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From Wikipedia, the free encyclopedia
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 dogs,
cats, 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.The hair can be divided into three different parts length-wise: (1)
the bulb, a swelling at the base which originates from the dermis (most
growth occurs in the bulb which contains hair stem cells); (2) the root,
which is the hair lying beneath the skin surface inside a protective
follicle; and (3) the shaft, which is considered the part of the hair above
the skin surface though it appears first in the epidermis. In cross-section,
there are also three parts: (1) the medulla, an area in the core which
contains loose cells and airspaces; (2) the cortex, which contains densely
packed keratin; and (3) the cuticle, which is a single layer of cells
arranged like roof shingles.
Curly Hair
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 scalp as they walked upright in the
intense African (equatorial) UV light (Jablonski, 2006). While some might
argue that, by this logic, Africans/humans should also express hairy
shoulders given that these body parts would putatively be exposed to similar
conditions. However, the protection of the head, the seat of the brain that
enabled humanity to become one of the most successful species on the planet
(which is also 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 which Homo erectus transitioned
from furry to naked skin, their 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, while the roots of straight
human hair may act as fiber optic tubes that allow UV light to pass into the
skin, 'kinks' in fiber optic tubes are known to prevent UV light from
passing through (note: this is due to the incident angle of the UV light
made to the reflective inner surface of the hair follicle approaching the
normal to the surface of the plane, reducing internal reflection). In this
sense, during the period in which humans were gradually losing their
straight body hair and thereby exposing initially the pale skin underneath
their fur to the sun, straight hair would have been an evolutionary
liability. Hence, tightly coiled or 'kinky' Afro-hair may have evolved to
prevent the entry of UV light into the body during the transition towards
dark, UV-protected skin.
Alternatively, some intuit 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, unless
it was an evolution to provide shade from the sun that was required as body
hair was reduced. However, anthropologists such as Nina Jablonski make the
opposite argument with regards to this hair texture. Specifically,
Jablonski's (2006) assertions suggest that the adjective "wooly" 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 (2006) argues, actually facilitates an increase in the circulation
of cool air onto the scalp. Further, Afro-hair does not respond as easily to
moisture and/or sweat as straight hair. Thus it does not stick to the neck
and/or scalp when wet. Rather, unless totally immersed/drenched, it tends to
retain its basic springy puffiness. In this sense, the trait may arguably
contribute to slightly enhanced comfort levels in intense equatorial
climates compared to straight hair (which, alternatively, tends to naturally
fall over the ears and neck to a degree that, arguably, may provide 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 too trivial for natural selection to have
played a role. They argue that Darwin's explanation was that sexual
selection may be responsible for such traits.[7] However, the concept of
"triviality" is a human value judgment. It has nothing to do with whether
physical traits are/were actually adaptive. In fact, while the sexual
selection hypothesis cannot be totally ruled out, the asymmetrical
distribution of this trait does not indicate that this was the primary
causal factor. Specifically, if hair texture were simply the result of
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 towards 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 paleoanthropological 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 (along with a small cluster of other physical features)
approaches ubiquity is 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 is, again, not indicative of the relatively
variable trends associated with human sexual aesthetics. |
