As promised in my previous post, I decided to explore the similar eye-shape in Khoisan Africans and East-South Asians. The former are said to be the "oldest", most diverse group of humans, that split from the original Homo sapiens group and remained isolated in Southern Africa for ovre 150,000 years. Yet they have some particular features not found in other African groups, but common in Eurasia (pale pigmented skin, and a typical Asian feature in their eyes, the epicanthic fold).
Few papers on eye-shape genes
Unexpectedly, even though tens of thousands of papers have been published on genes and their effect on human health and features, there are just a handful dealing with the epicanthic fold, and none of them have identified the genes originating it.
I have found comments on the Internet stating that it evolved independently in Africa and in Asia. That is is the result of convergent evolution (like the one that shaped mammal dolphins, fish sharks, and reptilian ichtyosaurs, or mammalian bats, birds and flying pterodactyl reptiles) where bodies are shaped to forms that are best adapted to their environments. But no in-depth analysis of genes producing these eye-shapes.
I wonder if the lack of publications is due to some unconscious bias, where scientists don't want to look into an Asian feature found in a small allegedly "ancestral" population, a fact that seems to go against the established Out of Africa origin of humans.
Why is it prevalent among the "ancient" (150 ky old) Khoisan in southern Africa, and the modern Out of Africa humans in Eastern Asia (40 ky old) and South East Asia?
Its presence elsewhere is due to migrations from Asia, moving into northern Europe, America and Polynesia.
Epicanthal Folds
The inner corner of the eye, or medial canthus has a distinct appearance in some human populations. It is quite frequent among East Asians, and also among Tibetans, Central Asians, South East Asians, American Natives, and even North European groups with ancient Asian origins, like the Sami and Finish people. Interestingly, the Khoisan people also have this feature. The map below shows its global distribution (but the prevalence is lower in America, Europe and South Asia, highest in South Africa and East Asia (Source). The map also provides some genetics to the different regions, but since it came from Reddit, and I haven't found research to back it, I am doubtful. It mixes mtDNA and Y-chromosome haplogroups with the EDAR variant.
One paper (Peng et al., 2015) studied Uyghur people, and found that EDAR V370A may be related to the fold, but it only seemed to be statistically significant for the left eye fold, but not for the right one. The authors also note that EDAR V370 plays a role in embryonic development and therefore affects ectodermal-derived features like teeth and face shape, hair thickness, sweat glands, etc. I have posted about the EDAR gene and the peopling of America. Below is the data from this paper:
The Fold itself
What is the Epicanthal fold: it is a skin fold on the upper eyelid with an oblique or vertical alignment that covers the medial canthus (inner corner) and gives the eye a distinct external appearance. It has no effect on the zize of shape of the eye itself, but it makes it appear as almond shape, more "slanted" and smaller.
"The genetic basis for epicanthus is not well known" (Source), but I came across some interesting facts about the development of this feature: all human embryos: "in all races during foetal life" (Source). Most babies lose them by the time of birth, but other babies are born "with epicanthic folds, In some people the folds are retained into adulthood, while in some people they reduce at an early age... and often grow out of it as the bridge of their nose grows" (Source).
The lower nose bridge a common feature in Asians and Khoisan people influences the muscles on their faces, and the tensions produced by them which may be one of the causes of these folds.
" It is normally most prominent during childhood. As the bridge of the nose gains more height with age, it pulls the skin away from the eye, diminishing and reshaping the fold." Source
Kwon and Nguyen, 2015, in a paper on epicanthic eyes, confirm the uncertain origin of this trait: "the developmental mechanism of Asian epicanthus has not been defined clearly" and attribute the eye shape to the thickening of a muscle (the preseptal orbicularis oculi muscle) caused by an environmental factors such as ultraviolet radiation, cold weather, dust in the air leading to "excessive frowning, and repeated excessive frowning can induce orbicularis muscle hypertrophy ... There would be a strong repeated contraction of upper orbicularis muscle and depressor supercilii muscle in the Asian eyelid from frowning. Excessive muscle contraction would be an unavoidable action in the protection of eyes from environmental harshness. Hence, environmental adaptation would be a basic cause for the formation of the epicanthus"
Those born with this trait would have been better adapted in these harsh enviornments, and the feature could have been fixed in these populations by natural selection. As the Khoisan live in the Kalahari desert, dust there could have selected for this variant. However, why is it not present among people living in the Sahara or in Saudi Arabia, also dusty, desertic, UV-radiated regions?
Asia, Africa and human origins
Interestingly, Yuan, 2019 proposing an Out of Asia and Into Africa theory, suggests an Asian origin for the distinct features of the Khoisan: "Fossils or traits indicating AMH [Anatomically Modern Humans] migration from East Asia into Africa or Europe have been noted before. First, native Africans such as Khoisans are well known to have certain East Asian features such as shoveling teeth, epicanthic fold, and lighter skins"
But how did this trait, so common in Asia reach South Africa and leave no traces in the intermediate regions (Iran, Middle East, South Asia, Caucasus, North Africa, Central Africa, Western Africa)?
Maybe the answer lies in the findings of a paper published by Krishna R Veeramah et al., 2001: the Khosian or people linked to them once occupied a much larger territory:
"Conventional thinking has tended toward a model where KhoeSan initially diverged from the ancestors of all other AMH groups and remained relatively isolated. However, the KhoeSan demonstrate deep genetic connections with other click-speaking peoples in Tanzania (Henn et al. 2011), with proposed time to the most recent common ancestor (TMRCA) estimates ranging from 35 to 110 kya (Chen et al. 2000; Knight et al. 2003; Gonder et al. 2007; Tishkoff et al. 2007). In addition, a genetic link with contemporary Ethiopian populations has also been proposed (Cruciani et al. 2002; Salas et al. 2002; Semino et al. 2002). This, along with linguistic evidence, suggests that the KhoeSan territory once covered a much larger area, extending further northwest toward the Great Rift Valley (Cavalli-Sforza et al. 1994; Blench 2006; Scheinfeldt et al. 2010). Recent autosomal-based analyses show a tendency for KhoeSan and Pygmies to cluster together and away from other sub-Saharan Africans (Zhivotovsky et al. 2003; Tishkoff et al. 2009; Sikora et al. 2011), leading to the hypothesis that the ancestors of these two populations may have once formed a proto-KhoeSan–Pygmy hunter-gatherer group that was geographically widespread before being encroached upon by expanding agricultural populations."
If their homeland reached further north, it is possible that a migration from Asia could have reached Eastern Africa and admixed with the ancestors of modern Khoisan. Then, other African groups, with archaic admixture advanced and pushed them into their current territory, closing the door for furhter mixing. These dark-skinned Africans erased the ancestral San people in East Africa.
Perhaps the Hadza and Sandawe people from Tanzania are a relict of the old San click-speaking people of East Africa
The EDAR gene mentioned further up also influences the shape of teeth, and in this, the teeth of Homo erectus and modern East Asians share the same shape! Research on the EDAR 1540C variant states that "the continuity of shovel-shaped incisors between Homo erectus and modern humans in East Asia was a rationale for the multiregional evolution theory of modern humans, although this theory is not generally supported at present" (Kimura et al., 2009).
So, a bolder alternative to its distribution could be that the ancestral origin is found among the Homo erectus, who originated in Asia, back-migrated into Asia and passed it on to the Khoisan people, with their click language. It entered the continent along a southern Asian route, leaving no trace in Southwestern Asia or North Africa. They entered Africa from the East, settling there, later modern humans picked it up from these erectus in China and in Africa, leading to its current distribution.
Let's see what further research brings.
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