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Natural Hairstyles
Natural hair, black hair, and afro-textured hair are terms used to refer to the texture of Black African hair that has not been altered chemically (by perming, relaxing, straightening, bleaching or coloring). Not all people of Black African descent have naturally afro-textured hair, although the overwhelming majority does. In particular, certain groups such as the Fulbe of West Africa have a few members with hair that ranges from straight to loosely coiled, due to admixture with non-Black African populations. Nonetheless, among non-admixed Black Africans, tightly coiled, Afro-hair is a ubiquitous trait. Adjectives such as “hard”, “kinky”, “nappy”, or “woolly” are also used to describe natural hair. This hair is typically tightly coiled and soft to the touch. Andamanese Negritos and most Melanesian people also have tightly coiled hair.
Afro hair texture global distribution
Afro hair is a predominant characteristic of Black 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. It also has to do with the retention of that which was adaptively essential at the equator (dark skin) and the loss of that which was no longer essential (Afro hair) following admixture. Specifically, after the migration of a group 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, in addition, 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 is estimated to have arisen. 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. This feature may have been advantageous in this context because coiled hair slows and/or prevents the entry of UV radiation (and possibly other forms of radiation such as heat/ultra red) into the skin. Once dark skin evolved ~1 million years however, Afro-hair texture was less crucial in terms of protection from UV rays, but it was likely sustained in the population because, as mentioned, it had the ability to prevent UV (and possibly heat) from entering the body (or in this case, head) and thus cool and protect the brain. Alternatively, the trait may have sustained high frequencies in the founding/original (African) human population simply due to the fact that, over the 1-2 million years of its existence, most of the various genes that determine it likely retained phenotypic monomorphism in the population due to the genetotypic dominance of coiled/curled hair genes over straight hair genes. In other words, an ancient selective sweep likely occurred for these genes while skin was darkening 1-2 million years ago which hasn't reversed itself due to sustained levels of inter-African admixture, and/or the genetic dominance of the factors determining the phenotype. Notably, sexual selection is another possible factor that cannot be completely ruled out (although it seems unlikely that the distinct populations occupying the entire sub-Saharan region of Africa would have the same taste with regards to hair form).
In sum, after the re-evolution of straight hair among those who migrated from Africa to northern Eurasia ~50-60,000 years ago, some of these northerners likely went southward (before the Holocene) to places like India. 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 northerners arrived, they intermixed with the dark skinned, Afro-haired inhabitants of the region giving rise to a (possibly sexual) selective sweep for straight hair and dark skin. In the case of Polynesians and South American Indians, archaeological and genetic evidence suggests that (straight haired) migrants of Northern East Asian descent were the first to populate these regions; making them the predominant inhabitants of this region 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 (or replacement of) 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 (when humans became hairless to enable perspiration). It should also be emphasized that Afro-hair texture is no longer essential to survival at the equator given the presence of dark skin. This explains why dark skinned straight haired (northern influenced) groups are able to survive in UV-rich regions (such as India, Australia, Polynesia, and South America) without tightly coiled (Afro) hair texture.