Hair


Hair
Hair
Gray945.png
Cross section of a hair
Latin unguis
Code TH H3.12.00.3.02001

Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class.[1]

The human body, apart from its 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.

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

  1. the part beneath the skin, called the hair follicle or when pulled from the skin, called 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; and in some cases,
  3. the medulla, a disorganized and open area at the fiber's center.[3]

Contents

Description

Strand of human hair at 200x magnification

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][6] The highly structured 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. Asian hair typically has a round fiber and is quite straight. Oval and irregularly-shaped fibers are generally more wavy or even curly.[5] 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 17 to 180 micrometers (0.00067 to 0.0071 in).[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".[5] The base of the root is called the bulb, which 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 erector pili muscles, which are responsible for causing hairs to stand up. In humans, with little body hair, the effect results in goose bumps.[9]

Natural Color

All natural hair colors are the result of two types of hair pigment. 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 dark-blond, brown, and black hair, while pheomelanin is dominant in red hair.[5] Blond hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin production decreases or stops.

Human hair growth

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

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

Polar bears use their fur for warmth and while their skin is black, their transparent fur appears white and provides camouflage while hunting and serves as protection by hiding cubs in the snow.

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' ('stiffening'), but is more commonly known as 'having goose bumps' in humans.[10] 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 lay flat on the skin which allows heat to leave.

Protection

Human hair may not compete with the painful spines of the porcupine, but much of the hair on the human body is suited to protect it. This natural armor cannot directly protect humans from potential predators, but it does help to keep the sense organs, such as the eyes, working properly.

Touch sense

Movements of hair shafts are detected by nerve receptors within the skin and by hair follicle receptors through displacement and vibration of hair shafts. Hairs can sense movements of air as well as touch by physical objects and are especially sensitive to the presence of insects. Some hairs, such as eyelashes, are especially sensitive to the presence of potentially harmful matter.[11][12][13]

Eyebrows and eyelashes

Eyelashes and eyebrows help to protect the eyes from dust, dirt, and sweat

The eyebrows provide some protection to the eyes from dirt, sweat, and rain. More importantly, they play a key role in non-verbal communication.[14] 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.[15] The eye reflexively closes as a result of this sensation.

Evolution

A 2008 study by the Medical University of Vienna traced the origins of hair to the common ancestor of mammals, birds, and lizards that lived 310 million years ago. The study found chickens, lizards, and humans all possessed a similar set of genes that was involved in the production of alpha keratin. In chickens and lizards, the α-keratin produced was found in their claws, but in mammals it was used to produce hair. The scientists involved continued searching for the mechanisms that allowed mammals to use the keratins of animal claws to produce hair.[16][17]

Human "hairlessness"

Humans seem hairless not because of a lack of follicles but because of the predominance of vellus fibers which are thinner, shorter, and more transparent than terminal fibers. The density of human hair follicles on the skin is about the average for an animal of equivalent size.[18] 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, cold temperatures, and UV radiation [19]. Additionally, it can be used as a communication tool and as a camouflage [20]. 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, pigs, whales and naked mole rats [21]. 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 would be experienced in Africa. Therefore, evidence of 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.

Dr. Alan R. Rogers, an evolutionary geneticist at the University of Utah, used mutations in the MC1R gene to estimate when human skin darkened. He said early humans may have gone through several genetic "clean sweeps" with light-skinned individuals dying off and dark-skinned individuals surviving. He estimates the last of these clean sweeps took place 1.2 million years ago.[22] Therefore, apparent bareness in humans, likely has existed at least since that time. Later when humans began to live in more northerly latitudes, however, light skin became an advantage for the production of vitamin D and the recessive gene began to become more prevalent in those populations.

Balding, where terminal hair switches to vellus hair, usually occurs at around thirty to forty years of age. In prehistoric times, most individuals were not as likely to live past 30, with few reaching their fourth decade,[23] and thus it wasn't likely that this trait was subject to selection.

The soft, fine hair found on many nonhuman mammals is typically called fur.

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.[24] 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.[24] 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 erector 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.

The thermoregulatory hypothesis (first proposed by Darwin in 1871 and later developed by Dr. Peter Wheeler, 1984, 1985) suggests that a need for decreased body hair originated as a response to climate change that began approximately 3 million years ago [25] [26]. 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 chimpanzee-lineage, 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 [27]. 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.[28] 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. Opponents of the thermoregulatory hypothesis would say that losing hair added an extreme weakness to cold, but, seeing as how humans figured out cutlery around 2.6 million years ago, our ancestors easily could have found clothing within the 1.4 million years between cutting up their kills and losing their hair.[clarification needed]

The aquatic ape hypothesis posits that sparsity of hair is an adaptation to a semi-aquatic environment, but the theory has yet to gain support among scientists.[29]

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.[30]. 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.

A final hypothesis is that human hair was reduced in response to ectoparasites.[31][32] 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 flees, 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. Additional evidence for this hypothesis comes from studies that show that most ectoparasites on modern humans are confined to the head and pubic regions. Early humans were able to compensate for the loss of warmth and protection provided by body hair with subcutaneous fat and clothing. Hairlessness could maintained in modern society by sexual selection for hairless partners in addition to natural selection to reduce parasite loads.

Evolutionary variation

Evolutionary biologists suggest that the genus Homo arose in East Africa approximately 2.5 million years ago.[33] They devised new hunting techniques.[33] The higher protein diet led to the evolution of larger body and brain sizes.[33] Jablonski[33] 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.[33] 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,[34] and Stephen Jay Gould published a short list in Ontogeny and Phylogeny.[35] 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

A 19th century Scottish woman with curly hair - Mrs Duncan Campbell of Inverneill.

Jablonski[33] asserts that it was evolutionarily advantageous for pre-humans to retain the hair on their heads in order to protect 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 given that 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). During the gradual process by which Homo erectus made a transition from furry to naked skin, their hair texture putatively changed gradually from straight[citation needed] (the condition of most mammals, including humanity's closest cousins—chimpanzees), to Afro-textured hair or 'kinky' (i.e. tightly coiled). This argument is based on the principle that curly hair impedes the passage of UV light into the body relative to straight hair (thus curly and/or coiled hair would be particularly advantageous for pale-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/skin) via the hair shaft. Specifically, the results of this study suggest that this phenomenon occurs in a manner similar to the way that light passes through fiber optic tubes (which do not function as effectively when kinked/sharply-curved/coiled). In this sense, during the period in which 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. Hence, tightly coiled or 'kinky' Afro-hair may have (initially) evolved to prevent the entry of UV light into the body during the transition toward dark, UV-protected skin.

By contrast, some intuit 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. They reason that although hair density in African peoples is much less than their European counterparts, in the intense sun the effective 'woolly hat' produced would have been a disadvantage. However, anthropologists such as Nina Jablonski make the opposite argument with regards to this hair texture. Specifically, Jablonski's assertions [33] suggest that the adjective "woolly" in reference to Afro-hair is a misnomer to the extent that it connotes 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 effect. This, in turn, Jablonski argues,[33] more likely facilitates an increase in the circulation of cool air onto the scalp. Further, Afro-hair does not respond as easily to moisture/sweat as straight hair. Thus it does not stick to the neck and scalp when wet. Rather, unless totally drenched, it tends to retain its basic springy puffiness. In this sense, the trait may enhance comfort levels in intense equatorial climates compared to 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.[36] 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.[dubious ] 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.[citation needed] 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/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.[33] 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.[33] 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.[33]

Man with straight hair

In this sense, the evidence with regard to the evolution of straight hair texture seems to support Jablonski's suggestions [33] 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 fur head/ear warmers 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.[37][38][39] 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

Even though growing hair is an inevitable part of being human, it is becoming a concern to many individuals, especially young adults who believe that it is unsightly and should be removed. Hair removal is almost always motivated by cosmetic reasons.

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 it.

Shaving

Many razors have multiple blades purportedly to ensure a close shave. While shaving initially will leave skin feeling smooth and hair free, new hair growth will appear a few hours after hair removal.

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.

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 five to nine 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.

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 will confuse what a haircut is versus what a trim is. 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.

Social role

Portrait of a Woman, Alessandro Allori (1535–1607; Uffizi Gallery): a plucked hairline gives a fashionably "noble brow"

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 colour 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. 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. 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.[40] 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. Many punks wear a hairstyle known as a Mohawk or other spiked and dyed hairstyles; skinheads have short-cropped or completely shaved heads. Mullet hairstyles stereotypically have been associated with rednecks.[citation needed]

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.[citation needed]

In the 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 dominant (northern/European) 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 ethnicities, one component being African. It is also important to note that Afro-textured hair is not universal in Africa as well, and hair texture varies among different African ethnicities.[citation needed]

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, the defendants had their heads shaved by the police, causing public outcry. During the appeal trial, they appeared in the dock wearing wigs.[citation needed]

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 Orthodox Christianity. 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 women for modesty reasons. Certain Hindu sects also wear head scarves for religious reasons. Sikhs generally keep their hair uncut 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.

See also

References

Notes

  1. ^ definition askoxford.com
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  3. ^ Feughelman, Max Mechanical properties and structure of alpha-keratin fibres: wool, human hair and related fibres, Sydney, UNSW Press (1996) ISBN 0868403598
  4. ^ a b Hair Structure and Hair Life Cycle
  5. ^ a b c d "How Does Hair Grow?" Web. 09 Feb. 2010
  6. ^ Topic 2: The Layers of the Hair
  7. ^ Ley, Brian (1999). "Diameter of a Human Hair". Archived from the original on 2010-06-28. http://www.webcitation.org/5qpTnR6HP. Retrieved 2010-06-28. 
  8. ^ "Inside the hair". Pantogar. http://www.pantogar.com/en/inside_the_hair.php. Retrieved 9 Feb. 2010. 
  9. ^ Hair follicle Wikipedia. 23 Feb. 2010
  10. ^ Bubenik, George A. (September 1, 2003). "Why do humans get "goosebumps" when they are cold, or under other circumstances?". Scientific American. http://www.scientificamerican.com/article.cfm?id=why-do-humans-get-goosebu. 
  11. ^ Neuroscience for Kids - Receptors. Faculty.washington.edu. Retrieved on 2010-12-14.
  12. ^ hair biology - functions of the hair fiber and hair follicle. Keratin.com. Retrieved on 2010-12-14.
  13. ^ Sabah, NH (1974). "Controlled stimulation of hair follicle receptors.". Journal of applied physiology 36 (2): 256–7. PMID 4811387. http://jap.physiology.org/cgi/reprint/36/2/256.pdf. 
  14. ^ Sound Medicine: Why Do We Have Eyebrows?
  15. ^ Thumbprints – The Human Eyelash
  16. ^ Viegas, Jennifer (2008-11-11). "Human hair linked to dinosaur claws: Origins of hair go back 310 million years to common ancestor". MSNBC. http://www.msnbc.msn.com/id/27663717/. 
  17. ^ Eckhard, L. et al. (2008). "Identification of reptilian genes encoding hair keratin-like proteins suggests a new scenario for the evolutionary origin of hair". Proceedings of the National Academy of Sciences of the United States of America 105 (47): 18419–23. doi:10.1073/pnas.0805154105. PMC 2587626. PMID 19001262. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2587626. 
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  23. ^ Steve Jones, Robert Martin & David Pilbeam, ed (1994). The Cambridge Encyclopedia of Human Evolution. Cambridge: Cambridge University Press. p. 242. ISBN 0-521-32370-3.  Also ISBN 0-521-46786-1 (paperback)
  24. ^ a b Montagna, William (1972). "The Skin of Nonhuman Primates". Integrative and Comparative Biology 12: 109. doi:10.1093/icb/12.1.109. 
  25. ^ Wheeler, P (1984). "The evolution of bipedality and loss of functional body hair in hominids". Journal of Human Evolution 13: 91. doi:10.1016/S0047-2484(84)80079-2. 
  26. ^ Darwin, Charles (1871). The Descent of Man, and Selection in Relation to Sex. London: John Murray. 
  27. ^ Jablonski, N. G.; G. Chaplin (2010). "Human skin pigmentation as an adaptation to UV radiation". Proceedings of the National Academy of Sciences of the United States of America 107: 8962–8968. 
  28. ^ Wheeler, P.E. (1985). "The loss of functional body hair in man: the influence of thermal environment, body form and bipedality". Journal of Human Evolution 14: 23. doi:10.1016/S0047-2484(85)80091-9. 
  29. ^ Meier, R (2003). The complete idiot's guide to human prehistory. Alpha Books. pp. 57–59. ISBN 0028644212. http://books.google.com/?id=r_Wit4KYSmMC&pg=PA57. 
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  31. ^ Pagel, Mark and Bodmer, Walter (2003). "A naked ape would have fewer parasites". Proceedings of the Royal Society B: Biological Sciences 270: S117–S119. doi:10.1098/rsbl.2003.0041. PMC 1698033. PMID 12952654. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1698033. 
  32. ^ Rantala, MJ (1999). "Human nakedness: adaptation against ectoparasites?". International journal for parasitology 29 (12): 1987–9. PMID 10961855. http://users.utu.fi/mjranta/reprints/1.%20Rantala1999.pdf. 
  33. ^ a b c d e f g h i j k l Jablonski, N.G. (2006). Skin: a natural history. Berkley, CA: University of California Press
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  35. ^ short-list of 25 characters reprinted in Gould, Stephen Jay (1977). Ontogeny and phylogeny. Harvard. p. 357
  36. ^ Descent of Man
  37. ^ Fujimoto, A; Kimura, R; Ohashi, J; Omi, K; Yuliwulandari, R; Batubara, L; Mustofa, MS; Samakkarn, U et al. (2008). "A scan for genetic determinants of human hair morphology: EDAR is associated with Asian hair thickness.". Human molecular genetics 17 (6): 835–43. doi:10.1093/hmg/ddm355. PMID 18065779. http://hmg.oxfordjournals.org/content/17/6/835.full.pdf. 
  38. ^ Fujimoto, A; Ohashi, J; Nishida, N; Miyagawa, T; Morishita, Y; Tsunoda, T; Kimura, R; Tokunaga, K (2008). "A replication study confirmed the EDAR gene to be a major contributor to population differentiation regarding head hair thickness in Asia.". Human genetics 124 (2): 179–85. doi:10.1007/s00439-008-0537-1. PMID 18704500. http://www.tulips.tsukuba.ac.jp/dspace/bitstream/2241/103672/1/HG_124-2.pdf. 
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  40. ^ Olmert, Michael (1996). Milton's Teeth and Ovid's Umbrella: Curiouser & Curiouser Adventures in History, p.53. Simon & Schuster, New York. ISBN 0-684-80164-7

Bibliography

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  • Jablonski, N.G. (2006). Skin: a natural history. Berkley, CA: University of Califiornia 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.
  • Tishkoff, S.A. (1996). Global patterns of linkage disequilibrium at the CD4 locus and modern human origins. Science. 271(5254), 1380–1387.

External links


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  • haïr — [ air ] v. tr. <conjug. : 10> • 1080; frq. °hatjan REM. Au Canada, [ ajir ] avec h muet : j hais. 1 ♦ Avoir (qqn) en haine. ⇒ détester, exécrer, honnir. « Ils nous haïssent de toute la haine du domestique pour le maître, du petit pour le… …   Encyclopédie Universelle

  • hair — W1S1 [heə US her] n [: Old English; Origin: hAr] 1.) [U] the mass of things like fine threads that grows on your head ▪ She put on her lipstick and brushed her hair . ▪ I must get my hair cut it s getting very long. ▪ You ve had your hair done… …   Dictionary of contemporary English

  • Hair — (h[^a]r), n. [OE. her, heer, h[ae]r, AS. h[=ae]r; akin to OFries. h[=e]r, D. & G. haar, OHG. & Icel. h[=a]r, Dan. haar, Sw. h[*a]r; cf. Lith. kasa.] 1. The collection or mass of filaments growing from the skin of an animal, and forming a covering …   The Collaborative International Dictionary of English

  • *haïr — ● haïr verbe transitif (francique hatjan) Avoir de la haine pour quelqu un, lui vouloir du mal, abhorrer, exécrer quelqu un : Haïr les dictateurs. Avoir de la répugnance, de l aversion, de l horreur pour quelque chose : Haïr l hypocrisie. ● haïr… …   Encyclopédie Universelle

  • hair — HAIR. v. a (l h s aspire.) Vouloir mal à quelqu un. Haïr ses ennemis, les haïr mortellement, cruellement. haïr quelqu un à mort, le haïr à la mort. haïr son prochain, le haïr. comme la peste, le haïr en diable. haïr les meschans. haïr les… …   Dictionnaire de l'Académie française

  • hair — [ her ] noun *** uncount the thing that grows on your head in a mass of thin fibers: long black hair a girl with dark shoulder length hair He wore his hair in a ponytail. You need to brush your hair before you go out. a. uncount hair that grows… …   Usage of the words and phrases in modern English

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