Thursday, January 22, 2015

Why do Asians Have Slanted/Small Eyes?


Why do Asians Have Slanted/Small Eyes?
Multiple Responses:
“The "slant" you are referring to is called an epicanthal fold. It was primarily an adaptive trait for those in Central Asia and Eastern Asia to block some of the sunlight that was reflecting off from the white surfaces, that being snow among many other related elements. This being the reason, such fold isn't signature to only Asians. Afro-Asians, the Inuits, people of Madagascar and Native Americans are documented to having this trait too.

As to why the epicanthal fold is dubbed as ugly and bigger eyes are seen as beautiful, it can be answered in two ways. Psychology research has shown that the fundamental reason as to why babies are viewed as cute is because they have larger eyes than that of adults. Ever noticed why cartoons aimed at children have large eyes as a way of funneling the "cute factor" to both kids and adults alike, e.g. Wall-E:
It's also why Manga is so popular nowadays because the eyes of most of the characters are bigger than any other artistic entities.
The other reason is that Eurocentrism and American Imperialism has plagued the world in the sense that Western standards of "beauty" have become universal, with even Africans and Indians lightening their skins to be tantamount to that of the "ideal".

You should be proud of what God gave you, and not of what Man wants you to be.”

“The small, almond-shaped look of the eyes has always been a feature that is uniquely Asian. Most people of non-East Asian descent have big and round eyes, so some Asians feel the need to "normalize" themselves. Not all Asians have small/slanted eyes though.”

“The trait that makes "Asian eyes" distinct from "non-Asian eyes" is epicanthic folds, which are little flaps of skin on the eyelid. Epicanthic folds aren't only found in Asians; they're also common in some European populations and in the Khoisan, an African population that forms the oldest branch on the modern human family tree.

There are also some birth defects that often cause epicanthic folds as a side effect. There isn't just one allele that causes epicanthic folds; lots of different loci can code for them. Because of this, the developmental pathways that produce epicanthic folds aren't very clear.”

“The unfortunate answer is that nobody really knows.

It was once thought that it was related to the plica semilunaris, a vestigial structure from the nictitating membrane, a so-called "second eyelid" that some animals have. It was thought that the epicanthic fold served to protect some against snow blindness. It sounds good, but it doesn't appear to be correct. The answer might even be somewhat opposite.

It can actually appear in any population, but it tends to appear in people with small noses. Specifically, the flatter the nasal bridge, the bony part of the nose, the more likely an epicanthic fold will appear. The populations in which the epicanthic folds most often appear, such as some Asians and Khoisans, tend to have flat noses.

Since most infants of whatever ethnicity are born with detectable epicanthic folds, it may simply be that as the nose gets bigger (when it does) the shape of the eyes changes.

That would raise the question of why Asians etc. have small noses. Maybe that's the wrong question, though. Maybe we should ask why other groups have bigger noses. That seems at least plausibly a more tractable problem, as there is evidence that bigger noses are associated with bigger sinuses, which serve to warm air in cold climates.

Still, that doesn't explain why Bantus, with fairly flat noses (my girlfriend is Bantu, and so I get to look up close) don't usually have prominent epicanthic folds.

Evolution is complex, though. Perhaps the bignose mutation simply didn't happen for certain populations exposed to the cold and snow but another adaptation for smaller eyes did, or an adaptation for bigger eyes didn't happen. Bigger eyes might seem to be an advantage in Africa, considering how much natural camouflage goes around. But that's also true in South America, and it didn't seem to happen there.

It's all rather unsatisfying, I'm afraid.”

“If you live in the tropics, dark skin is a helpful adaptation, because it protects your skin from sunlight (fewer sun burns and cancers).
If you live in the northern latitudes where there is significantly less sun certain times of the year, light skin is a helpful adaptation, because it allows more sunlight to penetrate the dermis, thereby creating more vitamin D.
The primary characteristic of east Asian eyes is called the epicanthic fold, and we believe it may have evolved to protect the eye from harsh winds, but the jury is still out on that one.”

“It was recently discovered that a collection of physical traits typical in Asian populations (thicker hair shafts, more sweat glands, teeth shape, smaller breasts) are all the result of a single gene mutation that occurred 35,000 years ago in what is now China. There are a few explanations as to why that may have spread through generations to become so widespread, but scientists aren't sure which is correct. It could be a combination of them. This article goes into detail:

extra sweat glands could have been the feature favored by natural selection, with all the other effects being dragged along in its train...the EDAR variant arose about 35,000 years ago in central China and that the region was then quite warm and humid. Extra sweat glands would have been advantageous to the hunter-gatherers who lived at that time.

Thick hair and small breasts are visible sexual signals which, if preferred by men, could quickly become more common as the carriers had more children.

each of the effects of the EDAR variant may have been favored by natural selection at a different time. A series of selections on different traits thus made the variant version so common among East Asians.

Of course these gene isn't responsible for the eye shape (epicanthic fold) common in Asian populations, but it's the same principle.

Melanin is the pigment in human skin that makes it dark. It protects from the harmful effects of the sun's UV radiation, keeping the UV from damaging skin and the DNA in our skin cells.

As early humans migrated out of Africa, they were all dark from having evolved near the equator and needing the protection of melanin from the fierce sun. At some point, a mutation arose in a population that had migrated out of Africa that caused lighter color skin.

Some scientists suggest that lighter skin offered a strong survival advantage for people who migrated out of Africa by boosting their levels of bone-strengthening vitamin D; others have posited that its novelty and showiness simply made it more attractive to those seeking mates.

EDIT: Vitamin D is made in your skin as a reaction to sunlight, so the Vitamin D theory of light skin goes that in the weaker sunlight of northern climates, having a light skin mutation gave you an evolutionary advantage because more of the sun's energy would penetrate the skin for form Vitamin D.

TLDR We don't know for sure but here's the "how" and some theories "why."”

Gaining a deep insight into human evolution, researchers have identified a mutation in a critical human gene as the source of several distinctive traits that make East Asians different from other races.

The traits — thicker hair shafts, more sweat glands, characteristically identified teeth and smaller breasts — are the result of a gene mutation that occurred about 35,000 years ago, the researchers have concluded.

The discovery explains a crucial juncture in the evolution of East Asians. But the method can also be applied to some 400 other sites on the human genome. The DNA changes at these sites, researchers believe, mark the turning points in recent human evolution as the populations on each continent diverged from one another.

The first of those sites to be studied contains the gene known as EDAR. Africans and Europeans carry the standard version of the gene, but in most East Asians, one of the DNA units has mutated.

Seeking to understand if the gene was the cause of thicker hair in East Asians with the variant gene, a team of researchers led by Yana G. Kamberov and Pardis C. Sabeti at the Broad Institute in Cambridge, Mass., decided to test the gene in mice, where its effects could be more easily explored.

Mice already have EDAR, an ancient mammalian gene that plays a leading role in the embryo in shaping hair, skin and teeth. The Broad team engineered a strain of mice whose EDAR gene had the same DNA change as the East Asian version of EDAR.

When the mice grew up, the researchers found they did indeed have thicker hair shafts, confirming that the changed gene was the cause of East Asians’ thicker hair. But the gene had several other effects, they report in Thursday’s issue of the journal Cell.

One was that the mice, to the researchers’ surprise, had extra sweat glands. A Chinese member of the team, Sijia Wang, then tested people in China and discovered that they, too, had more numerous sweat glands, evidently another effect of the gene.

Another surprise was that the engineered mice had less breast tissue, meaning that EDAR could be the reason that East Asian women have generally smaller breasts.

East Asians have distinctively shaped teeth for which their version of EDAR is probably responsible. But the mice were less helpful on this point; their teeth are so different from humans’ that the researchers could not see any specific change.

The finding that the gene has so many effects raises the question of which one was the dominant trigger for natural selection.

Dr. Sabeti said the extra sweat glands could have been the feature favored by natural selection, with all the other effects being dragged along in its train.

“We’re the only mammals to have changed their entire hair pattern. So the changes in teeth, hair and breasts — it’s very possible they are the passengers and thermoregulation is the key,” she said, referring to the role of sweat glands in cooling the body.

East Asians are sometimes assumed to have evolved in a cold environment because of their narrow nostrils, which conserve heat, and the extra eyelid fat that insulates the eye. But the Broad team calculates that the EDAR variant arose about 35,000 years ago in central China and that the region was then quite warm and humid. Extra sweat glands would have been advantageous to the hunter-gatherers who lived at that time.

But Joshua Akey, a geneticist at the University of Washington in Seattle, said he thought the more likely cause of the gene’s spread among East Asians was sexual selection. Thick hair and small breasts are visible sexual signals which, if preferred by men, could quickly become more common as the carriers had more children. The genes underlying conspicuous traits, like blue eyes and blond hair in Europeans, have very strong signals of selection, Dr. Akey said, and the sexually visible effects of EDAR are likely to have been stronger drivers of natural selection than sweat glands.

Yet a third view is held by Dr. Kamberov, who believes that each of the effects of the EDAR variant may have been favored by natural selection at a different time. A series of selections on different traits thus made the variant version so common among East Asians. About 93 percent of Han Chinese carry the variant, as do about 70 percent of people in Japan and Thailand, and 60 to 90 percent of American Indians, a population descended from East Asians.

The Broad team is studying EDAR as part of a larger plan to identify all the genetic variants responsible for recent human evolution. Many researchers, including Dr. Sabeti, have devised ways of scanning the human genome to detect the fingerprints of natural selection. But these scans have typically identified large chunks of the genome that contain many genes. There is often no way to tell which gene was the target of natural selection.

A team led by Dr. Sabeti and Sharon R. Grossman of the Broad Institute has now refined the usual scanning methods and identified 412 sites on the genome that have been under selection. Each site is small enough that it contains at most a single gene.

Each race has a different set of selected regions, reflecting the fact that the human population had dispersed from its African homeland and faced different challenges that led to genetic adaptation on each continent. About 140 of the sites affected by natural selection are in Europeans, 140 in East Asians and 132 in Africans, the authors report in another article published Thursday in Cell.

Inserting some of the other selected genes into mice might help explain why they were favored, and point to critical turning points in recent human evolution, Dr. Sabeti said.

In the case of EDAR, putting the gene into mice has only magnified the mystery of why it was selected. But the researchers are not discouraged. “A reflection of good science is that a step forward opens up a lot more questions,” Dr. Akey said.

“For the sake of answering, we will skip the fact that the illusion of "smaller" eyes largely applies to just EAST Asians, and not the many other peoples such as Iranians, Indians, Arabs, etc. East Asians don't really have "tiny" eyes; it is really the same size as non-East Asians. They simply have a patch of extra skin (called an epithelial fold) above their eyes, because that sort of thing combats the extreme cold and fierce winds of Tibet, where the ancestors of East Asians lived for many tens of thousands of years before migrating down into warmer places. Since the epithelial fold is a part of their DNA, it stayed, even though the need for it no longer exists for all Asians but the Tibetans.”

“Not all Asians have “smaller” eyes. This mainly applies to East Asians. Their eyes aren’t technically smaller, either. They simply have an extra patch of skin called an epithelial fold above their eyes.

That fold allows them to protect their eyes against extreme cold and fierce winds that occur in Tibet. Average annual snowfall is 18 inches and the land is severely dry 9 months out of the year.
The epithelial fold is simply the way they adapted to their environment. Tibet is where the ancestors of many East Asians lived for tens of thousands of years before migrating to warmer places. The epithelial fold, however, is part of the DNA now and so continues to be a trademark of East Asians.”

“I remember reading somewhere that it had to do with the glare of the sun off the flat steppes of Asia. Their eyes are more 'squinty' to block out more direct rays of sun. This theory could be completely wrong, but its seems to at least have some intuitive merit.”

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