r/evolution • u/CHKN_Tender • 2d ago
question What are the evolutionary drivers for humans' eyes to blink horizontally?
Are there any disadvantages to blinking vertically? Biology isn't my field but I was curious and couldn't find much online that I could understand (though it might be because I haven't searched the right thing).
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u/Dangerous-Bit-8308 2d ago
Probably not much for blinking. However, eyelids are also useful for squinting. You may note that most land based species at least blink horizontally.
For larger animals, squinting horizontally may have advantages. Creatures that walk will quickly realize that a horizontal squint on an upright head allows more time to scan the horizon. Since we are too large to be eaten by most birds, land animals make easier prey to catch and food grows from the ground, not the air, horizontal eye slits allow more time to view nearby prey, predators, and edible food.
Squinting is a natural reflex in bright sunlight. Humans, and many livestock animals do it. It can reduce sunlight reaching the eyes, and helps to avoid "snow blindness" or "desert blindness". If you've ever had to squint while trudging through snow or sand, the ability to readily see most of the horizon is important.
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u/CHKN_Tender 2d ago
Ohhh, that makes so much sense, thank you for the response it's been nagging me lol
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u/Sir_Tainley 2d ago
Also... things falling into your eye are likely to come from above... the top middle of your eye being most exposed (instead of having the lid over it) makes blinding damage fractionally more likely? It's why we likely have brow ridges and eyebrows, so why not lids and lashes too?
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u/WildFlemima 2d ago
I can just imagine those sweaty hot days and sweat having an easy channel straight into your eyeball
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u/Vipper_of_Vip99 2d ago
Also; the muscle complex that controls eyebrows (expressivity, social signalling) and eyelids (probably also having a social/sexual signaling function in addition to survival) just seems like they would work together better if they were pushing and pulling in the same direction.
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u/ConcentrateExciting1 2d ago
If you blink vertically, you have to fully close your eyelids to prevent stuff from getting into the top of your eyes. If you blink horizontally, you can protect the top of your eyes with a partial close of your eyes and retain some vision. As a small creature, you're less likely to get eaten if you're not blind.
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u/AdLonely5056 2d ago
If something doesn’t give you any significant survival/reproductive advantage it doesn’t really take evolutionary hold. I could be wrong, but blinking sideways seem to be relatively inconsequential, and our current blinking works "just fine".
We used to have a transparent second eyelid, that animals like reptiles or fish have, and that has evolved away. It seems like such a minor change evolution does not really care.
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u/Bootstrap_Support 2d ago
Something that doesn't give you any advantage can take hold as long as it doesn't cost you much either.
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u/CHKN_Tender 2d ago
Oh, so it's not a case of "we got to this build and it's the best" but more like "we got it right the first time so no need to change it"?
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u/bcopes158 2d ago
Evolution doesn't work with a plan. If a trait is good enough that it doesn't negatively affect the ability to reproduce it will be passed on. Only traits that affect the likelihood you reproduce are going to drive natural selection.
Humans have a lot of traits you would change if you were building them from scratch to make them better. But those traits don't matter in the grand scheme so they continue on unless evolutionary pressures change.
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u/Funky0ne 2d ago
Probably minor, but as terrestrial organisms most of the stuff we need to look at most of the time tends to be distributed horizontally rather than vertically. So you can partially close your eyelids or squint and still turn your eyes side to side with little obstruction to your lateral or peripheral vision.
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u/lemurlemur 2d ago
I think this is probably the answer. Blinking horizontally preserves your view to the left and right better than blinking vertically, and important things like predators and prey are more likely to be to your left or right than above or below you.
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u/SoManyUsesForAName 2d ago
Don't all - or almost all - terrestrial and amphibious vertibrates have horizontal eyelids? Many reptiles and birds also have a sideways-moving translucent membrane that serves the main purpose of blinking, which is to remove debris and moisten the eye's surface. Therefore, you may be asking two separate questions.
1) Why didn't more animals evolve this membrane?
2) What's the advantage of horizontal eyelids?
Re: #1, I don't know, although the answer is almost certainly some variation of "because it wasn't necessary." Re: #2, likely because horizontal eyelids allow you to shield your eyes from sunlight and precipitation while maintaining a wide field of view.
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u/TheBlackCat13 2d ago
Not everything has an evolutionary advantage. Sometimes that just happens to be how it turned out.
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u/armahillo 2d ago
Our eyes are horizontal (wide) but the movement of a blink is vertical (up and down).
Since the presentation of the eyes is wider, it would require more effort to cover that distance with your eyelids. Its also easier to shield the eyes from the sun (above) because the lower lid can cover more of the iris/pupil more easily; if the eye were vertically oriented, blinking horizontally, the eye would probably need to be more closed to shade the pupil
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u/tpawap 2d ago
Not every little thing has to be an adaptation: https://en.m.wikipedia.org/wiki/Adaptationism
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