Asian people have unique-looking eyes, and as I understand it, one of the reasons (and maybe the only reason, I don’t know) that there is an extra fold of skin in the eye-lid, giving the eye its unique shape and appearance.
I assume this was due to an evolutionary need… For some reason, that eye feature was needed by people who lived in a certain region of the world, and it was so beneficial, those humans in that area continued to evolve with this eye feature intact. I guess it could be the other way around, in that maybe eyes on caucasians, for example evolved their round shape and lost that extra fold of skin, but I am assuming the asian eye features evolved from the eyes that most of the rest of the world seem to have.
I have heard or read somewhere that the extra skin permits a tighter seal, protecting the eye from harsh winds and cold temperatures, but I have no idea if this is true.
My questions:
is there an explanation for this eye development, as far as why it evolved and what the benefit of the extra skin flap provides?
did any animals in the same area of the globe develop a similarly-equiped eye lid? I’m guessing no, but I don’t know anything about animal eye lids, so I thought i’d ask.
I would think that if something beneficial to a mammal’s sight (or protection of its sight) evolved for one mammal, it may evolve for all mammals in a similar environment. Or, would this be a trait that would evolve too far down the lines of evolution to only be seen on the humans of a particular region?
My apologies for anything that may be offensive in this question. Nothing was meant to be offensive, and I hope anyone who might be offended would accept this pre-emptive apology.
The trait is called an epicanthic fold. It is most prevalent in populations of northeast and central Asia. It’s also found in Inuits and Eskimos, and even some northern Europeans. A couple of isolated populations that show it are the San (Bushmen) of southern Africa and the Andameseof islands off South Asia. The trait in these groups may have separate origins.
Like many such traits, the adaptive value of the fold is speculative. It is usually postulated to be an adaptation to shield the eyes in a cold, dry climate (or in the case of the San, just dry.) However, this would be difficult to prove.
Animals in cold dry environments may show adaptations to protect the eyes such as thick eyelashes, but I am not aware of any that have a similar kind of fold, even cold-weather primates.
Not a definitive one. It should be noted that certain African population (the Bushman, for example) also have epicanthic folds, so this condition may have evolved in Africa, and not in harsher, Asian climates. Or it may have evolved more than once, like lighter skin appears to have done.
The trait could be homologous between the Bushmen and Andamanese, since the Andamanese are thought to be close to the first populations that migrated out of Africa. It’s not out of the question that it could be an ancestral survival in northern Asians, but this would be surprising since it is less prevalent in southeast Asians (which would presumably have had a more direct relationship with the populations ancestral to the Andamanese).
Epicanthic fold. All humans have it as fetuses. In white people sometimes it survives into childhood and disappears as the bridge of the face develops.
Warner Oland, who played Charlie Chan, was Swedish.
As has been mentioned, the fold is retained in Down’s Syndrome (which for this reason at one time was known as Mongolism) and in some other congenital conditions such as fetal alcohol syndrome.
Since in populations that don’t have the fold as adults it disappears as the bridge of the nose develops, this opens the possibility that the fold may not be adaptive in itself, but instead may be associated with differences in the nasal structure between some populations.
As I understand evolution theory, something that is tweaked in the gene pool will not propagate fast enough to take hold if it didn’t have some functional value…
So, if this was indeed just a random occurrence with no functional value, wouldn’t it have just disappeared within the population over time? Certainly it wouldn’t be likely to be seen more and more in subsequent generations, correct?
Genetically influenced physical structures don’t have to be beneficial. But if they are detrimental, they’ll be selected against. That doesn’t mean they’ll disappear, but they’re less likely to propagate.
A couple years ago I showed a friend a photo of a pretty Asian woman with beautiful eyes. She was showing her lower girly bits directly to the camera and holding the labia open with her hands. I said “One thing I love about Asian women is their epicanthic fold”. The guy had no idea what it was and assumed the fold it must be down there somewhere but did not want to ask and went along with the conversation uneasily. He finally admitted he did not know what I was talking about and smiled sheepishly when I told him what the epicanthic fold is.
I understand this point about eye color. However wouldn’t asians be considered to have homogeneous eye and hair color? Black hair, and either dark brown or black eyes is all i’ve ever seen. And is this the same as the epicanthic fold? So if the fold did not provide any advantage in the evolution of the asian population’s eye, how/why did it develop only in the asian population, and why is it observed in close to 100% of the population?
Because I would think that the fold was either a change in the asian genetic code and propagated through population selection, or the rest of the world’s people who don’t have the fold selected it out over time if it didn’t have any real advantage or disadvantage and they didn’t find it appealing
Is this a correct assumption?
I would understand this in comparison to eye color if only a small percentage of asian people had this trait… Say the same percentage of asian people who have green eyes. (i don’t know the population well enough to know if there are any variations… The only green eyed asian girl I ever saw was in the movie “Big Trouble In Little China” (starring Kurt Russell!)). If eye color doesn’t matter, why do almost all asians have dark eyes (I’d say extremely dark brown or black) and black hair? Was there an advantage to have these hair/eye colors for the region they populated, or was this a trait that was self-selected because it was appealing to the reproducing population?
Or is there no answer to this?
A [dominant] genetic change related to an independent phenotype characteristic that is not advantageous will be selected against.
A change that is neutral, or associated with more than one characteristic, or, for that matter, just recessive, will/might not be selected against.
Always remember that ‘survival of the fittest’ was an economic theory, and would lead to too many evolutionary dead ends. The most viable species have wide ranges of critical characteristics, to improve that chances of survival in shifting environments.
No, I never have seen any Ainu. Ever. I will do a search and see what you are referring to.
I understand what you are saying. And yes, a trait can be passed on without being advantageous. But over time, a trait like that could probably be something appealing to the reproducing pair.
I’m not sure if it’s been mentioned yet, but Asian eyes are actually two traits. one is epicanthic folds, which are from the fact that Asians have smaller noses. This could in part be a neotenous trait. Additionally, east Asians also have fat deposits on their eye lids. The combination of these two traits is what tends to give Asian eyes their distinctive look. The fat deposits are believed to have spread out of Mongolia from the last I heard. There’s some speculation that it could have something to do with keeping sand out of your eyes. But I’m not sure how much I buy it.
I have always wondered about it also. Here in the Yucatan peninsula, most of the indigenous people’s children have Asian eyes. As they get older, this feature disappears. I always assumed it was because their ancestors came via the Bering Strait.
