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Hair is a type of 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.

Cross section of a hair


The hair can be divided into three parts length-wise, (1) the bulb, a swelling at the base which originates from the dermis(most growth occurs in the bulb which contains hair stem cells), (2) the root, which is the hair lying beneath the skin surface is inside a protective follicle and (3) the shaft, which is the hair above the skin surface though it appears first in the epidermis. 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.

Evolution

A recent study by scientists from the Medical University of Viennamarker 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.

Human "hairlessness"

Though human skin is considered "hairless", humans actually have the same number of hair follicles per unit area as other primates. Human hair is barely visible as it is thinner, shorter and more transparent than the hair of other mammals. Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.

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 strong 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 was needed after the fur was gone.

Human hair under 200-times magnification
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 hypothesis 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). Some counter this argument by noting that among the most hairless people are Northern Europeans who live in a cold and relatively low sun environment. However, abundant genetic and archaeological evidence indicates that the hairlessness of those current-day modern humans whose immediate ancestors came to occupy Northern latitudes is attributable to the relatively recent origin of this species in equatorial, sub-Saharan Africa approximately 200,000 years ago, followed by an even more recent departure from Africa that was initiated approximately 60,000 years ago. Hence it is highly likely that the ancestors of Northern Europeans (et al. Northern groups) failed to develop fur due to a) their relatively recent entry into the area, and b) the fact that the high levels of intelligence that had evolved in the human lineage while in Africa enabled them to survive in the cold European climate by way of the practice of wearing animal furs. Hence the development of fur was rendered effectively unnecessary.

The soft, fine hair found on many nonhuman mammals is typically called fur.
Others hold that there are several problems with the Savanna 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). Hence 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 were not as likely to live past 30. Hence it wasn't as common a 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. ) 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 it is arguable that the Savanna Theory model provides the best explanation for the loss of fur experienced by the human lineage given the available evidence.

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), as well as a much smaller role of testosterone in women.

The aquatic ape hypothesis posits that sparsity of hair is an adaptation to an aquatic environment, but it has little support among scientists.

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 variation

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). Some note 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. However, it can be argued that these two species also pant; a device that compensates for inefficiencies in the evaporation of sweat due to fur. Some counter the argument that dark skin was necessary following loss of fur by suggesting that tanning on exposed skin in primates is also seen and possibly was a retained feature, while hyper-pigmentation as in Africans and Indians, as well as albinism are later mutations. However, this argument doesn't account for the fact that equatorial UV light is such that the relatively minor tanning that occurs among chimpanzees (who, it should be mentioned, spend most of their time protected from the sun by way of a forest canopy), is insufficient in terms of providing full protection.

In addition, while some individuals affirm the hypothesis concerning loss of hair via the evolution of sweat glands, they assert that the question remains as to why such a large surface area is required for cooling when other animals in these regions have much larger volumes to surface area, yet are still covered in thick fur and are able to cool solely by panting. They cite examples that include monkeys, lions and zebra, (though as previously mentioned, they acknowledge that both zebra and monkeys possess the ability to sweat). However, this assessment fails to account for the fact that the speed at which the human lineage changed in response to higher cognitive ability far outpaced that of other species. Specifically, the fairly sudden invention of stone tools by primitive humans ~2.8 million years ago rapidly transitioned the human lineage away from the simple scavenging of protein from the bone marrow derived from the kills of large African predators (a fairly passive endeavor), towards active hunting that entailed spending relatively long periods of time chasing wild game in the hot equatorial sun. Such a pace of change was unparalleled among other species who, instead, acquired their adaptations to the African heat over considerably longer periods of time during which many of them moved into the equatorial region at a gradual pace. Thus, the significantly greater urgency amongst the members of the human lineage for heat adaptations that could keep up with the huge nutritional benefits that they were accruing from the practice of hunting (leading to an avalanche effect in which increasing protein intake fueled increasing brain size/intelligence) may explain these stark differences.

Texture

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 scalp as they walked upright in the intense African (equatorial) UV light (Jablonski, 2006). While some might argue that, by this logic, Africans/humans should also express hairy shoulders given that these body parts would putatively be exposed to similar conditions. However, the protection of the head, the seat of the disproportionately large brain that enabled humanity to become one of the most successful species on the planet (which is also 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 transitioned from furry to naked skin, their 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, while the roots of straight human hair may act as fiber optic tubes that allow UV light to pass into the skin, 'kinks' in fiber optic tubes are known to prevent UV light from passing through (note: 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 initially the pale skin underneath their fur to the sun, straight hair would have been an evolutionary liability. Hence, tightly coiled or 'kinky' Afro-hair may have evolved to prevent the entry of UV light into the body during the transition towards 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, unless it was an evolution to provide shade from the sun that was required as body hair was reduced. However, anthropologists such as Nina Jablonski make the opposite argument with regards to this hair texture. Specifically, Jablonski's (2006) assertions suggest that the adjective "wooly" 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 (2006) argues, actually facilitates an increase in the circulation of cool air onto the scalp. Further, Afro-hair does not respond as easily to moisture and/or sweat as straight hair. Thus it does not stick to the neck and/or scalp when wet. Rather, unless totally immersed/drenched, it tends to retain its basic springy puffiness. In this sense, the trait may arguably contribute to slightly enhanced comfort levels in intense equatorial climates compared to straight hair (which, alternatively, tends to naturally fall over the ears and neck to a degree that, arguably, may provide 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 too trivial for natural selection to have played a role. They argue that Darwin's explanation was that sexual selection may be responsible for such traits. However, the concept of "triviality" is a human value judgment. It has nothing to do with whether physical traits are/were actually adaptive. In fact, while the sexual selection hypothesis cannot be totally ruled out, the asymmetrical distribution of this trait does not indicate that this was the primary causal factor. Specifically, if hair texture were simply the result of arbitrary human aesthetic preferences, one would expect that the global distribution of the various hair textures would be fairly random. Instead, the distribution of Afro-hair is strongly skewed towards 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 paleoanthropological 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. 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 the relatively variable trends associated with human sexual aesthetics.

Straight hair

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, 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).

Hence, evidence with regards to the evolution of straight hair texture seems to support Jablonski's (2006) suggestions that the need for vitamin D triggered the transition 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 facilitates 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 given that straighter hair ends tend to point downwards 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. And even this process, they argue, is hindered by the curvation at the base of the hair. Hence, coupled with the amount of skin covered by long head hair, these factors seem to mitigate against the adaptive usefulness of straight hair at Northern latitudes. They further argue that UV light is also poorly reflected from soil and dull surfaces. However, 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 may have also 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 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 (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, which is a time frame that covers from the earliest of the 'Out of Africa' migrations up to now.

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. Male pattern baldness is a 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. The Cavaliers and Roundheads are another example of politically-motivated hairstyles. Long hair on men had a resurgence in the 1960s. Some notable hairstyles include skinheads and mullet. Members of the Sikh religion don't cut their hair. 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.

Hair, power, punishment and status

Heads were shaved in concentration camps. Head-shaving was used as punishment, especially for women with long hair. Military haircuts, monastic tonsures. 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 in the 1920s cut their traditional long hair into short bob cuts to show their independence and sexual freedom. Hippies in the 1960s grew their hair long in order to illustrate their distance from mainstream society.

The film Easy Rider (1969) includes the description of a hippie forcibly having his head shaved with a rusty razor, to symbolise 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 public outcry. During the appeal trial, they appeared in the dock wearing wigs.

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 religious rituals in Orthodox Christianity. Orthodox Judaism endorses the use of wigs, scarves and other headcoverings for women for modesty reasons as in Islam. Hassidic Judaism, on the other hand, discourages the trimming of head hair, and male practitioners typically wear their hair in ringlets (peyos). Sikhs generally keep their hair uncut and tied in a bun on the head, which is then covered appropriately using a turban.

Shampooing

Washing hair is usually done with shampoo, however there are instances where it is washed with other materials. Washing hair is a subject of discussion with different parties arguing for and against the practice. The case for not washing hair has been championed by British broadcaster and journalist Andrew Marr. Matthew Parris is another who apparently does not wash his hair.

In his article in the Daily Mail Marr wrote:
Former Conservative MP and author Matthew Parris threw down the gauntlet last week when he announced that he hadn't washed his hair in a decade, and suffered no ill-effects, socially or otherwise.


Marr then argued the case advising non use of shampoo was natural as well as having environmental and economic benefits. Jessica Simpson has also been cited as a reduced shampooer rather than a non shampooer.

Hair pigment

All natural hair colours are the result of two types of hair pigment. Both these pigments are a type of melanin produced inside the hair follicle: Phaeomelanin is responsible for the yellowish-blond to red colors and Eumelanin is responsible for the brown to black shades. Gray hair occurs when these melanin molecules are no longer produced, so there is no pigment.

See also



Notes

References

  • 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.


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