestigial means reduced from an ancestral function. Establishing that the appendix hosts gut flora does not restore its role in fermenting cellulose, and does not explain why it sits at the end of a caecum that is large in animals which need one.

"Wisdom teeth and appendicitis are results of the modern diet"

Partly correct, and worth stating. Softer processed food likely contributes to smaller jaws and higher impaction rates, and appendicitis rates vary with diet and hygiene.

It does not remove the structural point. A jaw that cannot accommodate its own full complement of teeth, and a blind-ended tube that kills when obstructed, are the expected consequences of reduction from an ancestral configuration under new conditions. They are not what a structure designed for present conditions would look like.

Third molar agenesis is also relevant: a substantial fraction of people never develop them at all, and the frequency varies sharply between populations, which is the signature of a trait under relaxed selection.

"This is degeneration since the Fall, not evolution"

Internally consistent, and creationist authors accept that human bodies have deteriorated.

Degeneration and descent both predict loss. They differ on what the losses should correspond to.

Human reduced structures correspond specifically to functional structures in other primates and mammals — a tail-supporting sacrum, a fermenting caecum, a mobile pinna, a working vomeronasal organ. The correspondence extends to the genome, where the genes for several of these are broken in the pattern shared with other primates rather than in a human-specific way. Deterioration alone gives no reason for that alignment.

"Male nipples are just a developmental economy, not evidence"

Correct as embryology. All human embryos follow a common developmental route before sexual differentiation, and nipples form before it.

That is the argument rather than an answer to it. A shared developmental route producing a structure with no function in half the population is what descent from a common plan looks like. A design account has to treat it as a choice.

What the evidence shows#

Human anatomy contains structures that are reduced versions of structures functional in other primates and mammals, several with retained secondary uses that the definition anticipates.

In at least two cases the remnant can be measured rather than inferred: the ear muscles respond electrically to lateral sounds, and the vomeronasal organ's disabling mutation in TRPC2 is shared with other catarrhine primates.

The human body also carries routing decisions — the laryngeal nerve, the inguinal canal — that are intelligible as inherited and awkward as designed.

The general treatment is in Vestigial Structures and Atavisms; the genetic counterpart is in Pseudogenes in the Human Genome.