Hair is a filamentous outgrowth of protein, found only on mammals. It projects from the epidermis, though it grows from hair follicles deep in the dermis. Although many other organisms, especially insects, show filamentous outgrowths, these are not considered "hair". So-called "hairs" (trichomes) are also found on plants. The projections on arthropods, such as insects and spiders are actually insect bristles. 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 primary component of hair fiber is keratin. Keratins are proteins, long chains (polymers) of amino acids.
Human hair
Body hair
Historically, several ideas have been advanced to describe the reduction of human body hair. All were faced with the same problem that there's no fossil record of human hair to back up the conjectures nor to determine exactly when the feature evolved. However, recent research on the evolution of lice suggests that human ancestors lost their body hair approximately 3.3 million years ago.
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 savanna, including bipedal locomotion and an upright posture. There are several problems (including balding) with this theory, not least of which is that cursorial hunting is used by other animals that don't show any thinning of hair.
Another theory for the thin body hair on humans proposes that Fisherian runaway sexual selection played a role here (as well as in the selection of long head hair). Possibly this occurred in conjunction with neoteny, with the more juvenile appearing females being selected by males as more desirable; see types of hair and vellus hair.
The aquatic ape hypothesis posits that sparsity of hair is an adaptation to an aquatic environment, but it has little support amongst scientists and very few aquatic mammals are, in fact, hairless.
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.
Head hair
Head hair is a type of hair that's grown on the head (sometimes referring directly to the scalp).The most noticeable part of human hair is the hair on the head, which can grow longer than on most mammals and is more dense than most hair found elsewhere on the body. The average human head has about 100,000 hair follicles. (External Link
) Its absence is termed alopecia, commonly known as baldness. Anthropologists speculate that the functional significance of long head hair may be adornment, a by-product of secondary natural selection once other somatic hair had been lost. Another possibility is that long head hair is a result of Fisherian runaway sexual selection, where long lustrous hair is a visible marker for a healthy individual (with good nutrition, waist length hair—approximately 1 meter or 39 inches long—would take around 84 months, or about 7 years, to grow). Each follicle can grow about 20 individual hairs in a person's lifetime. (External Link) Average hair loss is about 100 strands a day. The average human scalp measures approximately 120 square inches (770 cm²). These values are also reported by Desmond Morris although it isn't clear if these are applicable to both men and women.
Average number of head hairs (Caucasian) (External Link)
| color |
number of hairs |
diameter |
| Blonde |
146,000 |
1⁄1500th to 1⁄500th inch |
17 to 51 micrometers |
| Black |
110,000 |
1⁄400th to 1⁄250th inch |
64 to 100 micrometers |
| Brunette |
100,000 |
variable |
variable |
| Red |
86,000 |
variable |
variable |
Types of hair
Humans 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.
Growth
Different parts of the human body feature different types of hair. From childhood onward,
vellus hair covers the entire human body regardless of
sex or
race except in the following locations: the
lips, the palms of
hands, the soles of
feet, certain external genital areas, the
navel and
scar tissue. The
density of the hairs (in hair follicles per
square centimeter) varies from one person to another.
The rising level of male
hormones (
androgens) during
puberty causes a
transformation process of vellus hair into
terminal hair on several parts of the body. The hair follicles respond to
androgens, primarily
testosterone and its derivatives; the hair in these locations can be thus termed
androgenic hair. The rate of hair growth and the weight of the hairs increase. However, different areas respond with different sensitivities. As testosterone levels increase, the sequence of appearance of androgenic hair reflects the gradations of androgen sensitivity. The
pubic area is most sensitive, and heavier hair usually grows there first in response to androgens.
Areas on the human body that develop terminal hair growth due to rising androgens in both sexes, men and women, are the
underarms and the
pubic area. In contrast, normally only men grow androgenic hair in other areas. There is a
sexual dimorphism in the amount and distribution of androgenic hair, with males having more terminal hair (particularly
facial hair,
chest hair,
abdominal hair and hair on
legs and
arms) and females having more vellus hair, which is less visible. The
genetic disposition determines the sex-dependent and individual rising of androgens and therefore the development of androgenic hair.
Increased body hair on women following the male pattern can be referred to as
hirsutism. An excessive and abnormal hair growth on the body of males and females is defined as
hypertrichosis. Considering an individual occurrence of body hair as abnormal doesn't implicitly depend on medical indications but also on cultural and social attitudes.
Individual hairs alternate periods of growth and dormancy. During the growth portion of the cycle, hair follicles are long and
bulbous, and the hair advances outward at about a third of a
millimeter per day. After three to six months, body hair growth stops (the pubic and armpit areas having the longest growth period). The follicle shrinks and the
root of the hair grows rigid. Following a period of
dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows for a long duration and to a great length before being shed. The rate of growth is approximately 15 millimeters, or about ⅝
inch, per month.
Texture
Hair
texture is measured by the degree of which one's hair is either fine or coarse, which in turn varies according to the diameter of each individual hair. There are usually four major types of hair texture:
fine,
medium,
coarse and
wiry. Within the four texture ranges hair can also be thin, medium or thick density and it can be straight, curly, wavy or kinky.
Hair conditioner will also alter the ultimate equation and can be healthy, normal, oily, dry, damaged or a combination. Hair can also be textured if
straighteners, crimpers,
curlers, etc are used to style hair. Also, an expert
hairdresser can change the hair texture with the use of special chemicals.
Hair is genetically programmed to be straight, curly or wavy, and it tends to 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 hair was curly 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 or straight is determined by the shape of the follicle itself and the direction in which each strand grows out of its follicle. Curly hair is shaped like an elongated oval and grows at a sharp angle to the scalp.
Curly hair has a different biological structure than 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, hard to manage, and often frizzy.
Hair, whether it's curly or straight, is affected by the amount of humidity in the air. It serves as a "truth serum" 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.
Hair texture variation is likely to have resulted from a significant event in human evolutionary history. Evolutionary biologists agree that the evidence suggests that genus Homo arose in East Africa approximately 2 million years ago. During this time body size increased in response to richer dietary intake. This increase was most likely a reflection of rapidly increasing brain size among members of this genus, which facilitated an increasing intellectual capacity that made more varied dietary access possible (for example via new hunting and scavenging techniques etc.). Jablonski et al (2004) postulate that as body size increase, it became evolutionarily necessary to expel heat from the body at a more rapid rate. As a result, humans developed the ability to sweat. They also lost body hair in order to facilitate sweat evaporation and hence cooling of the body.
Aging
Older people tend to develop grey hair because the
pigment in the hair is lost and the hair becomes colorless. 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 isn't actually grey; the grey head of hair is a result of a combination of the dark and white/colorless 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 color and afterward turns white. In fact, the grey or white appearance of individual hair
fibers is a result of light scattering from air
bubbles in the central medula of the hair fiber. 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're 50. The tendency toward
baldness is a trait shared by a number of other
primate species, and is thought to have evolutionary roots.
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.
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.
(External Link)
Cultural attitudes
Head 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 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.
The traditional hair styling in some parts of
Africa also gives interesting examples of how people dealt with their head hair. The
Maasai warriors tied the front hair into sections of tiny
braids while the back hair was allowed to grow to waist length. Women and non-warriors, however, shaved their heads. Many tribes dyed the hair with red earth and
grease; some stiffened it with
animal dung.
Contemporary social and cultural conditions have constantly influenced popular hair styles. 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.
(External Link)
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 didn't 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 don't 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).
In
Western societies it became a public trend during the late twentieth century, particularly for women, to reduce or to remove their body hair. The
bikini and
Brazilian waxing fashion as well as the sexual imagery in advertising and movies are major reasons for this development. This media trend began in the
United States and is becoming ever more popular throughout other Western countries. It was also beginning to gain currency among men, among whom shaving or trimming one's body hair is sometimes jokingly called "
manscaping".
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 in
Western societies.
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 has had social and sexual significance in a number of societies, as a sign of
masculinity in men, and
femininity in women when in the "right" place, and as a sign of
effeminacy in men and unfemininity in women when in the "wrong" place. Where the right and wrong places are differs from one culture to another.
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:
Some groups, for example Sikhs and male orthodox Jews, never cut some or all of their hair
Growing and removing
Romans, Greeks and shaving
Hair as item of female beauty
Hair length for men: Cavaliers and Roundheads, long hair in the 1960s, skinheads, mullets and other hairstyles, the uncut hair of Sikhs
Hair length for women: trends and fashions
Hairy arms and legs, regional variations in hirsutism
Depilation, eyebrow plucking, unibrow, etc.
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
Extremely long hair of some Indian holy men
Regular hairdressing as sign of wealth
The dreadlocks of the Rastafari movement
Own removal of hair in order to liberate oneself from their past, usually after a trying time in one's life.
Tightly curled Afros are sometimes worn among Blacks as a symbol of racial pride
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 Orthodox Judaism
Hair ornaments
Keeping pubic hair hidden or shaven
In Hindu culture, it's believed that the base of the hair shaft contains certain hormones that stimulate the opposite sex. This, combined with the notion that the woman's hair is the most attractive part of her body, was the reason behind tonsuring (shaving) of a woman after her husband's death, so that no person would be attracted to her and thereby secure her chastity. This is the reason why the son tonsures his head after his parents' death, it instills a sense of detachment from worldly pleasures in him for the duration of mourning.
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