Imagine a species that is very much like humans in shape, size and physiology. But they are able to stare at the sun for long periods without suffering any harm, and without needing any artificial device.

Suppose the evolutionary pressure for this is that they come from a much brighter world.

How could they evolve sun resistant eyes?

I would like them to have visual 20/20 vision, and acuity and color resolution equal or close to ours - but that is NOT a constraint. If the capacity to stare at the sun requires their sight to be worse than humans' in some aspect or another, so be it.


7 Answers 7


There are four ways to mitigate bright light which both biology and photography are employing.

  1. Diaphragm (pupil). Your species can shrink their pupils to a very tiny size, filtering out as much light as necessary. In addition to that, their eye color should be black, otherwise the light bleeding from the iris would overwhelm the light coming through the pupil;

  2. Filter ("third eyelid" membrane). Extra light can be filtered out by a Nictitating membrane, which is present in a number of animals and vestigial in humans. To protect from bright sunlight, it has to be dark grey in color;

  3. Sensitivity. The retina itself can be accustomed to bright light and have mechanisms to avoid damage. This is probably the hardest way to achieve desired results, because human retina already has quite wide dynamic range;

  4. Exposition. Your species should be blinking in the bright light. In humans, blinking is compromising clear vision, but there is no real reason why species can not adapt to it.

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    $\begingroup$ "Your species should be blinking in the bright light. I am not sure that is acceptable." I don't see why it would be unnaceptable. In fact that might be easier to evolve IMO. Nice answer, +1. $\endgroup$ Commented Jul 9, 2019 at 23:41
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    $\begingroup$ Number 4 is brilliant (no pun intended). #1 and #4 together should fit perfectly the requirements with very little evolutionary help. +1 $\endgroup$
    – LSerni
    Commented Jul 10, 2019 at 0:16
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    $\begingroup$ I would imagine 2 and 3 being the most effective. The human eye constricts a lot already, you will reach a point in which no light would get into the eye. 2 and 3 are evolutionary adaptations to the worlds conditions. If the sun is metal poor, then it probably be a lot brighter and evolution would have to adapt to that condition. there are examples here on Earth that have a protective membrane or have differing light sensitivity. $\endgroup$
    – Sonvar
    Commented Jul 10, 2019 at 3:07
  • $\begingroup$ I agree with the other comments, this is the best answer thus far but a complete answer should be either #1 + #4 or #2 + #3 $\endgroup$ Commented Jul 10, 2019 at 13:56
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    $\begingroup$ This answer only address light intensity. It does not solve the issue of UV damage. Maybe #2 could address it by making it clear that the membrane blocks certain wavelengths and is capable of replacing itself or repairing the UV damage that it sustains while shielding the eye itself. $\endgroup$
    – user43774
    Commented Jul 10, 2019 at 15:34

Star Trek's Vulcans have evolved a nictitating membrane that shields their eyes (Spock's World by Diane Duane and TOS episode "Operation -- Annihilate!"). But this is a bit of an emergency measure.

It would also be possible to have a photochromic layer on the cornea, and it could even be active. That is: when hit by ultraviolet rays, or by a strong enough light, the cornea darkens. Also, a small biologic current is ran through the vitreous body, aligning special protein chains so that they act like a shutter.

The two phenomenons are independent and can compound each other, allowing a continuous regulation of light. Brain adaptation can then compensate color skew, giving 20/20 visus and optical and chromatic resolution similar to that of humans.

Nematic cholesterol-like chains can offer light attenuation up to 95%, and I think that a "darkened" cornea should be able to easily provide another 20%. This means your aliens should be able to achieve comfortable photopic vision at illuminations of more than two million lux, and still see perfectly well in Earth-normal room lighting (their luminance range would go from around 10-2 to 108; they'd probably fall to scotopic vision at early dusk, and be unable to see on a clear starry night).

On the other hand, an overcast day would probably look to them quite dark, because they'd get the 20% attenuation from the cornea due to the UV component unfiltered by the clouds.


Polarized Lenses.

Making your creature's eyes' lenses polarized would help to block sunlight and glare without having to have extra eyelids or tiny pupils which will be detrimental to its eyesight. Here is a quote about polarization of sunglasses:

Today’s sunglasses—with or without prescription—can be modified with a technique called polarization to reduce light glare and improve vision and safety in the sun.

Not only are polarized lenses not detrimental to eyesight, they are supposed to enance it.

Polarized lenses not only reduce glare, they make images appear sharper and clearer, increasing visual clarity and comfort.

Also, you could use two lenses instead of a dilating pupil. The way polarization works, you could have two polarized lenses on each eye of your creature. Instead of opening/closing its pupils to block light, it would merely need to rotate one of the lenses to immediately adjust its eyes to a bright light. As the lenses' polarizing filters intersect more directly, they block more of the light.

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    $\begingroup$ And as a bonus feature, they could then very easily detect polarization as well, as an additional attribute of color--particularly if each eye, or different portions of the lens, were polarized in different directions. $\endgroup$ Commented Jul 10, 2019 at 4:11
  • $\begingroup$ They'd be good at spear-fishing - one use case of polarised lenses is cutting out reflections $\endgroup$
    – Baldrickk
    Commented Jul 10, 2019 at 11:00
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    $\begingroup$ I'm not sure polarisation helps that much when looking directly at the sun. Polarized sunglasses help because most glare is light reflected from horizontal surfaces (like the sea, or a wet road), while light from the sun is not polarized. Using two polarized filters is neat, but rotating a filter would be at least as complex as an extra eyelid with a simple dark filter. $\endgroup$ Commented Jul 10, 2019 at 11:02
  • $\begingroup$ @Robin Bennett, sunlight/artificial light's waves are angled in all directions, so polarization would remove all but one direction, greatly reducing the brightness. The second polarized lens would block enough of the remaining light that it would not damage the retinas. Also, a rotating filter would be as complex as an extra eyelid, but it would be able to immediately adjust to any light level, which makes it work slightly better. $\endgroup$ Commented Jul 10, 2019 at 14:56

Do you mean "How to evolve?" literally?

Evolution isn't something that tries to achieve a goal. In general, it is a meaningless random process that takes a large gene pool of many variations of genes, and then kills off individuals that lack the specific genes that will help them to survive long enough to produce children.

The number and frequency of genetic variations (mutations) can be increased by radiation or chemicals, but those same factors can also be fatal, and the mutations can produce many strange and undesirable results.

So, assuming there are sufficient variations available, you will need something that will cause people that have sensitive eyes to tend to die before they reproduce. That something can be natural or artificial.

For a natural process, perhaps for some long but limited period of time the sunlight becomes much brighter than usual and people with sensitive eyes end up with very poor vision or even blindness. They will tend to die early or be found unsuitable as mates, and so not pass on the "bad" genes. For the most part, only those individuals that have genes that allow them to tolerate bright sunlight will propagate their genes to the next generation.

For an artificial process, perhaps society requires a rite of passage ceremony during which people of a certain age, whether 8 days, 12 years, or whatever, are forced to stare at the sun for a full minute (or some other duration). Again, those that carry the sensitive genes will end up blind or with damaged vision and will be unlikely to have children of their own.

Whatever it is that increases mutations, and whatever it is that kills off the unwanted attributes, are of little importance. The selection process for who gets to pass their genes on to the next generation is all that counts.

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    $\begingroup$ "forced to stare at the sun for a full minute ... those that carry the sensitive genes will end up blind or with damaged vision and will..." Be killed by the Sun god/superstitious tribe $\endgroup$ Commented Jul 10, 2019 at 13:48
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    $\begingroup$ I guess that if you blinded mere 10% of the tribe, it wouldn't even survive winter. Such evolutionary pressure would have to come MUCH earlier - say water-based organisms didn't require it because they could simply go deeper, but all land-based animals do. $\endgroup$ Commented Jul 10, 2019 at 20:40

First, your humans need a mutation that for some reason provides an evolutionary advantage to be able to stare at the sun or a co-evolutionary mutation that is co-joined with that mutation. As an example, a mutation that results in a lens that under muscle strain polarizes -- attenuating the light.

If that mutation spreads into the gene pool and flourishes and somehow provides an advantage in spreading the mutation then future mutations can promote and increase that ability.

Somewhere there will need to be an improved tolerance for UV radiation. It causes cataracts. So if humans are going to be staring at the sun before we invent telescopes, that feels like it needs to be more resistant for the individuals to benefit from staring at the sun

Another mutation might be that the viscous goo in the eye might be self-polarizing or become less opaque in the presence of strong light.

Our eyelids could become thinner, allowing us to see bright things through them

Any or all of these of millennia of natural selection might combine to let humans star at the sun.

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    $\begingroup$ Maybe there are (or were) really large birds of prey that have learned to attack humans from 'out of the sun'? $\endgroup$ Commented Jul 10, 2019 at 11:04
  • $\begingroup$ @Robin Bennet, that's interesting. But, I'd think their shadow would alert their prey of the attack. $\endgroup$
    – EDL
    Commented Jul 10, 2019 at 20:24

They Were Prey

I will try to explain the why and not so much the how.

On this planet these humanoids were not at the top of the food chain. There were multiple flying predators which loved to feast on them. These predators learned / evolved to dive bomb when they were directly between the sun and their prey. Their prey not being able see them would have little chance of escaping.

Over many generations and mutations this humanoid species eyes started getting a feature that helped them look closer and closer to the sun. Those who could look close to the sun were more likely to avoid being lunch for these predators. Eventually this feature was refined to allow the humanoid to directly look at the sun with no ill affect.

This feature could be an extra eyelid with the same properties as sunglasses or new sensory cell(s) that are impervious to the harmful rays of the sun.


It's not possible. You killed this possibility yourself with the stipulation: "a species that is very much like humans in shape, size and physiology"

No species on Earth that meets this stipulation has a need for staring at the sun for long periods. Not a survival need, which means that evolution could not, by definition, EVER act to produce such an individual ANYWHERE on ANY PLANET.

IF you remove that stipulation and say something like "a species that LOOKS like humans but has a completely oddball physiology that for some inane reason requires staring at the sun" then yes, you could create some evolutionary issue that might produce this.

But I have to warn you - staring at a bright light source for a long time leaves that individual vulnerable to predators. So if for example you created a specie that had to "charge up by staring at the sun" for an hour every day - then that specie would quickly be eliminated because some OTHER specie on that planet would rapidly evolve the ability to NOT have to stare at the sun - so that they could run up and kill off and eat the specie that did have to stare at the sun in the morning while it was busy doing it.

By extension this applies to staring at ANYTHING for a long period of time. Not just the sun.

This is why when mammals evolved they out competed the reptiles and killed most reptile species off - because they could kill and eat the reptiles in the early morning when the reptile was still sluggish and warming itself. Thus the adage "early bird gets the worm" (this is an "idea evolvement" that created the phrase which in a general sense means that if you go out hunting early in the morning you are more apt to catch prey that is still sluggish in the morning. Worms are no more accessible to birds in the early morning than anytime else in the day)

The requirement for not moving and staring at something for a long time or doing something for a long time is an evolutionary disadvantage and once Nature figured out how to dispense with that, the individuals who didn't have that restriction killed off all the individuals that did. It's why we invented Drive Thrus, son. ;-)

What you have to keep in mind is this - when using the "rules of nature" that are in force on Earth you must remember that humans are the end result of hundreds of millions of years of evolution following those rules - and we are very very successful, if not the most successful specie. So you have a VERY high bar to use those rules to produce a "human-like" specie that is going to have significant differences from real humans. Nature has been working on it for hundreds of millions of years and we is what she came up with. So no triple-breasted women, sorry!!!

Earth couldn't even keep 2 separate "human-like" species going - Homo-Sapiens and Neanderthals - for much more then a couple hundred thousand years before the evolutionary advantage of Homo-Sapiens killed them off.

If you want to say "Earth's rules of nature" are absolutes anywhere in the Universe then in order to get something significantly different than a human you are going to have to fundamentally alter the environment - make the planet bigger or remove most carbon or something of that nature - in which case you CANNOT naturally evolve ANY specie that is going to be "like humans" in physiology.

One of my favorite SF authors is Harry Harrison and in his Deathworld trilogy he took wonderful liberties describing Meta as among the most beautiful woman the protagonist had ever seen - the extra heavy gravity brought out her firm strap-like muscles and so on - in reality the most successful female on Deathworld would have been a squat, fat, tremendously muscled apish female that waddled around.

But it was fun to read so don't let my throwing cold water on the idea dissuade you! :-)

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    $\begingroup$ Welcome to the site, Ted. This post contains a lot of bold assertions - many of them not in the scope of the question - with no supporting evidence. You've also made some assumptions that are not in the OP (that the species evolved on Earth, that the species requires staring at the sun). $\endgroup$
    – K. Morgan
    Commented Jul 10, 2019 at 17:24
  • $\begingroup$ @K.Morgan well, that point is actually valid: Usually no species has no evolutionary reason to stare at the sun regardless of general brightness level, so the brightness adaptations of the eye usually stop right before looking at it. This would probably change if there were a flying predator that likes attacking from the sun, so Ted's assumption that the question's assumptions are wrong is still mistaken. $\endgroup$
    – toolforger
    Commented Jul 10, 2019 at 20:17
  • $\begingroup$ Welcome to world-building, Ted. Your response is at risk of being deleted since it doesn't try to answer the question but focuses, instead, on why there is no answer to the question. You can frame your response as a frame-challenge to keep it from being deleted, or you can edit your answer to address the OP's interest. $\endgroup$
    – EDL
    Commented Jul 10, 2019 at 21:39
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    $\begingroup$ Staring at stuff is a trait commonly found in reptiles and ambush hunters. Crocodiles and snakes are some of the most successful hunters on the planet and have survived since the dinosaurs. The fact that reptiles still exist show that waiting to charge up is an effective tactic. The tactic of waiting also applies to many insect species and bird species that are hunters. Also birds are not mammals. You are using that expression incorrectly. If you are the early bird, there are more worms because the other birds have not yet eaten them. $\endgroup$
    – Shadowzee
    Commented Jul 11, 2019 at 2:52
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    $\begingroup$ Welcome ted. The capacity to do something does not equate with the capacity having evolved for that particular task - thumbs didn't evolve to specifically use smartphones. At the moment you seem to be writing a critique on the question rather than answering it, could you use comments to do this instead. (From review) $\endgroup$ Commented Jul 11, 2019 at 4:33

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