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Traditionally, a Cyclops has one eye, as well as some other aliens in fiction.

My question is simple: Would there be any situations, Earth-like or otherwise, where a creature would benefit from having only one eye?

As far as I know, creatures with eyes on earth all have more than one.

This question is not a duplicate of Anatomically correct Cyclops as this question focuses on all kinds of creatures, not only primates. Also the other question needs an Earth-like environment, something this question specifically does not require. Answers to this question would not necessarily be applicable to the other.

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    $\begingroup$ 1 eye means less brain power spent on interpreting the signals from the eye. It also may mean substantially less brain power in producing stero images for 3D perception. This brain power could arguably translate to faster reaction times, or a high 'frame rate' so the eye can dart around more quickly. $\endgroup$
    – FraserOfSmeg
    Mar 1, 2016 at 10:13
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    $\begingroup$ Actually one-eyed animals exist - en.wikipedia.org/wiki/Cyclops_(genus) $\endgroup$
    – enkryptor
    Mar 1, 2016 at 11:21
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    $\begingroup$ @Lostinfrance I concur. A worm is a valid answer here but not on the cyclops. $\endgroup$
    – Angelo Fuchs
    Mar 1, 2016 at 13:55
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    $\begingroup$ Get a look at the Unioc from Schlock Mercenary. $\endgroup$
    – MakorDal
    Mar 2, 2016 at 7:33
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    $\begingroup$ @FraserOfSmeg I challenge your assumption. The (Terran) brain does not work like that, and in fact 99% or so of the image processing involves identifying objects of interest and "alerting" your response systems to that. $\endgroup$
    – Carl Witthoft
    Mar 2, 2016 at 14:05

18 Answers 18

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As a rule, evolution favors simplicity over complexity when additional complexity does not add anything useful. Creatures that live for many generations in environments without light (such as cave salamanders and fish) often lose their eyes entirely; one fewer structure to expend energy growing or sustaining, one fewer opening for parasites to invade.

The hard part is figuring out a case where vision is still useful, but two eyes are not. The advantages to having two eyes over one are:

  1. Depth perception (if the eyes are positioned close together, typical for predators)

  2. Peripheral vision (if the eyes are positioned far apart, typical for prey)

  3. Redundancy in case one eye is lost

So if you can think of a creature that would have no use for depth perception or peripheral vision, and the danger of eye-invading parasites is more significant than the danger of having an eye poked out and losing vision entirely, but vision conveys enough benefit that having an eye is better than having no eyes, then such a creature will indeed benefit from having a single eye.

One possible circumstance I can think of is if the light is too dim or the environment too cloudy to see anything clearly anyway, but detecting shifts in light or dark is still helpful (recognizing day/night cycles or the shadow of an approaching predator). Such a creature would probably have a simple eye or eyespot akin to those of simple animals like worms, rather than the complex, human-like one cyclopes are generally depicted with.

Another interesting possibility is if the creature originally had two eyes, but started using those eyes for separate functions to the point where one eye turned into something completely different. Chameleons, for instance, usually use one eye to watch where they are going and another to keep watch on prey. What if this distinction further developed to the point where one eye moved to the bottom of the head and the other eye moved to the top, and further still until the two eyes hardly even resembled each other? Then, if its descendant lost the use for one of its specialized eyes, it might lose that eye and wind up with only the other. It would probably have a very hard time competing with its binocular counterparts, but if it lived on an island with no competition it might survive.

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    $\begingroup$ To further the point about the chameleon, they actually achieve a form of depth perception with only one eye: en.wikipedia.org/wiki/… So it's possible a creature could evolve with only one eye that acts like a chameleon's, and never really need a second (except for redundancy, but if the eye is well-protected then that's not as much of an issue) $\endgroup$
    – thanby
    Mar 1, 2016 at 22:19
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    $\begingroup$ I'd also like to add that if the creature had another sense it relied on for resolving its environment (like echolocation) it would have less reason to even worry about depth perception and be less likely to have evolved two eyes to begin with. That system could form the basis of its "visual" redundancy; it could go blind but not be crippled entirely. $\endgroup$
    – thanby
    Mar 1, 2016 at 22:25
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    $\begingroup$ @thanby That bit about chameleon depth perception is interesting! It seems there is more than one solution to 3-d vision (snakes "sway" before striking in order to get a bearing on their target, for example, though this is susceptible to a target that is also swaying, which is how "snake charming" works). So a cyclops could have depth perception by quickly rolling its eye back and forth. Binocular vision is still superior, but one chameleon-like eye isn't quite as crippling as it seems. $\endgroup$
    – IndigoFenix
    Mar 8, 2016 at 8:53
  • $\begingroup$ To expand on the single eye - perhaps the second eye changes which wavelengths it can see - becoming a more general UV detector to allow a UV sensitive creature to move out of the sun, or to work at infrared wavelengths with a much larger field of view (think wide angle lens) to detect food. It's not really an "eye" anymore. $\endgroup$
    – Jon Story
    Apr 27, 2016 at 14:10
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There certainly are benefits to having two eyes, as listed by Chris G in his answer: stereoscopic vision, redundancy, wide field of vision. Nonetheless it is not necessary for there to be any benefit of one eye over multiple eyes for a creature to evolve only one eye.

Evolution occurs when natural variations among a population have a differential effect on how successful the animals or people are at reproducing. But natural selection can only "work" with what it's presented with by natural variation. And what natural variation (mostly mutation) throws up is a matter of chance.

Here on earth it seems that the common ancestor of insects, fish, mammals, birds, etc. happened to be bilaterally symmetrical and hence all these creatures now have two eyes. Something about that common ancestor caused its genes to be successful, but that something wasn't necessarily anything to do with its having two "eyespots" (the photo-receptive patches that later evolved into eyes) - and wasn't even necessarily anything to do with its bilateral symmetry. It could even have been a quality that was evolutionarily favourable then but has since become "evolutionary baggage" or disappeared entirely.

But one can easily envisage that evolution went differently on another planet, or in an alternative timeline of this one. A creature happened to be born there with a quality (never mind what) that causes extraordinary evolutionary success, but as it happens this creature has a body shape which only has one location where the chance occurrence of a patch of light-sensitive cells can provide it with immediate advantage. When populations are small, chance plays a bigger role. This creature's genes spread rapidly and "corner the market" on most evolutionary niches. Two-eyed creatures never get started.

A less dramatic possibility is one-eyed creatures coexisting with two-eyed creatures but evolving along a separate branch where some other advantage (which always means local, immediate advantage) was so great that the disadvantage of only one eye didn't matter. Evolution only has to be "good enough". We know stereoscopic vision isn't all-conquering. Some creatures on earth have evolved poor vision when their ancestors had better vision, some have no eyes at all. We also know that redundancy isn't the be-all and end-all. We'd be safer with two hearts but we have only one.

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    $\begingroup$ Upvoted for pointing out that two eyes might not necessarily show up in the first place. Evolution is a tricky subject; it needs the random mutation first to select for "more successful", it can't just go "this is a good idea, let's make it that way". That being said, a bilaterally symetrical organism has a high probability of having two eyes, so cyclopses would need some explaining. (Not impossible though.) $\endgroup$
    – DevSolar
    Mar 1, 2016 at 10:37
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    $\begingroup$ @DevSolar why does a bilaterally symetrical organism have a high probability of having two eyes? It has just one mouth, after all... I'm talking without knowing here, I'm really interested in an answer. $\endgroup$
    – Ray O'Kalahjan
    Mar 1, 2016 at 11:38
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    $\begingroup$ @RayO'Kalahjan: Difficult to explain within a comment. Bilateral symetry results from how organisms grow. The technical terms here are Embyrogenesis, Gastrulation, and Neurulation. This makes it rather unlikely for anything not related to digestion or central nervous system to come in other numbers than two. At least for any bilaterally symetrical organism we know. ;-) I am not saying "impossible", just "unlikely". $\endgroup$
    – DevSolar
    Mar 1, 2016 at 11:57
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    $\begingroup$ @JanDvorak: From the Embryogenesis perspective, yes. ;-) The lungs, for example, form as extensions of the digestive tract. No kidding. ;-) $\endgroup$
    – DevSolar
    Mar 2, 2016 at 5:37
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    $\begingroup$ Though we don't have two hearts, for some reason we have two kidneys and two lungs. I wonder why. $\endgroup$
    – Nayuki
    Mar 2, 2016 at 7:10
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In very narrow tunnels, when species develop more like Worms then Vertebrate and light is introduced late into the development.

When you have a species that creates tunnels that it maintains. And if this species and all others in its biosphere don't come near light, ever, they will not develop eyes (at first) because being photo-sensitive is of no advantage.

Now, by chance, some bio luminescent species start colonizing this tunnels. The situation changes and any form of vision will be favorable versus no vision. So some species (including our worms) happen to develop photo-sensitive cells on their skin. If they happen to cluster on the front facing side and start to build some form of eyeball that would be favorable to two eyes.

  1. Its better then no eye because seeing whats in front of you is helpful in non-collapsing tunnels.
  2. Having two eyes is of no advantage because there is no depth to be perceived anyway, the tunnels are narrow and windy. And two eyes mean two places of infection. Also worms (unlike many other species) are "round" not "symmetrical". The value of the left vs. right or up vs. down is sero (until they develop an eye, then mouth-half and eye-half of the front facing side will start to be distinguishable, maybe.

Also, as non-symmetrical species their genes don't favor even numbered things over odd number of things (as do a lot of species that have eyes on earth).

You still have a lot of problems for this biosphere (where does the energy come from, for example, no light means no sun. No sun is bad. But maybe on the cooling surface of hot plamebo?), but it would be possible.

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  • $\begingroup$ "Having two eyes is of no advantage because there is no depth to be perceived anyway" - one in the front and one in the back, perhaps? $\endgroup$
    – John Dvorak
    Mar 2, 2016 at 3:52
  • $\begingroup$ @JanDvorak Perhaps. $\endgroup$
    – Angelo Fuchs
    Mar 2, 2016 at 8:02
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No reason at all, and I assume the only reason we generally have 2 eyes is for either wide field vision (in the case of prey) or distance-determining vision (in the case of predators).

However, in an imaginary world, there's no reason some other form of determining your location would appear instead of a passive light sensor. Take dolphins or bats and their echo-location senses. It could be possible a species would develop in (perhaps gloomy conditions) where a light-emitting sense would be more useful than a passive light-detecting sense: a species that would flash a light and detect the results using a single eye, maybe the light emission is more directed (to avoid detection from predators) and so multiple detecting sensors would not be useful.

So you can have sharks with lasers on their heads!

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    $\begingroup$ +1 A single "vision" organ could "perceive depth" if the organism that possesses it also emits radiation in the same frequency band that the organ detects. Either the brain could decode the delay time between emission and detection or the organ itself could do emission, detection, and decoding. Depending on the sophistication of the organ and the frequencies that are emitted and detected, there could be many advantages over our binocular vision. $\endgroup$
    – Todd Wilcox
    Mar 1, 2016 at 20:49
  • $\begingroup$ No, the reason we have 2 eyes is the profound bias toward bilateral symmetry built into our basic body plan genes. Even in the case of, for instance, flounders, the young start out with two eyes and then one eye migrates to the top of the body as the body adapts to life as a flatfish. $\endgroup$
    – WhatRoughBeast
    Mar 1, 2016 at 23:53
  • $\begingroup$ @WhatRoughBeast is this bilateral symmetry the reason why I have 1 nose and 1 mouth? $\endgroup$
    – gbjbaanb
    Mar 2, 2016 at 8:49
  • $\begingroup$ @gbjbaanb - No, but it's the reason you have two nostrils and 32 teeth, evenly distributed with 16 per side.. $\endgroup$
    – WhatRoughBeast
    Mar 2, 2016 at 14:15
  • $\begingroup$ @WhatRoughBeast you miss my point, I can have one central eye just as easily as 1 central mouth. Symmetry doesn't preclude singular sense organs; we have 2 eyes that are positioned where they are to detect things that matter to us (as 1 eye would not give wide field of view or distance) , but we could just as easily have a single one centrally positioned if we lived in a world where that worked for us too, or two noses - one either side of our ears. $\endgroup$
    – gbjbaanb
    Mar 2, 2016 at 14:26
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I'm going to say no

Unless you have one HUGE eye, it will highly impractical for you to see 90 degrees or more without leaving your eye highly exposed to damage.

With a single eye, there are no redundant systems or back-ups if it gets damaged.

Eyes have evolved to help with specific functions. Two eyes in front help with depth perception and with hunting/targeting prey. Eyes on the side allow for more vision of surroundings. Less depth perception and more sensing movement and trying to avoid death. One eye has the significant downside of no real benefit.

There are very few animals that have a single eye, and the ones I have found are nearly microscopic, and i don't think this would evolve into anything larger, even on an alien planet.

In order for a species to evolve to have only one eye, either they would have a system that functions as sight for them, thus making the single eye enough supplementary vision, or a single eye would have to present an advantage over dual eyes. I can't find any theories where a single eye is more beneficial, so that leaves the redundant systems idea.

One option is for the creature to have an "eye" that's just a photo-receptor to determine time of day for defense purposes. They can navigate using other senses, such as echolocation, sensing water pressure, sensing earth tremors, or smell (such a with bats, some fish, and moles).

I'm also a bit new to this reality check thing, so if you want sources or anything I can provide some, but this is all based upon some knowledge from my biology classes and some quick google searches for examples.

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You can have depth perception with a single lens if you have the sensing cells arranged at sub-wavelength steps, giving a light-field camera. Although obviously more complicated than the ordinary eye, structures at such scale are not biologically impossible ( e.g iridescent scales on butterfly wings ). There is an advantage of such optical systems are 'good for imaging fast moving objects where auto focus may not work well', so if your Cyclops hunts something moving faster than conventional eyes can focus, then such eyes could evolve, and having evolved having two of them is unnecessary.

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  • $\begingroup$ Especially if said eye were armored/protected somehow so redundancy became less necessary $\endgroup$
    – thanby
    Dec 28, 2016 at 18:18
  • $\begingroup$ Depth perceptIon is unnecessary for herbivores. Look at sheep, pigeons etc. Their eyes point sideways to give them an almost 360 degree field of view to spot approaching predators. But there is almost no overlap of visual field between eyes so little scope for depth perception. Note that the way birds move their heads to and fro is an alternative way to acquire depth information. They are using parallax. (As you will attempt to do when you visit the grand canyon ... and fail, it's too big!) $\endgroup$
    – nigel222
    Dec 29, 2016 at 11:51
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    $\begingroup$ BTW my father was blind in one eye. He did not regard that as a significant handicap. He soon learned to substitute parallax for depth perception. Note also that jewellers often perform delicate work on small objects using a monocle ie work one-eyed by choice. $\endgroup$
    – nigel222
    Dec 29, 2016 at 12:05
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In a water environment, if vision is only required for orientation 'which way is up?' or when to rise to/migrate from the surface for predation purposes. I suppose these factors may also apply to something that lives just under the soil surface, if particular wavelengths can penetrate some way through the medium.

ETA: these are evolutionary reasons why a single eye might develop in the first place, which has to be the start point for any consideration of why there is just one eye. To take this further, additional complexity might develop to introduce rod/cone equivalents in concentric rings at the back of the eye which - with a fixed focus lens - would provided distance measurement and direction identification. This addresses why the binary eye system wouldn't be needed. And if the lens is faceted, as in a fly's eye, then it could become even more efficient.

ETA2: And some reasons why it could become more efficient than two eyes? Well, if the lens protrudes from the body surface (and is sufficiently robust - would need either: (a) tough tegument, (b) tough eyelids, (c) ability to retract/extend) could provide greater range of coverage up to 180 degrees all-round with a fish-eye lens. Without eyelids or retract/extend then there would be no need for the development of muscles to even move the eye, and all processing would be done by the brain.

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    $\begingroup$ Hi Noisy, welcome to the site. You don't really answer the question, which is what advantage a single eye give compared to more..? $\endgroup$
    – clem steredenn
    Mar 1, 2016 at 12:43
  • $\begingroup$ I was addressing the 'reasons why' part of the question. From an evolution point of view, there has to be a start point for the initial development of an eye, and if evolution starts in water then the reasons I posit are a good starting argument. I'll expand my suggestion to make that clear, and then address on-going evolution paths for more complexity. $\endgroup$
    – Noisy
    Mar 1, 2016 at 13:49
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    $\begingroup$ Hey Noisy. To elaborate on what @bilbo_pingouin was saying, answers are expected to directly address the question being asked as well as providing support/evidence for the proposed solution. The format around here can be tricky to get used to, it takes a little time. I would note that in time as you gain rep you will be able to comment on any post, which is where a shorter version of your post would make more sense. If you have questions check out help center and Worldbuilding Chat $\endgroup$
    – James
    Mar 1, 2016 at 14:58
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The real question is : "What is an eye ?" Are we talking of human-like eye ? A fly has very different "eyes" than most of the other animals.

Compound eyes are made up of thousands of individual visual receptors, called ommatidia. Each ommatidium is a functioning eye in itself, and thousands of them together create a broad field of vision for the fly (http://animals.mom.me/flies-see-out-compound-eye-5361.html)

They can see different things :

houseflies have the ability to see polarized light, but humans cannot differentiate between polarized and unpolarized light. Polarized light is light in which the waves travel only in one plane.

A fly need this 360° vision to survive because, well, it's just a fly.. And the fly's eye has many disadvantages, but I used it as an example, your creature can have a different type of eye.

We need two eyes for for depth perception because they are quite simple. What if there was two or four pupils in the same eye ?

The now extinct Trilobite is an interesting one :

Trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied: some trilobites had only one, while some had thousands of lenses in a single eye.

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If we consider an "eye" to be a single biological structure which has the purpose of encoding an image of the world such that its owner can perceive and act within that world based on the image, then this can cover a broad range of sensory organs.

There can be good reasons why an organism - even one that relies heavily on vision - might evolve to have only one "eye" rather than two or more, provided that the organism has the right type of eye.

If we consider the human eye, it has a single lens that focuses light onto an area of photosensitive cells, giving two-dimensional perception.

If we consider an insect's compound eye it is made up of many ommatidia, each of which represents one or seven pixels in the insect's field of view.

Now consider something in between these two types of eye. If we have a compound eye made up of many subunits that are more like a human eye - each subunit having a lens that focuses light onto an area containing many photosensitive cells, then each subunit would form is own image with a fair amount of detail. By combining the images obtained from all the adjacent subunits, each of which would be pointing in a slightly different direction, and is also slightly offset from its neighbours, we get an optical array that is similar to that found in a light-field camera.

This means that unlike both human and insect eyes, depth perception does not require two or more structures, but only one. By placing an array of eye subunits on part of the organism's body, potentially forming a spherical structure, the organism could gain a 360° 3D view of its surroundings that is in-focus at any range. The organism could house processing structures within the eye's structure, and pass a processed image to its brain.

While made up of multiple subunits, each of which could be an eye, the structure operates as a whole. This form of eye has the advantage that the organism has equally good vision in all directions at all distances simultaneously, and gains a true 3D concept of its environment. Unlike human eyes, there is no delay for focusing or tracking. This would be of advantage to a creature that could be attacked from any direction by quick airborne and land-based enemies.

However, this form of light-field eye would most likely suffer from the problem that it has a lower resolution, sacrificing perception of fine detail for broad coverage at any range, unless the organism has a particularly large portion of its brain devoted to visual processing.

Now, to address the original question: Would it be more advantageous to have only one of these light-field eyes, or more advantageous to have more than one?

The answer would be that under most circumstances it would be advantageous to have only one. Each light-field eye would require a significant amount of processing power, and each spherical light-field eye could have a field of view that, if placed at the top of the organism's body, would cover the organism's entire surroundings with the exception of into the structure supporting the eye. If there were two, they would require additional processing structures with their associated metabolic costs, they could conceivable obstruct each-other's field of view, and they would be unlikely to provide much advantage, as each alone produces a 3D image that is in-focus at any range.

Under such circumstances, an organism with only one 360° light-field eye that covers all potential avenues of attack would have a metabolic advantage over an organism with more than one, hence favouring cyclopia. There is the issue that the eye would lack redundancy, however if the organism's predators are a sufficient threat that being captured is typically fatal, this would tend to favour the system with the lowest metabolic cost. Additionally, since the light-field eye is made up of many subunits, some subunits could be lost without greatly degrading the acuity of the eye as a whole, unless several adjacent subunits were lost.

An evolved light-field eye would be quite similar to the Lytro Immerge camera, though could cover the area directly overhead in a way that the Immerge doesn't appear to do.

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Following with the answer that indicates that evolving two eyes is not a given.

My suggestion would be that the first time an eye evolved in this environment, it happen to be an structure that provided 360 degrees vision. For example, a medusa like creature for which skin cells also are photosensitive, so the skin and eye are the same organ.

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  • $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$
    – JDługosz
    Dec 31, 2016 at 7:25
  • $\begingroup$ @JDługosz "My question is simple: Would there be any situations, Earth-like or otherwise, where a creature would benefit from having only one eye?" - man 360 degree vision(!), sure give me two. $\endgroup$
    – MolbOrg
    Dec 31, 2016 at 18:07
  • $\begingroup$ I read it as a further comment on the other post. You point out that it might be just fine to only have one (if that happened) in this case. But this Answer donesnt give another reasin why it would be benefitial to only have one. He lacks the rep to post a comment so made it an Answer instead. $\endgroup$
    – JDługosz
    Dec 31, 2016 at 22:09
  • $\begingroup$ I understand some people may interpret this is a comment. But it does intend to be a separate answer, while referring to the other answer without elaborating what was already discussed there. $\endgroup$
    – Bruno Guardia
    Jan 2, 2017 at 18:33
  • $\begingroup$ There's a sensible trade-off that makes sense here: a more complex, more capable eye evolved first restricting the possibility of a second one developing later on because of lack of energy/the one present is already good enough. $\endgroup$
    – Three Diag
    Apr 15, 2019 at 19:50
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The perceived frequency would matter. Visible light is roughly around 500nm; if a species evolved to be able to view lower frequency, longer wavelength electromagnetic radiation, presumably they would need a larger lens and focal length, and hence a larger eye. If the eye needs to be large enough, the benefit of having a second eye might not outweigh the expense.

This would perhaps enable seeing into the far infrared, or maybe even into the microwave or radio spectrum. There would be a large biological cost, but I could imagine benefits from that as well.

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Aside from theoretical speculations, looking at evolution it appears that once a species has evolved two eyes, it is unlikely to go back to one. Even in animals like the flounder, the useless eye moves to a useful location rather than disappearing.

For species where general light-awareness is useful, but eyes are not, there is no need for a (relatively complex) eye to evolve at all. Simple light-sensitivity of the skin (or parts of it) would serve the same purpose for a much reduced evolutionary cost.

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  • $\begingroup$ In a flounder the whole skull twists around, bringing the eye along. $\endgroup$
    – JDługosz
    May 3, 2016 at 1:41
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This may be a possible scenario: Protection of the eye is more important

a) stereoscopic vision is not particularly important in their environment for some reason, for example if they don't have to do much hunting or fighting and most of their food comes from plants which could be easily identified and foraged. Perhaps they have good passive defenses (like a Hedgehog's spikes or just really tough armour) which discourages predators.

Or perhaps the evolution of their brain could never get the hang of processing it.

b) the eye could perceive EM frequencies that penetrate objects that would be opaque to us. (e.g. they have X-ray vision) and their environment has a lot of waves of these frequencies flying around. They'll need it because...

c) protection of the eye is so important that it's buried deep within their tough and thick skull and only have a single aperture into the skull to see through. Perhaps they are vulnerable to sand-storms or small flying pests. (Though as has been pointed out, redundancy is good protection too, maybe in their environment, it's not as good). Trying to peer through a narrow hole would not be much good for an eye, but they get their peripheral vision (albeit not as clear but still useful to a limited degree) from (b) above. They have limited vision through their skull.

This hole is the only point of entry to the inside of the skull that doesn't go through bone, so having just a single one (and therefore a single eye) makes all of the contents of the skull easier to protect.

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    $\begingroup$ Good answer, but I think you should elaborate more on A and B. In what kind of environment would it be unimportant to have stereoscopic vision? Why would seeing those frequencies make it better for the creature to have on eye? $\endgroup$
    – Xandar The Zenon
    Apr 27, 2016 at 14:58
  • $\begingroup$ @XandarTheZenon I've modified the answer to explain this a bit better. You need to see those frequencies because otherwise the small aperture would be too restrictive to be useful. $\endgroup$
    – komodosp
    May 3, 2016 at 8:41
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The key here is not whether 1 eye benefits the creature, but whether 2 eyes do.

Evolution tends to favor simplicity, and thus the most efficient use of resources given. Now if the species in question evolved 2 eyes, there's nothing you can really do. But if before your species evolved eyes, they went into an area where depth perception doesn't matter, like a cave, then another eye would be nothing more than another part that can get damaged. So, using the logic of evolution 1 eye would be equally matched with 2, if not favored.

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In Sundiver, spoiler alert,

The species evolved with only one eye but had the ability to both use and emit coherent light. That is, project holograms and sense phase and direction of incoming light, which was a return from his own emitted pulses.

That is, the being has something more like high-def lidar than a camera.

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  • $\begingroup$ If there is a spoiler alert, wouldn't it be appropriate to use the spoiler markdown? $\endgroup$
    – Secespitus
    Jun 26, 2017 at 8:51
  • $\begingroup$ I didn't think it was that important since it's a well-known book. I had debated whether to say “spoiler” at all, IIRC. $\endgroup$
    – JDługosz
    Jun 26, 2017 at 8:54
  • $\begingroup$ I haven't heard of it until now. It just struck me as weird to write "spoiler alert" and not use the appropriate markdown in that case. The book seems to be quite old, so it's probably not very likely that many people are reading it at any given moment. $\endgroup$
    – Secespitus
    Jun 26, 2017 at 8:59
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You'd have to be in an environment in which having two eyes is a liability instead of an asset, so that evolution would favor creatures with a single eye.

Some ideas:

  • A world in which light is so bright that creatures must use the blind spot of their eye and create an eclipse-like effect in order to look at something.
  • A world full of things and creatures emitting coherent (laser-like) beams of light. Catching more than one at a time could be too cumbersome or require too much cognitive processing to be worth it evolutionary.
  • Conversely, a world in which light is bright and washing everything away, so that information is only conveyed in variations in ambient light.
  • A non-Euclidean world in which light takes such convoluted paths that having two eyes / viewpoints is more troubling than helpful.
  • Creatures that live in one-dimensional bright environments, like intestinal parasites of a bio-luminescent creature or optic fiber network dwellers.
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There is that old saying; in the last of the blind, the one eyed man is king.

If you are talking about an "Earth Like" planet, the scenario where a creature with a single eye would have an advantage. They would have an advantage over all creatures with no eyes.

I suspect the main evolutionary reason that we (and pretty much all creatures on this planet) have two eyes is that the bulk of living things on our planet exhibit bilateral symmetry. That is to say we, butterflies, pigs, fish, pretty much everything, are essentially have a creature that's mirrored down the middle.

On a planet where creatures are not created with bilateral symmetry, it would be more likely to see a creature with a single eye 1. evolving and 2. having an advantage (assuming everything else didn't have a functioning eye, maybe they just had a few light sensitive cells).

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  • $\begingroup$ The reason for having 2 or more eyes is depth perception. A 1-eyed creature cannot judge distance. Note: I count an insect's compound eye as more than one eye... and outside the scope of the question since Cyclops had a human eye. $\endgroup$
    – SRM
    Dec 25, 2016 at 4:34
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    $\begingroup$ I would argue that the reason for having two eyes is bilateral symmetry which exists in almost all earth creatures. An advantage to having two over one is depth perception, however a planet where creatures didn't feature bilateral symmetry, a single rues creature would have an advantage over a no-eyes creature. Weather complex beings depend on bilateral symmetry is open to debate. Note sure why the downvote and it's surely legitimate to suggest bilateral symmetry logically spawns two eyes creatures so that is what you'd need rid of to have a single eyed creature naturally evolve. $\endgroup$
    – Steve Matthews
    Dec 26, 2016 at 9:22
  • $\begingroup$ Another reason insects have two eye-sets is for peripheral vision, but again that could be solved. I think a lack of bilateral symmetry is a perfectly valid suggestion, especially if the world contains mostly blind competitors and prey. $\endgroup$
    – thanby
    Dec 28, 2016 at 18:24
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If an organism is simple enough then it may not be able to afford a second eye. Cyclops bicuspidatus exists on Earth and is an example of this.

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    $\begingroup$ ChronoD’s answer mentions it but doesn’t name the species. Maybe this should be a comment or edit on the older answer instead? $\endgroup$
    – JDługosz
    Dec 31, 2016 at 5:02

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