Hair

Hair
Cross section of a hair
Strand of human hair at 200× magnification.
Identifiers
MeSH	D006197
Code	TH H3.12.00.3.02001
TA	A16.0.00.014
FMA	70752
Anatomical terminology [edit on Wikidata]

Hair is a protein filament that grows from follicles found in the dermis, or skin. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types and hair care, but hair is also an important biomaterial primarily composed of protein, notably keratin. Attitudes towards hair, such as hairstyles and hair removal, vary widely across different cultures and historical periods, but it is often used to indicate a person's personal beliefs or social position, such as their age, gender, or religion.^[1]

Overview

The word "hair" usually refers to two distinct structures:

1. the part beneath the skin, called the hair follicle, or, when pulled from the skin, the bulb. This organ is located in the dermis and maintains stem cells, which not only re-grow the hair after it falls out, but also are recruited to regrow skin after a wound.^[2]
2. the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones.

Hair fibers have a structure consisting of several layers, starting from the outside:

1. the cuticle, which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles,
2. the cortex, which contains the keratin bundles in cell structures that remain roughly rod-like.
3. the medulla, a disorganized and open area at the fiber's center.^[3]

Description

Each strand of hair is made up of the medulla, cortex, and cuticle.^[4] The innermost region, the medulla, is not always present and is an open, unstructured region.^[5] The highly structural and organized cortex, or middle layer of the hair, is the primary source of mechanical strength and water uptake. The cortex contains melanin, which colors the fiber based on the number, distribution and types of melanin granules. The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. People with straight hair have round hair fibers. Oval and other shaped fibers are generally more wavy or curly. The cuticle is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water.^[4] The diameter of human hair varies from .017 to .18 millimeters (0.00067 to 0.00709 in).^[6] There are two million small, tubular glands and sweat glands that produce watery fluids that cool the body by evaporation. The glands at the opening of the hair produce a fatty secretion that lubricates the hair.^[7]

Hair growth begins inside the hair follicle. The only "living" portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered "dead". The base of a hair's root (the "bulb") contains the cells that produce the hair shaft.^[8] Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the arrector pili muscles, which are responsible for causing hairs to stand up. In humans with little body hair, the effect results in goose bumps.

Natural color

All natural hair colors are the result of two types of hair pigments. Both of these pigments are melanin types, produced inside the hair follicle and packed into granules found in the fibers. Eumelanin is the dominant pigment in brown hair, and black hair, while pheomelanin is dominant in red hair. Blond hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin production decreases or stops, while poliosis, typically in spots is hair (and often the skin to which the hair is attached) that never possessed melanin at all in the first place, or ceased for natural genetic reasons, generally in the first years of life.

Human hair growth

Hair grows everywhere on the external body except for mucus membranes and glabrous skin, such as that found on the palms of the hands, soles of the feet, and on the lips.

Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each has specific characteristics that determine the length of the hair. All three occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.

The body has different types of hair, including vellus hair and androgenic hair, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection.

Texture

Hair exists in a variety of textures. Three main aspects of hair texture are the curl pattern, volume, and consistency. The derivations of hair texture are not fully understood. All mammalian hair is composed of keratin, so the make-up of hair follicles is not the source of varying hair patterns. There are a range of theories pertaining to the curl patterns of hair. Scientists have come to believe is that the shape of the hair shaft has an effect on the curliness of the individual's hair. A very round shaft allows for less disulfide bonds to be present in the hair strand. This means the bonds present are directly in line with one another, resulting in straight hair.^[9] The flatter the hair shaft becomes, the curlier hair gets, because the shape allows more cysteines to become compacted together resulting in a bent shape that, with every additional disulfide bond, becomes curlier in form.^[9] As the hair follicle shape determines curl pattern, the hair follicle size determines thickness. While the circumference of the hair follicle expands, so does the thickness of the hair follicle. An individual's hair volume, as a result, can be thin, normal, or thick. The consistency of hair can almost always be grouped into three categories: fine, medium, and coarse. This trait is determined by the hair follicle volume and the condition of the strand.^[10] Fine hair has a small circumference in relation to medium and coarse strands; coarse hair having the largest circumference.^[10] Coarse hair has a more open cuticle than thin or medium hair causing it to be the most porous.^[10]

Classification systems

There are various systems that people use to classify their curl patterns. Being knowledgeable of an individual's hair type is a good start to knowing how to take care of one's hair. There is not just one method to discovering one's hair type.

Andre Walker system

This hair typing system is the most widely used system to classify hair. The system was created by the hairstylist of Oprah Winfrey, Andre Walker. According to this system there are four types of hair: straight, wavy, curly, kinky.

• Type 1 is straight hair, which reflects the most sheen and also the most resilient hair of all of the hair types. It is hard to damage and immensely difficult to curl this hair texture. Because the sebum easily spreads from the scalp to the ends without curls or kinks to interrupt its path, it is the most oily hair texture of all.
• Type 2 is wavy hair, whose texture and sheen ranges somewhere between straight and curly hair. Wavy hair is also more likely to become frizzy than straight hair. While type A waves can easily alternate between straight and curly styles, type B and C Wavy hair is resistant to styling.
• Type 3 is curly hair known to have an S-shape. The curl pattern may resemble a lowercase "s", uppercase "S", or sometimes an uppercase "Z". This hair type is usually voluminous, "climate dependent (humidity = frizz), and damage prone." Lack of proper care causes less defined curls.
• Type 4 is kinky hair.

Andre Walker hair types

TYPE 1: Straight
1a	Straight (Fine/Thin)	Hair tends to be very soft, shiny, oily, poor at holding curls but difficult to damage.
1b	Straight (Medium)	Hair characterised by volume and body.
1c	Straight (Coarse)	Hair tends to be bone-straight and difficult to curl. Common in Asian women.
TYPE 2: Wavy
2a	Wavy (Fine/Thin)	Hair has definite "S" pattern and is usually receptive to a variety of styles.
2b	Wavy (Medium)	Can tend to be frizzy and a little resistant to styling.
2c	Wavy (Coarse)	Frizzy or very frizzy with thicker waves; often more resistant to styling.
TYPE 3: Curly
3a	Curly (Loose)	Curly hair that usually presents a definite "S" pattern and tends to combine thickness, fullness, body and/or frizziness.
3b	Curly (Tight)	As 3a but with tighter curling.
TYPE 4: Kinky
4a	Kinky (Soft)	Hair tends to be very fragile, tightly coiled and can feature curly patterning.
4b	Kinky (Wiry)	As 4a but with less visible (or no) curly patterning.

FIA system

This is a method which classifies the hair by curl pattern, hair-strand thickness and overall hair volume.

FIA hair classification

Curliness
Straight
1a	Stick-straight.
1b	Straight but with a slight body wave adding some volume.
1c	Straight with body wave and one or two visible S-waves (e.g. at nape of neck or temples).
Wavy
2a	Loose with stretched S-waves throughout.
2b	Shorter with more distinct S-waves (resembling e.g. braided damp hair).
2c	Distinct S-waves, some spiral curling.
Curly
3a	Big, loose spiral curls.
3b	Bouncy ringlets.
3c	Tight corkscrews.
Very ("Really") curly
4a	Tightly coiled S-curls.
4b	Z-patterned (tightly coiled, sharply angled)
Strands
F	Fine Thin strands that sometimes are almost translucent when held up to the light. Shed strands can be hard to see even against a contrasting background; similar to hair found on many people of Scandinavian descent. You can also try rolling a strand between your thumb and index finger. Fine hair is difficult to feel or it feels like an ultra-fine strand of silk.
M	Medium Strands are neither fine nor coarse; similar to hair found on many Caucasians. You can also try rolling a strand between your thumb and index finger. Medium hair feels like a cotton thread. You can feel it, but it isn't stiff or rough. It is neither fine or coarse.
C	Coarse Thick strands whose shed strands usually are easily identified against most backgrounds; similar to hair found on many people of Asian or Native American descent. You can also try rolling a strand between your thumb and index finger. Coarse hair feels hard and wiry. As you roll it back and forth, you may hear it.
Volume by circumference of full-hair ponytail
i	Thin	circumference less than 2 inches (5 centimetres)
ii	Normal	... from 2 to 4 inches (5 to 10 centimetres)
iii	Thick	... more than 4 inches (10 centimetres)

Function

Many mammals have fur and other hairs that serve different functions. Hair provides thermal regulation and camouflage for many animals; for others it provides signals to other animals such as warnings, mating, or other communicative displays; and for some animals hair provides defensive functions and, rarely, even offensive protection. Hair also has a sensory function, extending the sense of touch beyond the surface of the skin. Guard hairs give warnings that may trigger a recoiling reaction.

Warmth

While humans have developed clothing and other means of keeping warm, the hair found on the head serves as primary sources of heat insulation and cooling (when sweat evaporates from soaked hair) as well as protection from ultra-violet radiation exposure. The function of hair in other locations is debated. Hats and coats are still required while doing outdoor activities in cold weather to prevent frostbite and hypothermia, but the hair on the human body does help to keep the internal temperature regulated. When the body is too cold, the arrector pili muscles found attached to hair follicles stand up, causing the hair in these follicles to do the same. These hairs then form a heat-trapping layer above the epidermis. This process is formally called piloerection, derived from the Latin words 'pilus' ('hair') and 'erectio' ('rising up'), but is more commonly known as 'having goose bumps' in English.<ref name="Why Do Humans Get "goosebumps" When They Are Cold, or under Other Circumstances?: Scientific American.">Bubenik, George A. (September 1, 2003). "Why do humans get "goosebumps" when they are cold, or under other circumstances?". Scientific American. </ref> This is more effective in other mammals whose fur fluffs up to create air pockets between hairs that insulate the body from the cold. The opposite actions occur when the body is too warm; the arrector muscles make the hair lie flat on the skin which allows heat to leave.

Protection

In some mammals, such as hedgehogs and porcupines, the hairs have been modified into hard spines or quills. These are covered with thick plates of keratin and serve as protection against predators.

Touch sense

Displacement and vibration of hair shafts are detected by hair follicle nerve receptors and nerve receptors within the skin. Hairs can sense movements of air as well as touch by physical objects and they provide sensory awareness of the presence of ectoparasites.^[11] Some hairs, such as eyelashes, are especially sensitive to the presence of potentially harmful matter.^{[12][13][14][15]}

Eyebrows and eyelashes

The eyebrows provide moderate protection to the eyes from dirt, sweat and rain. They also play a key role in non-verbal communication by displaying emotions such as sadness, anger, surprise and excitement.^[16] In many other mammals, they contain much longer, whisker-like hairs that act as tactile sensors.

The eyelash grows at the edges of the eyelid and protects the eye from dirt. The eyelash is to humans, camels, horses, ostriches etc., what whiskers are to cats; they are used to sense when dirt, dust, or any other potentially harmful object is too close to the eye.^[17] The eye reflexively closes as a result of this sensation.

Evolution

Hair has its origins in the common ancestor of mammals, the synapsids, about 300 million years ago. It is currently unknown at what stage the synapsids acquired mammalian characteristics such as body hair and mammary glands, as the fossils only rarely provide direct evidence for soft tissues. Skin impression of the belly and lower tail of a pelycosaur, possibly Haptodus shows the basal synapsid stock bore transverse rows of rectangular scutes, similar to those of a modern crocodile.^[18] An exceptionally well-preserved skull of Estemmenosuchus, a therapsid from the Upper Permian, shows smooth, hairless skin with what appears to be glandular depressions.^[19] The oldest known fossil showing unambiguous imprints of hair however, is the Callovian (late middle Jurassic) Castorocauda, an early mammal.^[20] Some modern mammals have a special gland in front of each orbit used to preen the fur, called the harderian gland. Imprints of this structure is found in the skull of the small early mammals like Morganucodon, but not in their cynodont ancestors like Thrinaxodon.^[21]

The hairs of the fur in modern animals are all connected to nerves, and so the fur also serves as a transmitter for sensory input. Fur could have evolved from sensory hair (whiskers). The signals from this sensory apparatus is interpreted in the neocortex, a chapter of the brain that expanded markedly in animals like Morganucodon and Hadrocodium.^[22] The more advanced therapsids could have had a combination of naked skin, whiskers, and scutes. A full pelage likely did not evolve until the therapsid-mammal transition.^[23] The more advanced, smaller therapsids could have had a combination of hair and scutes, a combination still found in some modern mammals, such as rodents and the opossum.^[24]

Human hairlessness

The general hairlessness of humans in comparison to related species may be due to loss of functionality in the pseudogene KRTHAP1 (which helps produce keratin) in the human lineage about 240,000 years ago.^[25] On an individual basis, mutations in the gene HR can lead to complete hair loss, though this is not typical in humans.^[26] Humans may also lose their hair as a result of hormonal imbalance due to drugs or pregnancy.^[27]

In order to comprehend why humans are essentially hairless, it is essential to understand that mammalian body hair is not merely an aesthetic characteristic; it protects the skin from wounds, bites, heat, cold, and UV radiation.^[28] Additionally, it can be used as a communication tool and as a camouflage.^[29] To this end, it can be concluded that benefits stemming from the loss of human body hair must be great enough to outweigh the loss of these protective functions by nakedness.

Humans are the only primate species that have undergone significant hair loss and of the approximately 5000 extant species of mammal, only a handful are effectively hairless. This list includes elephants, rhinoceroses, hippopotamuses, walruses, some species of pigs, whales and other cetaceans, and naked mole rats.^[29] Most mammals have light skin that is covered by fur, and biologists believe that early human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as explained in the Out of Africa hypothesis. Therefore, evidence of the time when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin was needed after the fur was gone.

It was expected that dating the split of the ancestral human louse into two species, the head louse and the pubic louse, would date the loss of body hair in human ancestors. However, it turned out that the human pubic louse does not descend from the ancestral human louse, but from the gorilla louse, diverging 3.3 million years ago. This suggests that humans had lost body hair (but retained head hair) and developed thick pubic hair prior to this date, were living in or close to the forest where gorillas lived, and acquired pubic lice from butchering gorillas or sleeping in their nests.^[30][31] The evolution of the body louse from the head louse, on the other hand, places the date of clothing much later, some 100,000 years ago.^[32][33]

The process of balding, where terminal hair switches to vellus hair, usually occurs at around thirty to forty years of age. In prehistoric times, most individuals did not survive to adulthood, let alone reaching their fourth decade^[34] and therefore balding tends to act as a signal of maturity. In women survival to such an advanced age is usually coupled with a decrease in fertility (see menopause), but in men fertility is retained beyond middle-age. The persistence (but non-ubiquity) of balding in men, coupled with its general absence in women, suggests that there was a selection pressure against balding in women, but variations in hair patterns among men did not prevent their reproductive success leading to stable polymorphisms (perhaps representing different mating strategies); for example some men could have benefitted from baldness by signalling advanced maturity and social status; while other men simulated the appearance of youth and vigor by retaining their hair.

Most species evolved as the climate in Africa changed, to adjust their thermoregulation to the intense UV and sunlight at the equator, mostly by panting. Early hominids likely possessed fur similar to other large apes, but about 2.5 million years ago they developed a greater distribution of sweat glands that enabled them to perspire over most of the body. It is not clear whether the change in body hair appearance occurred before or after the development of sweat glands. Humans have eccrine sweat glands all over their bodies.^[35] Aside from the mammary glands that produce a specialized sweat called milk, most mammals just have apocrine sweat glands on their armpits and loin. The rest of their body is covered in eccrine glands. There is a trend in primates to have increased eccrine sweat glands over the general surface of the body.^[35] It is unclear to what degree other primates sweat in response to heat, however.

The sweat glands in humans could have evolved to spread from the hands and feet as the body hair changed, or the hair change could have occurred to facilitate sweating. Horses and humans are two of the few animals capable of sweating on most of their body, yet horses are larger and still have fully developed fur. In humans, the skin hairs lie flat in hot conditions, as the arrector pili muscles relax, preventing heat from being trapped by a layer of still air between the hairs, and increasing heat loss by convection.

Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.

Several hypotheses explained hairlessness as a thermoregulatory adaptation to hot and dry savanna. The most known thermoregulatory hypothesis in modern paleoanthropology was proposed by Peter Wheeler (1984, 1985). He suggests that a need for decreased body hair originated as a response to climate change that began approximately 3 million years ago.^[36][37] At this time, the earth entered a period of global cooling that had a dehumidifying effect on the main early human habitats in East and Central Africa. Lush, wooded forests gave way to dry, grassland savannah; because of this, early humans were required to travel farther in search of food and water. As early humans diverged from their 'pre-chimpanzee like', rather the ancestral lineage shared with chimpanzees, they also became omnivorous in order to maximize calorie intake, an important distinction in a nutrient-scarce environment. Prey, however, are moving targets, and though early humans changed the traditionally ape-like appearance of the australopithecines and adapted long, strong legs to facilitate sustained running, dense, hairy coats still posed a potentially fatal risk of causing overheating during the chase.^[29] It is posited that thick hair got in the way of the sweat evaporating, so humans evolved a sparser coat of fur. Although hair provides protection against harmful UV radiation, since our hominin ancestors were bipedal, only our heads were exposed to the noonday sun.^[38] Humans kept the hair on our head which reflects harmful UV rays, but our body hair was reduced. The rise in eccrine glands occurred on the genes that determine the fate of epidermal stem cells in human embryonic development.

Another hypothesis for the thick body hair on humans proposes that Fisherian runaway sexual selection played a role (as well as in the selection of long head hair), (see types of hair and vellus hair), as well as a much larger role of testosterone in men. Sexual selection is the only theory thus far that explains the sexual dimorphism seen in the hair patterns of men and women. On average, men have more body hair than women. Males have more terminal hair, especially on the face, chest, abdomen, and back, and females have more vellus hair, which is less visible. The halting of hair development at a juvenile stage, vellus hair, would also be consistent with the neoteny evident in humans, especially in females, and thus they could have occurred at the same time.^[39] This theory, however, has significant holdings in today's cultural norms. There is no evidence that sexual selection would proceed to such a drastic extent over a million years ago when a full, lush coat of hair would most likely indicate health and would therefore be more likely to be selected for, not against, and not all human populations today have sexual dimorphism in body hair.

A further hypothesis is that human hair was reduced in response to ectoparasites.^[40][41] The "ectoparasite" explanation of modern human nakedness is based on the principle that a hairless primate would harbor fewer parasites. When our ancestors adopted group-dwelling social arrangements roughly 1.8 mya, ectoparasite loads increased dramatically. Early humans became the only one of the 193 primate species to have fleas, which can be attributed to the close living arrangements of large groups of individuals. While primate species have communal sleeping arrangements, these groups are always on the move and thus are less likely to harbor ectoparasites. Because of this, selection pressure for early humans would favor decreasing body hair because those with thick coats would have more lethal-disease-carrying ectoparasites and would thereby have lower fitness. However, early humans were able to compensate for the loss of warmth and protection provided by body hair with clothing, and no other mammal lost body hair to reduce parasite loads.

Another view is proposed by James Giles, who attempts to explain hairlessness as evolved from the relationship between mother and child, and as a consequence of bipedalism. Giles also connects romantic love to hairlessness.^[42]

Another hypothesis is that humans use of fire caused, or initiated the reduction in human hair.^[43]

Evolutionary variation

Evolutionary biologists suggest that the genus Homo arose in East Africa approximately 2.5 million years ago.^[44] They devised new hunting techniques.^[44] The higher protein diet led to the evolution of larger body and brain sizes.^[44] Jablonski^[44] postulates that increasing body size, in conjunction with intensified hunting during the day at the equator, gave rise to a greater need to rapidly expel heat. As a result, humans evolved the ability to sweat: a process which was facilitated by the loss of body hair.^[44] A major problem with this theory, however, is that it does not explain why males are larger, hairier, and were more active in hunting than females. The female-male size differential among other closely associated primates is much greater than among humans, however, so it might have been reduced during evolution.

Other primates have sweat gland in their armpits that function as those of humans, and thus it is probable that human sweat glands evolved from a similar distribution, spreading to more areas of the body, rather than occurring through evolution of a new trait. It is not known whether the increased distribution of sweat glands occurred before, during, or after, the change in body hair, or even whether the two are related developments. Horses also sweat, and they are larger, hairier, and expend more energy running than human males, so there may not be any connection between the ability to sweat and the apparent hairlessness of humans.

Another factor in human evolution that also occurred in the prehistoric past was a preferential selection for neoteny, particularly in females. The idea that adult humans exhibit certain neotenous (juvenile) features, not evinced in the great apes, is about a century old. Louis Bolk made a long list of such traits,^[45] and Stephen Jay Gould published a short list in Ontogeny and Phylogeny.^[46] In addition, paedomorphic characteristics in women are widely acknowledged as desirable by men. For instance, vellus hair is a juvenile characteristic. However, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation, women do not, leaving their vellus hair visible.

Texture

Curly hair

Jablonski^[44] asserts head hair was evolutionarily advantageous for pre-humans to retain because it protected the scalp as they walked upright in the intense African (equatorial) UV light. While some might argue that, by this logic, humans should also express hairy shoulders because these body parts would putatively be exposed to similar conditions, the protection of the head, the seat of the brain that enabled humanity to become one of the most successful species on the planet (and which also is very vulnerable at birth) was arguably a more urgent issue (axillary hair in the underarms and groin were also retained as signs of sexual maturity). Sometime during the gradual process by which Homo erectus began a transition from furry skin to the naked skin expressed by Homo sapiens, hair texture putatively gradually changed from straight hair (the condition of most mammals, including humanity's closest cousins—chimpanzees) to Afro-textured hair or 'kinky' (i.e. tightly coiled). This argument assumes that curly hair better impedes the passage of UV light into the body relative to straight hair (thus curly or coiled hair would be particularly advantageous for light-skinned hominids living at the equator). It is substantiated by Iyengar's (1998) findings that UV light can enter into straight human hair roots (and thus into the body through the skin) via the hair shaft. Specifically, the results of that study suggest that this phenomenon resembles the passage of light through fiber optic tubes (which do not function as effectively when kinked or sharply curved or coiled). In this sense, when hominids (i.e. Homo Erectus) were gradually losing their straight body hair and thereby exposing the initially pale skin underneath their fur to the sun, straight hair would have been an adaptive liability. By inverse logic, later, as humans traveled farther from Africa and/or the equator, straight hair may have (initially) evolved to aid the entry of UV light into the body during the transition from dark, UV-protected skin to paler skin.

Some conversely believe that tightly coiled hair that grows into a typical Afro-like formation would have greatly reduced the ability of the head and brain to cool because although African people's hair is much less dense than its European counterpart's, in the intense sun the effective 'woolly hat' that such hair produced would have been a disadvantage. However, such anthropologists as Nina Jablonski oppositely argue about this hair texture. Specifically, Jablonski's assertions^[44] suggest that the adjective "woolly" in reference to Afro-hair is a misnomer in connoting the high heat insulation derivable from the true wool of sheep. Instead, the relatively sparse density of Afro-hair, combined with its springy coils actually results in an airy, almost sponge-like structure that in turn, Jablonski argues,^[44] more likely facilitates an increase in the circulation of cool air onto the scalp. Further, wet Afro-hair does not stick to the neck and scalp unless totally drenched and instead tends to retain its basic springy puffiness because it less easily responds to moisture and sweat than straight hair does. In this sense, the trait may enhance comfort levels in intense equatorial climates more than straight hair (which, on the other hand, tends to naturally fall over the ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair).

Further, some interpret the ideas of Charles Darwin as suggesting that some traits, such as hair texture, were so arbitrary to human survival that the role natural selection played was trivial. Hence, they argue in favor of his suggestion that sexual selection may be responsible for such traits. However, inclinations towards deeming hair texture "adaptively trivial" may root in certain cultural value judgments more than objective logic. In this sense the possibility that hair texture may have played an adaptively significant role cannot be completely eliminated from consideration. In fact, while the sexual selection hypothesis cannot be ruled out, the asymmetrical distribution of this trait vouches for environmental influence. Specifically, if hair texture were simply the result of adaptively arbitrary human aesthetic preferences, one would expect that the global distribution of the various hair textures would be fairly random. Instead, the distribution of Afro-hair is strongly skewed toward the equator. Further, it is notable that the most pervasive expression of this hair texture can be found in sub-Saharan Africa; a region of the world that abundant genetic and paleo-anthropological evidence suggests, was the relatively recent (~200,000-year-old) point of origin for modern humanity. In fact, although genetic findings (Tishkoff, 2009) suggest that sub-Saharan Africans are the most genetically diverse continental group on Earth, Afro-textured hair approaches ubiquity in this region. This points to a strong, long-term selective pressure that, in stark contrast to most other regions of the genomes of sub-Saharan groups, left little room for genetic variation at the determining loci. Such a pattern, again, does not seem to support human sexual aesthetics as being the sole or primary cause of this distribution.

Straight hair

According to the recent single origin hypothesis, anatomically modern humans arose in East Africa approximately 200,000 years ago. Then, ~150,000 years later (i.e. around 50,000 years ago), sub-groups of this population began to expand our species' range to regions outside of, and (later) within, this continent (Tishkoff, 1996). For those members of this group who migrated far north (i.e. to northern Eurasia, etc.), the UV light of these regions was too weak to penetrate the highly pigmented skin of the initially (relatively) dark-skinned migrants so as to provide enough vitamin D for healthy bone development.^[44] Malformed bones in the pelvic area were especially deadly for women because they interfered with the successful delivery of babies, leading to the death of both the mother and the infant during labor. Hence, those with less pigmented skin survived and had children at higher rates because their skin allowed more UV light for the production of vitamin D.^[44] Thus, the skin of those in the group that left the African continent and went far north gradually developed adaptations for relatively greater translucence compared to equatorial hues. This enabled the passage of more UV light into the body at high latitudes, facilitating the natural human body-process of manufacturing vitamin D (which is essential for bone development) in response to said light.^[44]

In this sense, the evidence with regard to the evolution of straight hair texture seems to support Jablonski's suggestions^[44] that the need for vitamin D triggered the transition from dark to pale, translucent skin among modern humans. Specifically, the distribution of this trait suggests that this need may have (initially) grown so intense at certain (early) points that those among said (initially more deeply pigmented skinned) Northern-migrants with mutations for straighter hair survived and had children at (somewhat) higher rates. This early change in texture was likely subsequently followed by the accumulation of adaptively advantageous genetic changes that led to the above-mentioned skin-translucence. This argument is made based on the principle that straight fibers better facilitate the passage of UV light into the body relative to curly hair. It is substantiated by Iyengar's (1998) findings that UV light can pass through straight human hair roots in a manner similar to the way that light passes through fiber optic tubes (Iyengar, 1998).

Nonetheless, some argue against this stance because straighter hair ends tend to point downward while fiber optics requires that light be transmitted at a high angle to the normal of the inner reflective surface. In light of this, they suggest that only light reflected from the ground could successfully enter the hair follicle and be transmitted down the shaft. Even this process, they argue, is hindered by the curvature at the base of the hair. Therefore, coupled with the amount of skin covered by long head hair, these factors seem to militate against the adaptive usefulness of straight hair at northern latitudes. They further argue that UV light also is poorly reflected from soil and dull surfaces. These ideas can be countered by the fact that during the winter, the time of year in which UV light is most scarce at northern latitudes, the ground is often covered with white snow. Given that white is the most effective color in terms of facilitating the reflection of ground light, the hypothesis that straight hair could have been adaptively favorable, cannot be fully discounted in this regard. In addition, as mentioned in the previous section, straight hair also may have contributed to enhanced comfort levels in the north. This is evident in the extent to which, relative to curly hair, it tends to provide a layer of protection for ears and necks against the cold.

The latter hypothesis seems the more plausible evolution determinant as the surface area of the head is minute compared to the remainder of the body, thus the energy required in producing long hair for the express purpose of "optical" amplification of UV light reflected from the snow seems counterproductive (however, it's very likely that the trait was sustained due to a nuanced combination of multiple influences, given that human hunting-skills and ingenuity were such by 50,000 years ago that said benefits in terms of 'comfort' could have alternatively been derived from constructing head and ear warmers of fur from prey, etc.). Scientists point to the fact that straight hair found in many ethnic groups is denser as well and has a greater ability to "show" as it does not coil, hence providing more warmth as the likely deterministic factor for the evolution of straight long hair. Some scientists argue that since the head and appendages are the greatest areas for heat loss from the body, the ability to grow long hair on the crown of the head as well as the face provides a distinct advantage in a cold climate. Since the main sensory organs are anatomically located on the head, long hair provides the necessary warmth and protection in a cold climate that allows the use of these organs by exposing them to the elements to "sense", in for example a hunt, yet still providing necessary warmth and protection to sustain prolonged exposure. It may be argued, therefore, that the ability to grow long, straight, densely packed hair provides a distinct evolutionary advantage in cold climate; however, it would be a distinct disadvantage in a hot climate, when compared to loosely packed, spongy, closely cropped hair.

The EDAR locus

A group of studies have recently shown that genetic patterns at the EDAR locus, a region of the modern human genome that contributes to hair texture variation among most individuals of East Asian descent, support the hypothesis that (East Asian) straight hair likely developed in this branch of the modern human lineage subsequent to the original expression of tightly coiled natural afro-hair.^[47][48][49] Specifically, the relevant findings indicate that the EDAR mutation coding for the predominant East Asian 'coarse' or thick, straight hair texture arose within the past ~65,000 years, which is a time frame that covers from the earliest of the 'Out of Africa' migrations up to now.

Removal practices

Depilation is the removal of hair from the surface of the skin. This can be achieved through methods such as shaving. Epilation is the removal of the entire hair strand, including the part of the hair that has not yet left the follicle. A popular way to epilate hair is through waxing.

Shaving

Shaving is accomplished with bladed instruments, such as razors. The blade is brought close to the skin and stroked over the hair in the desired area to cut the terminal hairs and leave the skin feeling smooth. Depending upon the rate of growth, one can begin to feel the hair growing back within hours of shaving. This is especially evident in men who develop a five o'clock shadow after having shaved their faces. This new growth is called stubble. Stubble typically appears to grow back thicker because the shaved hairs are blunted instead of tapered off at the end, although the hair never actually grows back thicker.

Waxing

Waxing involves using a sticky wax and strip of paper or cloth to pull hair from the root. Waxing is the ideal hair removal technique to keep an area hair-free for long periods of time. It can take three to six weeks for waxed hair to begin to resurface again. Hair in areas that have previously been waxed also is known to grow back finer and thinner, especially compared to hair that has been shaved with a razor.

Laser removal

Laser hair removal is a cosmetic method where a small laser beam pulses selective heat on dark target matter in the area that causes hair growth without harming the skin tissue. This process is repeated several times over the course of many months to a couple of years with hair regrowing less frequently until it finally stops; this is used as a more permanent solution to waxing or shaving. Laser removal is practiced in many clinics along with many at-home products.

Cutting and trimming

Because the hair on the head is normally longer than other types of body hair, it is cut with scissors or clippers. People with longer hair will most often use scissors to cut their hair, whereas shorter hair is maintained using a trimmer. Depending on the desired length and overall health of the hair, periods without cutting or trimming the hair can vary.

Many people confuse haircuts and trims. A haircut is usually performed in order to change one's hairstyle, while a trim helps to keep away split ends and keep the hair well-groomed. Cutting hair tends to take off more hair than trimming hair does. When hair is trimmed, only the first few centimeters need to be removed, whereas haircuts can sometimes result in the loss of many inches of hair.

Cut hair may be used in wigs. Global imports of hair in 2010 was worth $US 1.24 billion.^[50]

Social role

Hair has great social significance for human beings. It can grow on most external areas of the human body, except on the palms of the hands and the soles of the feet (among other areas). Hair is most noticeable on most people in a small number of areas, which are also the ones that are most commonly trimmed, plucked, or shaved. These include the face, ears, head, eyebrows, legs, and armpits, as well as the pubic region. The highly visible differences between male and female body and facial hair are a notable secondary sex characteristic.

Indication of status

Healthy hair indicates health and youth (important in evolutionary biology). Hair color and texture can be a sign of ethnic ancestry. Facial hair is a sign of puberty in men. White hair is a sign of age or genetics, which may be concealed with hair dye (not easily for some), although many prefer to assume it (especially if it is a poliosis characteristic of the person since childhood). Male pattern baldness is a sign of age, which may be concealed with a toupee, hats, or religious and cultural adornments. Although drugs and medical procedures exist for the treatment of baldness, many balding men simply shave their heads. In ancient China, the queue was a male hairstyle worn by the Manchus from central Manchuria and the Han Chinese during the Qing dynasty; hair on the front of the head was shaved off above the temples every ten days, mimicking male-pattern baldness, and the rest of the hair braided into a long pigtail. Hair whorls have been discovered to be associated with brain development.

Hairstyle may be an indicator of group membership. During the English Civil War, the followers of Oliver Cromwell decided to crop their hair close to their head, as an act of defiance to the curls and ringlets of the king's men.^[51] This led to the Parliamentary faction being nicknamed Roundheads. Having bobbed hair was popular among the flappers in the 1920s as a sign of rebellion against traditional roles for women. Female art students known as the "cropheads" also adopted the style, notably at the Slade School in London, England. Regional variations in hirsutism cause practices regarding hair on the arms and legs to differ. Some religious groups may follow certain rules regarding hair as part of religious observance. The rules often differ for men and women.

Many subcultures have hairstyles which may indicate an unofficial membership. Many hippies, metalheads and Indian sadhus have long hair, as well many older indie kids. Many punks wear a hairstyle known as a mohawk or other spiked and dyed hairstyles; skinheads have short-cropped or completely shaved heads. Long stylized bangs were very common for emos, scene kids and younger indie kids in the 2000s and early 2010s, among people of both genders.

Heads were shaved in concentration camps, and head-shaving has been used as punishment, especially for women with long hair. The shaven head is common in military haircuts, while Western monks are known for the tonsure. By contrast, among some Indian holy men, the hair is worn extremely long.

In the time of Confucius (5th century BCE), the Chinese grew out their hair and often tied it, as a symbol of filial piety.

Regular hairdressing in some cultures is considered a sign of wealth or status. The dreadlocks of the Rastafari movement were despised early in the movement's history. In some cultures, having one's hair cut can symbolize a liberation from one's past, usually after a trying time in one's life. Cutting the hair also may be a sign of mourning.

Tightly coiled hair in its natural state may be worn in an Afro. This hairstyle was once worn among African Americans as a symbol of racial pride. Given that the coiled texture is the natural state of some African Americans' hair, or perceived as being more "African", this simple style is now often seen as a sign of self-acceptance and an affirmation that the beauty norms of the (eurocentric) dominant culture are not absolute. It is important to note that African Americans as a whole have a variety of hair textures, as they are not an ethnically homogeneous group, but an ad-hoc of different racial admixtures.

The film Easy Rider (1969) includes the assumption that the two main characters could have their long hairs forcibly shaved with a rusty razor when jailed, symbolizing the intolerance of some conservative groups toward members of the counterculture. At the conclusion of the Oz obscenity trials in the UK in 1971, the defendants had their heads shaved by the police, causing public outcry. During the appeal trial, they appeared in the dock wearing wigs.^[52] A case where a 14-year-old student was expelled from school in Brazil in the mid-2000s, allegedly because of his fauxhawk haircut, sparked national debate and legal action resulting in compensation.^[53][54]

Religious practices

Women's hair may be hidden using headscarves, a common part of the hijab in Islam and a symbol of modesty required for certain religious rituals in Eastern Orthodoxy. Russian Orthodox Church requires all married women to wear headscarves inside the church; this tradition is often extended to all women, regardless of marital status. Orthodox Judaism also commands the use of scarves and other head coverings for married women for modesty reasons. Certain Hindu sects also wear head scarves for religious reasons. Sikhs have an obligation not to cut hair (a Sikh cutting hair becomes 'apostate' which means fallen from religion)^[55] and men keep it tied in a bun on the head, which is then covered appropriately using a turban. Multiple religions, both ancient and contemporary, require or advise one to allow their hair to become dreadlocks, though people also wear them for fashion. For men, Islam, Orthodox Judaism, Orthodox Christianity, Roman Catholicism, and other religious groups have at various times recommended or required the covering of the head and sections of the hair of men, and some have dictates relating to the cutting of men's facial and head hair. Some Christian sects throughout history and up to modern times have also religiously proscribed the cutting of women's hair. For some Sunni madhabs, the donning of a Kufi or Topi (cap) is a form of sunnah.^[56]

See also

References

Notes

Bibliography

• Iyengar, B. (1998). "The hair follicle is a specialized UV receptor in human skin?". Bio Signals Recep 7 (3): 188–194. doi:10.1159/000014544. 
• Jablonski, N. G. (2006). Skin: a natural history. Berkeley, CA: University of California Press. 
• Rogers, Alan R.; Iltis, David; Wooding, Stephen (2004). "Genetic variation at the MC1R locus and the time since loss of human body hair". Current Anthropology 45 (1): 105–108. doi:10.1086/381006. 
• Tishkoff, S. A.; Dietzsch, E.; Speed, W.; Pakstis, A. J.; Kidd, J. R.; Cheung, K.; Bonne-Tamir, B.; Santachiara-Benerecetti, A. S.; et al. (1996). "Global patterns of linkage disequilibrium at the CD4 locus and modern human origins". Science 271 (5254): 1380–1387. Bibcode:1996Sci...271.1380T. doi:10.1126/science.271.5254.1380. PMID 8596909. 

External links

Wikimedia Commons has media related to Hair.

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Look up hair in Wiktionary, the free dictionary.

• How to measure the diameter of your own hair using a laser pointer
• Instant insight outlining the chemistry of hair from the Royal Society of Chemistry
• The dictionary definition of hairsbreadth at Wiktionary

Human hair List of hairstyles / facial hairstyles Classification by type Lanugo Androgenic Terminal Vellus by location Head Nose Ear Eyebrow Eyelash Underarm Chest Abdominal Pubic Leg Hairstyles Afro Afro-textured hair Asymmetric cut Bangs Beehive Big hair Blowout Blunt hair Bob cut Bouffant Bowl cut Braid or Plait Brush cut Bun (odango) Bunches Burr Businessman cut Butch cut Buzz cut Caesar cut Chignon Chonmage Chupryna Comb over Conk Cornrows Crew cut Crochet braids Cropped hair Croydon facelift Curtained hair Devilock Dice Bob Dido flip Digital perm Dreadlocks Duck's ass Emo Extensions Fade Fauxhawk Feathered hair Finger wave Flattop Flipped hair Fontange French braid French twist Full crown Half crown Harvard clip High and tight Highlights Hime cut Historical Christian hairstyles Hi-top fade Induction cut Ivy League Jewfro Jheri curl Kiss curl Layered hair Liberty spikes Long hair Marcelling Mod cut Mohawk Mop-top Mullet 1950s 1980s Pageboy Part Payot Perm Pigtail Pixie cut Pompadour Ponytail Punch perm Princeton Professional cut Queue Quiff Rattail Razor cut Regular haircut Regular taper cut Ringlets Shag Shape-Up Short back and sides Short brush cut Short hair Spiky hair Standard haircut Surfer hair Taper cut Tonsure Updo Undercut Waves Weave Wings Facial hair Beard Chin curtain Chinstrap Designer stubble Goatee Moustache Fu Manchu handlebar horseshoe pencil toothbrush walrus Neckbeard Shenandoah Sideburns Soul patch Van Dyke Hair loss Cosmetic Removal waxing threading plucking chemical electric laser IPL Shaving head leg Razor safety straight other Alopecia areata totalis universalis Male-pattern hair loss Glabrousness Hypertrichosis Management Trichophilia Trichotillomania Pogonophobia Related topics Backcombing Bearded lady Barber (pole) Hairdresser Hairstyle Hair color / coloring Hair crimping Hair fetishism (pubic) Hair growth Hair length Hair straightening

Skin and related structures Skin Epidermis Stratum corneum Stratum lucidum Stratum granulosum Stratum spinosum Stratum basale Basement membrane Basal keratinocyte Lamina lucida Lamina densa Dermis Papillary Dermal papillae Reticular Subcutaneous tissue Panniculus/Pannus (Panniculus adiposus Panniculus carnosus) Membranous layer Loose connective tissue Superficial fascia Adnexa Skin glands Sweat glands: Apocrine sweat gland Eccrine sweat gland Sebaceous Hair Root sheath Outer root sheath Inner root sheath Henle's layer Huxley's layer Hair shaft Cuticle Cortex Medulla Bulb with matrix cells Hair follicle Muscle Arrector pili muscle Pilosebaceous unit Hair sebaceous gland Nail Nail matrix Nail plate Lunula Eponychium Paronychium Hyponychium

Authority control LCCN: sh85058305 GND: 4128008-8 BNF: cb119311767 (data) NDL: 00575445 BNE: XX525867