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Topic ClosedThe Evolution of Afro-Textured Hair

 
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Direct Link To This Post Topic: The Evolution of Afro-Textured Hair
    Posted: Dec 27 2010 at 9:47am
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Direct Link To This Post Posted: Dec 27 2010 at 9:48am

Adaptation

Tightly coiled hair is typically described as being an evolutionary adaptation to the warm, tropical climates of sub-Saharan Africa and Melanesia (although it does not exist in tropical regions such as India or indigenous South America and thus may have evolved for a different original purpose). Some hold that tightly coiled hair may allow for greater cooling because sweat is able to remain on the scalp.[1] Furthermore it has been asserted that, when the sun is directly overhead, the top surface of the hair heats up while leaving a barrier of cooler air between the scalp and the top surface of the hair. This barrier of cooler air assists in cooling the brain. This effect is most prominent in people with 'frizzy' (Afro-textured) hair.[2]

The context in which Afro hair texture arose

Evolutionary biologists suggest that the genus Homo arose in East Africa approximately 2.5 million years ago.[2] During this time new hunting techniques were innovated.[2] The higher protein diet led to the evolution of larger body and brain sizes.[2] In Skin: A Natural History, Nina Jablonski 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.[2] 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, may have enabled humans to warm themselves at night using animal skins. Furthermore, as the furless condition slowly developed, genetic evidence suggests that dark skin color may have gradually evolved to protect the body from UV during the day (and thus compensate for the sparse hair coverage). Thus, it is still possible that pre-humans lost fur mostly for the purpose of facilitating the evaporation of sweat and the corresponding cooling of the body.

The evolution of Afro hair texture

Jablonski agrees that it was evolutionarily advantageous for pre-humans (Homo erectus of ~2 million years ago) to retain the hair on their heads in order to protect the skin there as they walked upright in the intense African (equatorial) UV light. Auxiliary hair (in the groin and 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 being straight (the condition of most mammals, including humanity's closest cousin--the chimpanzee), to Afro-like or 'kinky' (ie tightly coiled). This is made clear in a study by Iyengar (1998) which has provided evidence that the roots of straight human hair may act as optic fibers that allow UV light to pass into the skin. In this sense, during the period in which pre-humans were gradually losing most of their straight body hair (fur) and thereby exposing the probably pale skin underneath their fur to the sun (Rogers et al, 2004), straight hair would have been an evolutionary liability. 'Kinks' in fiber optic tubes are known to prevent UV from passing through. Hence, tightly coiled or 'kinky' hair may have evolved to prevent the entry of UV light during the gradual transition period towards the evolution of dark skin and the simultaneous transition from hairiness to virtual nudity. Later, after a group of modern humans left Africa approximately 60,000 years ago, straight hair re-evolved among those members who migrated to northern Eurasia (see hair texture).

The global distribution of Afro hair texture

Afro hair is a predominant characteristic of sub-Saharan Africans, Andaman Islanders, and Melanesians. It is often posited that this hair texture (which is unique among all mammals--and most humans) is an adaptation to tropical climates. However, as mentioned, many (dark skinned) straight haired people have also been found to thrive in similar types of warm equatorial environments. Thus the distribution of the trait likely has more to do with the migration patterns of those who left Africa to populate the rest of the world within ~60,000 years ago (Quintana-Murci et al, 2004). It also has to do with the retention of that which was adaptively essential at the equator (ie dark skin) and the loss of that which was no longer essential (ie Afro hair) following admixture. Specifically, after the migration of modern humans out of Africa, those who settled in warm sunny regions similar to sub-Saharan Africa, like the Andaman Islands and Melanesia (and also remained isolated from straight haired northern migrants) did not experience adaptive (nor admixture) pressure for their hair to straighten. Thus it remained Afro-like.

Nonetheless, in places like India, South America, Australia, and Polynesia, hair is straight despite warm, UV-rich climates. This is explainable given the context in which Afro-hair likely arose (which is described above). To reiterate, this unique texture likely evolved 1-2 million years ago, just before the time that dark skin (Harding et al, 2000) is estimated to have arisen (as indicated by human MC1R genetic demographic patterns--see Harding 2000). The trait may have reached high frequency in order to compensate for the gradual process in which relatively pale hominid (pre-human) skin was being exposed to the African equatorial sun as it lost its protective fur (as indicated by Iyengar 1998--also see Rogers et al 2004). Once dark skin evolved however, the Afro-hair texture was no longer essential for protection. Nevertheless, it was not a liability either (by contrast, it was indeed a liability for those who migrated to northern Eurasia--see hair texture). Therefore, there was no pressure within Africa to re-evolve straight hair.

In addition, because intermixture within the large sub-Saharan African population remained relatively high for a significant portion of its pre-history (compared to those who migrated outside of this region), severe, sustained isolation and its associated intensive inbreeding (ie bottlenecks) did not occur (Tishkoff, 1996). Thus, straight hair also did not naturally arise by way of random mutation and isolation (ie genetic drift) among any of the subgroups of the sub-Saharan region. In fact, it is very likely, given the basically ubiquitous distribution of the Afro-hair trait among contemporary sub-Saharan Africans (the most genetically diverse macroethnicgroup on earth whose direct ancestors comprised the orignal population from which all of humanity derived), that at one time the Afro-hair trait characterized the entire human population (i.e. before the exodus from Africa and the settlement of some of the migrants in northern Eurasia).

In sum, after the re-evolution of straight hair among those who migrated from Africa to northern Eurasia ~60-50,000 years ago (see hair texture), some of these northerners then went southward (before the Holocene) to places like India (Quintana-Murci et al, 2004). Polynesian, Australian, and South American populations are also known to have been influenced (either slightly or entirely) by Northern migrants. For example, in the case of Australia (and likely India), when the northerners arrived, they intermixed with the dark skinned, Afro-haired inhabitants of the region (Redd & Stoneking, 1999). In the case of Polynesians and South American Indians, archaeological evidence suggests that (straight haired) migrants of Northern East Asian descent were to first to populate these regions (Diamond, 2005); making them the predominant inhabitants until modern times. Straight hair (combined with dark skin) thus came to dominate among these groups likely due to pre-Holocene waves of migration into these areas from the north and (in the case of India and Australia) subsequent admixture with the dark skinned, Afro-haired inhabitants.

Afro-hair is, in this sense, most likely a reminder/remnant of a crucial time in hominid evolutionary history (ie when humans became hairless). Also, while the unique texture may arguably provide additional comfort in hot areas, it did not initially evovle simply for this purpose. Furthermore, it should be emphasized that Afro-hair texture is, by far, not essential to survival at the equator given expression of dark skin. Thus, dark skinned, straight haired groups are able to survive there without this trait.

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Direct Link To This Post Posted: Dec 27 2010 at 3:23pm
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Direct Link To This Post Posted: Dec 27 2010 at 6:07pm
Originally posted by babycheex babycheex wrote:

innerresttinggg
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Direct Link To This Post Posted: Dec 27 2010 at 6:16pm
Geek hmmmm
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Direct Link To This Post Posted: Dec 27 2010 at 6:18pm
That is wayyyyy too long for my non-reading self 
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Direct Link To This Post Posted: Dec 27 2010 at 11:16pm
Originally posted by Zaharaluv Zaharaluv wrote:

That is wayyyyy too long for my non-reading self 

D:< READ A BOOK!
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Direct Link To This Post Posted: Dec 27 2010 at 11:32pm
what?
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Direct Link To This Post Posted: Dec 27 2010 at 11:34pm
interesting indeed. Thumbs Up
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Direct Link To This Post Posted: Jan 13 2011 at 1:23am
This is actually really interesting

Its true that our bodies (including hair), adapt to our environments and means of survival in that environment
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