Hair
Hair is a protein filament that grows from follicles found in the dermis, or skin. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types and hair care, but hair is also an important biomaterial primarily composed of protein, notably keratin. Attitudes towards hair, such as hairstyles and hair removal, vary widely across different cultures and historical periods, but it is often used to indicate a person's personal beliefs or social position, such as their age, gender, or religion.
Overview
** the part beneath the skin, called the hair follicle or when pulled from the skin, called the bulb. This organ is located in the dermis and maintains stem cells, which not only re-grow the hair after it falls out, but also are recruited to regrow skin after a wound.
** the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones.
Hair fibers have a structure consisting of several layers, starting from the outside:
** the cuticle, which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles,
** the cortex, which contains the keratin bundles in cell structures that remain roughly rod-like.
** the medulla, a disorganized and open area at the fiber's center.
Description
Each strand of hair is made up of the medulla , cortex, and cuticle. The innermost region, the medulla, is not always present and is an open, unstructured region. The highly structural and organized cortex, or middle layer of the hair, is the primary source of mechanical strength and water uptake. The cortex contains melanin, which colors the fiber based on the number, distribution and types of melanin granules. The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. People with straight hair have round hair fibers. Oval and irregularly shaped fibers are generally more wavy or even curly. The cuticle is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water.The diameter of human hair varies from .017 to .18 millimeters (0.00067 to 0.00709 in). There are two million small, tubular glands and sweat glands that produce watery fluids that cool the body by evaporation. The glands at the opening of the hair produce a fatty secretion that lubricates the hair.
Hair growth begins inside the hair follicle. The only "living" portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered "dead". The base of a hair's root (the "bulb") contains the cells that produce the hair shaft. Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the arrector pili muscles, which are responsible for causing hairs to stand up. In humans with little body hair, the effect results in goose bumps.
Natural color
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| A young woman with brown hair. |
All natural hair colors are the result of two types of hair pigments. Both of these pigments are melanin types, produced inside the hair follicle and packed into granules found in the fibers. Eumelanin is the dominant pigment in brown hair, and black hair, while pheomelanin is dominant in red hair. Blond hair and black hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin production decreases or stops, while poliosis, typically in spots is hair (and often the skin to which the hair is attached) that never possessed melanin at all in the first place, or ceased for natural genetic reasons, generally in the first years of life.
Human hair growth
Main article: Human hair growth
Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each has specific characteristics that determine the length of the hair. All three occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.
The body has different types of hair, including vellus hair and androgenic hair, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection.
Human hairlessness
The general hairlessness of humans in comparison to related species may be due to loss of functionality in the pseudogene KRTHAP1 (which helps produce keratin) in the human lineage about 240,000 years ago. On an individual basis, mutations in the gene HR can lead to complete hair loss, though this is not typical in humans.Humans may also lose their hair as a result of hormonal imbalance due to drugs or pregnancy.In order to comprehend why humans are essentially hairless, it is essential to understand that mammalian body hair is not merely an aesthetic characteristic; it protects the skin from wounds, bites, heat, cold, and UV radiation. Additionally, it can be used as a communication tool and as a camouflage. To this end, it can be concluded that benefits stemming from the loss of human body hair must be great enough to outweigh the loss of these protective functions by nakedness.
Humans are the only primate species that have undergone significant hair loss and of the approximately 5000 extant species of mammal, only a handful are effectively hairless. This list includes elephants, rhinoceroses, hippopotamuses, walruses, pigs, whales and other cetaceans , and naked mole rats. Most mammals have light skin that is covered by fur, and biologists believe that early human ancestors started out this way also. Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to the strong UV radiation as explained in the Out of Africa hypothesis. Therefore, evidence of the time 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.
It was expected that dating the split of the ancestral human louse into two species, the head louse and the pubic louse, would date the loss of body hair in human ancestors. However, it turned out that the human pubic louse does not descend from the ancestral human louse, but from the gorilla louse, diverging 3.3 million years ago. This suggests that humans had lost body hair (but retained head hair) and developed thick pubic hair prior to this date, were living in or close to the forest where gorillas lived, and acquired pubic lice from butchering gorillas or sleeping in their nests. The evolution of the body louse from the head louse, on the other hand, places the date of clothing much later, some 100,000 years ago.
The process of balding, where terminal hair switches to vellus hair, usually occurs at around thirty to forty years of age. In prehistoric times, most individuals did not survive to adulthood, let alone reaching their fourth decade and therefore balding tends to act as a signal of maturity. In women survival to such an advanced age is usually coupled with a decrease in fertility , but in men fertility is retained beyond middle-age. The persistence (but non-ubiquity) of balding in men, coupled with its general absence in women, suggests that there was a selection pressure against balding in women, but variations in hair patterns among men did not prevent their reproductive success leading to stable polymorphisms (perhaps representing different mating strategies); for example some men could have benefitted from baldness by signalling advanced maturity and social status; while other men simulated the appearance of youth and vigor by retaining their hair.
The soft, fine hair found on many nonhuman mammals is typically called fur.
The sweat glands in humans could have evolved to spread from the hands and feet as the body hair changed, or the hair change could have occurred to facilitate sweating. Horses and humans are two of the few animals capable of sweating on most of their body, yet horses are larger and still have fully developed fur. In humans, the skin hairs lie flat in hot conditions, as the arrector pili muscles relax, preventing heat from being trapped by a layer of still air between the hairs, and increasing heat loss by convection.
Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.
Several hypotheses explained hairlessness as a thermoregulatory adaptation to hot and dry savanna. The most known thermoregulatory hypothesis in modern paleoanthropology was proposed by Peter Wheeler (1984, 1985). He suggests that a need for decreased body hair originated as a response to climate change that began approximately 3 million years ago. At this time, the earth entered a period of global cooling that had a dehumidifying effect on the main early human habitats in East and Central Africa. Lush, wooded forests gave way to dry, grassland savannah; because of this, early humans were required to travel farther in search of food and water. As early humans diverged from their 'pre-chimpanzee like', rather the ancestral lineage shared with chimpanzees, they also became omnivorous in order to maximize calorie intake, an important distinction in a nutrient-scarce environment. Prey, however, are moving targets, and though early humans changed the traditionally ape-like appearance of the australopithecines and adapted long, strong legs to facilitate sustained running, dense, hairy coats still posed a potentially fatal risk of causing overheating during the chase. It is posited that thick hair got in the way of the sweat evaporating, so humans evolved a sparser coat of fur. Although hair provides protection against harmful UV radiation, since our hominin ancestors were bipedal, only our heads were exposed to the noonday sun. Humans kept the hair on our head which reflects harmful UV rays, but our body hair was reduced. The rise in eccrine glands occurred on the genes that determine the fate of epidermal stem cells in human embryonic development.
Another hypothesis for the thick body hair on humans proposes that Fisherian runaway sexual selection played a role (as well as in the selection of long head hair), as well as a much larger role of testosterone in men. Sexual selection is the only theory thus far that explains the sexual dimorphism seen in the hair patterns of men and women. On average, men have more body hair than women. Males have more terminal hair, especially on the face, chest, abdomen, and back, and females have more vellus hair, which is less visible. The halting of hair development at a juvenile stage, vellus hair, would also be consistent with the neoteny evident in humans, especially in females, and thus they could have occurred at the same time. This theory, however, has significant holdings in today's cultural norms. There is no evidence that sexual selection would proceed to such a drastic extent over a million years ago when a full, lush coat of hair would most likely indicate health and would therefore be more likely to be selected for, not against, and not all human populations today have sexual dimorphism in body hair.
A further hypothesis is that human hair was reduced in response to ectoparasites. The "ectoparasite" explanation of modern human nakedness is based on the principle that a hairless primate would harbor fewer parasites. When our ancestors adopted group-dwelling social arrangements roughly 1.8 mya, ectoparasite loads increased dramatically. Early humans became the only one of the 193 primate species to have fleas, which can be attributed to the close living arrangements of large groups of individuals. While primate species have communal sleeping arrangements, these groups are always on the move and thus are less likely to harbor ectoparasites. Because of this, selection pressure for early humans would favor decreasing body hair because those with thick coats would have more lethal-disease-carrying ectoparasites and would thereby have lower fitness. However, early humans were able to compensate for the loss of warmth and protection provided by body hair with clothing, and no other mammal lost body hair to reduce parasite loads.
Another view is proposed by James Giles, who attempts to explain hairlessness as evolved from the relationship between mother and child, and as a consequence of bipedalism. Giles also connects romantic love to hairlessness.
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