Evolutionary leftovers in the human body

In 1871, Charles Darwin devoted several pages of The Descent of Man to cataloguing anatomical structures in the human body that appeared to serve no useful purpose but closely resembled functional organs in other animals.¹ He called these "rudimentary" structures and argued that they were among the strongest lines of evidence for human descent from earlier forms of life. More than 150 years later, the list has only grown longer and better documented. Modern anatomy, comparative biology, and molecular genetics have confirmed that the human body is a palimpsest—a document written over but never fully erased—in which the traces of ancestral anatomy remain clearly visible.^{2, 3}

A vestigial structure, properly defined, is not necessarily a structure with no function whatsoever. It is a structure that has lost the primary function for which it originally evolved, even if it has been co-opted for minor secondary roles.² The human coccyx, for example, serves as an attachment point for certain pelvic muscles, but it no longer functions as a tail. The appendix may harbour immune tissue, but it no longer digests cellulose. Vestigiality is about the loss of the original adaptive significance, not the absence of any function at all.³

The vermiform appendix

The human vermiform appendix is a narrow, finger-like pouch that extends from the cecum, the blind-ended pouch where the small intestine meets the large intestine. In humans it is typically 5 to 10 centimeters long and has no significant digestive function.⁴ In many herbivorous mammals, by contrast, the cecum is an enormous fermentation chamber—in horses it can hold over 30 litres—where symbiotic bacteria break down cellulose from plant material.⁵ The human appendix is homologous to this large fermenting cecum, reduced to a vestige as the human lineage shifted from a heavily herbivorous diet to one increasingly dominated by fruit, meat, and cooked food.^{5, 6}

Darwin himself noted this homology in The Descent of Man, writing that the appendix is "a rudiment of a structure which was of service in the earliest progenitors of man" for digesting leafy vegetation.¹ Comparative anatomical studies by Smith and colleagues, who surveyed the cecal appendix across 533 mammalian species, confirmed that the appendix has evolved independently at least 32 times across mammals but has been lost far fewer times, suggesting it may have been co-opted for immune functions even as its original digestive role disappeared.⁶ A 2016 morphological analysis by Lemâitre and colleagues found that in primates and closely related taxa, appendix size is inversely correlated with cecum size, consistent with the appendix representing a reduced remnant of a once-larger organ.⁷

The appendix does contain lymphoid tissue, particularly in younger individuals, and a 2007 hypothesis by Bollinger and colleagues proposed that it may serve as a "safe house" for beneficial gut bacteria during episodes of diarrheal disease.⁸ This secondary immune function, however, does not negate its vestigial status with respect to its original digestive role. Hundreds of millions of people have had their appendices surgically removed (appendectomy) with no measurable long-term health consequences, underscoring how marginal any current function is.⁴

Wisdom teeth

The third molars, commonly known as wisdom teeth, are the last teeth to erupt in the human mouth, typically appearing between the ages of 17 and 25. In a substantial proportion of modern humans, they fail to erupt properly or become impacted—trapped beneath the gum line or wedged against adjacent teeth—because the modern human jaw is simply too small to accommodate them.^{9, 10} This mismatch between tooth size and jaw size is a hallmark of vestigiality: the teeth are inherited from ancestors with larger jaws, but the jaws have since shrunk while the genetic instructions for producing third molars remain largely intact.¹⁰

Daniel Lieberman's research on craniofacial evolution at Harvard has shown that the reduction of the human face and jaw over the past several million years is linked to dietary changes—the adoption of cooking, food processing, and softer diets—which reduced the mechanical loading on the jaws during development.¹¹ Earlier hominins like Australopithecus and Paranthropus had massive jaws and large molars adapted for grinding tough plant foods. As the hominin diet shifted toward higher-quality, softer foods, selective pressure for large jaws relaxed, and the face retracted beneath the braincase.^{11, 12}

The clinical consequences are dramatic. Studies of third molar impaction in modern populations find rates ranging from 30% to over 70%, depending on the population studied.⁹ A 2021 review in the Journal of Clinical Medicine found that the retromolar space—the gap behind the second molar where wisdom teeth must erupt—is significantly smaller in modern humans than in earlier hominin species, directly explaining the high impaction rate.⁹ Furthermore, agenesis of the third molar (congenital absence of one or more wisdom teeth) affects an estimated 20 to 25% of the global population, with higher rates in certain populations, indicating that natural selection may be in the process of eliminating these teeth entirely.^{10, 13}

The coccyx

The human coccyx, or tailbone, consists of three to five fused vertebrae at the very end of the vertebral column, below the sacrum. It is the vestigial remnant of the tail that is present in most other mammals and in many other primates.^{14, 15} In tailed mammals, the caudal vertebrae are numerous, unfused, and mobile, controlled by a suite of muscles that allow the tail to be wagged, curled, or used for balance. In humans and the other great apes, these caudal vertebrae have been reduced to a small, fused remnant that cannot move independently.¹⁴

The muscles that once moved the ancestral tail have not disappeared entirely. The coccygeus muscle (also called ischiococcygeus) attaches to the coccyx and, in tailed mammals, is the primary muscle responsible for lateral tail movement.¹⁵ In humans, it has been repurposed as part of the pelvic floor, helping to support the pelvic organs. Similarly, the levator ani muscle, which in tailed mammals assists in tail wagging and curling, has been co-opted in humans to form the bulk of the pelvic diaphragm, playing a critical role in continence and support of the abdominal and pelvic viscera.^{15, 16} These muscles thus exemplify a key principle of vestigiality: the structure has changed function, but its homology to the tail-wagging muscles of other mammals is unmistakable from comparative anatomy.²

Perhaps the most striking evidence that the coccyx is a remnant tail comes from rare developmental anomalies. Human infants are occasionally born with true vestigial tails—soft, boneless projections containing muscle, blood vessels, and nerves, arising from the coccygeal region. A 1984 review in Human Pathology by Dao and Netsky documented 33 reported cases of true human tails and pseudotails in the medical literature, with true tails containing adipose tissue, striated muscle, blood vessels, and nerves.¹⁷ These atavistic structures are understood as the reactivation of a developmental programme that is normally suppressed but never entirely lost from the genome.^{3, 17}

Arrector pili muscles and goosebumps

Attached to every hair follicle in the human skin is a tiny smooth muscle called the arrector pili. When these muscles contract—triggered by cold, fear, or strong emotion—they pull the hair follicle upright, producing the familiar bumps on the skin known as goosebumps (cutis anserina).¹⁸ In densely furred mammals, this piloerection response serves two important functions: it traps a thicker layer of insulating air against the skin for thermoregulation, and it makes the animal appear larger to potential predators or rivals.^{18, 19} A porcupine raising its quills and a cat puffing up its fur are both using their arrector pili muscles to dramatic effect.¹⁸

In humans, whose body hair has been reduced to fine, nearly invisible vellus hairs over most of the body, piloerection accomplishes neither of these goals. The hairs are too sparse and short to trap meaningful insulation, and raising them does not make a human appear larger or more threatening.¹⁹ The arrector pili muscles thus contract in response to the same sympathetic nervous system signals as in other mammals, but the functional output—effective piloerection—has been lost along with the dense fur it was designed to raise.^{2, 18}

Interestingly, a 2020 study published in Cell by Shwartz and colleagues at Harvard discovered that arrector pili muscles form a structural bridge connecting sympathetic nerves to hair follicle stem cells. The sympathetic nerve fibres that trigger goosebumps also directly stimulate stem cells in the hair follicle bulge, promoting hair growth.²⁰ This finding suggests that while the thermoregulatory and threat-display functions of piloerection are vestigial in humans, the arrector pili muscles have been co-opted into a signalling role within the hair follicle stem cell niche—yet another example of a vestigial structure acquiring a secondary function that differs entirely from its original adaptive purpose.²⁰

Vestigial muscles

Two muscles in the human limbs are so reduced from their ancestral state that they are absent in a significant fraction of the population with no detectable functional consequence: the palmaris longus of the forearm and the plantaris of the calf.

The palmaris longus is a slender muscle of the anterior forearm that, when present, inserts into the palmar aponeurosis of the hand. A comprehensive review by Ioannis and colleagues, surveying studies across multiple ethnic populations, found that the muscle is congenitally absent in approximately 14% of people overall, with rates varying from as low as 1.5% in some Asian populations to over 25% in some European populations.²¹ Crucially, its absence produces no measurable loss of grip strength or hand function.^{21, 22} When present, the palmaris longus is so dispensable that it is one of the most commonly harvested tendons for reconstructive surgery elsewhere in the body.²² In other primates and in many non-primate mammals, the homologous muscle is more robust and plays a more significant role in wrist flexion and, in arboreal species, in gripping branches.²

The plantaris is a small muscle in the posterior compartment of the leg, with a very long, thin tendon that runs alongside the much larger Achilles tendon. It is absent in approximately 7 to 20% of the human population, again with no functional deficit.²³ In arboreal primates such as macaques and chimpanzees, the plantaris is considerably more robust and contributes to foot-gripping during climbing and branch locomotion.^{23, 24} In humans, who walk upright on flat surfaces and do not grip with their feet, the muscle has dwindled to a vestige—so thin that it is sometimes called "the freshman's nerve" by anatomy students who mistake its tendon for a nerve during dissection.²⁴

Prevalence of absence for selected vestigial muscles^{21, 23}

The ear and the eye

Two small but telling vestigial features are found on the human ear and eye, each pointing to ancestral structures that remain fully functional in other animals.

Darwin's tubercle is a small thickening or nodule sometimes found on the junction of the upper and middle thirds of the helix (outer rim) of the human ear. Darwin described it in The Descent of Man as a vestige of the pointed ear tip found in many other mammals, where it served as part of the mobile, funnel-shaped pinna used to localize sounds.¹ Its prevalence varies widely across populations, from roughly 10% in some groups to over 50% in others.²⁵ A 2016 review by Loh and Cohen in Dermatologic Surgery confirmed that the tubercle represents the morphological remnant of the mammalian ear tip and has no known auditory function in humans.²⁵ Humans also retain the auricular muscles—the anterior, superior, and posterior auricular muscles—that in many mammals rotate the ear toward sounds, but in most humans these muscles are too weak to produce visible ear movement.²

The plica semilunaris is a small, crescent-shaped fold of conjunctiva located in the medial corner (inner canthus) of the human eye. It is the vestigial remnant of the nictitating membrane, or third eyelid, which is fully developed in birds, reptiles, amphibians, sharks, and many mammals including camels, polar bears, and seals.²⁶ In those animals, the nictitating membrane sweeps horizontally across the eye to clean and protect the corneal surface while maintaining visibility. In humans, the plica semilunaris retains no sweeping ability and cannot cover the eye; it assists only in the drainage of tears and the movement of the eyeball.^{26, 27} Koz and colleagues studied the histological structure of the human plica semilunaris at different developmental stages and found that it contains smooth muscle fibres homologous to those that power the nictitating membrane in other vertebrates, but these fibres are too reduced to produce any membrane movement.²⁷

Male nipples

Male nipples are sometimes cited as a vestigial structure, though their explanation is more accurately one of developmental constraint. In all mammals, the embryonic body plan is initially sexually undifferentiated. Nipple and mammary gland precursors begin to form during the first several weeks of embryonic development, before the sex-determining gene SRY on the Y chromosome activates and triggers the production of testosterone that drives male sexual differentiation.^{28, 29} By the time hormonal signals begin to masculinize the embryo, the nipple primordia are already established, and there is no developmental mechanism to remove them.²⁹

Stephen Jay Gould argued in a 1991 essay that male nipples are best understood not as vestigial organs in the strict Darwinian sense, but as a by-product of the shared developmental programme between males and females.³⁰ Natural selection acts strongly on nipples in females, where they are essential for lactation and infant survival, and this selective pressure maintains the developmental pathway that produces nipples in both sexes. Because male nipples impose no significant fitness cost, there has been no selective pressure to evolve a mechanism for suppressing them in males.^{28, 30} The result is that roughly half of all humans carry non-functional mammary apparatus—a vivid illustration of how evolution works with inherited developmental programmes rather than designing organisms from scratch.³⁰

Evolutionary significance

The vestigial structures scattered throughout the human body constitute a powerful line of evidence for evolution by natural selection. Each one represents an inherited feature that made sense in the context of an ancestral body plan but has been rendered unnecessary by subsequent changes in anatomy, diet, locomotion, or habitat.^{2, 3} A designer working from scratch would have no reason to equip humans with a remnant tail, a fermenting chamber repurposed as a lymphoid pouch, muscles for raising nonexistent fur, or the traces of a third eyelid. These features are precisely what descent with modification predicts: structures inherited from ancestors in which they served clear adaptive functions, gradually losing their original purpose as the lineage evolved in new directions.^{1, 2}

The pattern extends well beyond anatomy. The human genome contains thousands of pseudogenes—broken copies of genes that were once functional in ancestral species but have accumulated disabling mutations.³ These genomic "fossils" parallel the anatomical vestiges: both are inherited relics that evolutionary theory predicts and that no alternative hypothesis explains as parsimoniously. The convergence of evidence from gross anatomy, histology, embryology, comparative morphology, and genomics all point to the same conclusion: the human body was not designed from a blank slate but was incrementally modified from the bodies of earlier primates, earlier mammals, and ultimately from far more distant ancestors whose anatomy we still carry, quietly, within us.^{2, 3}

Summary of vestigial structures in the human body^{2, 3}