Why Do Asians and Caucasians Have Different Eye Shapes?

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It's widely believed that all humans originated from Africa, which makes the modern physical variations between different races of humans pretty fascinating. How did we evolve to all be so different?

BabyMetal. Not only are they fun music performers, but also own traditionally 'Asian' eyes.

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Now, of course, curiosity about why some humans evolved differently than others has taken science down some damned dark places in the past. For example, when it intermingles with racist social prejudices and concepts of 'evolutionary superiority' you get eugenics and naziism. It would be wrong to approach this kind of subject and not acknowledge this past.
But if you accept that really none of these traits are superior to one another, you can learn something really interesting about how different populations of humans adapted to their different environments, which makes it a pretty fascinating branch of science.

Take, for example, the presence of epicanthic folds. Or, rather 'Asian' eyes vs 'Caucasian' eyes. Why did they appear? What function do they serve?


Biffy Clyro. Also fun music performers, but this time with traditionally 'Caucasian' eyes.



John David Ward of Quora explains why.


"I know this is going to sound really strange and clinical, but please bear with me. 


That distinctive Asiatic eye look is caused by a variety of things, including strong, forward projecting zygomatic arches (the cheek bones just under the eyes), relatively large epicanthic folds, smoothness caused by fat around the eye socket, and a flatter nose bridge. Not everyone has all of these traits, or has them to the same degree, but these traits all tend to reinforce each other visually, leading to a distinctive effect. 

In theory, these effects are presumably caused by a combination of continentality and sexual selection for neoteny (that is, for cuteness). The exact way this happened is still the subject of some controversy, but the correlation is well established.

The story goes that ancestors of the Asiatic peoples (that is, the people who live in modern day East Asia) migrated, in prehistoric times, from somewhere around Central Asia or Siberia to their current locations. That area is the place on Earth which is the farthest inland, and as a result experiences large seasonal swings in temperature. They would have adapted to deal with dry, cold winters, hot, bright summers, and dust. They would have had to deal with dust in the summer and glare from sunlight reflecting off snow in the winter, under conditions where survival was fairly difficult.

If environmental effects were all that mattered, moving south-east like this would have resulted in these Asiatic people losing their distinctive features as an adaptation to their now more temperate environment. But that wouldn't be instantaneous and environment is not the only factor in play. When these prehistoric Asiatic people reached more temperate and coastal areas, which were capable of supporting a larger population, they underwent a population explosion, which allowed sexual selection to take over. The Yellow River Civilization, for instance, which is the ancestor to the modern nation of China, had a relatively high population for many thousands of years.

Sexual selection is mating preference. That is, cuter people (as judged subjectively by the pool of potential mates) are more desired as reproductive partners and therefore are more likely to pass on their genes. Social selection in connection with infanticide in times of famine might also have played a role.

So we can say that features which originally arose as adaptations to an extreme, dusty climate were probably exapted through sexual selection.

Edit: I want to add that epicanthic folds are pretty common around the world, especially in children (they protect one's eyes while they're still developing), and the majority of people have low or flat nose bridges. So it's important to consider who you're comparing these Asiatic eyes to. 

If you're comparing them to Caucasians—which is not synonymous with "white" or "European" but is a collective term for the people who form the majority of the population in countries of the Middle East, Anatolia, Northern Africa, India, Europe, Russia, and former European colonies, who have commonalities, including facial features, regardless of their skin color—keep in mind that Caucasian eyes are not neutral. That is, they do not represent an unchanged original state; they have a distinctive look, too, and there's nothing obvious or inevitable about comparing East Asian eyes to Caucasian eyes rather than anyone else. The eyes of East Asian people look particularly different when you compare them to Caucasians, because the Caucasian peoples went through just about the opposite sort of evolution: their ancestors came from, and for the most part continue to live in, areas where the climate was stabilized by the presence of a nearby body of water (which, due to its tremendous specific heat capacity, tends to mediate year-old temperatures by emitting heat in the winter and absorbing it in the summer).

If you look at an atlas, you'll see for yourself that the north-western quadrant of the Old World is much craggier than the north-eastern quadrant, with a lot of coastline, full of islands (like the British isles and the islands belonging to Italy and Greece), peninsulas (like Denmark, Italy, Little Britain, Iberia, and Scandinavia), bays (like the Bay of Biscay), seas (like the Caspian Sea, the Black Sea, the Red Sea, the North Sea, and the Baltic Sea, and the Mediterranean, most of which are remnants of the ancient inland Tethys Sea before it closed up), and gulfs (like the Persian Gulf and the Gulf of Aden).



Image from Wikipedia, if you didn't recognize the style


So when someone is described as having Asian looking eyes or Caucasian looking eyes, often all we're really saying is that they look like their prehistoric ancestors came from a highly continental area (like the middle of the continent of Asia) or a highly coastal area (like the area around the remains of the Tethys sea), just as when we describe someone as white or black, we're really talking about, via skin color, whether they look like their ancestors came from high latitudes (closer to the poles) or low latitudes (closer to the equator). It doesn't necessarily have anything to do with where their relatives are actually from.

For instance, South Africa is relatively continental (Africa is a much bigger continent than people think). The indigenous people of South Africa, because they live in and have adapted to a location far enough from the equator to have pronounced seasons, and where the coastline is relatively smooth, have eyes that are frequently described as looking "Asian." You can see this in this image of Nelson Rolihlahla Mandela, who is Xhosa, a ethnic group from South Africa:





Meanwhile, Melanesians and Australian aborigines, who migrated into their current locations by island-hopping through Southeast Asia, have eyes that look more "Caucasian," as you can see in this picture of Australian pop singer Christine Anu, who is a Torres Strait islander, and who had a brief role in The Matrix Reloaded:





Likewise, since Japan is an island country, is perhaps not surprising that the Ainu, who are the indigenous minority people of Japan, have facial features that have been described as "Caucasian," despite not being particularly closely related to any of the people of Europe, the Middle East, India, or North Africa.

See this picture of a random Ainu man:





Meanwhile, there are people in Europe who are (presumably) descended from the native people of the more continental region of Europe, called the Russian plain or the East European Plain, who have Asian-looking eyes, including quite pronounced epicanthic folds. Most of these people were, at some point, driven up into Finnland (or restricted to Finnland, since Finnland is sometimes considered part of the East European Plain itself) by the territorial expansion of the Eastern Slavs, so these sorts of features most commonly pop up in Scandinvia, but it can also be found occasionally in Poland (going by Wikipedia). For some reason I've never bothered to research, before routine contact with the Orient was established, European writers tended to describe these as "Tataric" features, despite the fact that they're not really present in Crimean Tatars, at least not today.

The most commonly cited example of Asian-looking eyes among Europeans is Björk Guðmundsdóttir, who, despite being of Icelandic nationality, has, through the genetic lottery, inherited via her Scandinavian ancestors a fairly impressive case:





Keep all this in mind when you think about the case of the anthropologists who examined the remains of Kinnewick Man found in Washington state. They were widely misreported as having stated that he was Caucasian. What they discovered, rather, was that measurement of Kinnewick Man's skull showed evidence of morphological adaptations to a coastal, rather than continental, climate which made him resemble that picture of an Ainu man more than that picture of Björk. That doesn't mean that he wasn't actually Native America. Osteology isn't everything. There's a lot more to a person than their facial features."



Source
- Your local blogger's taste in music :p


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