mainly composed of keratin proteins and keratin-associated proteins (krtaps). the human genome encodes 54 different keratin proteins which are present in various amounts in hair. similarly, humans encode more than 100 different krtaps which crosslink keratins in hair. the content of krtaps ranges from less than 3% in human hair to 30–40% in echidna quill. functions many mammals have fur and other hairs that serve different functions. hair provides thermal regulation and camouflage for many animals; for others it provides signals to other animals such as warnings, mating, or other communicative displays; and for some animals hair provides defensive functions and, rarely, even offensive protection. hair also has a sensory function, extending the sense of touch beyond the surface of the skin. guard hairs give warnings that may trigger a recoiling reaction. warmth polar bears use their fur for warmth and while their skin is black, their transparent fur appears white and provides camouflage while hunting and serves as protection by hiding cubs in the snow. while humans have developed clothing and other means of keeping warm, the hair found on the head serves primarily as a source of heat insulation and cooling (when sweat evaporates from soaked hair) as well as protection from ultra-violet radiation exposure. the function of hair in other locations is debated. hats and coats are still required while doing outdoor activities in cold weather to prevent frostbite and hypothermia, but the hair on the human body does help to keep the internal temperature regulated. when the body is too cold, the arrector pili muscles found attached to hair follicles stand up, causing the hair in these follicles to do the same. these hairs then form a heat-trapping layer above the epidermis. this process is formally called piloerection, derived from the latin words 'pilus' ('hair') and 'erectio' ('rising up'), but is more commonly known as 'having goose bumps' in english. this is more effective in other mammals whose fur fluffs up to create air pockets between hairs that insulate the body from the cold. the opposite actions occur when the body is too warm; the arrector muscles make the hair lie flat on the skin which allows heat to leave. protection in some mammals, such as hedgehogs and porcupines, the hairs have been modified into hard spines or quills. these are covered with thick plates of keratin and serve as protection against predators. thick hair such as that of the lion's mane and grizzly bear's fur do offer some protection from physical damages such as bites and scratches. touch sense displacement and vibration of hair shafts are detected by hair follicle nerve receptors and nerve receptors within the skin. hairs can sense movements of air as well as touch by physical objects and they provide sensory awareness of the presence of ectoparasites. some hairs, such as eyelashes, are especially sensitive to the presence of potentially harmful matter. eyebrows and eyelashes eyelashes and eyebrows help to protect the eyes from dust, dirt, and sweat. the eyebrows provide moderate protection to the eyes from dirt, sweat and rain. they also play a key role in non-verbal communication by displaying emotions such as sadness, anger, surprise and excitement. in many other mammals, they contain much longer, whisker-like hairs that act as tactile sensors. the eyelash grows at the edges of the eyelid and protects the eye from dirt. the eyelash is to humans, camels, horses, ostriches etc., what whiskers are to cats; they are used to sense when dirt, dust, or any other potentially harmful object is too close to the eye. the eye reflexively closes as a result of this sensation. evolution hair has its origins in the common ancestor of mammals, the synapsids, about 300 million years ago. it is currently unknown at what stage the synapsids acquired mammalian characteristics such as body hair and mammary glands, as the fossils only rarely provide direct evidence for soft tissues. skin impression of the belly and lower tail of a pelycosaur, possibly haptodus shows the basal synapsid stock bore transverse rows of rectangular scutes, similar to those of a modern crocodile, so the age of acquirement of hair logically couldn't have been earlier than ~299 ma, based on the current understanding of the animal's phylogeny. an exceptionally well-preserved skull of estemmenosuchus, a therapsid from the upper permian, shows smooth, hairless skin with what appears to be glandular depressions, though as a semi-aquatic species it might not have been particularly useful to determine the integument of terrestrial species. the oldest undisputed known fossils showing unambiguous imprints of hair are the callovian (late middle jurassic) castorocauda and several contemporary haramiyidans, both near-mammal cynodonts, giving the age as no later than ~220 ma based on the modern phylogenetic understanding of these clades. more recently, studies on terminal permian russian coprolites may suggest that non-mammalian synapsids from that era had fur. if this is the case, these are the oldest hair remnants known, showcasing that fur occurred as far back as the latest paleozoic. some modern mammals have a special gland in front of each orbit used to preen the fur, called the harderian gland. imprints of this structure are found in the skull of the small early mammals like morganucodon, but not in their cynodont ancestors like thrinaxodon. the hairs of the fur in modern animals are all connected to nerves, and so the fur also serves as a transmitter for sensory input. fur could have evolved from sensory hair (whiskers). the signals from this sensory apparatus is interpreted in the neocortex, a section of the brain that expanded markedly in animals like morganucodon and hadrocodium. the more advanced therapsids could have had a combination of naked skin, whiskers, and scutes. a full pelage likely did not evolve until the therapsid-mammal transition. the more advanced, smaller therapsids could have had a combination of hair and scutes, a combination still found in some modern mammals, such as rodents and the opossum. the high interspecific variability of the size, color, and microstructure of hair often enables the identification of species based on single hair filaments. naked mole-rat (heterocephalus glaber) in a zoo. in varying degrees most mammals have some skin areas without natural hair. on the human body, glabrous skin is found on the ventral portion of the fingers, palms, soles of feet and lips, which are all parts of the body most closely associated with interacting with the world around us, as are the labia minora and glans penis. there are four main types of mechanoreceptors in the glabrous skin of humans: pacinian corpuscles, meissner's corpuscles, merkel's discs, and ruffini corpuscles. the naked mole-rat (heterocephalus glaber) has evolved skin lacking in general, pelagic hair covering, yet has retained long, very sparsely scattered tactile hairs over its body. glabrousness is a trait that may be associated with neoteny. the soft, fine hair found on many nonhuman mammals is typically called fur. evolutionary variation evolutionary biologists suggest that the genus homo arose in east africa approximately 2.5 million years ago. they devised new hunting techniques. the higher protein diet led to the evolution of larger body and brain sizes. jablonski 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 evolved the ability to sweat: a process which was facilitated by the loss of body hair. another factor in human evolution that also occurred in the prehistoric past was a preferential selection for neoteny, particularly in females. the idea that adult humans exhibit certain neotenous (juvenile) features, not evinced in the other great apes, is about a century old. louis bolk made a long list of such traits, and stephen jay gould published a short list in ontogeny and phylogeny. in addition, paedomorphic characteristics in women are often acknowledged as desirable by men in developed countries. for instance, vellus hair is a juvenile characteristic. however, while men develop longer, coarser, thicker, and darker terminal hair through sexual differentiation, women do not, leaving their vellus hair visible. curly hair yellow curly hair and scalp from body which had long black wig over hair. parts of wig plait remains. from egypt, gurob, probably tomb 23. 18th–19th dynasty. the petrie museum of egyptian archaeology, london man with curly hair (david luiz) global hair texture map jablonski asserts head hair was evolutionarily advantageous for pre-humans to retain because it protected the scalp as they walked upright in the intense african (equatorial) uv light. while some might argue that, by this logic, humans should also express hairy shoulders because these body parts would putatively be exposed to similar conditions, the protection of the head, the seat of the brain that enabled humanity to become one of the most successful species on the planet (and which also is very vulnerable at birth) was arguably a more urgent issue (axillary hair in the underarms and groin were also retained as signs of sexual maturity). sometime during the gradual process by which homo erectus began a transition from furry skin to the naked skin expressed by homo sapiens, hair texture putatively gradually changed from straight hair (the condition of most mammals, including humanity's closest cousins—chimpanzees) to afro-textured hair or 'kinky' (i.e. tightly coiled). this argument assumes that curly hair better impedes the passage of uv light into the body relative to straight hair (thus curly or coiled hair would be particularly advantageous for light-skinned hominids living at the equator). it is substantiated by iyengar's findings (1998) that uv light can enter into straight human hair roots (and thus into the body through the skin) via the hair shaft. specifically, the results of that study suggest that this phenomenon resembles the passage of light through fiber optic tubes (which do not function as effectively when kinked or sharply curved or coiled). in this sense, when hominids (i.e. homo erectus) were gradually losing their straight body hair and thereby exposing the initially pale skin underneath their fur to the sun, straight hair would have been an adaptive liability. by inverse logic, later, as humans traveled farther from africa and/or the equator, straight hair may have (initially) evolved to aid the entry of uv light into the body during the transition from dark, uv-protected skin to paler skin. jablonski's assertions suggest that the adjective "woolly" in reference to afro-hair is a misnomer in connoting the high heat insulation derivable from the true wool of sheep. instead, the relatively sparse density of afro-hair, combined with its springy coils actually results in an airy, almost sponge-like structure that in turn, jablonski argues, more likely facilitates an increase in the circulation of cool air onto the scalp. further, wet afro-hair does not stick to the neck and scalp unless totally drenched and instead tends to retain its basic springy puffiness because it less easily responds to moisture and sweat than straight hair does. in this sense, the trait may enhance comfort levels in intense equatorial climates more than straight hair (which, on the other hand, tends to naturally fall over the ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair). further, it is notable that the most pervasive expression of this hair texture can be found in sub-saharan africa; a region of the world that abundant genetic and paleo-anthropological evidence suggests, was the relatively recent (≈200,000-year-old) point of origin for modern humanity. in fact, although genetic findings (tishkoff, 2009) suggest that sub-saharan africans are the most genetically diverse continental group on earth, afro-textured hair approaches ubiquity in this region. this points to a strong, long-term selective pressure that, in stark contrast to most other regions of the genomes of sub-saharan groups, left little room for genetic variation at the determining loci. such a pattern, again, does not seem to support human sexual aesthetics as being the sole or primary cause of this distribution. straight black hair the edar locus a group of studies have recently shown that genetic patterns at the edar locus, a region of the modern human genome that contributes to hair texture variation among most individuals of east asian descent, support the hypothesis that (east asian) straight hair likely developed in this branch of the modern human lineage subsequent to the original expression