History of hair

History of hair

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. Asian hair typically has a round fiber and is quite straight. 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 17 to 180 micrometers (0.00067 to 0.0071 in).

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 the root is called the bulb, which 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 erector pili muscles, which are responsible for causing hairs to stand up. In humans, with little body hair, the effect results in goose bumps.

Hair-follicle cyclingHair grows in cycles of various phases: anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase. Each phase has several morphologically and histologically distinguishable sub-phases. Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one of several hairs that might arise from a single follicle exits. Normally up to 90% of the hair follicles are in anagen phase while, 10–14% are in telogen and 1–2% in catagen. The cycle’s length varies on different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hairs have a much shorter length limit compared to hairs on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor.

Natural ColorHuman hair color

All natural hair colors are the result of two types of hair pigment. 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 dark-blond, brown hair, and black hair, while pheomelanin is dominant in red hair. Blond hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin production decreases or stops.

Curly hair , Jablonski asserts , that it was evolutionarily advantageous for pre-humans to retain the hair on their heads in order to protect 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 given that 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). During the gradual process by whichHomo erectus made a transition from furry to naked skin, their hair texture putatively changed gradually from Afro-textured hair or ‘kinky’ (i.e. tightly coiled) to straight hair (the condition of most mammals, including humanity’s closest cousins—chimpanzees) This argument is based on the principle that curly hair impedes the passage of UV light into the body relative to straight hair (thus curly or coiled hair would be particularly advantageous for dark-skinned hominids living at the equator). It is substantiated by Iyengar’s (1998) findings 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 this study suggest that this phenomenon occurs in a manner similar to the way that light passes through fiber optic tubes (which do not function as effectively when kinked or sharply curved or coiled). In this sense, during the period in which 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 as hominoids traveled farther from Africa or The Equator. Hence, 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 pale-white skin.

By contrast, some believe that tightly coiled hair that grows into a typical Afro-like formation would have greatly reduced the ability of the head and brain to cool. They reason that although hair density in African peoples is much less than their European counterparts, in the intense sun the effective ‘woolly hat’ produced would have been a disadvantage. However, anthropologists such as Nina Jablonski make the opposite argument with regards to this hair texture. Specifically, Jablonski’s assertions suggest that the adjective “woolly” in reference to Afro-hair is a misnomer to the extent that it connotes 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 effect. This, in turn, Jablonski argues, more likely facilitates an increase in the circulation of cool air onto the scalp. Further, Afro-hair does not respond as easily to moisture and sweat as straight hair. Thus it does not stick to the neck and scalp when wet. Rather, unless totally drenched, it tends to retain its basic springy puffiness. In this sense, the trait may enhance comfort levels in intense equatorial climates compared to 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, some interpret the ideas of Charles Darwin as suggesting that some traits, such as hair texture, were so arbitrary to human survival that the role natural selection played was trivial. Hence, they argue in favor of his suggestion that sexual selection may be responsible for such traits. However, inclinations towards deeming hair texture “adaptively trivial” may root in certain cultural value judgments more than objective logic. In this sense the possibility that hair texture may have played an adaptively significant role cannot be completely eliminated from consideration. In fact, while the sexual selection hypothesis cannot be ruled out, the asymmetrical distribution of this trait vouches for environmental influence. Specifically, if hair texture were simply the result of adaptively arbitrary human aesthetic preferences, one would expect that the global distribution of the various hair textures would be fairly random. Instead, the distribution of Afro-hair is strongly skewed toward the equator. 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 hair Straight black hair

According to the recent single origin hypothesis, anatomically modern humans arose in East Africa approximately 200,000 years ago. Then, ~150,000 years later (i.e. around 50,000 years ago), sub-groups of this population began to expand our species’ range to regions outside of, and (later) within, this continent (Tishkoff, 1996). For those members of this group who migrated far north (i.e. to northern Eurasia, etc.), the UV light of these regions was too weak to penetrate the highly pigmented skin of the initially (relatively) dark-skinned migrants so as to provide enough vitamin D for healthy bone development. Malformed bones in the pelvic area were especially deadly for women because they interfered with the successful delivery of babies, leading to the death of both the mother and the infant during labor. Hence, those with less pigmented skin survived and had children at higher rates because their skin allowed more UV light for the production of vitamin D. Thus, the skin of those in the group that left the African continent and went far north gradually developed adaptations for relatively greater translucence compared to equatorial hues. This enabled the passage of more UV light into the body at high latitudes, facilitating the natural human body-process of manufacturing vitamin D (which is essential for bone development) in response to said light.

In this sense, the evidence with regard to the evolution of straight hair texture seems to support Jablonski’s suggestions that the need for vitamin D triggered the transition from dark to pale, translucent skin among modern humans. Specifically, the distribution of this trait suggests that this need may have (initially) grown so intense at certain (early) points that those among said (initially more deeply pigmented skinned) Northern-migrants with mutations for straighter hair survived and had children at (somewhat) higher rates. This early change in texture was likely subsequently followed by the accumulation of adaptively advantageous genetic changes that led to the above-mentioned skin-translucence. This argument is made based on the principle that straight fibers better facilitate the passage of UV light into the body relative to curly hair. It is substantiated by 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 (Iyengar, 1998).

Man with straight hair

Nonetheless, some argue against this stance because straighter hair ends tend to point downward while fiber optics requires that light be transmitted at a high angle to the normal of the inner reflective surface. In light of this, they suggest that only light reflected from the ground could successfully enter the hair follicle and be transmitted down the shaft. Even this process, they argue, is hindered by the curvature at the base of the hair. Therefore, coupled with the amount of skin covered by long head hair, these factors seem to militate against the adaptive usefulness of straight hair at northern latitudes. They further argue that UV light also is poorly reflected from soil and dull surfaces. These ideas can be countered by the fact that during the winter, the time of year in which UV light is most scarce at northern latitudes, the ground is often covered with white snow. Given that white is the most effective color in terms of facilitating the reflection of ground light, the hypothesis that straight hair could have been adaptively favorable, cannot be fully discounted in this regard. In addition, as mentioned in the previous section, straight hair also may have contributed to enhanced comfort levels in the north. This is evident in the extent to which, relative to curly hair, it tends to provide a layer of protection for ears and necks against the cold.

The latter hypothesis seems the more plausible evolution determinant as the surface area of the head is minute compared to the remainder of the body, thus the energy required in producing long hair for the express purpose of “optical” amplification of UV light reflected from the snow seems counterproductive (however, it’s very likely that the trait was sustained due to a nuanced combination of multiple influences, given that human hunting-skills and ingenuity were such by 50,000 years ago that said benefits in terms of ‘comfort’ could have alternatively been derived from constructing head and ear warmers of fur from prey, etc.). Scientists point to the fact that straight hair found in many ethnic groups is denser as well and has a greater ability to “show” as it does not coil, hence providing more warmth as the likely deterministic factor for the evolution of straight long hair. Some scientists argue that since the head and appendages are the greatest areas for heat loss from the body, the ability to grow long hair on the crown of the head as well as the face provides a distinct advantage in a cold climate. Since the main sensory organs are anatomically located on the head, long hair provides the necessary warmth and protection in a cold climate that allows the use of these organs by exposing them to the elements to “sense”, in for example a hunt, yet still providing necessary warmth and protection to sustain prolonged exposure. It may be argued, therefore, that the ability to grow long, straight, densely packed hair provides a distinct evolutionary advantage in cold climate; however, it would be a distinct disadvantage in a hot climate, when compared to loosely packed, spongy, closely cropped hair.

The EDAR LocusA 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 of tightly coiled natural afro-hair. Specifically, the relevant findings indicate that the EDAR mutation coding for the predominant East Asian ‘coarse’ or thick, straight hair texture arose within the past ~65,000 years, which is a time frame that covers from the earliest of the ‘Out of Africa’ migrations up to now.