This is a guest post by Angarika Deb.
In a lineage of ancestors, humans are the only species left without a coat of body hair. Keeping in mind thermoregulation of bare skin, we speculate conditions for evolution of nakedness. Can it be coupled with bipedality?
One of the salient features of the Mammals group is possession of body hair. Well, most of them at least. But we stand living proof against that. How, where and why did our body hair disappear and nakedness evolve? While Darwin argued that nakedness evolved for sexual ornamental purposes, Andersson disagrees on the premise that, if sexual traits like a shiny plumage are indicative of good health, skin devoid of hair would convey poor health and won’t attract mates. It is important to determine the initial step of this denudation. A coat of body hair prevents too much heat reaching the body in daytime as well as shielding from cold at night. Protection from wind, wounds, bites, and UV radiation also feature in the advantages. Why then, did Homo sapiens end up losing one great layer of protection? If one believes in ‘Survival of the Fittest’, the benefits stemming from near disappearance of human body hair must surely be great enough to outweigh the costs of these protective functions. The repository of hypotheses trying to explain this step of evolution is still growing.
Our ancestors came from a forested environment, gradually moving to open landscapes (the savanna). Australopithecus gave way to Homo species. This is the period when nakedness is said to have started arising in humans. Scientists are trying to narrow own the environmental conditions and period in history when denudation might have occurred, to be able to predict elective forces for its proliferation. But whether it was a forested habitat or the savanna is a topic of great debate and there is equal support for both. Up until last decade, heavier approval lay on the evolution of nakedness in the savanna. But as calculations grow detailed, we see views shifting towards the idea of naked man first arising in a forested environment, and paleontological evidence supports this.
Hairy to naked skin calls for major thermal changes in the body that need to be compensated for, to ensure optimal survival of the individual. A comparison for potential advantages in both environmental conditions have been drawn by Amaral based on calculations by Wheeler. Analysis of Wheeler’s paper, points hat naked skin is at an overall disadvantage when ambient temperature is higher than the body temperature. This is true for the savanna, where temperatures are high due to direct sunlight and minimum shade. Also, night temperatures drop rapidly, making it very chilly. Naked skin receives more sunlight and external heat during the day and loses heat quicker at night, as compared to furry skin. It has been calculated that the thermal load on naked skin is three times higher. This can be partly compensated for by raising the sweating capacity, which is generally higher in humans. But at the same time, airy animals can also increase their sweating capacity almost as high, without losing the hair, and thus have greater fitness. In fact, this trend has been noted in baboons and patas monkeys who have a sweating capacity as high as humans and have developed a denser mantle which reduces the incoming thermal load and provides warmth at night. Thus, the ideal characteristics for an open landscape have been calculated to be retention of body hair whilst increasing the sweating capacity. And this is widely exemplified by numerous hairy species thriving in the savanna. A naked primate additionally suffers losses in water levels of the body due to transpiration from exposed skin. With these disadvantages, emergence of naked man seems unlikely. It is worth mentioning a statement made by Newman in his paper here, which points out that nakedness is a primary disadvantage in an open environment since it requires a compensatory adaptation, sweating. Therefore, it must have stemmed from other selective forces or preceded the move into savanna, at least for its inception.
Amaral further explores the consequences of nakedness in bipeds. Both traits are a remarkable shift from our ancestral structure and their evolution is often speculated to be linked, since they could provide beneficial pre-adaptations for the other to arise. General consensus is that nakedness is more favourable if he animal walks on two legs bipedal) than walking on all fours (quadrupedal). But which one came first is still a subject under study and great conjecture.
A method to address this issue is comparison of the two possible intermediate stages – a naked quadruped and a haired biped – with the initial and final stages – hairy quadruped and naked biped. Wheeler’s calculations give possibilities for both intermediates. A biped animal is highly favoured in a hot savanna setting due to lower body surface exposed to direct sunlight and thus minimal solar flux on body. But nakedness having great disadvantages in such an environment (as discussed above), cannot originate in the savanna and thus a hairy biped should serve as a fitter population alternative. This means, if bipedality arises first, emergence f nakedness would become unlikely. On the other hand, if nakedness originates first in a tropical setting before bipedality, it bestows great profits. A naked quadruped dwelling in forested areas enjoys perks of easier thermal regulation as incoming eat is less and there is no direct sunlight. Additionally, loss of hair would allow the animal to wade more easily in shallow waters (found around forests) for collection of food. It also solves the problem of parasites that harbour on hairy skin. A naked quadruped thus seems a better intermediate stage which would gradually develop bipedality on facing selective pressures – increased thermal load due to movement and settlement in the savanna domain. Amaral suggests that a naked quadruped would eventually develop bipedality as it also starts suffering from lower reproductive fitness. Body hair reduction would lead to infants falling from mother due to inefficient clinging. These arguments correspond more losely to the initial stage of evolution as also proposed by Newman. Wheeler in his paper, suggests bipedality as a pre-adaptation for nakedness to evolve but an analysis of his results contradicts his own contention. Results brought forth by Amaral thus shift the time and place of denudation of humans to an earlier period when our ancestors lived in a forested habitat.
Though there is still neither certainty nor unanimity in views regarding the origin of nakedness, it should be remembered that the answer might lie in stitching several hypotheses together. Evolution is a complex process nd a wider angle provides a more acceptable view. It is suggested thus, that a wide repository of theories may contribute decisively to an image of human evolution if they are assigned to their correct period. Unfortunately, fossils cannot help us when it comes to studying differences in skin and hair. But it seems acceptable from our analysis that nakedness might have evolved in a more closed, forested habitat and preceded bipedality. To support this, many other theories competing in the past may be harmonised, to yield important contributions to the understanding of why we stand naked today.
Angarika Deb has completed her MSc in Evolutionary and Behavioural Ecology from the University of Exeter (Department of Biosciences), UK.
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