In the context of human evolution

Human evolution

Human evolution refers to the evolutionary history of the genus Homo, including the emergence of Homo sapiens as a distinct species and as a unique category of hominids and mammals...

, human vestigiality involves those characters (such as organs

Organ (anatomy)

In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

 or behavior

Behavior

Behavior or behaviour refers to the actions and mannerisms made by organisms, systems, or artificial entities in conjunction with its environment, which includes the other systems or organisms around as well as the physical environment...

s) occurring in the human

Human

Humans are the only living species in the Homo genus...

 species that are considered vestigial—in other words having lost all or most of their original function

Function (biology)

A function is part of an answer to a question about why some object or process occurred in a system that evolved through a process of selection. Thus, function refers forward from the object or process, along some chain of causation, to the goal or success...

 through evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

. Although structures usually called "vestigial" often appear functionless, a vestigial structure may retain lesser functions or develop minor new ones. In some cases, structures once identified as vestigal simply had an unrecognized function

Vestigial characteristics occur throughout nature, one example being the vestigial hind limbs of whale

Whale

Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...

s and snake

Snake

Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s. Many human characteristics are also vestigial in other primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s and related animals. The following characteristics have been or still are considered vestigial in humans.

History

In 1893, Robert Wiedersheim

Robert Wiedersheim

Robert Wiedersheim was a German anatomist who is famous for publishing a list of 86 “vestigial organs” in his book 'The Structure of Man: An Index to His Past History'....

 published a book on human anatomy and its relevance to man's evolutionary history. This book contained a list of 86 human organs that he considered vestigial, or as Wiedersheim himself clarified "Organs having become wholly or in part functionless, some appearing in the Embryo alone, others present during Life constantly or inconstantly. For the greater part Organs which may be rightly termed Vestigial." This list included many of the examples on this page as well as others then mistakenly believed to be vestigial, such as the pineal gland

Pineal gland

The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...

, the pituitary gland

Pituitary gland

In vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...

, and the thymus gland. Some of these organs later had their functions elucidated; the production of melatonin and regulation of the circadian rhythm in the case of the pineal, immune system functions by the thymus, and for the pituitary, the secretion of numerous hormones with roles in a wide variety of biological processes.

Appendix

The vermiform appendix

Vermiform appendix

The appendix is a blind-ended tube connected to the cecum , from which it develops embryologically. The cecum is a pouchlike structure of the colon...

 is a vestige of the cecum

Cecum

The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...

, an organ that would have been used to digest cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 by humans' herbivorous ancestors. Analogous organs in other animals similar to humans continue to perform that function, whereas other meat-eating animals may have similarly diminished appendices. In line with the possibility of vestigial organs developing new functions, some research suggests that the appendix may guard against the loss of symbiotic bacteria that aid in digestion.

Coccyx

The coccyx

Coccyx

The coccyx , commonly referred to as the tailbone, is the final segment of the vertebral column. Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between...

, or tailbone, is the remnant of a lost tail

Tail

The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...

. All mammals have a tail at one point in their development; in humans, it is present for a period of 4 weeks, during stages

Carnegie stages

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo....

 14 to 22 of human embryogenesis

Human embryogenesis

Human embryology is the study of human development during the first eight weeks from gametogenesis pre-conception through fertilization up to and including the 8th week after implantation of the zygote in the uterus...

. This tail is most prominent in human embryos 31–35 days old. The tailbone, located at the end of the spine, has lost its original function in assisting balance and mobility, though it still serves some secondary functions, such as being an attachment point for muscles, which explains why it has not degraded further.

In rare cases congenital defect results in a short tail-like structure being present at birth. Twenty-three cases of human babies born with such a structure have been reported in the medical literature since 1884.

Wisdom teeth

Wisdom teeth

Wisdom tooth

A wisdom tooth, in humans, is any of the usually four third molars. Wisdom teeth usually appear between the ages of 17 and 25. Most adults have four wisdom teeth, but it is possible to have fewer , or more, in which case they are called supernumerary teeth...

 are vestigial third molars that human ancestors used to help in grinding down plant tissue. The common postulation is that the skulls of human ancestors had larger jaws with more teeth, which were possibly used to help chew down foliage to compensate for a lack of ability to efficiently digest the cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 that makes up a plant cell wall. As human diets changed, smaller jaws were selected by evolution, yet the third molars, or "wisdom teeth," still commonly develop in human mouths. Currently, wisdom teeth have become useless and even harmful to the extent where surgical procedures are often done to remove them.

Agenesis

Agenesis

In medicine, agenesis refers to the failure of an organ to develop during embryonic growth and development due to the absence of primordial tissue...

 of wisdom teeth in human populations ranges from zero in Tasmanian Aboriginals to nearly 100% in indigenous Mexicans. The difference is related to the PAX9

PAX9

Paired box gene 9, also known as PAX9, is a protein which in humans is encoded by the PAX9 gene. It is also found in mammals generally.- Function :This gene is a member of the paired box family of transcription factors...

 gene (and perhaps other genes).

Vomeronasal organ

In some animals the vomeronasal organ

Vomeronasal organ

The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813....

 (VNO) is part of a second, completely separate sense of smell, known as the accessory olfactory system

Olfaction

Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

. Many studies have been performed to find if there is an actual presence of a VNO in adult human beings. Trotier et al. estimated that around 92% of their subjects that had no septal surgery had at least one intact VNO. Kjaer and Fisher Hansen, on the other hand, stated that the VNO structure disappeared during fetal development as it does for some primates. However, Smith and Bhatnagar (2000) asserted that Kjaer and Fisher Hansen simply missed the structure in older fetuses. Won (2000) found evidence of a VNO in 13 of his 22 cadavers (59.1%) and in 22 of his 78 living patients (28.2%). Given these findings, some scientists have argued that there is a VNO in adult human beings. However, most investigators have sought to identify the opening of the vomeronasal organ in humans, rather than identify the tubular epithelial structure itself. Thus it has been argued that such studies, employing macroscopic observational methods, have sometimes missed or even misidentified the vomeronasal organ.

Among studies that use microanatomical methods, there is no reported evidence that human beings have active sensory neurons like those in working vomeronasal systems of other animals. Furthermore, there is no evidence to date that suggests there are nerve and axon connections between any existing sensory receptor cells that may be in the adult human VNO and the brain. Likewise, there is no evidence for any accessory olfactory bulb in adult human beings, and the key genes involved in VNO function in other mammals have become pseudogene

Pseudogene

Pseudogenes are dysfunctional relatives of known genes that have lost their protein-coding ability or are otherwise no longer expressed in the cell...

s in human beings. Therefore, while the presence of a structure in adult human beings is debated, a review of the scientific literature by Tristram Wyatt concluded, "most in the field ... are sceptical about the likelihood of a functional VNO in adult human beings on current evidence."

Ear

The ears of a Macaque

Macaque

The macaques constitute a genus of Old World monkeys of the subfamily Cercopithecinae. - Description :Aside from humans , the macaques are the most widespread primate genus, ranging from Japan to Afghanistan and, in the case of the barbary macaque, to North Africa...

 monkey, and most other monkeys, have far more developed muscles than those of humans and therefore have the capability to move their ears to better hear potential threats. Humans and other primates such as the orangutan

Orangutan

Orangutans are the only exclusively Asian genus of extant great ape. The largest living arboreal animals, they have proportionally longer arms than the other, more terrestrial, great apes. They are among the most intelligent primates and use a variety of sophisticated tools, also making sleeping...

 and chimpanzee

Chimpanzee

Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...

 however have ear muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s that are minimally developed and non-functional, yet still large enough to be identifiable. A muscle attached to the ear that cannot move the ear, for whatever reason, can no longer be said to have any biological function. In humans there is variability in these muscles, such that some people are able to move their ears in various directions, and it has been said that it may be possible for others to gain such movement by repeated trials. In such primates the inability to move the ear is compensated mainly by the ability to turn the head

Head

In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....

 on a horizontal plane, an ability which is not common to most monkeys—a function once provided by one structure is now replaced by another.

The outer structure of the ear also shows some vestigial features, such as the node or point on the helix of the ear known as Darwin's tubercle

Darwin's tubercle

Darwin's tubercle is a congenital ear condition which often presents as a thickening on the helix at the junction of the upper and middle thirds. The feature is present in approximately 10.4% of the population...

 which is found in around 10% of the population, this feature is labelled (a) in the accompanying figure.

Eye

The plica semilunaris is a small fold of tissue on the inside corner of the eye. It is the vestigial remnant of the nictitating membrane

Nictitating membrane

The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...

 (the "third eyelid") which is present in other animals such as bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, reptile

Reptile

Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s, and fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

. It is rare in mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, mainly found in monotreme

Monotreme

Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...

s and marsupial

Marsupial

Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

s. Its associated muscles are also vestigial. The plica semilunaris of Africans

African people

African people refers to natives, inhabitants, or citizen of Africa and to people of African descent.-Etymology:Many etymological hypotheses that have been postulated for the ancient name "Africa":...

 and Indigenous Australians

Indigenous Australians

Indigenous Australians are the original inhabitants of the Australian continent and nearby islands. The Aboriginal Indigenous Australians migrated from the Indian continent around 75,000 to 100,000 years ago....

 are slightly larger than in other peoples. Only one species of primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

, the Calabar Angwantibo

Calabar Angwantibo

The Calabar angwantibo , also known as the Calabar potto, is a strepsirrhine primate from the Lorisidae family. It shares the genus Arctocebus with the golden angwantibo...

, is known to have a functioning nictitating membrane.

Musculature

A number of muscles in the human body are thought to be vestigial, either by virtue of being greatly reduced in size compared to homologous muscles in other species, by having become principally tendonous or by being highly variable in their frequency within or between populations.

Head

The Occipitalis Minor is a muscle in the back of the head which normally joins to the auricular muscles

Auricular muscles

The auricular muscles are the three muscles surrounding the auricula or outer ear:* anterior auricular muscle* superior auricular muscle* posterior auricular muscle-Action:...

 of the ear. This muscle is very sporadic in frequency—always present in Malays, in 56% of Africans, 50% of Japanese, 36% of Europeans, and is nonexistent in the Khoikhoi

Khoikhoi

The Khoikhoi or Khoi, in standardised Khoekhoe/Nama orthography spelled Khoekhoe, are a historical division of the Khoisan ethnic group, the native people of southwestern Africa, closely related to the Bushmen . They had lived in southern Africa since the 5th century AD...

 people of southwestern Africa and in Melanesia

Melanesia

Melanesia is a subregion of Oceania extending from the western end of the Pacific Ocean to the Arafura Sea, and eastward to Fiji. The region comprises most of the islands immediately north and northeast of Australia...

ns. Other small muscles in the head associated with the occipital region and the post-auricular muscle complex are often variable in their frequency.

Face

In many non-human mammals the upper lip and sinus area is associated with whiskers or vibrissae which serve a sensory function. In humans these whiskers do not exist but there are still sporadic cases where elements of the associated vibrissal capsular muscles or Sinus hair muscles can be found. Based on histological studies of the upper lips of 20 cadavers, Tamatsu et al. found that structures resembling such muscles were present in 35% (7/20) of their specimens.

Arm

The palmaris longus muscle

Palmaris longus muscle

The palmaris longus is seen as a small tendon between the flexor carpi radialis and the flexor carpi ulnaris, although it is not always present. The muscle is absent in about 14 percent of the population, however this varies greatly with ethnicity...

 is seen as a small tendon between the flexor carpi radialis and the flexor carpi ulnaris, although it is not always present. The muscle is absent in about 14% of the population, however this varies greatly with ethnicity. One study has shown the prevalence of palmaris longus agenesis in 500 Indian patients to be 17.2% (8% bilateral and 9.2% unilateral). The palmaris is a popular source of tendon material for grafts and this has prompted studies which have shown the absence of the palmaris does not have any appreciable effect on grip strength.

The levator claviculae muscle

Levator claviculae muscle

In human anatomy, the levator claviculae is a very rare accessory and vestigial skeletal muscle in the posterior triangle of the neck. It originates on the transverse processes of the upper cervical vertebrae and is inserted in the lateral half of the clavicle...

 in the posterior triangle of the neck

Posterior triangle of the neck

-Boundaries:It has the following boundaries:Apex: Union of the sternocleidomastoid and the trapezius muscles at the superior nuchal line of the occipital boneAnterior: Posterior border of the sternocleidomastoideus...

 is a supernumerary muscle present in only 2–3% of all people but nearly always present in most mammalian species, including gibbons and orangutans.

Torso

The pyramidalis muscle

Pyramidalis muscle

The pyramidalis is a small and triangular muscle, anterior to the Rectus abdominis, and contained in the rectus sheath.-Attachments and actions:...

 of the abdomen is a small and triangular muscle, anterior to the rectus abdominis, and contained in the rectus sheath

Rectus sheath

The Rectus sheath is formed by the aponeuroses of the Obliqui and Transversus. It contains the Rectus abdominis and Pyramidalis muscles.It can be divided into anterior and posterior laminae....

. It is absent in 20% of humans and when absent the lower end of the rectus then becomes proportionately increased in size. Anatomical studies suggest that the forces generated by the pyramidalis muscles are relatively small.

Leg

The plantaris muscle

Plantaris muscle

Plantaris is a vestigial structure and one of the superficial muscles of the posterior crural compartment of the leg.It is innervated by the tibial nerve ....

 is composed of a thin muscle belly and a long thin tendon. The muscle belly is approximately 2–4 inches long, and is absent in 7–10% of the human population. It has some weak functionality in moving the knee and ankle but is generally considered redundant and is often used as a source of tendon for grafts. The long, thin tendon of the plantaris is humorously called "the freshman's
nerve," as it is often mistaken for a nerve by first-year medical students.

Breasts

See also : Accessory breast

Accessory breast

Accessory breasts, also known as polymastia, supernumerary breasts, or mammae erraticae, is the condition of having an additional breast...

and Extra nipple

Extra nipples or extra breasts sometimes appearing along the mammary lines appearing as a remnant to mammalian ancestors whom possessed more than 2 breasts or nipples.

Sensory

Although the sense of smell or olfaction is highly important for many animals in avoiding predators

Predation

In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

, finding food

Prey detection

Prey detection is the process by which predators are able to detect and locate their prey via sensory signals. This article treats predation in its broadest sense, i.e. where one organism eats another.-Evolutionary struggle and prey defenses:...

, and other functions, it is far less essential to human survival, as humans have for the most part no predators, and obtain food mostly by agriculture

Agriculture

Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

. There is great variation

Genetic variation

Genetic variation, variation in alleles of genes, occurs both within and among populations. Genetic variation is important because it provides the “raw material” for natural selection. Genetic variation is brought about by mutation, a change in a chemical structure of a gene. Polyploidy is an...

 in olfactory sensitivity from person to person, which is common in vestigial characteristics. It has been observed that native South Americans, American Natives, and African peoples have a highly developed sense of smell, such that they may be able to identify others in the dark by their odor alone. This does not mean that having any olfactory ability at all is vestigial, for example it may save a person from inhaling toxic fumes. A characteristic may degenerate despite being of some use if there is very little or no selection pressure

Evolutionary pressure

Any cause that reduces reproductive success in a proportion of a population, potentially exerts evolutionary pressure or selection pressure. With sufficient pressure, inherited traits that mitigate its effects - even if they would be deleterious in other circumstances - can become widely spread...

 on the genes associated with it. In other words, having a good sense of smell may be something a person would desire, but unless those without such abilities have a lower reproductive success or fitness

Fitness (biology)

Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

, there is no barrier to its degeneration.

Behavioral

Humans also bear some vestigial behaviors and reflexes. For example, the formation of goose bumps

Goose bumps

Goose bumps, also called goose flesh, goose pimples, chill bumps, chicken skin, funky spots, Dasler Bumps, chicken bumps or the medical term cutis anserina, are the bumps on a person's skin at the base of body hairs which may involuntarily develop when a person is cold or experiences strong...

 in humans under stress

Stress (medicine)

Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...

 is a vestigial reflex

Reflex action

A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

; a possible function in human evolutionary ancestors was to raise the body's hair, making the ancestor appear larger and scaring off predators. Raising the hair is also used to trap an extra layer of air, keeping an animal warm. Due to the diminished amount of hair in humans, the reflex formation of goose bumps when cold is also vestigial.

The palmar grasp reflex is supported to be a vestigial behavior in human infants. When placing a finger or object to the palm of an infant, it will securely grasp it. This grasp is found to be rather strong. Some infant

Infant

A newborn or baby is the very young offspring of a human or other mammal. A newborn is an infant who is within hours, days, or up to a few weeks from birth. In medical contexts, newborn or neonate refers to an infant in the first 28 days after birth...

s—37% according to a 1932 study—are able to support their own weight from a rod, although there is no way they can cling to their mother. The grasp is also evident in the feet too. When a baby is sitting down, its prehensile feet assume a curled-in posture, much like what is observed in an adult chimp. An ancestral primate would have had sufficient body hair to which an infant could cling unlike modern humans, thus allowing its mother to escape from danger, such as climbing up a tree in the presence of a predator without having to occupy her hands holding her baby.

Molecular

There are also vestigial molecular structures in humans, which are no longer in use but may indicate common ancestry with other species. One example of this is L-gulonolactone oxidase

L-gulonolactone oxidase

L-gulonolactone oxidase is an enzyme that catalyzes the reaction of D-glucuronolactone with oxygen to L-xylo-hex-3-gulonolactone and hydrogen peroxide. It uses FAD as a cofactor...

, a gene that is functional in most other mammals and produces an enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 that synthesizes Vitamin C

Vitamin C

Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

. In humans and other members of the suborder Haplorrhini

Haplorrhini

The haplorhines, the "dry-nosed" primates , are members of the Haplorhini clade: the prosimian tarsiers and the anthropoids...

, a mutation disabled the gene and made it unable to produce the enzyme. However, the remains of the gene are still present in the human genome

Human genome

The human genome is the genome of Homo sapiens, which is stored on 23 chromosome pairs plus the small mitochondrial DNA. 22 of the 23 chromosomes are autosomal chromosome pairs, while the remaining pair is sex-determining...

 as a vestigial genetic sequence called a pseudogene

Pseudogene

Pseudogenes are dysfunctional relatives of known genes that have lost their protein-coding ability or are otherwise no longer expressed in the cell...

.

Functioning Vestigial Structures

Some scientific experiments have proved that certain vestigial structures have functions, in some cases vital functions. Non-coding DNA is termed "junk DNA" because it has no known function. Later, important functions were discovered for some of the non-coding DNA.