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[5]. 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.
