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As a series of anatomically correct myths, here we have the Cyclops. One of the most similar creatures to man but with 'one' noticeable difference, one eye. It seems that land animals love their two theme; two eyes, two ears, two sets of limbs, etc. Having two eyes helps with depth perception and a backup in case one fails. So it begs the question why would a cyclops ever evolve. All that is needed for an accepted answer is a primate (or primate-like creature) that has evolved one eye. Is there a realistic way that a cyclops can evolve? Using Earth or near-Earth biology, how close could I get to the cyclops design?

A list of all of the Anatomically Correct questions can be found here

Anatomically Correct Series

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  • $\begingroup$ There are many instances where a mutation resulted in only one eye. This phenomenon is called Cyclopia. To clarify, that is not what you want for a realistic cyclops? You want an actual species of them? Similar to Cyclops? $\endgroup$ – DoubleDouble Nov 20 '15 at 19:52
  • $\begingroup$ @DoubleDouble yes it has to evolve it. also your link is wrong $\endgroup$ – TrEs-2b Nov 20 '15 at 20:20
  • $\begingroup$ @DoubleDouble after reading the answers I realize that they are not wrong, sorry. $\endgroup$ – TrEs-2b Nov 20 '15 at 20:36
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    $\begingroup$ You could have stereo vision with a single eye socket, I guess. It would either move and refocus quickly or have multiple pupils somehow. $\endgroup$ – Crissov Nov 20 '15 at 23:00
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There are some small crustaceans known as copepods that possess a single unpaired eye in the center of their heads. This eye is known as the naupliar eye, which is sometimes retained and sometimes lost depending on the species.

In the species Pontella spinipes, the naupliar eye contains three lenses with six photoreceptors behind them. The photoreceptors in the male naupliar eye are arranged in a way that suggests that it's used to detect the colour pattern of females, which are blue and yellow.

They have this singular eye even though they still contain bilateral symmetry (antennae/antennules.)

So since we have a species that does have a single eye, we have to figure out why a primate would have one. Primates have forward-looking eyes with stereoscopic vision to give them depth perception, along with strong manipulators and prehensile feet.

It uses its digits to manipulate and hunt on the land. It doesn't have the benefit of floating in dark waves, eating phytoplankton and not requiring a lot of complexity.

In order for a primate to have a single unpaired eye, it seems that you would have to drastically change its environment. Perhaps if it lived in darkness and perceived things differently, such as sensitivity to sound, vibrations, etc. You have to remove its need to have forward looking stereostopic vision and its need to manipulate or move in the way it does.

Give it a (maybe) dark environment, filled with plenty of food sources that it doesn't have to expend the energy and tactics to hunt. It might even need to abandon the trees, because it's rather difficult to leap from branch to branch without depth perception. The ocean has tons of phytoplankton, reefs, and algae. Land has plants and insects. If the primate can conceivably live off of a plentiful terrestrial alternative without threat from predators where the stereoscopic vision would enable its survival, it might retain an unpaired eye if there's no biological reason to give it another one.

This is all conjecture, though. It seems to me that even if it were possible, with all of these necessary changes to the environment and how it might affect the rest of the primate's biology, you probably won't be able to call it a primate anymore.

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Not a primate, but Cyclops is the appropriately named genus that features a single eye.

A single eye birth defect cyclopia found in any number of species, including humans. From evolutionary theory, nothing would prevent such a birth defect from becoming isolated and forming a separate species. This particular birth defect often results in miscarriage and usually associated with a defective nose and results in suffocation. So, this particular form of birth defect is very unlikely to result in a separate breeding population. A different defect that results in a single eye without the other health issues would be more likely to breed effectively.

Even if a trait is a disadvantage, if it breeds true, it can be the basis of a new species. For example, one-eyed are considered unfit mates, so they don't interbreed with the two eyes population. Genetic isolation is essentially the definition of a new species. Further genetic drift would be expected over time.

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It's not just one eye - it's one BIG eye.

The Cyclops is a large hominid species that has evolved as a specialized nomadic shepherd. In order to decide where to lead its flock next, being able to see long distances - and as such, where it might find the best pastures and smallest amount of competition from other cyclopes or wild flocks - is more important than depth perception, since it doesn't need to hunt anyway. Large eyes are also good for night vision, which a shepherd will need to keep watch for nocturnal predators.

The animal with the most comparable adaptation is the ostrich, a bird famous for having eyes larger than its brain. The cyclops, however, is an intelligent species. There is a lot of intraspecific competition between them - when deciding where to lead their flock, they also have to anticipate where other cyclopes are likely to lead their flocks, in order to avoid unwanted competition. As such, they need to be capable of long-term planning coupled with a strong theory of mind. They may also have to keep track of agreements or truces regarding particular areas of land. As such, it needs at least one big, long-distance eye, and a big brain.

There isn't enough room in its head for two oversized eyes and a big brain. So the best trade-off is to lose one eye.

One might think that losing one eye is problematic for a shepherd because it reduces peripheral vision and therefore could miss sight of a predator, but remember that grazing animals normally keep their heads low to the ground in order to graze, and as such need the ability to spot predators out of the corner of their eyes quickly. A cyclops, by contrast, can stand guard on a nearby hill and slowly rotate to get a clear view of the landscape, including all potential predators, for miles around - it has nothing better to do. Also, if a predator gets through, the cyclops doesn't have to be concerned about its own life, the worst that could happen is that it will lose a few sheep. So good distance vision and intelligence trump peripheral vision in this case.

The early cyclopes were likely binocular hominid shepherds that started to develop one of their eyes to be bigger than the other, using their larger eye to see further away. Over time, the smaller, less-used eye atrophied and eventually disappeared, and the main eye became more centralized.

This lifestyle also explains the cyclops' large size - height for seeing farther, strength for fighting off predators and wrangling its flock, and as a shepherd they have easy access to meat and protein. Of course the colossal size they are typically depicted with is an exaggeration, but maybe nine or ten feet tall with a wide, squat shape could be fairly realistic.

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If a species needs vision but not two eyes you can assume that one eye will be dominant, then start to atrophy while the remaining, dominant eye moves towards the center. So what you need is a list of reasons to have two eyes and explanation why they do not apply to the Cyclops.

Wider field of vision and having a spare.

Many prey animals need two eyes to observe their surroundings for predators. Many animals need two eyes not to be blinded when something happens to one of them.

The Cyclops has no predators that we know of and displayed ability to act even after losing its one eye based on its hearing and scent. This actually makes sense since it supposedly lived in a cave where relying on light would be impractical. We can assume its primary sense is hearing with ability to echo-locate objects in caves and close range with high accuracy. While catching insects or small fish requires ultrasound, a Cyclops probably could get away using lower frequencies, maybe the sounds of its breathing and movement would be sufficient.

And for all we know the breathing of a Cyclops produces loud but inaudible to humans ultrasonic whine. Or the Cyclops can rely on passive hearing to detect changes and the sounds of its movement to first detect static shapes. Since the Cyclops could not locate humans by their breathing despite otherwise being able to act fairly effectively, I'd say ultrasound based solution is most reasonable. Although without the sheep to mask both sounds and scent it would probably have been a different story.

Depth perception

Instead of stereoscopic vision the Cyclops could simply rely on its echo-location ability for depth perception at close range. At longer ranges phase or contrast detection could be used. Or a hybrid of the two. If the Cyclops start from close distance (which is in echo-location range) and then focuses farther until the target image is sharp, its depth perception would be good enough with one eye. (Because you usually only need accuracy up close.) And the eye would give valuable information from beyond echo-location range so the one eye would be retained and probably have excellent vision. Since a Cyclops lives by hunting and pastoralism in a mountainous region seeing moving things at a large distance would be useful.

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I can think of two ways that an animal only needs one eye. The first is that it is not a primary sensor. Say they evolved mostly in caves and so touch and sound would be much more important senses. Sight would be a back up for the 'border' lands. It might have started evolving eyes, and then moved into the twilight area, and other senses became much more important.

The other is an idea I'm stealing from David Brin. He had an arboreal humanoid that had one eye. How the species evolved was it's eye worked like bat's ears. Sonar, but instead of sound, it produced a laser for distance. Kind of like hunting range finders, Not only could this work, but it could be a weapon if developed far enough (not likely). Most likely it would be in the infra-red range, so if you really made one mad, and it stared at you, you might start feeling a little warm! New meaning to a 'burning gaze'.

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Compound eyes don't need to come in two, so if your primate developed two and they merged together (like your rear skull bones) or developed as one (as your frontal skull bone) it could be a primate with only one eye.

Other then our Spherical lensed eyes compound eyes can work perfectly well as a single eye if its large enough. It doesn't even need to be spherical but could be oval and still work fine. As the various Ommatidia feed their information directly to the brain its not required to be two separate entities.

A Compound eye can produce depth perception all on its own. If your species is (or was in its evolutionary past) a fast reproducer, its not overly important that individuals die when their only eye breaks. Redundancy here is in numbers of individuals anyway.

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