All about health: Hair

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.[1] 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, the bulb, a swelling at the base which originates from the dermis(most growth occurs in the bulb which contains hair stem cells), the root, which is the hair lying beneath the skin surface is inside a protective follicle and 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, the medulla, an area in the core which contains loose cells and airspaces the cortex, which contains densely packed keratin and the cuticle, which is a single layer of cells arranged like roof shingles.
A recent study by scientists from the 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.

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.

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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[who?] 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[citation needed]. 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.[dubious – discuss] 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.[dubious – discuss]) 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[citation needed] (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.

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All about health

суббота, 28 ноября 2009 г.

Hair

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