Hair

For other uses, see Hair (disambiguation).

Human hair under 200-times magnification

Transverse section of hair follicle

Hair is a keratinised protein filament that grows through the epidermis from follicles deep within the dermis. Found exclusively in mammals, it is one of the of the defining characteristic of the mammalian class. Although many other organisms, 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. The hair of non-human mammal species is commonly referred to as fur. 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. Keratins are proteins: long chains (polymers) of amino acids.

Body hair

In animals, the hair is referred to as fur.

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 have been hairless at least since that time.

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). 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 (ie cool the body). Furthermore, balding usually occurs at around 30 - 40 years of age. In prehistoric times, most individuals did not live past 30. 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 ). 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.

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).

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

In reality, there may be little to explain. Humans, like all primates, are part of a trend toward sparser hair in larger animals; 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.

Characteristics

Traditional Hopi hair style, photo by Edward S. Curtis, 1922

Types of hair

Human beings have three different types of hair:

• Lanugo, the fine hair that covers nearly the entire body of fetuses.
• Vellus hair, the short, fine, "peach fuzz" body hair that grows in most places on the human body in both sexes.
• Terminal hair, the fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair.

Texture

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, curly, 'kinky' or wavy, 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.

Context in which human head (and auxiliary) hair texture variation may have arisen

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). 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). Hence the former hypothesis concerning loss of hair via the evolution of sweat glands is still quite viable.

The evolution of 'Afro' hair texture

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). 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. However, it is notable that 'kinks' in fiber optic tubes are known to prevent UV light from passing through. 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.

The re-evolution of straight hair texture among modern humans

According to the recent single origin hypothesis (the one most supported by the empirical data), anatomically modern humans (Homo sapiens) arose in East Africa approximately 200,000 years ago (Tishkoff, 1996). Anatomically modern behavior in terms of innovation in hunting instruments and artistic expression arose within the past 65,000-70,000 years in Africa. It was during this time that modern humans began to expand their range to regions outside of (and within) Africa (Tishkoff, 1996). Among those in the group who left the African continent, some migrated to northern regions such as central and northeast Asia. These groups initially faced a special dilemma. Their dark African skin and 'kinky' African hair, both of which had evolved to minimize entry of UV light into the body, 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. Malformed bones in the pelvic area were especially deadly for women in that they interfered with the successful delivery of babies. Hence, lighter skin gradually evolved to allow UV light into the skin (Jablonski, 2006). It is probable that, during the transition period from dark to light skin, the need for vitamin D grew so intense that the hair of these northerners simultaneously straightened so that UV light could pass into the body. This, again, is in accord with 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. Furthermore, the need for the change from 'kinky' to straight hair is consistent with the fact that UV light cannot as easily pass through 'kinked' fiber optic tubes.

Aging

Older people tend to develop grey hair because their hair follicles produce less pigment and the hair becomes colourless. 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 ageing 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 (incidentally human scalp hair grow at a rate four times that of human nails. Human nails grow at a rate of 0.1 mm/day). It is commonly claimed that hair and nails will continue growing for several days after death. This is a myth; the appearance of growth is actually caused by the retraction of skin as the surrounding tissue dehydrates, making nails and hair more prominent.

Pathological impacts on hair

Drugs used in cancer chemotherapy frequently cause a temporary loss of hair, 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.

The hair shafts may also store certain poisons for years, even decades, after death. In the case of Col. Lafayette Baker, who died July 3, 1868, use of an atomic absorption spectrophotometer showed the man was killed by white arsenic. The prime suspect was Wally Pollack, Baker's brother-in-law. According to Dr. Ray A. Neff, Pollack had laced Baker's beer with it over a period of months, and a century or so later minute traces of arsenic showed up in the dead man's hair. Mrs. Baker's diary seems to confirm that it was indeed arsenic, as she writes of how she found some vials of it inside her brother's suitcoat one day.

Width

According to The Physics Factbook, the diameter of human hair ranges from 17 to 181 µm.

Hair care and hair loss

Main articles: Hair care, Baldness, and Baldness treatments

Cultural attitudes

Head hair

People from different cultures have invented various ways to arrange, or "style" their hair.

The remarkable head hair of humans has gained an important significance in nearly all present societies as well as any given historical period throughout the world. The haircut has always played a significant cultural and social role.

In ancient Egypt head hair was often shaved, especially amongst children, as long hair was uncomfortable in the heat. Children were often left with a long lock of hair growing from one part of their heads, the practice being so common that it became the standard in Egyptian art for artists to depict children as always wearing this "sidelock". Many adult men and women kept their heads permanently shaved for comfort in the heat and to keep the head free of lice, while wearing a wig in public.

In ancient Greece, ancient India and ancient Rome men and women already differed from each other through their haircuts. The head hair of women was long and pulled back into a chignon. Many dyed their hair red with henna and sprinkled it with gold powder, often adorning it with fresh flowers. Men’s hair was short and even occasionally shaved. In Rome hairdressing became ever more popular and the upper classes were attended to by slaves or visited public barber shops.

Maasai warriors with their traditional hair styling

The traditional hair styling in some parts of Africa also gives interesting examples of how people traditionally represent their head hair. During a certain phase of their lives (ie the warrior phase), traditional Maasai warriors tie the front hair into sections of tiny braids while the back hair was allowed to grow long. Women and non-warriors, however, shave their heads. Many tribes (such as those in Namibia) dye the hair with red earth and grease; some stiffen it with animal dung.

Despite the general trend among straight haired groups for women to have longer hair than men, social and cultural conditions have influenced popular hair styles such that this has not always been the case. From the 17th century into the early 19th century it was the norm in Western culture for men to have long hair often tied back into a ponytail. Famous long-haired men include René Descartes, Giacomo Casanova, Oliver Cromwell and George Washington. During his younger years Napoleon Bonaparte had a long and flamboyant head of hair. Before World War I men generally had longer hair and beards. The trench warfare between 1914 and 1918 exposed men to lice and flea infestations, which prompted the order to cut hair short, establishing a norm that has persisted.

It has also been advanced that short hair on men has been enforced as a means of control, as shown in the military and police and other forces that require obedience and discipline. Additionally, slaves and defeated armies were often required to shave their heads, in both pre-medieval Europe and China.

Long hair was almost universal among women in Western culture until World War I. Many women in conservative Pentecostal groups abstain from trimming their hair after conversion (and some have never had their hair trimmed or cut at all since birth). The social revolution of the 1960s led to a renaissance of unchecked hair growth. Hair length is measured from the front scalp line on the forehead up over the top of the head and down the back to the floor. Standard milestones in this process of hair growing are waist length, hip length, classic length (midpoint on the body, where the buttocks meet the thighs), thigh length, knee length, ankle length and even beyond. It takes about seven years, including occasional trims, to grow one's hair to waist length. Terminal length varies from person to person according to genetics and overall health.

A thriving salon culture in Detroit gave rise to the Detroit Hair Wars in 1991. Using the medium of human and synthetic hair, elaborate fantastical head pieces, such as spider webs, flowers and flying "hair-y copters", have been made by participants.[1]

The average human adult male has approximately 150,000 hair follicles on his head.

Xie Qiuping has the world's longest hair at 5.627 m (18 ft 5.54 in).

Body hair

Mark Twain, Shirtless. A human male with body hair.

The attitudes towards hair on the human body also vary between different cultures and times. In some cultures profuse chest hair on men is a symbol of virility and masculinity; other societies display a hairless body as a sign of youthfulness.

In ancient Egypt, people regarded a completely smooth, hairless body as the standard of beauty. An upper class Egyptian woman took great pains to ensure that she did not have a single hair on her body, except for the top of her head (and even this was often replaced with a wig). The ancient Greeks later adopted this smooth ideal, considering a hairless body to be representative of youth and beauty. This is reflected in Greek female sculptures which do not display any pubic hair. Islam stipulates many tenets with respect to hair, such as the covering of hair by women and the removal of armpit and pubic hair (see five physical characteristics traits of fitrah).

Hair as business factor

Hair care for humans is a major world industry with specialized tools, chemicals and techniques. The business of various products connected with human hair has become an important industrial and financial factor globally.

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.

File:Allori Portrait.jpg

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

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 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
• Hairstyle can be an indicator of group membership:
  • Metalhead long-hair for headbanging, also symbolic of a metalhead belonging to the metal world.
  • Beatle "mop-top" haircuts
  • Punk mohawk haircuts
  • Skinhead haircut
  • Mullet hairstyle
  • Deathhawk A larger, fuller version of a mohawk - popular in the gothic sub-culture
  • Undercut where the sides and back of the head are shaved short or bald, and the top hair is allowed to grow long. Common with metalheads and in the Cybergoth subculture, especially with women, although it is accepted as a unisex hair style.
  • Fascinator 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).
• Some groups, for example Sikhs and male orthodox Jews, never cut or shave some or all of their hair.
• Some groups, such as women in the Muslim and orthodox Jewish communities, cover their hair as part of religious observance.
• It is found that hair whorl is associated with brain development.

Growing and removing

• 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.
• Hairy arms and legs, regional variations in hirsutism

Hair, power, punishment, and status

File:Hair-punishment.jpg

French civilians shave the head of a young woman as punishment for wartime collaboration, August 29 1944

• 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.
• Own removal of hair in order to liberate oneself from their past, usually after a trying time in one's life.
• Tightly coiled hair in its natural state can be worn in an Afro. This hairstyle was once worn among Blacks as a symbol of racial pride. Given that the coiled texture is the natural state of most Black people's 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.

Concealing and revealing

• Keeping women's hair hidden: headscarves, the hijab in Islam, head-shaving and wigs in Orthodox Judaism etc.
• Keeping men's hair hidden: 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

See also

• Hypotrichosis, the state of having a less than normal amount of hair on the head or body

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.

External links

• Nutrient for hair
• Discussion about shaving and cultures
• Answers to several questions related to hair from curious kids
• How to measure the diameter of your own hair using a laser pointer
• Instant insight outlining the chemistry of hair from the Royal Society of Chemistry
• About Fur: An Imaginary Portrait of Diane Arbus,the recent film which describes only a part of the life and work of the photographer, particularly her relationship to a man who was born with a peculiarly hair-covered skin