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 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.
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 (i.e. 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.
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.
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.
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.
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.
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.
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).
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 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.
* 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.