The Conundrum of Human Nakedness

ABSTRACT

Human beings are the only primates without thick body hair. Evolution has played its part in reducing the thickness and density of body hair from our prime ancestors. To understand the causality of this, it is wise to explore three other characteristics which distinguish Homo sapiens from other primates; bipedalism, practical nakedness, and the family reproductive unit. The evolution of hairlessness, combined with the need for efficient eccrine sweat production for cooling, provided evolutionary pressure to protect the skin from ultraviolet damage by developing cutaneous pigmentation. There was a subsequent loss of pigmentation as humans journeyed to northern latitudes. The relationship between nakedness, bipedality, the establishment of family reproductive unit, and the appearance of melanin in the skin all constitute a fascinating challenge to understand. It is indeed intriguing that while nature prefers the Homo sapiens to be relatively naked and hairless, the Homo sapiens is so obsessively fascinated with the preservation of persistence of hair. These issues are indeed very relevant for all physicians who treat hair, such as dermatologists and hair transplant surgeons.

INTRODUCTION

Homo sapiens having evolved from apes exhibit many dissimilarities from their ancestors, nakedness being the most prominent. It is evident that the main difference does not just lie in the quantity of hair, but also in the quality of it.

There are several theories for human hairlessness.

One explanation for hairlessness comes from the aquatic ape theory.[1,2] According to this theory, change in the body hair was secondary to prolonged exposure to aquatic environments. Pagel and Bodmer[3] suggest that human hairlessness evolved to free our ancestors from external parasites such as fleas, ticks, and lice.

A further theory is that human ancestors lost their fur as a way of dealing with overheating.[4] This overheating occurred because our ancestors moved out into the savannah, hunted, and incorporated meat into their diet. The major problem with this theory is that stone tools and a few butchered bones are hardly any evidence to show that human ancestors were in a situation that resembled that of carnivorous animals.[5] According to Harris,[6] hairlessness was selected for by a parental choice based on cultural standards of beauty. Hairless infants were preferred to hairy infants (who were abandoned or killed).

Darwin[7] suggested that hairlessness could not have evolved through natural selection as it had no obvious advantage. He claims that human hairlessness evolved through sexual selection where males came to see the nakedness of the female skin as being sexually attractive. This account is important because it recognizes the sexual significance of naked skin, especially naked female skin. However, it still lacks an explanation for why females might have a preference for hairless males and why female hairlessness was selected in the first place. The difficulties facing these theories show that human hairlessness has not been fully explained.

Three major human characteristics, which have a bearing on the puzzle of the nakedness of humans need to be explained.

Upright bipedalism[8]

Upright bipedalism appears to be a significant characteristic which separates it from other primates. The attainment of bipedalism is regarded as the key factor of humanization. Many hypotheses and ideas have been advanced to explain the origin of bipedalism, but no common understanding has been established. Some of these ideas include that upright posture and bipedal gait were useful for vigilance against predators; Hominins were scavengers and bipedalism was a necessary adaptation to procure sufficient food; and finally, bipedal walking was less expensive than quadrupedalism energetically.[8] It is a possibility that bipedalism is explainable by a single hairless mutation that worked as a driving force of upright walking while holding a baby with both hands.[9]

Practical hairlessness[8]

Hairlessness is possibly related to bipedalism. The major utility of hair in primates is its role as a grip for babies.[8] A hairless mother had to hold her baby with a hand or more safely with both hands, forcing her to walk upright.[8]

Family as a reproductive and social unit

The three major basic characteristics, i.e., bipedalism, hairlessness, and a family as a reproductive and social unit, possibly share the same root at the origin and be associated intrinsically and inseparably with the origin of human development.[9] The unique sexual and reproductive behavior of monomorphic humans, including monogamous and seasonless mating, lack of an externally recognizable estrous cycle, and continual receptivity are explainable inseparably as a result of hairlessness.[8]

The generally accepted view is that modern humans originated from Africa.[10,11] The earliest members of our lineage were members of the genus Ardipithecus and lived mostly in woodland environments.[12] Later protohumans, also referred to as hominids, in the genus Australopithecus, lived in a wider range of environments but have also only been recovered from Africa.[13] Data from fossil records to study climate change indicate that approximately 2.6 million years ago, as the Pleistocene Epoch began, the habitat of protohumans changed as the earth entered the period of climatic oscillations known as the ice ages. Perhaps in response to the increased variance in the environment, approximately 2.6 million years ago the archeological evidence indicates that our ancestors started to use stone tools to butcher animal bones.[14] Meat is a richer source of nutrients than vegetable matter but it is scarce and mobile. In addition to the stone tools, these early members of the genus Homo also evolved modern human-like skeletal proportions that reflect a more efficient long-distance walking ability.[15] With the greater energy expenditure of daytime long-distance walking came the need for greater thermal regulation, particularly cooling. Bipedalism carries significant competitive advantages of speed, height, and the use of tools. Homo erectus, 1.6 million years ago, was the first hominid to have elongated limbs capable of sustained walking and running. Therefore, the switch from less hair to an efficient cooling system was underway 1.6 million years ago.[4]

Hairlessness and melanin

The central equatorial African savanna is a high ultraviolet (UV) environment and a lack of hair results in UV-induced damage through a lack of sun protection.[16] The effect of climate change producing a drier environment also placed stress on the epidermal barrier. Therefore, evolutionary pressures worked to provide the needed protection, and there are several theories concerning this, including the need to protect against UV-induced skin cancer and UV-induced barrier dysfunction as well as the requirement for folic acid protection.[16] Folic acid is an essential vitamin needed for numerous biological functions, including DNA synthesis and repair, red blood cell production, and spermatogenesis.[16] UV-induced folate degradation has been demonstrated in light-skinned patients exposed to natural sunlight.[16] Therefore, there was a strong evolutionary pressure to protect the skin and this was achieved by the production of melanin.[17]

As humans migrated away from the central equatorial African climate to different latitudes, the exposure to UVB diminished and therefore the ability to produce sufficient Vitamin D also diminished.[16] Highly melanized skin requires longer exposure to UVB when the intensity is reduced to produce sufficient Vitamin D. Rickets is a serious complication of Vitamin D deficiency which would have resulted in deformed pelvises that were inadequate for successful childbirth in prehistory if the evidence of recent history is reviewed.[16] In 1956, approximately 15% of African–American women had significantly deformed pelvises due to childhood Vitamin D deficiency, which reflected the cohort of women born before the widespread adoption of dietary Vitamin D supplements in the 1930s.[18] The obstetric complications included an abnormal presentation, umbilical cord prolapse, and impossible vaginal delivery in 10% of cases, with fetal mortality at 10%–15% in the absence of cesarean section.[18] The lighter skin of females may be due to the evolutionary pressure to produce greater quantities of Vitamin D during pregnancy and lactation.[17] The development of clothing by ancient humans is another example of the adoption of the use of technologies that include scraping, cutting pelts, and the ability to pierce and sew, which has the unintended consequence of compounding potential Vitamin D deficiency. The study of human lice Pediculosis humanus capitis (head lice) and Pediculosis humanus (body lice) has shed light on the first use of clothing. These two lice are morphologically similar but inhabit different body sites. Head lice live on the scalp and feed more frequently than the body lice that live in clothing and move to the body to feed once or twice a day.[19] Sucking lice parasitizing mammals are generally host specific,[20] and body lice are derived from head lice.[21] With human evolution and the loss of body hair, the louse needed to evolve and diverge into the two types.[22]

Belt[23] was the first to suggest that a reduction of body hair would have partially alleviated Homo sapiens of ectoparasites. Aspects of our evolutionary history, i.e., group living and a fixed home base, meaning we were potentially prone to high ectoparasite loads. Consequently, a reduction in body hair may be an adaptation to remove ectoparasitic refuges on the body[3] and make parasites easier to find and remove.

CONCLUSION

What does this change on the thickness and length of hair signify? What is its role in the process of evolution? Does it imply that the future generations will have thinner and shorter hair and eventually none of it? It is crucial to understand the theories of hairlessness and ponder on its implications.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.