Welcome to a Scintilla of Playful Musings

Welcome to my new blog, noos anakainisis, translated literally as mind renewal. The primary obsessions are neuroscience, computation, information, structure, form, art and history of science. Some environmental, political, and technological developments will also be included.

I hope your neurons are sufficiently stimulated...

Tuesday, September 7, 2010

The Bare Skin Hypothesis

A paraphrase of the hypothesis offered by Professor Nina G. Jablonksi:

Starting 3 million yrs ago, earth entered into a phase of global cooling that had a drying effect in East and Central Africa, where our human ancestors lived. The decline in regular rainfall changed woodlands into open savanna grasslands. The dwindling resources of fruits, leaves, tubers and seeds as well as drinking water forced our ancestors to abandon leisurely foraging habits for sustained activity of walking/running many miles to stay hydrated and obtain enough calories. Around this time, hominids also began incorporating meat into their diet, as revealed by the appearance of stone tools and butchered animal bones around 2.6 million yrs ago.

Homo ergaster evolved essentially modern body proportions that would have permitted prolonged walking/running and details of the joint surfaces of the ankle, knee and hip make clear that these hominids actually exerted themselves in this way.  The increase in walking and running builds up heat internally in the muscle and would have required that hominds both enhance their eccrine sweating ability (2-5 million watery glands close to skin surface that can produce up to 12 liters of sweat a day, rather than oily apocrine and sebaceous glands associated with deeper hair follicles, all of which develop from the same unspecialized epidermal stem cells) and lose their body hair to avoid overheating in the hot open savannas.  This combination of naked skin and watery sweat that sits directly atop it rather than collecting in the fur allows humans to eliminate excess heat very efficiently.  For furry animals, the effectiveness of cooling diminishes as an animal's coat become wet and matted with this thick, oily sweat.  Under conditions of duress, heat transfer is inefficient (evaporation occurs at the tips of the fur rather than the surface of the skin), requiring that the animal drink large amounts of water, which may not be readily available, in which case, the animal will collapse from heat exhaustion.  Human cooling system is so superior that in a marathon on a hot day, a human could outcompete a horse.  

MC1R gene is one of the genes responsible for producing skin pigmentation.  A specific gene variant always found in Africans with dark pigmentation originated ~1.2 million years ago.  Early human ancestors are believed to have had pinkish skin covered with black fur, much like chimps, so the evolution of permanently dark skin was a presumed requisite evolutionary follow-up to the loss  of our sun-shielding body hair.  

Comparison of human and chimp DNA reveals that one of the most significant differences are in the genes that code for proteins controlling properties of the skin (waterproofness, scuff-resistance).  The outermost skin layer--the stratum corneum of the epidermis--is composed of flattened, brick-like dead cells--corneocytes--which contain a unique combination of proteins, including novel types of keratin and involucrin, and are surrounded by ultrathin layers of lipids that act like mortar. Most genes directing SC development are ancient and highly conserved among vertebrates, so the human mutations signify that they were important to survival.

Maintenance of hair in armpits and groins despite loss elsewhere must serve to propagate pheromones (chemicals that serve to elicit behavioral responses from other individuals) and to help keep these areas lubricated during locomotion.  Hair on the head was most likely retained to help shield against excess heat on the top of the head (a barrier layer of air between sweating scalp and hot surface of the hair, with tightly curled hair being the optimum for max thickness of this airspace).  Other hairtypes/body types evolved as humans dispersed out of tropical Africa.

Daniel E. Lieberman and Dennis M. Bramble. (2007) The Evolution of Marathon Running: Capabilities in Humans.  Sports Medicine 37(4-5): 288-290.

Alan R. Rogers, D. Iltis, S. Wooding. (2004) Genetic Variation at the MC1R Locus and the Time since Loss of Human Body Hair.  Current Anthropology, 45(1): 105-108.

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