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'{{Other uses}} {{Infobox anatomy | Name = Hair | Latin = unguis | GraySubject = | GrayPage = | Image = Gray945.png | Caption = Cross section of a hair | Image2 = | Caption2 = | Map = | MapPos = | MapCaption = | Precursor = | System = | Artery = | Vein = | Nerve = | Lymph = | MeshName = | MeshNumber = | Code = [[Terminologia Histologica|TH]] H3.12.00.3.02001 }} '''Hair''' is a [[Protein filament|filament]]ous [[biomaterial]], that grows from [[hair follicle|follicles]] found in the [[dermis]]. The human body, apart from its [[Glabrous skin#Glabrous skin|glabrous skin]], is covered in follicles which produce thick [[terminal hair|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]]. Found exclusively in [[mammals]], hair is one of the defining characteristics of the mammalian [[class (biology)|class]].<ref>[http://www.askoxford.com/concise_oed/hair?view=uk definition] askoxford.com</ref> Hair-like structures, called [[cilia]], but which are not classified as hair, are visible in organs such as the [[nose]] and [[ear]] and these structures also occur in many other organs of mammals, other animals, and plants. Although non-mammals, especially [[insects]], show filamentous outgrowths, these are not considered "hair" either, in the scientific sense. These so-called "hairs" ([[trichome]]s) also are found on [[plant]]s. The projections on [[arthropod]]s, such as [[insect]]s and [[spider]]s, are classified as [[bristle]]s, which are composed of a [[polysaccharide]] called [[chitin]]. There are varieties of [[cat]]s, [[dog]]s, and [[mouse|mice]] bred to have little or no visible hair (fur). In some species, hair is absent at certain stages of life. 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;<ref>{{cite journal|last1= Krause|first1= K|last2= Foitzik|first2= K|title= Biology of the Hair Follicle: The Basics|journal= Seminars in Cutaneous Medicine and Surgery|volume= 25|page= 2|year= 2006|doi= 10.1016/j.sder.2006.01.002}}</ref> and 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. 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 contain 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.<ref>Feughelman, Max [http://books.google.com/books?id=PSNIYKwKu8kC&printsec=frontcover Mechanical properties and structure of alpha-keratin fibres: wool, human hair and related fibres], Sydney, UNSW Press (1996) ISBN 0868403598</ref> ==Description== ===Construction of the root and strand=== [[File:Hair follicle-en.svg|thumb|left|Hair follicle structure]] Hair is composed mainly of the protein [[keratin]]. Keratin assembles into rope-like ike yo mamas dick[[intermediate filaments]]. The structure of these filaments provides strength to the hair shaft. Hair growth begins inside the [[hair follicle]].The only "living" bithes with holes 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".<ref name="How Does Hair Grow?">[http://www.hairlosshelp.com/hair_loss_research/hair.cfm "How Does Hair Grow?"] Web. 09 Feb. 2010</ref> The base of the root is called the bulb, which contains the cells that produce the hair shaft.<ref name="Inside The Hair">{{cite web|url=http://www.pantogar.com/en/inside_the_hair.php|title=Inside the hair|publisher=Pantogar|accessdate=9 Feb. 2010}}</ref> 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]].<ref name="Hair Follicle">[[Hair follicle]] Wikipedia. 23 Feb. 2010</ref> [[File:Menschenhaar 200 fach.jpg|thumb|Strand of human hair at 200x magnification]] Each strand of hair is made up of the [[Medulla (hair)|medulla]], [[Cortex (hair)|cortex]], and [[Cuticle (hair)|cuticle]].<ref name="Hair Structure and Hair Life Cycle">[http://www.follicle.com/hair-structure-life-cycle.html Hair Structure and Hair Life Cycle]</ref> The innermost region, the medulla, is not always present and is an open, unstructured region.<ref name="How Does Hair Grow?"/><ref name="Topic 2: The Layers of the Hair">[http://www.texascollaborative.org/hildasustaita/module%20files/topic2.htm Topic 2: The Layers of the Hair]</ref> 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.<ref name="How Does Hair Grow?"/> 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.<ref name="Hair Structure and Hair Life Cycle"/> The diameter of human hair varies from {{convert|17|to|180|um|abbr=on}}.<ref>{{cite web| last = Ley|first = Brian|title = Diameter of a Human Hair|year = 1999|url = http://hypertextbook.com/facts/1999/BrianLey.shtml|accessdate = 2010-06-28|archiveurl = http://www.webcitation.org/5qpTnR6HP|archivedate = 2010-06-28}}</ref> ===Hair pigment=== {{Main|Human hair 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.<ref name="How Does Hair Grow?"/> Blond hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin decreases or disappears. ===Human hair growth=== {{Main|Human hair growth}} Hair grows everywhere on the external body except for mucus membranes and [[Glabrous skin#Glabrous skin|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 phase has specific characteristics that determine the length of the hair. All three phases 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=== [[File:Ursus maritimus Steve Amstrup.jpg|thumb|left|Polar bears use their fur for warmth and while their skin is black, their transparent fur appears white and provides camouflage while hunting and for 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 (zoology)|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.<ref name="Why Do Humans Get "goosebumps" When They Are Cold, or under Other Circumstances?: Scientific American.">{{Cite journal|last=Bubenik|first=George A.|date=September 1, 2003|title=Why do humans get "goosebumps" when they are cold, or under other circumstances?|journal=Scientific American|url=http://www.scientificamerican.com/article.cfm?id=why-do-humans-get-goosebu}}</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 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 [[predator]]s, 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.<ref>[http://faculty.washington.edu/chudler/receptor.html Neuroscience for Kids - Receptors]. Faculty.washington.edu. Retrieved on 2010-12-14.</ref><ref>[http://www.keratin.com/aa/aa031.shtml hair biology - functions of the hair fiber and hair follicle]. Keratin.com. Retrieved on 2010-12-14.</ref><ref>{{cite journal|pmid=4811387|last1=Sabah|first1=NH|year=1974|pages=256–7|issue=2|volume=36|title=Controlled stimulation of hair follicle receptors.|journal=Journal of applied physiology|url=http://jap.physiology.org/cgi/reprint/36/2/256.pdf}}</ref> ====Eyebrows and eyelashes==== [[File:Eyelashes.jpg|thumb|right|Eyelashes and eyebrows help to protect the eyes from dust, dirt, and sweat]] The [[eyebrows]] are situated above the [[eyelids]] on the [[forehead]]. In many other mammals they contain much longer hairs that are upright, functioning as tactile sensors. While they have little significance to the survival of humans, they remain a part of the body's [[physiological]] makeup because of the role they play in protecting the [[eyes]] from [[dirt]], [[sweat]], and [[rain]], as well as non-verbal [[communication]].<ref name="Sound Medicine: Why Do We Have Eyebrows?">[http://soundmedicine.iu.edu/segment/324/redirect?seg=324 Sound Medicine: Why Do We Have Eyebrows?]</ref> People have developed other means, such as [[hat]]s, [[umbrellas]], and [[goggles]] to serve the purpose of eyebrows more effectively, but the presence of eyebrows may suggest that they were once important to earlier species' survival.<ref name="Sound Medicine: Why Do We Have Eyebrows?"/> The [[eyelash]] grows at the edges of the eyelid and protects the eye from dirt entering the eye. Camels have particularly long and thick eyelashes. 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.<ref name="Thumbprints - The Human Eyelash.">[http://ion.asu.edu/liv41_eyelash/liv41_thumb.htm Thumbprints – The Human Eyelash]</ref> The eye reflexively closes as a result of this [[sensation (psychology)|sensation]]. ==Evolution== {{See also|Evolution of hair}} 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 [[keratin|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.<ref>{{Cite news|url=http://www.msnbc.msn.com/id/27663717/|title=Human hair linked to dinosaur claws: Origins of hair go back 310 million years to common ancestor|last=Viegas|first=Jennifer|date=2008-11-11|work=[[MSNBC]]}}</ref><ref>{{Cite journal|doi=10.1073/pnas.0805154105 |year=2008|author=Eckhard, L. et al.|title=Identification of reptilian genes encoding hair keratin-like proteins suggests a new scenario for the evolutionary origin of hair|pmid=19001262|volume=105|issue=47|pages=18419–23|pmc=2587626|journal=Proceedings of the National Academy of Sciences of the United States of America}}</ref> ===Human "hairlessness"=== {{synthesis|date=November 2010}} Human body hair is barely visible as it is thinner, shorter, and more translucent than the hair of other mammals. Humans are part of a trend toward sparser hair in larger animals (fewer follicles per square inch of exposed skin). There are several African mammals that have very sparse fur, including the elephant and the hippopotamus, both at the upper end of this trend. The ''density'' of human hair follicles on the skin is about the average for an animal of equivalent size.<ref>{{cite journal|last1=Schwartz|first1=GG|last2=Rosenblum|first2=LA|title=Allometry of primate hair density and the evolution of human hairlessness.|journal=American journal of physical anthropology|volume=55|issue=1|pages=9–12|year=1981|pmid=6789685|doi=10.1002/ajpa.1330550103}}</ref> It still is not clear why so much of human hair is short, under-pigmented [[vellus hair]], rather than [[terminal hair]], and the effect of testosterone on the hair follicles in both human and other mammalian species. Because most human hair is less apparent vellus hair, this gives the appearance of being hairless, especially in females. 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.<ref name=MC1R>{{Cite news| title=Why Humans and Their Fur Parted Ways|first=Nicholas|last=Wade|publisher=New York Times|date=2003-08-09|accessdate=2008-07-25|url=http://query.nytimes.com/gst/fullpage.html?res=9C03E0DE1030F93AA2575BC0A9659C8B63&sec=health&spon=&pagewanted=2}}</ref> 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 absorption 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,<ref name="encylopediahumanevolution">{{Cite document |title=The Cambridge Encyclopedia of Human Evolution|editors=Steve Jones, Robert Martin & David Pilbeam|date=1994|publisher= Cambridge University Press|place= Cambridge |isbn= 0-521-32370-3|page=242}} Also ISBN 0-521-46786-1 (paperback)</ref> and thus it wasn't likely that this trait was subject to selection. [[File:Vulpes vulpes sitting.jpg|thumb|The soft, fine hair found on many nonhuman mammals is typically called fur.]] <!-- The following is hidden - rewritten for clarity and to remove unsubstantiated assumptions stated as fact --><!-- Nevertheless, other species likely migrated to Africa by way of a gradual process. This provided them with time to adjust to the intense UV and sunlight by way of other means (such as [[panting]]). Hominids, on the other hand, originally possessed fur, but, due to a relatively sudden change in behavior 2.5 million years ago (due to hominid inventiveness/technological innovation) that involved intense hunting{{cite}} during the day, they developed sweat glands that enabled them to perspire. This change necessitated the loss of most body hair in order to facilitate sweat evaporation (i.e. cool the body). -->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]]{{cite}} 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 our body.<ref name="Montagna, W 1972">{{cite journal|last1=Montagna|first1=William|title=The Skin of Nonhuman Primates|journal=Integrative and Comparative Biology|volume=12|page=109|year=1972|doi=10.1093/icb/12.1.109}}</ref> 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.<ref name="Montagna, W 1972"/> 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 (developed by Dr. Peter Wheeler, 1984, 1985) suggests that when human ancestors started living on the savanna, humans began sweating more to stay cool.<ref>{{cite journal|last1=Wheeler|first1=P|title=The evolution of bipedality and loss of functional body hair in hominids|journal=Journal of Human Evolution|volume=13|page=91|year=1984|doi=10.1016/S0047-2484(84)80079-2 }}</ref> 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.<ref>{{cite journal|last1=Wheeler|first1=P.E.|title=The loss of functional body hair in man: the influence of thermal environment, body form and bipedality|journal=Journal of Human Evolution|volume=14|page=23|year=1985|doi=10.1016/S0047-2484(85)80091-9 }}</ref> Humans kept the hair on our head which reflects harmful UV rays, but our body hair was reduced. 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. {{Clarify|date=May 2011}} 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.<ref name = Meier>{{Cite book| last = Meier|first = R|title = The complete idiot's guide to human prehistory|publisher = Alpha Books|year = 2003|isbn = 0028644212|pages = 57–59 |url=http://books.google.com/?id=r_Wit4KYSmMC&pg=PA57}}</ref> 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|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 [[Androgenic hair|body hair]] than women. Males have more [[terminal hair]], especially on the [[facial hair|face]], [[chest hair|chest]], [[abdominal hair|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.<ref>Dixson, A.F. ''Sexual Selection and the Origins of Human Mating Systems.'' Oxford University Press, USA; 1 edition (2009) ISBN 0199559422</ref> A final hypothesis is that human hair was reduced in response to [[ectoparasites]].<ref name=Pagel&Bodmer2003>{{Cite journal|year=2003 |author=Pagel, Mark and Bodmer, Walter |title=A naked ape would have fewer parasites |journal=[[Proceedings of the Royal Society|Proceedings of the Royal Society B: Biological Sciences]] |pmid=12952654 |volume=270 |pages= S117–S119 |pmc=1698033 |doi=10.1098/rsbl.2003.0041}}</ref><ref>{{cite journal|pmid=10961855|last1=Rantala|first1=MJ|year=1999|pages=1987–9|issue=12|volume=29|title=Human nakedness: adaptation against ectoparasites?|journal=International journal for parasitology|url=http://users.utu.fi/mjranta/reprints/1.%20Rantala1999.pdf}}</ref> Getting rid of our hair might have reduced the risk of [[fleas]], [[ticks]], [[lice]], and other biting insects. ===Evolutionary variation=== Evolutionary biologists suggest that the [[genus Homo]] arose in [[East Africa]] approximately 2.5 million years ago.<ref name=Jablonski06>Jablonski, N.G. (2006). ''[http://books.google.com/books?hl=nl&lr=&id=EYi9S3VtIGsC&oi=fnd&pg=PP13 Skin: a natural history].'' Berkley, CA: University of California Press</ref> They devised new hunting techniques.<ref name=Jablonski06/> The higher protein diet led to the evolution of larger body and brain sizes.<ref name=Jablonski06/> Jablonski<ref name=Jablonski06/> 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.<ref name=Jablonski06/> 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,<ref>{{Cite book|last=Bolk|first=L.|year=1926|title=Das Problem der Menschwerdung|publisher=Fischer|location=Jena|language=German}}</ref> and [[Stephen Jay Gould]] published a short list in ''[[Ontogeny and Phylogeny (book)|Ontogeny and Phylogeny]]''.<ref>short-list of 25 characters reprinted in Gould, Stephen Jay (1977). ''Ontogeny and phylogeny''. Harvard. p. 357</ref> In addition, [[paedomorphic]] characteristics in women are widely [[Physical attractiveness#Determinants of female physical attractiveness|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<ref name=Jablonski06/> agrees 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|date=September 2010}} (the condition of most mammals, including humanity's closest cousins—chimpanzees), to Afro-like or 'kinky' (i.e. tightly coiled). In this sense, during the period in which humans were gradually losing their straight body hair and thereby exposing initially the 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 evolved to prevent the entry of UV light into the body during the transition toward dark, UV-protected skin. Alternatively, 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 <ref name=Jablonski06/> 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,<ref name=Jablonski06/> more likely facilitates an increase in the circulation of cool air onto the scalp. Further, Afro-hair does not respond so easily to moisture and 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. They argue that Darwin's explanation which was that sexual selection may be responsible for such traits.<ref>[[Descent of Man]]</ref> However, the concept of "triviality" is a human value judgment, and hair ''texture'' still may have played a role. 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|date=September 2010}} 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 (along with a small cluster of other physical features) approaches ubiquity is this region. {{Citation needed|date=September 2010}} 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 is, again, not indicative of human sexual aesthetics. ====Straight hair==== According to the [[recent single origin hypothesis]], anatomically modern humans arose in East Africa approximately 200,000 years ago (Tishkoff, 1996). Then, ~150,000 years later, modern humans began to expand their range to regions outside of (and within) this continent (Tishkoff, 1996). The skin of those in the group that left the African continent had developed the ability to manufacture [[vitamin D]] (which was essential for bone development) upon exposure to UV light.<ref name=Jablonski06/> However, the UV light of northern regions was too weak to penetrate the highly pigmented skin of the initial migrants in order to provide enough vitamin D for healthy bone development.<ref name=Jablonski06/> 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 lighter skin survived and had children at higher rates because their skin allowed more UV light for the production of vitamin D.<ref name=Jablonski06/> [[File:Vald - O Tour de la Bulle - P1210522.jpg|thumb|right|Man with straight hair]] In this sense, the evidence with regard to the evolution of straight hair texture seems to support Jablonski's suggestions <ref name=Jablonski06/> that the need for vitamin D triggered the transition back from dark to light skin. Specifically, the distribution of this trait suggests that this need may have grown so intense at certain points that Northerners with mutations for straighter hair survived and had children at higher rates. 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. 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]].<ref>{{cite journal|pmid=18065779|last1=Fujimoto|first1=A|year=2008|pages=835–43|issue=6|volume=17|last2=Kimura|first2=R|journal=Human molecular genetics|last3=Ohashi|first3=J|last4=Omi|first4=K|last5=Yuliwulandari|first5=R|last6=Batubara|first6=L|last7=Mustofa|first7=MS|last8=Samakkarn|first8=U|last9=Settheetham-Ishida|first9=W|title=A scan for genetic determinants of human hair morphology: EDAR is associated with Asian hair thickness.|url=http://hmg.oxfordjournals.org/content/17/6/835.full.pdf|doi=10.1093/hmg/ddm355}}</ref><ref>{{cite journal|pmid=18704500|last1=Fujimoto|first1=A|year=2008|pages=179–85|issue=2|volume=124|last2=Ohashi|first2=J|journal=Human genetics|last3=Nishida|first3=N|last4=Miyagawa|first4=T|last5=Morishita|first5=Y|last6=Tsunoda|first6=T|last7=Kimura|first7=R|last8=Tokunaga|first8=K|title=A replication study confirmed the EDAR gene to be a major contributor to population differentiation regarding head hair thickness in Asia.|url=http://www.tulips.tsukuba.ac.jp/dspace/bitstream/2241/103672/1/HG_124-2.pdf|doi=10.1007/s00439-008-0537-1}}</ref><ref>{{cite journal|last1=Mou|first1=C|last2=Thomason|first2=HA|last3=Willan|first3=PM|last4=Clowes|first4=C|last5=Harris|first5=WE|last6=Drew|first6=CF|last7=Dixon|first7=J|last8=Dixon|first8=MJ|last9=Headon|first9=DJ|title=Enhanced ectodysplasin-A receptor (EDAR) signaling alters multiple fiber characteristics to produce the East Asian hair form.|journal=Human mutation|volume=29|issue=12|pages=1405–11|year=2008|pmid=18561327|doi=10.1002/humu.20795}}</ref> 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== Though growing hair is an inevitable part of being human, many 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=== [[File:Shaving-system-2blade.jpg|thumb|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 [[Manual hair clippers|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== {{See also|Hairstyle}} [[File:Alessandro Allori - Portrait of Bianca Cappello.jpg|thumb|right|''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 being]]s. It can grow on most external areas of the [[human body]], except on the palms of the [[hand]]s and the soles of the [[foot|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, [[Plucking (hair removal)|plucked]], or [[shaving|shaved]]. These include the [[face]], [[ears]], [[head (anatomy)|head]], [[eyebrow]]s, [[leg]]s, and [[armpit]]s, 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 whorl]]s have been discovered to be associated with [[neural development|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.<ref>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</ref> This led to the Parliamentary faction being nicknamed [[Roundhead]]s. Having [[Bob_cut|bobbed hair]] was popular among the [[flapper]]s 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 [[hippie]]s, [[Heavy metal subculture|metalheads]], and Indian [[sadhus]] have [[long hair]]. Many [[punk fashion|punks]] wear a hairstyle known as a [[Mohawk Hairstyle|Mohawk]] or other spiked and dyed hairstyles; [[skinhead]]s have short-cropped or completely shaved heads. [[Mullet (haircut)|Mullet]] hairstyles stereotypically have been associated with [[redneck]]s.{{Citation needed|date=August 2010}} Heads were shaved in [[concentration camp]]s, 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|date=August 2010}} The ethnically [[Manchu people|Manchu]] [[Qing Dynasty]], beginning in the late seventeenth century China, ordered all Chinese citizens to adopt Manchurian hairstyles by shaving the front of their head and adopting a [[Queue (hairstyle)|queue]].{{Citation needed|date=August 2010}} 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-American|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 mixture, but an Ad-Hoc of many ethnicities, one component being African. It is also to note that "Afro Hair" is not universal in Africa as well, although is common among many "West African" ethnicities.{{Citation needed|date=August 2010}} 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 trial|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|date=August 2010}} ===Religious practices=== Women's hair may be hidden using [[headscarf|headscarves]], a common part of the ''[[hijab]]'' in [[Islam]] and a symbol of modesty required for certain religious rituals in [[Orthodox Christianity]]. [[Orthodox Judaism]] endorses the use of scarves and other head coverings for women for modesty reasons as in Islam. [[Sikhism|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. ==See also== *[[Hypotrichosis]], the state of having a less than normal amount of hair on the head or body *[[Hypertrichosis]], the state of having an excess of hair on the head or body *[[Natural afro-hair]] *[[Hair follicle]] *[[Cowlick]] ==References== ===Notes=== {{columns-list|2| {{Reflist}} }} {{Refimprove|date=December 2007}} ===Bibliography=== * <cite id=Iyengar1998>Iyengar, B. (1998). The hair follicle is a specialized UV receptor in human skin? Bio Signals Recep, 7(3), 188–194.</cite> * <cite id=Jablonski2006>Jablonski, N.G. (2006). ''Skin: a natural history.'' Berkley, CA: University of Califiornia Press.</cite> * <cite id=Rogers2004>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.</cite> * <cite id=Tishkoff1996>Tishkoff, S.A. (1996). Global patterns of linkage disequilibrium at the CD4 locus and modern human origins. Science. 271(5254), 1380–1387.</cite> ==External links== {{Commons category|Hair}} {{wikiquote}} {{Wiktionary|hair}} *[http://answers.google.com/answers/threadview?id=122411 Discussion about shaving and cultures] *[http://www.scribd.com/doc/2463/Curious-about-Nails-and-Hairs/ Answers to several questions related to hair from curious kids] *[http://www.fysikbasen.dk/English.php?page=HairMeasure How to measure the diameter of your own hair using a laser pointer] *[http://www.rsc.org/Publishing/Journals/cb/Volume/2007/7/hair_is_the_news.asp Instant insight] outlining the chemistry of hair from the [[Royal Society of Chemistry]] {{Human_hair footer}} {{integumentary system}} {{human anatomical features}} [[Category:Articles with inconsistent citation formats]] [[Category:Hair| ]] [[am:ፀጉር]] [[ar:شعر (تشريح)]] [[an:Pelo]] [[arc:ܡܙܬܐ]] [[gn:Tague]] [[ay:Ñik'uta]] [[az:Saç]] [[bn:চুল]] [[bg:Коса]] [[ca:Pèl]] [[cs:Chlup (zoologie)]] [[cy:Blew]] [[da:Hår]] [[de:Haar]] [[el:Τρίχα]] [[es:Pelo]] [[eo:Haro]] [[eu:Ile]] [[fa:مو]] [[fr:Pilosité humaine]] [[ga:Gruaig]] [[gd:Falt]] [[gl:Cabelo]] [[gu:વાળ]] [[ko:털]] [[hi:बाल]] [[hr:Kosa]] [[io:Pilo]] [[id:Rambut]] [[iu:ᓄᔭᐃᑦ/nujait]] [[ik:Nuyaq]] [[is:Hár]] [[it:Capelli]] [[he:שיער]] [[jv:Rambut]] [[pam:Buak]] [[ka:თმა]] [[kk:Шаш]] [[kv:Си]] [[ht:Cheve]] [[la:Pilus]] [[lv:Mati]] [[lt:Plaukas]] [[hu:Haj]] [[ml:മുടി]] [[mr:केस]] [[ms:Rambut]] [[nl:Haar (zoogdier)]] [[ne:कपाल]] [[ja:毛 (動物)]] [[no:Hår]] [[pi:केसा]] [[pag:Buek]] [[pnb:وال]] [[pl:Włos]] [[pt:Pelo]] [[ro:Păr]] [[qu:Chukcha]] [[ru:Волосы]] [[sa:केशः]] [[simple:Hair]] [[sk:Chlp (cicavce)]] [[sl:Las]] [[so:Tin]] [[sr:Коса]] [[sh:Kosa]] [[su:Rambut]] [[fi:Karva]] [[sv:Hår]] [[tl:Buhok]] [[te:వెంట్రుక]] [[th:ผม]] [[tg:Мӯй]] [[chr:ᎤᏍᏗᏰᎬ]] [[tr:Saç]] [[uk:Волосся]] [[ur:بال]] [[ug:چاچ]] [[vec:Caveji]] [[vi:Lông]] [[fiu-vro:Karv]] [[yi:האר]] [[yo:Irun]] [[zh-yue:毛]] [[diq:Porr]] [[zh:頭髮]]'