Body Hair Topics

• sweat evaporation
• body fur
• sweat glands
• brain sizes
• human ancestors
• thermal conductivity
• protein diet
• thinning of hair
• uv radiation
• naked skin
• human origin
• hominids
• dark skin
• technological innovation
• tropical regions
• human skin
• prehistoric times
• water content
• postulates

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Body Hair, Body Hair

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Human hair is barely visible as it is thinner, shorter and more transparent than the hair of other mammals.Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species. However, recent research on the evolution of lice suggests that human ancestors lost their body hair approximately 3.3 million years ago. Wade, N. (2007). In Lice, Clues to Human Origin and Attire.

Dark skin probably evolved after humans lost their body fur, because the naked skin was vulnerable to harsh African UV radiation. Therefore, evidence of when human skin darkened has been used to date the loss of human body hair, assuming that the dark skin would not have been needed until after the fur was gone.

Some hold that there are several problems with this theory (including balding), not least of which is that cursorial hunting is used by other animals that do not show any thinning of hair. Nevertheless, other species likely migrated to Africa by way of a gradual process. This provided them with time to adjust to the intense UV and sunlight by way of other means (such as panting). Hominids, on the other hand, originally possessed fur, but, due to a relatively sudden change in behavior 2.5 million years ago (due to hominid inventiveness/technological innovation) that involved intense hunting during the day, they developed sweat glands that enabled them to perspire. This change necessitated the loss of most body hair in order to facilitate sweat evaporation (i.e. cool the body), although body hair would allow for a greater surface area for sweat to evaporate from coupled with hairs thermal conductivity allowing more heat to be lost from more tropical regions where the water content of the air is greater. Furthermore, balding usually occurs at around 30 40 years of age. In prehistoric times, most individuals did not live past 30.

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), it must also be noted that primates and horses have armpits that sweat like those of humans and so this was not a new evolution, rather a possible preferential selection of perspiration over body hair. 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), although tanning on exposed skin in primates is also seen and possibly a retained feature, where hyper-pigmentation as in Africans and Indians, as well as albinism are later mutations. Hence the former hypothesis concerning loss of hair via the evolution of sweat glands is still quite viable.

In addition, axillary 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 cousinschimpanzees), 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, while it is notable that 'kinks' in fiber optic tubes are known to prevent UV light from passing through. 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 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. However, tightly coiled hair that grows into a typical afro-like formation would have greatly reduced the ability of the head and brain to cool, 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.

Source: Wikipedia > Hair