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Hair is a protein filament that grows through the epidermis from follicles deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class. Although other non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" in the scientific sense. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods such as insects and spiders are actually insect bristles, composed of a polysaccharide called chitin. There are varieties of cats, dogs, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life. The main component of hair fiber is keratin.
The hair can be divided into three parts length-wise, (1) the bulb, a swelling at the base which originates from the dermis, (2) the root, which is the hair lying beneath the skin surface, and (3) the shaft, which is the hair above the skin surface. In cross-section, there are also three parts, (1) the medulla, an area in the core which contains loose cells and airspaces (2) the cortex, which contains densely packed keratin and (3) the cuticle, which is a single layer of cells arranged like roof shingles.
Human beings have three distinct types of hair:
- Lanugo is fine hair that covers nearly the entire body of a fetus. Unless born prematurely, the fetus loses this layer of hair before birth. Lanugo also sometimes returns in cases of malnutrition or extreme anorexia nervosa, as the starved body attempts to insulate itself.
- Vellus hair is extremely short, fine, scarcely noticeable hair that covers most of the human body in both sexes.
- Terminal hair is fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair.
Balding and greying
The tendency of older people to develop grey hair is due to "a massive build up of hydrogen peroxide due to wear and tear of our hair follicles ...[which] winds up blocking the normal synthesis of melanin, our hair's natural pigment."  Grey hair is considered to be a characteristic of normal aging. The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has grey hair, and in general men tend to become grey at younger ages than women.
It should be noted however, that grey hair in itself is not actually grey; the grey head of hair is a result of a combination of the dark and white/colourless hair forming an overall 'grey' appearance to the observer. As such, people starting out with very pale blond hair usually develop white hair instead of grey hair when aging. Red hair usually doesn't turn grey with age; rather it becomes a sandy colour and afterward turns white. Some degree of scalp hair loss or thinning generally accompanies aging in both males and females, and it's estimated that half of all men are affected by male pattern baldness by the time they are 50. The tendency toward baldness is a trait shared by a number of other primate species, and is thought to have evolutionary roots.
Human scalp hair normally grows at a rate of 0.4 mm/day. Hair does not grow after death; the appearance of growth is actually caused by the retraction of skin as the surrounding tissue dehydrates, making nails and hair more prominent.
Hair texture is described as fine, medium, coarse or wiry, depending on the hair diameter. Within the four texture ranges hair can also be thin, medium or thick density and it can be straight, curly, 'kinky' (tightly coiled), or wavy. Hair conditioner also affects hair texture. Hair can be healthy, normal, oily, dry, damaged or a combination. Hair texture can also be affected by hair styling equipment such as straighteners, crimpers, or curlers. Also, a hairdresser can change hair texture with chemicals.
Hair is genetically programmed to be straight, wavy, curly, or 'kinky,' and it can change over time.
For many years, it was believed that the shape of a person’s hair was determined by the individual hair shafts, and that curly and 'kinky' hair get their shape because the cross-section of the hair shaft was flatter and had more intertwined layers than straight hair, which was round. But scientists have determined that whether your hair is curly, 'kinky', or straight is determined by the shape of the follicle itself and the direction in which each strand grows out of its follicle. Curly and/or 'kinky' hair is shaped like an elongated oval and grows at a sharp angle to the scalp. This growth pattern, in turn, determines the cross-section of the shafts.
Curly and/or 'kinky' hair has a different biological structure from straight hair. It tends to be much drier than straight hair because the oils secreted into the hair shaft by the sebaceous glands can more easily travel down the shaft of straight hair. People with very curly hair may find that this hair type can be dry and often frizzy.
Hair, whether it is curly or straight, is affected by the amount of humidity in the air. It serves as a restoring force for the hair, forcing water back into the hair fiber and forcing hair shaft to return to its original structure. This may be more noticeable in somebody with curly hair because it tends to get frizzy when the humidity rises.
By using hair-water partitioning, the density of hair is estimated to 1.32 kg/L. Thus, considering hair as cylindrical, the weight of a single hair is between 3 and 340 microgram per cm, for a diameter of 17 and 181 µm respectively.
The diameter of an average human eyelash ranges from 0.05 mm to 0.20 mm.
Drugs used in cancer chemotherapy frequently cause a temporary loss of hair (alopecia universalis), noticeable on the head and eyebrows, because they kill all rapidly dividing cells, not just the cancerous ones. Other diseases and traumas can cause temporary or permanent loss of hair, either generally or in patches. Patients with Hyperthyroidism or Hypothyroidism can experience hair loss until their hormone levels are regulated.
In recent years, a University of Toronto laboratory pioneered the use of neonatal hair to show exposure to drugs during pregnancy. Drugs such as cocaine, methamphetamine, heroin, etc., accumulate in the neonatal hair during its growth and thus serve as a long term “memory” of what happened during fetal life. The hair analysis method used by Dr. G. Koren's research group revolutionized the diagnosis of in utero exposure to drugs.
- See also: Evolution of hair
A recent study by scientists from 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 . In chickens and lizards, the keratin produced was found in their claws, but in mammals it was used to produce hair. The scientists involved were still searching for the mechanisms that allowed mammals to use the keratins of animal claws to produce hair  
Historically, some ideas have been advanced to explain the small amount of body hair in humans, as compared to other species. However, recent research on the evolution of lice suggests that human ancestors lost their body hair approximately 3.3 million years ago.
Most mammals have light skin that is covered by fur, and biologists believe that human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to harsh African UV radiation. Therefore, evidence of when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin would not have been needed until 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 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. Therefore, humans, in part, have been hairless at least since that time, as body hair does still remain in human populations.
The Savanna Theory suggests that nature selected humans for shorter and thinner body hair as part of a set of adaptations to the warm plains of the African savanna (in addition to bipedal locomotion and an upright posture), however it should be noted that among the most hairless people are Northern Europeans who live in a cold and relatively low sun environment. Some hold that there are several problems with this theory (including balding), not least of which is that cursorial hunting is used by other animals that do not show any thinning of hair. 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 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), although body hair would allow for a greater surface area for sweat to evaporate from coupled with hairs thermal conductivity allowing more heat to be lost from more tropical regions where the water content of the air is greater. Furthermore, balding usually occurs at around 30 – 40 years of age. In prehistoric times, most individuals did not live past 30. [dubious — see talk page] Hence it wasn't a common trait. Also, dark pigmentation of the skin could have compensated for premature baldness (although such a condition would have still been somewhat uncomfortable relative to having hair [dubious — see talk page]). Finally, there are indeed other African mammals that have lost fur due to equatorial heat. These include the African (and Indian) elephant, as well as the hippopotamus. Thus this theory remains the best explanation of human hair loss despite the persistence of those advocating the lice hypothesis et al. [How to reference and link to summary or text]
Another theory for the thin body hair on humans proposes that Fisherian runaway sexual selection played a role. (as well as in the selection of long head hair). Possibly this occurred in conjunction during fetal/early child development neoteny such that more juvenile appearing females being selected by males as more desirable (see types of hair and vellus hair) (however, this conclusion may be more of a reflection of current standards of beauty rather than prehistoric ones), as well as a much smaller role of testosterone in women.
Humans, like all primates, are part of a trend toward sparser hair in larger animals, possibly correlating to the lack of hair on elephants of African and Indian origin as opposed to an evolution due to the sunlight; the density of human hair follicles on the skin is actually about what one would expect for an animal of equivalent size. The outstanding question is why so much of human hair is short, underpigmented vellus hair, rather than terminal hair and the role of testosterone on the hair follicles to instigate their terminalisation in both human and other mamillian species.
Evolutionary biologists suggest that the genus Homo arose in East Africa approximately 2.5 million years ago (Jablonski, 2006). During this time new hunting techniques were innovated (Jablonski, 2006). The higher protein diet led to the evolution of larger body and brain sizes (Jablonski, 2006). Jablonski (2006) 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 developed the ability to sweat and thus lost body hair to facilitate this process (Jablonski, 2006), it must also be noted that primates and horses have armpits that sweat like those of humans and so this was not a new evolution, rather a possible preferential selection of perspiration over body hair. Notably, Pagel et al. (2003) argue against this hypothesis, stating that hominids without fur would not have been able to warm themselves as efficiently at night, nor protect themselves well enough from the sun during the day. However, it is likely that increased intelligence, combined with sophisticated hunting techniques, enabled humans to warm themselves at night using animal skins. Furthermore, assuming that hair loss evolved gradually, dark skin color likely developed to protect the skin during the day (Rogers et al. 2004), although tanning on exposed skin in primates is also seen and possibly a retained feature, where hyper-pigmentation as in African's and Indian's, aswell as albinism are later mutations. Hence the former hypothesis concerning loss of hair via the evolution of sweat glands is still quite viable, however does question why such a large surface area is required for cooling where animals of much larger volumes to surface area are still covered in thick fur in these regions and are able to cool souly by panting such as monkeys lions and zebra, though as previously mentioned both zebra and monkeys possess the ability to sweat.
Tightly coiled hair
Jablonski (2006) agrees that it was evolutionarily advantageous for pre-humans (Homo erectus) to retain the hair on their heads in order to protect the skin there as they walked upright in the intense African (equatorial) UV light (Jablonski, 2006), although this questions why Africans do not have hairer shoulders as this would also have been under the same conditions. In addition, axilliary hair (in the underarms) was likely retained as a sign of sexual maturity. During the process of going from fur to naked skin, hair texture putatively changed gradually from straight (the condition of most mammals, including humanity's closest cousins--chimpanzees), to Afro-like or 'kinky' (i.e. tightly coiled). This is supported by Iyengar's (1998) findings that the roots of straight human hair may act as fiber optic tubes that allow UV light to pass into the skin, while it is notable that 'kinks' in fiber optic tubes are known to prevent UV light from passing through. This is due to the incident angle of the UV light made to the reflective inner surface of the hair follicle approaching the normal to the surface of the plane, reducing internal reflection. In this sense, during the period in which humans were gradually losing their straight body hair and thereby exposing the pale skin underneath their fur to the sun, straight hair would have been an evolutionary liability. Hence, tightly coiled or 'kinky' natural afro-hair may have evolved to prevent the entry of UV light into the body during the gradual transition period towards the evolution of dark skin. However, tightly coiled hair that grows into a typical afro-like formation would have greatly reduced the ability of the head and brain to cool, although hair density in african peoples is much less than their european counterparts, aswell as their cranial index being lower, suggesting less heat being produced by the brain, in the intense sun the effective 'wooly hat' produced would have been a disadvantage, unless it was an evolution to provide shade from the sun that was required as body hair was reduced.
According to the recent single origin hypothesis, anatomically modern humans (Homo sapiens) 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). Among those in the group who left the African continent, some migrated to northern regions such as central and northeast Asia. It is hypothesized that, given their original sub-Saharan origin, these groups initially faced a special dilemma. Their dark African skin and 'nappy' African hair, both of which likely evolved to minimize entry of solar radiation into the body (Jablonski, 2006), were ill-suited to the weak sunlight of these regions. This is because, some time during the period in which humanity was in Africa, their skin had developed the ability to manufacture vitamin D (which was essential for bone development) upon exposure to UV light (Jablonski, 2006). 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 (Jablonski, 2006). Malformed bones in the pelvic area were especially deadly for women in that they interfered with the successful delivery of babies; possibly leading to the death of both the mother and the infant during labor. Hence, those with lighter skin gradually survived and had children at higher rates because their skin allowed more UV light for the production of vitamin D (Jablonski, 2006). Of course this is somewhat questionable as an earlier migration out of Africa was that of the Innuit's whose skin pigmentation is much less than their African counterparts at that time. Also questionable is whether this migration outward was parrallel or before the evolution of kinky hair. With regards to hair texture, it is likely that, during the transition period from dark to light skin, the need for vitamin D grew so intense that northerners with mutations for straighter hair survived and had children at higher rates. For, straight fibers better facilitate the passage of UV light into the body compared to 'kinky' hair. This is supported 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. Straighter hair ends also tend to point downwards, due to fibre optics requiring light to be transmitted at a high angle to the normal of the inner reflective surface, only light reflected from say the ground, would sucessfully enter the hair follicle and be transmitted down the shaft, again hindered by the curvation at the base of the hair, this coupled with the amount of skin covered by say long head hair, lends favour against this hypothesis. UV light is also poorly reflected from soil and dull surfaces.
The EDAR locus and the evolution of straight 'coarse' hair in East Asia
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 (Mou, 2008; Fujimoto, 2008; Fujimoto, 2008b). 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. This indicates that the initial expression of this trait among modern humans occurred only after the migration of humans out of sub-Saharan Africa. Though assuming a homonid descent, similar to that of the chimpanzee, straight hair would have already been evoluted and more likely to have been retained, although hair density on such animals is in opposition to that of modern man, ie less hair in armpits, chest and groin, as well as a lack of knowledge of the hair type of their counterparts who remained in Africa as they migrated out of Africa.
It is important to remember that all of the above has been discovered by extrapolation from modern genetic sources, and other theories, such as the UV transmission of light of hair has been performed under ideal scientific conditions, rendering the application of such erroneous within context of natural hair placement. The times of evolutions are always questionable as with any evolution, or rather, mutation, occurs randomly, and its prominence and spread are due to sexual selection according to sexual preference as well as environmental advantages. Ultimately it may be very possible that the cousin of the chimpanzee from which modern humans had evolved, looked very different from the chimpanzee, as despite the high correlation of genetic similarly, somewhere around 98%, 2% of the human genome equates to around 600 or so genes, with respect to gene configuration and chromosomal placement, the effects of such can vary greatly.
Social role of hair
Hair has great social significance for human beings. It can grow on most areas of the human body, except on the palms of the hands and the soles of the feet (among other areas), but hair is most noticeable in 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, nose, ears, head, eyebrows, eyelashes, 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.
Hair as indicator
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, which can be concealed with hair dye. A well known old wives' tale often told to children claims that eating a large portion of bread crusts can make hair curly. Male pattern baldness is sign of age, which can be concealed with a toupee, hats or religious/cultural adornments. In modern times, it can be reversed in some men with minoxidil (marketed as Rogaine or Regaine) or finasteride (marketed as Propecia); see Baldness treatments. Rather than these options, many men simply shave their heads for a clean look. Males in some religious groups, for example Sikhs and Orthodox Jews, may follow certain rules regarding hair as part of their faith, e.g. never cut their hair, or shave some or all of it. Some groups, such as women in the Muslim and orthodox Jewish communities, cover their hair as part of religious observance. Hair whorls have been discovered to be associated with brain development.
Hairstyle can be an indicator of group membership:. Metalheads can often feature long hair for headbanging, although long hair is commonplace for many men and women outside of heavy metal (ex: Indian sadhus, the hippie subculture, etc). Beatle "mop-top" haircuts. Mohawk haircuts, often associated with punk rock and the punk subculture. Skinhead haircuts, where the head is often shaved completely bald, or "buzzed". Mullet hairstyles, which have stereotypically been portrayed as pertaining to rednecks. Deathhawk A larger, fuller, back combed version of a mohawk - popular in the gothic sub-culture, and heavily featured in deathrock and gothic rock bands in the 1980s. Undercut where the sides and back of the head are shaved short or bald, and the top hair is allowed to grow long. Common among so-called "cybergoths" and followers of Industrial and heavy electronic music scenes. This is especially true of women in these subcultures, although the undercut is accepted as a unisex hair style. Fascinator (hair style) where the hair is short at the back and long at the front and the front forms itself into a point. It is similar to a mullet in reverse (also known as a frullet) or a devil lock. Hair that is usually short with a long side fringe [American: bangs] is a cut often associated with emo music and its fan basis. It is often dyed black or vibrant and contrasting colours such as pink or blue. It is considered a unisex haircut and often appears similar to the mop-top.
Growing and removing
In Western society, men's hair is generally kept short. This is due in part to 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. The Cromwell followers won. Hair length for men: Cavaliers and Roundheads, long hair in the 1960s, skinheads, mullets and other hairstyles, the uncut hair of Sikhs. 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. Hairy arms and legs, regional variations in hirsutism.
Hair, power, punishment, and status
Samson and Delilah. Shaved heads in concentration camps. Head-shaving as punishment - especially for women with long hair. Military haircuts, monastic tonsures.Kovstro and his Seven Hounds. Extremely long hair of some Indian holy men. Regular hairdressing as sign of wealth. The dreadlocks of the Rastafari were despised early in the movement's history. Having one's own hair cut in order to liberate oneself from their past, usually after a trying time in one's life. Having hair cut as a sign of mourning, which was practiced in a number of cultures. Yoko Ono famously cut her very long hair after the assassination of her husband John Lennon, saying, "John loved my long hair, so I gave it to him.". Tightly coiled hair in its natural state can 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 most African Americans' hair, 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. Flappers of the 1920s cut their traditional long hair into short bob cuts to show their independence and sexual freedom. Hippies of the 1960s grew their hair long in order to illustrate their distance from mainstream society. The film Easy Rider (1969) includes the description of one Hippie forcibly having his head shaved with a rusty razor to indicate the intolerance of some conservative groups towards the Hippie movement. At the conclusion of the Oz obscenity trials in the UK, the defendants had their heads shaved by the police, causing an outcry. During the appeal trial, they appeared in the dock wearing wigs.
Concealing and revealing
Keeping women's hair hidden: headscarves, the hijab in Islam, head-shaving and wigs in Orthodox Judaism etc. Keeping unshorn hairs, tied in a bun on the head and covered appropriately: the turban in Sikhism. Displaying women's hair: hair fashions in Western society. Displaying men's hair: facial hair in Islam, ringlets in Hassidic Judaism. Hair ornaments. Keeping pubic hair hidden or shaven.
- Hair pulling aka trichillomania
- Hypotrichosis, the state of having a less than normal amount of hair on the head or body
- Scalp (anatomy)
- Skin (anatomy)
- ↑ definition askoxford.com
- ↑ Why Hair Turns Gray Is No Longer A Gray Area: Our Hair Bleaches Itself As We Grow Older, sciencedaily, Fef. 24, 2009
- ↑ "Uncovering the bald truth about hair losser." Springfield News-leader, May 10, 2005. "Half of men" estimate is made by the American Academy of Dermatology and specifically estimates prevalence in the U.S. population, though this should reflect prevalence in other populations.
- ↑ Vreeman, Rachel C, Carroll, Aaron E (2007). Medical myths. British Medical Journal 335: 1288-1289.
- ↑ Ley, Brian (1999). Width of a Human Hair. The Physics Factbook.
- ↑ The World of Hair an on-line reference by Dr. John Gray,provided by the P&G Hair Care Research Center.
- ↑ Partitioning of Polycyclic Aromatic Hydrocarbons in the Hair-Air System Benjalak Karnchanasest; Des Connell; Michael Moore; Peter Vowles
- ↑ Graham K, Koren G, Klein J, Schneiderman J, Greenwald M. Determination of gestational cocaine exposure by hair analysis. JAMA 1989; 262: 3328-30.
- ↑ Human hair linked to dinosaur claws Origins of hair go back 310 million years to common ancestor
- ↑ Eckhard (2008). Identification of reptilian genes encoding hair keratin-like proteins suggests a new scenario for the evolutionary origin of hair.
- ↑ Wade, N. (2007). In Lice, Clues to Human Origin and Attire. New York Times, 156(53877), A17.
- ↑ includeonly>Wade, Nicholas. "Why Humans and Their Fur Parted Ways", 2003-08-09. Retrieved on 2008=07-25.
- ↑ Shwartz, G. G. & Rosenblum, L. A. (?). Allometry of primate hair density and the evolution of human hairlessness. American Journal of Physical Anthropology, 55(1), pp. ??
- ↑ Olmert, Michael (1996). Milton's Teeth and Ovid's Umbrella: Curiouser & Curiouser Adventures in History, p.53. Simon & Schuster, New York. ISBN 0684801647.
- Iyengar, B. (1998). The hair follicle is a specialized UV receptor in human skin? Bio Signals Recep, 7(3), 188-194.
- Jablonski, N.G. (2006). Skin: a natural history. Berkley, CA: University of Califiornia Press.
- Pagel, M. & Bodmer, W. (2003). A naked ape would have fewer parasites. Proceedings of the Royal Society of London. (http://www.anthro.utah.edu/~rogers/pubs/Pagel-BL-270-S117.pdf)
- 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.
- Discussion about shaving and cultures
- Answers to several questions related to hair from curious kids
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