为什么人类眼睛的颜色仅限于棕色、蓝色、绿色，以及罕见情况下的红色，而不是其他颜色?
ELI5: Why are human eye colours restricted to brown, blue, green, and in extremely rare cases, red, as opposed to other colours?
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网友：好问题。我是眼科医生。眼睛的颜色取决于虹膜，虹膜有两层，一层是前层，一层是富含色素的后层。这种色素就是黑色素，黑色素使我们的皮肤呈现出不同的颜色......
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sixsidepentagon
Great question. Eye doctor here.
Eye color is in the Iris, and the Iris has two layers, a back layer that's always fully pigmented, and front layer.
The pigment is melanin, same that makes our skin different colors.
Now when the front layer is densely pigmented, it appears dark, like brown or even black (though shine a bright enough light and you'll see black eyes are just very dark brown).
If it's not pigmented, it appears light blue. Why? Same reason the sky's blue, light scatters in it. Light scattering is a different topic, but basically short wavelengths (ie blue) bounce differently in the fluffy front layer without pigment.
What about in between? Well it turns out if there's pigment but it's not super dense, it's a bit of a lighter brown or dark orange. We call these amber eyes.
If it's between amber and blue, then it's like a cross between light orange and blue. That's how you get green eyes.
So that's how you get the spectrum: from blue to green to amber to brown to black.
So what about "red" or "violet" eyes? Turns out I lied in the beginning; that back layer I mentioned that's always pigmented isn't in a medical condition: Albinism. These folks unfortunately have a defect in producing melanin pigment, which is why they all have fair skin and light hair and such.
So why red? It's the same reason you can get red eyes in flash photography; the red is the color of the retina in the back of the eye. Flash photography causes you to accidentally image peoples retinas.
Now without the pigment in the back layer of the iris, the iris almost functionally becomes like Saran Wrap; it's clear and doesn't block out light anymore. So when you see red (or violet) eyes, you're seeing retina through their Iris.
Side note: this is one reason why folks with albinism have poor vision. The purpose of the iris is to act like a camera aperature. Without pigment, it can't block out light like an aperature, removing a whole element of the focusing system of the eye.
好问题。我是眼科医生。
眼睛的颜色取决于虹膜，虹膜有两层，一层是前层，一层是富含色素的后层。
这种色素就是黑色素，黑色素使我们的皮肤呈现出不同的颜色。
当虹膜前层拥有密集的色素时，眼镜颜色就会变暗，变成棕色，甚至是黑色(在足够明亮的光线下，你会看到黑色的眼睛其实只是深棕色)。
如果虹膜前层没有黑色素，那么眼睛就是浅蓝色的。为什么?和天空是蓝色的原因一样，短波光线(即蓝光)在没有色素的虹膜前层发生了散射。
如果黑色素少一点，你的眼睛就会变成绿色。
那么如果虹膜前层黑色素不是非常密集，眼睛颜色就会是浅棕色或深橙色。我们称之为琥珀眼睛。
所以这就是人类眼睛颜色的变化规律，随着黑色素增多，从蓝色到绿色到琥珀色到棕色再到黑色。
那么“红色”或“紫色”眼睛是怎么来的呢?还记得我上面提到的富含色素的虹膜后层吗？每个人的虹膜后层都拥有黑色素，但除了一种情况:白化病。
不幸的是，白化病患者在产生黑色素方面有缺陷，这就是为什么他们都有白皙的皮肤和浅色的头发等等。
为什么会出现红色眼睛?这和用闪光灯拍照时眼睛会发红是一个道理;红色是眼睛后部视网膜的颜色。闪光灯会让你拍出人眼视网膜的图像。
由于白化病患者没有了虹膜后层的色素，虹膜就成了一个透明的保鲜膜，无法挡住光线。所以当你看到红色(或紫色)眼睛时，你实际上是看到了他们虹膜后面的视网膜。
附注:这是白化病患者视力差的一个原因。虹膜的作用就像照相机的光圈。如果没有色素，它就不能像光圈一样挡住光线，眼睛也就失去了聚焦系统。
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A friend of mine lost his pigment due to an adverse reaction to a medication. His skin color changed from dark brown to white but his eye color also changed.
我的一个朋友因为药物的不良反应失去了黑色素。他的肤色从深棕色变成了白色，眼睛的颜色也发生了变化。
My eyes changed from brown to green after puberty. Anyone know why?
青春期后，我的眼睛从棕色变成了绿色。有人知道为什么吗?
My eyes went from “black” at birth and lightened to hazel as a kid. Around middle school they started getting lighter and are a shade of deep green now. I’m 30.
Edit: I was also born with almost black hair, it lightened to a mousy brown as a kid, and now it’s a richer brown with red undertones.
我的眼睛从出生时的黑色变成了儿童时的淡褐色。在中学时，它们开始变浅，现在变成了深绿色。我已经30岁了。
附注:我出生的时候头发几乎是黑色的，儿童时期头发变成了灰褐色，现在是深棕色和暗红色的结合体。
I can explain this one. Collagen is the answer, it is part of those structures and it's ammount and positioning can make the light scatter differently even though there's Melanin causing your eyes to be brown, making your eyes look green. So when you hit puberty the ammount o collagen in your eyes changed for some reason, causing your brown eyes to look green, that is a very common thing in Brazil's northeasters.
Another common eye color change is among babies with blue eyes which turn green/brown later,.
我可以解释这个。胶原蛋白就是答案，它的数量和位置会使光产生不同的散射。即使你虹膜上的黑色素导致你的眼睛原本应该是棕色的，但胶原蛋白使你的眼睛最终看起来是绿色的。所以当你进入青春期时，你眼睛里的胶原蛋白会因为某种原因发生变化，使你的棕色眼睛变成了绿色。
另一种常见的眼睛颜色变化发生在婴儿时期，刚出生的婴儿眼睛是蓝色的，随后变成了绿色/棕色，这是由于生命头几个月黑色素增加的缘故。
I’m pretty sure eye color is polygenetic to a point where .On your point of albinos having poorer eyesight because of lack of pigment as a filter: does this mean that brown eyed people possibly have the best built in protection against light and the sun?
我很确定很多基因决定了眼睛的颜色，你认为白化病导致视力变差是因为缺少色素作为滤光器。
这是否意味着棕色眼睛可能拥有最好的抗强光能力?
Short answer, yes.
简单说：是的。
I have blue eyes and always need sunglasses if it's even remotely bright out (even on overcast days), while my fiance, who has very dark eyes, doesn't need them unless it's, like, blinding.
我有一双蓝色的眼睛，光线稍微亮一点(即使是阴天)我也需要太阳镜，而我的未婚夫有一双非常黑的眼睛，除非失明，否则他不需要太阳镜。
My husband makes fun of me for wearing sunglasses when it's overcast, early morning, or dusk. It's just so bright still that I have to squint if I don't and I don't want wrinkles when I'm only 23. Haha
我丈夫嘲笑我在阴天、清晨或黄昏的时候戴墨镜。如果我不戴墨镜就得眯着眼睛，我可不想在只有23岁的时候就有皱纹。哈哈
Blue eyes here too. I get blinding headaches without sunglasses and have to wear them even when its overcast and nobody else is wearing them. Blue eyes are extra light sensative.
我也是蓝眼睛。我不戴太阳镜就会头疼得要命，甚至在阴天没人戴太阳镜的时候我也得戴。蓝眼睛对光线特别敏感。
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I knew a girl with light brown eyes, and when she'd cry they'd turn to emerald green. What's up with that?
我认识一个女孩，她的眼睛是浅棕色的，当她哭的时候，眼睛会变成翠绿色。这是怎么回事?
It is no coincidence that people in bright regions (steppes, savannas, deserts etc.) are predominantly brown-haired and consequently brown-eyed (and darker-skinned). All these darkness protects against the sun that burns with the fury of a thousand angry fires.
Those scandinavians are spared from the wrathful sun and have a higer count of bright-skinned blondes with blue eyes.
生活在光线强烈地区(草原、稀树大草原、沙漠等)的人大多是棕色头发，因此也就有了棕色眼睛(和深色皮肤)，这并非巧合。所有这些都在保护人类免受阳光的伤害。
斯堪的纳维亚人远离暴烈的阳光，所以肤色更浅，头发呈金色。
If u go to iceland or scandinavia, actually the ice and the snow reflect so much light is annoying to look at it, theres much more light there than in other places of the world, in some places the sun cames out like really soon.
Im actually curious about why the darkest eyes ar the native indian eyes, since some of this tribes live in the jungle when there is no much light.
When people who live in the savanna have clearer eyes, all brown but not as dark as indians.
如果你去冰岛或者斯堪的那维亚，实际上那里冰雪反射的光线更刺眼，那里的光线比世界上其他地方更多，有些地方的太阳很早就出来了。
事实上，我很好奇为什么土著印第安人的眼睛最黑，因为有些印第安部落生活在没有太多光线的丛林里。
生活在热带草原上的人的眼睛都是棕色的，不像印第安人那么黑。
just because the sun 'comes out like really soon' doesn't mean the STRENGTH of the sun is the same everywhere on earth, and also, it will set like really soon too. This means the farther north you go, the ever more oblique angle the sun makes with the ground, and reducing its flux. That's why its colder. Think of a shadow drawing out as the sun sets. The size of you didn't change, but the area of the shadow grew larger, if you were somehow pumping a set amount of energy into the 'shadow area' the energy per cm of the shadow area would decrease as the sun sets.
仅仅因为太阳很快就出来了，并不意味着地球上所有地方的光照强度都一样，而且太阳出来的早，落下的也早。这意味着你越往北走，太阳与地面的倾斜角度就越大，平均光照强度就会减少。这就是为什么越往北天气越冷。想象一下，太阳落山西沉，你身体正面的大小没有改变，但身后影子的面积却在逐渐变大。照在你身上的阳光很多，但平均分摊到你身后的影子里，就会变少。
Because these mutations take millennia to actually affect eye colour. Scandinavians hail from the Germanic tribes that populated the areas up north less than 10000 years ago, so their eye colour actually developed in the environment with much less snow and bright light. If you look at indigenous Scandinavians, the Lapland/Sami people, they are dark-haired and brown-eyed (same goes for Canadian/Greenland natives).
因为一些突变需要几千年才能真正影响眼睛的颜色。斯堪的纳维亚人来自日耳曼部落，他们在不到10000年前迁往北欧，所以他们眼睛的颜色实际上是在少雪、光线明亮的环境中形成的。如果你看看斯堪的纳维亚的土著，如拉普兰人和萨米人，他们都是黑头发和棕色眼睛(加拿大人和格陵兰人也是如此)。
people in the northern hemisphere have lighter eyes because overall the sun is in the sky less than in the south. It isn't as necessary to protect eyes from UV light damage in Europe, so the natural sextion for dark eyes was looser.
Another interesting related topic: why did humans who migrated to north America not lose their darker skin tone, hair & eye color, even though they were near the same latitudes as Europe (for 15K years)? Most likely because ancient Europeans gained the trait for fair skin and light eyes through cross-breeding with Neanderthals, who had red/blond hair and pale eyes. More info:
北半球人的眼睛颜色更浅，因为总的来说，天空中的太阳光线比南半球少。在欧洲，人们没有必要保护眼睛免受紫外线的伤害，所以自然选择是比较宽松的。
另一个有趣的相关话题是:为什么迁移到北美的人类没有失去他们的深色皮肤，头发和眼睛颜色，即使他们与欧洲在同一纬度（各自生活了15000年)?
这很可能是因为古代欧洲人通过与拥有红色/金色头发和浅色眼睛的尼安德特人杂交而获得了白皙皮肤和浅色眼睛。
It is believed that the ancient human ancestors all had dark brown or nearly black colored eyes and very dark hair (which is also controlled by lixed genes to eye color and skin color). Even though brown eyes are still considered mostly dominant overall eye colors, there are several different eye colors readily seen now in the global population of human beings. So where did all of these eye colors come from?
直射阳光，强紫外线环境给人带来生存压力，深肤色成了自然筛选的结果，比如皮肤中的黑色素，可以保护皮肤免受有害太阳射线的伤害。这使深色皮肤个体活得更长，他们会繁殖并将深色皮肤的基因传给后代。
远古人类的祖先都有深棕色或接近黑色的眼睛和深色的头发(这也是由与眼睛颜色和皮肤颜色相关的基因控制的)。尽管棕色眼睛仍然被认为是眼睛颜色中最主流的颜色，但在全球人类中，仍然可以看到几种不同的眼睛颜色。那么这些眼睛的颜色是从哪里来的呢?
The color of human eyes is not dictated by a single gene like many of the other traits. It is instead considered a polygenic trait, meaning there are several different genes on various chromosomes that carry information about what eye color an individual should possess. These genes, when expressed, then blend together to make various shades of different colors. Relaxed sextion for dark eye color also allowed more mutations to take hold. This created even more alleles available to combine together in the gene pool to create different eye colors.
Individuals who can trace their ancestors to Western European countries generally have a lighter skin color and lighter eye color than those from other parts of the world. Some of these individuals also have shown parts of their DNA that were very similar to those of the long-extinct Neanderthal lineage. Neanderthals were thought to have lighter hair and eye colors than their Homo sapien cousins.
The Continuing of Evolution
New eye colors could possibly continue to evolve as mutations build up over time. Also, as individuals of various shades of eye colors breed with one another, the blending of those polygenic traits may also result in the emergence of new shades of eye color. Sexual sextion may also explain some of the different eye colors that have popped up over time. Mating, in humans, tends to be non-random and as a species, we are able to choose our mates based on desirable characteristics. Some individuals may find one eye color much more appealing over another and choose a mate with that color of eyes. Then, those genes are passed down to their offspring and continue to be available in the gene pool.
大多数科学家都认为，就像自然筛选出了深肤色一样，不同的自然环境也筛选出了浅色的眼睛。当人类祖先开始向世界各地迁移时，深肤色的优势不那么强烈了。对于定居在现在西欧国家的人类祖先来说，深肤色和深色眼睛不再是生存的必要条件。这些高纬度地区有不同的季节，也没有非洲大陆赤道附近的直射阳光。由于选择压力不再那么强烈，突变基因更有可能遗传下去。
人类眼睛的颜色不像其他许多特征那样由单个基因决定。相反，它被认为是一种多基因特征，这意味着在不同的染色体上，有几个不同的基因携带着决定一个人眼睛颜色的基因信息。这些基因一旦表达出来，就会混合在一起，形成各种深浅不同的颜色。
而更多的基因突变。产生了更多的等位基因，在基因库中组合在一起，创造出不同的眼睛颜色。
西欧国家的人，通常比世界其他地方人的肤色和眼睛颜色更浅。其中一些人的基因也显示出与早已灭绝的尼安德特人有非常相似的部分。相比他们的智人表亲，尼安德特人被认为拥更浅的头发和眼睛颜色。
随着时间的推移，随着变异的积累，人类的眼睛颜色可能会继续进化。此外，当不同眼睛颜色深浅的个体互相繁殖时，这些多基因特征的混合也可能导致新的眼睛颜色的出现。
性选择也可以解释随着时间的推移，眼睛出现的不同颜色。人类的交配往往不是随机的，作为一个物种，有些人可能会认为一种眼睛的颜色比另一种眼睛的颜色更吸引人，就会选择这种眼睛颜色的伴侣。然后，这些基因会遗传给它们的后代，并在基因库中继续存在。
This seems suspect to me given that East Asians are estimated to have an even higher percentage of Neanderthal DNA on average than Europeans.
在我看来，这一结论似乎有些可疑，因为据估计，东亚人的尼安德特人基因平均比例甚至高于欧洲人。
Basically humans evolved light skin and light eyes to absorb more vitamin d from the sun when they emmigrated to areas with less sunlight.
基本上，当人类移居到阳光较少的地区时，他们进化出浅色的皮肤和眼睛，以便从阳光中吸收更多的维生素d。
I have 3 kids. I have blue eyes (and my whole family does - not a dark eye in the bunch), husband has brown eyes. We were curious how this would turn out, but when we lived in Florida we had 2 babies that had blue eyes until they were about 2. Then their eyes turned brown (one is a darker brown, one a kind of lighter brown that also looks green sometimes, but on official paperwork we say brown).
Then we moved to Germany and I had a baby. Again a blue-eyed toddler. But then her eyes stayed blue. I was curious about it, but not enough to really look for an answer, and one day came across something that talked about how exposure to certain rays impact eye color. When you live in a place with little sun, you need that light and blue eyes makes it easier to see/absorb/whatever it is. So my Florida babies...b/c of genetics, I guess...ended up with more pigment to protect from the abundance of light. My German baby needed less pigment to allow more light.
我有3个孩子。我的眼睛是蓝色的(我的家族都是蓝眼睛—不是黑眼睛)，丈夫的眼睛是棕色的。我们很好奇孩子的眼睛会是什么颜色，当我们住在佛罗里达的时候，我们的两个孩子两岁之前都是蓝眼睛，然后他们的眼睛变成了棕色(一个是深棕色，一个是浅棕色，有时看起来像绿色，但大多数时间里是棕色)。
后来我们搬到了德国，我又生了个孩子。还是个蓝眼睛的小孩。但现在她的眼睛依然是蓝色的。我对此很好奇，但还没有找到答案，但有些说法是，如果长期呆在特定的光照环境，会影响眼睛的颜色。当你生活在阳光很少的地方，你的身体需要阳光，而蓝色的眼睛更容易吸收阳光。我的前两个还在在佛罗里达长大，他们需要更多的黑色素来抵御充足的光线。
而我在德国出生的孩子则需要更少的色素来吸收更多的光线。
I did a paper about skin color in class. The tribes that moved out of Africa and into colder climate (Europe and northern Asia) were exposed to the elements leading to the wearing of animal furs and lodgings that were mostly caves to avoid the wind as well. Over time skin became less and less exposed to the sun on a regular basis which led to whiter/lighter skin. Dark skins evolutionary trait was to avoid sunburn and therein skin cancer. White skin is more of a byproduct of less sunlight, not an evolutionary advantage to less sunlight.
我写了一篇关于肤色的论文。人类离开非洲，进入寒冷气候环境(欧洲和亚洲北部)的部落，穿着动物的皮毛，并在洞穴中住宿。随着时间的推移，皮肤定期暴露在阳光下的次数越来越少，导致皮肤变白/变浅。深色皮肤的优势是避免晒伤，从而避免皮肤癌。白皙皮肤更多是日照不足的产物，而不是进化优势。
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True "violet" eyes are questionable. Most of them are actually blue, but only appear purple.
从来没有真正的紫色眼睛。它们实际上是蓝色的，只是在特定光线下看起来是紫色的。
Right! Blue + red = purple.
没错!蓝色+红色=紫色。
My girlfriend's daughter has blue eyes that occasionally appear purple. They're pretty amazing.
我女朋友的女儿有一双蓝色的眼睛，但偶尔会呈现紫色。很神奇。
It's really only brown or blue. Blue is the colour it scatters, so it's why the sky is blue or why some butterflies have the colours they have. Then theres melanin which gives brown colour. If you have only a little melanin, it isnt enough to "drown out" the blue so to speak, so the weak yellowish brown mixes with the blue and it looks more green to us. Then the more melanin someone has the less you see of the blue, so their eyes are brown.
实际上眼睛颜色只有棕色或蓝色。蓝色是散射颜色，参照为什么天空是蓝色的，或者为什么有些蝴蝶是蓝色的。黑色素使眼睛呈现出棕色。如果你有一点黑色素，那是不足以“淹没”蓝色，淡黄褐色和蓝色混合在一起，就成了绿色。那么一个人的黑色素越多，蓝色就越少，直到眼睛变成棕色。
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I have sectoral heterochromia, one eye is brown ish on top but the bottom is green/hazel. My other eye is mostly brown with some rings of yellow. When I cry they turn green. My daughter has such brown eyes they look redish, she's also Auburn hair and light skin w freckles. My son has dark eyes, one looks like someone spilt black paint in a spot.
我有虹膜异色症，一只眼睛上部是浅棕色，但底部是绿色/淡褐色。我的另一只眼睛大部分是棕色的，有一些边缘是黄色的。当我哭的时候，眼睛的颜色就变成了绿色。我女儿的眼睛是棕色的，看起来发红，她的头发是赤褐色的，肤色很浅，还有雀斑。我儿子有一双黑眼睛，看起来像是把黑颜料倒上去了一样。
Just going to reply this to you as a small adjustment to make it easier to understand. Your general eye color is derived from three things. Eumelanin and Pheomelanin make up the pigments, the resulting eye color is based on what mixture of the two you have. More eumelanin leads to darker eyes, and more pheomelanin leads to lighter eyes.
Another important note is that blue eyes don't always result from a lack of melanin. Sky blue eyes tend to have the lowest levels of overall melanin, but darker shades of blue eyes, or blueish green eyes tend to have plenty of melanin, it's just that the ratio leans much more heavily toward pheomelanin than eumelanin.
为了让你更容易理解，我来回答一下。你眼睛的颜色由三个因素决定。真黑色素和褐黑色素构成眼睛的色素，最终的眼睛颜色是基于你拥有的这两种色素的混合。真黑色素越多眼睛颜色越深，褐黑色素越多眼睛颜色越浅。
另一个重要的注意事项是，蓝眼睛并不总是等于缺乏黑色素。天蓝色眼睛的黑色素水平是最低的，但是深蓝色眼睛，或蓝绿色眼睛往往有大量的黑色素，只是比例上褐黑色素更多。
My grandmother and dad have taken eye drops for glaucoma and the chemicals in that have made their eyes change color.
我的祖母和父亲曾用眼药水治疗青光眼，其中的化学物质使他们的眼睛变色。
I know a handfull of people who have eyes that appear black. Like their pupil and iris are almost indistinguishable.
My boyfriend's eyes are a very icy blue with no apparent melanin in them. They look grey blue.
我认识一些黑色眼睛的人。他们的瞳孔颜色和虹膜颜色几乎无法分辨。
我男朋友的眼睛是非常冰冷的蓝色，里面没有明显的黑色素。看起来更像是灰蓝色。
Once a group of kids got really scared because the sun was shining directly into my eyes and you could see that they're super yellow. They thought I was a vampire (thank you, twilight).
Btw, my sister has black eyes.
有一次，我的眼睛让一群孩子非常害怕，因为阳光直射到我的眼睛里，我的眼睛变成了金黄色。他们以为我是吸血鬼(谢谢《暮光之城》)。
顺便说一下，我姐姐的眼睛是黑色的。
I went from an azure color where everyone would constantly point out how blue my eyes were (especially in the summer) to a very dull grayish blue color around puberty. My eyes are still considered "blue", but they are very different shades of it.
我小时候每个人都说我的眼睛很蓝(尤其是在夏天)，青春期我的眼睛由天蓝色变成了浅灰色。我的眼睛仍然被认为是“蓝色的”，但两者非常不同。
My eyes went from green to brown at puberty and my hair went from being red and curly to almost black and fairly straight.
在青春期，我的眼睛从绿色变成了棕色，我的头发从红色卷发变成了几乎黑色的直发。
Same here. I had deep brown eyes as a kid, only realized they had changed when someone else described my eyes as green.
我也一样。我小时候有一双深棕色的眼睛，直到别人说我的眼睛是绿色时，我才意识到它们变了。
The hormones in your body at puberty can activate genes to begin transcribing proteins that previously had not been produced, which can cause changes in hair and eye color
青春期时体内的荷尔蒙会激活基因，开始转录以前没有产生的蛋白质，从而导致头发和眼睛的颜色发生变化。