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Many creatures - humans among them - have binocular vision, where two eyes side by side allow good depth perception. There are quite a few other advantages over one eye, including a larger field of view, as well as redundancy (an injury leading to blindness in one eye won't cripple the creature's sight entirely).

I want to develop an Earth-like world where a group of humanoid creatures evolve with a sort of double binocular vision, with eyes in both the front and backs of their heads. Besides this, they would have relatively human-like cranial structure - only one nose, one mouth, one set of ears, and a fairly normal brain. The eyes in the back are perhaps slightly less developed than the front ones, but still quite functional.

I'm assuming that this sort of adaptation occurs in many other creatures in this world that would otherwise have normal binocular vision. An extra set of eyes requires a more complicated system of nerves and muscles, and it seems likely that this evolved quite slowly over time. Therefore, I'd bet that more than one species has evolved with this sort of property.

My question, then, is this: Under what sort of relatively Earth-like conditions would humanoids develop eyes in the backs of their heads? If you think there's no possibility for this to happen, then an answer supporting that would be, in my opinion, valid (and helpful!).

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    $\begingroup$ I don't think that having two sets of eyes confers enough of an advantage to be selected for. The closest thing that exists in vertebrates is the light sensing spot on the top of some lizards heads. $\endgroup$ – sphennings Jan 8 '18 at 16:29
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    $\begingroup$ @sphennings If you can support that in detail, I'd say that's a valid answer. $\endgroup$ – HDE 226868 Jan 8 '18 at 16:30
  • $\begingroup$ I have to say, this is the first question I've ever seen whose answers justify the existence of the Owlbear. Well done. $\endgroup$ – Jules Jan 9 '18 at 7:45
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    $\begingroup$ I'd just like to request that people stop posting joke answers about moms, teachers and little kids in the comments. They're just going to keep getting deleted. They might have been funny the first time, but at this point, they're a bit repetitive. Thanks. $\endgroup$ – HDE 226868 Jan 9 '18 at 13:57
  • $\begingroup$ I'm not really satisfied with any of the answers here. The idea of an already upright, tribal, tool using humanoid then developing extra eyes is not gonna happen. You'd be better off with say a spider species (many eyes) evolving to walk upright, they could lose/repurpose some limbs as they do so. In short, apes only have two eyes, they also have ears, swivelling heads, and friends and family to watch their backs - so don't start with an ape. $\endgroup$ – Grimm The Opiner Jan 10 '18 at 8:14

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If it did evolve it would probably form as a development of the parietal eye. This is a third light sensing organ that exists in some species of fish, reptiles, and amphibians. This organ is missing from all extant birds and mammals.

For such an organ to evolve into a second pair of eyes there would need to be a very strong selective pressure that would encourage it's growth. Since in many some species the lens of the parietal organ becomes opaque as the animal matures there seems to be an active pressure encouraging the lack of use of this organ.

Having a second fully developed set of eyes would require a lot of additional infrastructure. Humans would need another visual cortex to process the information they were receiving from this extra set of eyes. There would also need to be a capability to focus on two separate images at once and make sense of these disparate images. A much less developed system could provide a good enough solution to predators approaching from behind.

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    $\begingroup$ Any idea what the active pressure discouraging the parietal might be? $\endgroup$ – Bellerophon Jan 8 '18 at 17:05
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    $\begingroup$ @Bellerophon I don't mean to speak for sphennings but a big part of the reason why is stored his answer. A fully developed second set of eyes will requires both the resources to create and maintain these eyes and the resources to process the information from the second set of eyes. Our brain is large, consuming an outsized amount of energy from our body, and the part of the brain that handles visual control and processing is relatively large. You probably wouldn't need to double that size for a second set of eyes, but it would come close. $\endgroup$ – Dent7777 Jan 8 '18 at 17:14
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    $\begingroup$ @Bellerophon A human brain requires 20% of the calories used by a resting human, (about 300 a day). We'd need an extra visual cortex constantly running to support an extra set of eyes. That's extra infrastructure and extra food needed. It's also rather to see behind ourself we just need to turn our head. $\endgroup$ – sphennings Jan 8 '18 at 18:17
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    $\begingroup$ I guess this is all about environmental awareness. Ears allow us to sense our surroundings in a fairly reliable 360° fashion, it isn't perfect, but then again good enough. Eyes allow for a more in depth analysis of a given region of said environment. Moving your head and identifying a potential threat is fast and easy. If that isn't good enough, evolutionary pressure already found a solution better than another set of eyes. Move them to the side of the head, as in most creatures ranking lower in the food chain. This creates a very large field of vision, without requiring another set of eyes. $\endgroup$ – r41n Jan 9 '18 at 7:30
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    $\begingroup$ If were only light-sensitive with no acuity, it would still prevent the possessor from being snuck up on in some conditions (meaning less attrition while drinking, feeding and mating). $\endgroup$ – Will Crawford Jan 9 '18 at 13:59
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You need to constantly threaten those humanoids from the back - for example big silent owls and manual tasks that require a lot of time

The eyes need to provide an advantage for these humanoids, so there needs to be something they excel at because of their eyes at the back. The first thing that comes to mind are very silent predators.

If your humanoids are gathering something, for example herbs, and this process takes quite a long time predators could attack them. Normally we have our ears to detect stuff like that, so the next thing would be to make sure that whatever is attacking is silent. Owls are exceptionally quiet:

feathers adapted for silent flight

Imagine your humanoids needing to farm something that is rooted quite deep in the ground. And it needs to provide a lot of nutrition, like Bonegrass seeds, so it's incredibly valuable to get that stuff. Now while your humanoids are concentrating on un-rooting those yummy seeds a bigger owl attacks them. These are adapted owls that normally attack in groups or are so large that they can kill one of your humanoids.

The humanoids won't hear them coming - but they can see them. And if their eyes are adapted well enough they might see them soon enough to be able to run away and survive the attack. That might mean that these eyes are better at detecting something in dim light or darkness - maybe they have far more rod cells, which work better in dim light, than we and far less cone cells, which are used for color vision, to make place for the rods. So the back-eyes are colorblind, but can detect something coming even at night when owls are more active.

If they are following the humanoids the eyes again show how they are better than anything else - something attacking from behind can be evaded, even if you can't hear it.

This might also have other impacts/implications/advantages on/for your humanoids. To quote a comment from Bellerophon:

These humanoids would possibly also be less social than humans. If you are a social creature then the solution would be to have a lookout while you collect the yummy seeds. These humanoids seem to be adapted to be able to work alone so are probably solitary or live in very small groups.

You can go searching for these seeds far easier alone if you already have someone to watch your back with you - your back-eyes. Without those you would need another human being.

Collecting seeds faster is a good way to outplay other humanoids that need to send at least two huamnoids where your species only needs to send one.

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    $\begingroup$ These humanoids would possibly also be less social than humans. If you are a social creature then the solution would be to have a lookout while you collect the yummy seeds. These humanoids seem to be adapted to be able to work alone so are probably solitary or live in very small groups. $\endgroup$ – Bellerophon Jan 8 '18 at 16:34
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    $\begingroup$ @Bellerophon Yes, that's a good point. $\endgroup$ – Sec SE - clear Monica's name Jan 8 '18 at 16:37
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    $\begingroup$ "These are adapted owls that normally attack in groups or are so large that they can kill one of your humanoids." Relating to this, some of the points brought up in answers to my question What might cause (physically) large predatory birds to form large (in terms of numbers) flocks? may be of interest. $\endgroup$ – a CVn Jan 8 '18 at 20:02
  • $\begingroup$ @MichaelKjörling I totally forgot about that question. Thanks, that is indeed interesting. $\endgroup$ – Sec SE - clear Monica's name Jan 8 '18 at 20:08
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    $\begingroup$ "something attacking from behind can be evaded"—or counter-attacked. Might be useful for hunting, too—my brother-in-law came into the house laughing. Said four hunters were sitting in the field with their guns in their laps, staring at the forest. While several deer were eating grass in the field a few meters behind them. $\endgroup$ – WGroleau Jan 10 '18 at 7:21
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How accurate do you want to be? Let's look at the evolution of creatures, and how that applies to eyes.

All creatures of the subphylum vertebrata, have two eyes. That's a very basic template that hasn't varied across over 500 million years of evolution.

It isn't until you look higher up in taxonomy, to the basic kingdom of animals, that you start finding creatures with more or less than two eyes, as in a spider's eight eyes, or no eyes at all, as in an earthworm. Anything other than two eyes are very far removed from humans.

What has happened in vertebrates is evolution in the capability and location of the two eyes to better suit the creature's purpose. Some herbivores, such as rabbits, have the eyes located more on the side of the skull than the front. This severely limits binocular vision, but gives near 360 degree vision coverage, better for spotting predators. Raptors have evolved a magnified spot in the center of their eyes... good wide vision for a general search for prey, the magnified spot to fully identify a potential target. Some creatures have nearly non functional eyes, like whales, who do most of their 'seeing' with sound reflections.

A more likely evolution for rear vision in a human like species would be a head that could rotate 180 degrees quickly, such as horses, cows, and deer have. That could happen without variations to the basic vertebrate template, just lengthen the neck. Better than the rabbit's side placement, this would keep the acuity and depth perception of binocular vision, just add the capability to look in more directions quickly.

In the case of night predatory attackers, like a very large owl, that evolution might also include better night vision to spot the approach (more rods than cones in the retina), possibly an enhanced sense of smell to detect the odor of nearby predators, or as humans tend to be intelligent, the knowledge of where the large owls tend to hunt, and avoiding those locations and times, or possibly developing disguises, or even countermeasures like the thrown spear.

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    $\begingroup$ "All creatures of the subphylum vertebrata have two eyes": that not true. Actually the lineage which eventually gave rise to humans and birds and crocodiles and lizards and frogs had three eyes, and it may be that in very distant times our ancestors had four (as an primitively unpaired structure in the head is unlikely). The third eye eventually evolved into our pineal gland. $\endgroup$ – AlexP Jan 8 '18 at 20:47
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    $\begingroup$ There are also verterbrates with no eyes. $\endgroup$ – Bob says reinstate Monica Jan 9 '18 at 15:24
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    $\begingroup$ Actually, blind cave fish have eyes. They just don't work. Same reason you have tailbones, but no tail to wag. $\endgroup$ – tj1000 Jan 9 '18 at 19:42
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Don't start from mammals or chordates, because that implies late panspermia.

Instead, build a world (this is Worldbuilding, after all!) Have all major animals on this world have four eyes.

Quadrilateral symmetry would make this more likely, but "quadrilateral symmetry" is a slippery fish. You could have the symmetry last only for a time as they develop in the womb, as ours does: even mammals are arguably, at some very low level, quadrilaterally symmetric - two limbs above, and two below.

So if their symmetry lasted just a little longer, such animals could evolve into bipeds with manipulative arms, or animals designed to move quickly in one direction (our quadrupeds), and so on, but that their design is more quadrilaterally symmetric could inform your design of them, beyond the fact that four eyes was the typical pattern.

Prey animals would naturally tend to have the four eyes at four corners around their head, rather than paired. This would grant them stereo vision for 360 degrees. They wouldn't have to make the tradeoff that earth prey make, trading binocular sight for angular resolution. The rear eyes might migrate out on stalks/horns, the better to see around the body as they eat.

Predator animals would tend to have the rear eyes migrate forwards, up over the head or around the sides, granting them better peripheral vision for hunting, and perhaps being co-opted for other uses that the main eyes did not fill. Extended out on stalks to help with cooling, the extra eyes could be sensitive into the infra-red range.

Scavengers, opportunist feeders, would tend to vary between the two, and a state where the eyes were arranged in two pairs, front and back, could make a lot of sense, since it would still give about 360 vision, but binocular only "ahead" and "behind", which requires a whole lot less processing, allowing other parts of the brain to develop more, instead. For bipeds, the ahead/behind distinction might not make much sense: they might have one direction in which they can move more rapidly, but if they can look in both directions, it's likely that their arms would be arranged such that they could manipulate items on either face of their body with roughly equal facility.

Perhaps one side would be more capable at fine work, and one more capable of hard physical work and fast movement. Perhaps one set of eyes would naturally lean towards long-sightedness, and one to short-sightedness.

Perhaps, like left/right handedness, they could be dorsal/ventral-favored, favoring one or the face (which could lead to various social complexities, biases, genders, etc).

Perhaps they would have a subtly different concept of left/right than we do ("on the left as you walk down the road" is unambiguous; "your left arm" is ambiguous to something facing in both directions).

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    $\begingroup$ +1 for quadrilateral symmetry. Such a creature could become "humanoid" via a process similar to how a flounder moves both eyes to one side of its head, or it could have four arms and four legs with two of each perhaps being vestigial. $\endgroup$ – Perkins Jan 9 '18 at 23:39
  • $\begingroup$ Ooh, I was thinking of the floundering, but the vestigial idea is a great one! $\endgroup$ – Dewi Morgan Jan 10 '18 at 14:50
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Bottom line is that you simply wouldn't get a group of humanoids evolving a second set of eyes, any more than they'd evolve a second set of legs. The basic vertebrate body plan is too deeply imbedded in the HOX genes for that.

If you want humanoids with 360 degree vision, you have two alternatives. First, if this is a completely alien world, you have everything in that world's vertebrate-analogue lineage have four eyes. (Perhaps becoming vestigal in some branches.)

If you are going from an Earth-descended creature, you're basically limited to repositioning the eyes. They could for instance become like the eyes of horses, located to the sides of the head, and with an oblong pupil, giving nearly 360 degree vision (but binocular vision only in a narrow cone): https://en.wikipedia.org/wiki/Equine_vision

This wouldn't be all that difficult to arrange, evolutionarily. Just have your ancestral humanoids spend a lot of time evolving on the open plains, with a lot of predators eager to eat them. They'd likely acquire other horselike (or antelope & other plains creatures - I'm just more familiar with horses) attributes such as being good runners.

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    $\begingroup$ Humans did evolve on open, rolling grasslands and Humans are exceptionally good runners $\endgroup$ – Dent7777 Jan 8 '18 at 20:14
  • $\begingroup$ @Dent7777: Humans and their primate ancestors didn't spend that long on the grasslands, in evolutionary terms. While humans are pretty good endurance runners, they really aren't all that great at speed. For instance, I can easily keep pace with my horse at a trot; but not at a canter or gallop. Likewise my dog (descendant of the horse's predators) can keep up at a canter, but not a full gallop. $\endgroup$ – jamesqf Jan 8 '18 at 23:26
  • $\begingroup$ @jamesqf However, you can outrun the horse over a great enough distance. Sure there are plenty of animals capable of going faster than us on a short distance, but given enough time - humans can chase those animals to die of exhaustion (hence we fair so well on plains). We're in many ways more suited to surviving in the open - where we can start to move away from danger long before it reaches us. A dense jungle is much worse for us, where escaping danger relies more on sprinting - as the predator is already close when we see it. $\endgroup$ – Bilkokuya Jan 9 '18 at 17:27
  • $\begingroup$ @Bilkokuya: 1) Running really fast for a (comparatively) short distance is exactly what's needed to escape most predators. Or as the saying goes, I don't have to outrun the bear, I just have to outrun YOU :-) 2) If humans are so well-suited to life on the open plains, why don't we find large numbers of them living there, and not in e.g. forested areas? AFAIK, plains cultures - the Mongols, the American Plains Indian tribes - didn't really become successful until they obtained horses. $\endgroup$ – jamesqf Jan 9 '18 at 20:17
  • $\begingroup$ @Dent7777 - True, except there weren't all that many predators that were "eager" to eat humans. Opportunistically, sure. But e.g., four dudes with spears isn't a very good opportunity for anything, except for those four dudes. $\endgroup$ – Mazura Jan 10 '18 at 0:12
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Why the Parietal Eye?

I'm going to echo @sphennings and go with the parietal eye. I have chosen to follow this path because, as far as I can tell, the parietal eye is the only occurrence of extra eyes in creatures with spines. Unless the new set of eyes forms from a completely novel mutation (possible but by my estimation so unlikely as to be impossible) or genetic engineering, it seems like the best option.

Furthermore, I'm going to assume that, if humans have the new set of eyes, so do a vast array of other mammals. Mammals evolved from relatively small reptiles (living relatives of which do have the parietal eye), which would have certainly had a good reason to keep (not select away) the rearward facing eye. If humans were the only living mammals with the rearward eye, them you would have to explain why humans kept the eye while all other mammals selected it away.

Why Keep the Eye?

So why would humans, who are a relatively dominant species within our ecosystem, have such a pressing evolutionary need to sense what is behind us? I'll agree with @Secespitus and go with the predator theory. We would have needed to have dominant, constant depredation from species that attacked us from behind and above.

Predators

Now, what preys on your humans (and their great ape ancestors, and their simian ancestors all the way down the line) can vary to fit your story. Again, as far as I can tell, these predators would have to be ambush predators coming from above.

If the area of your world where humans evolved is covered in forest, cliffs, and ravines, then you have a little more flexibility. If it is out on the plains, then you have a little less.

With structures to leap from, you could have any sort of predator that fills the jaguar, drop bear ecological niche. This could be a jaguar or a tree-dwelling lizard or a species we don't have back here on earth.

With or without structures to leap from, you could have any sort of predator that fills the silent flight, giant Owl niche. This could be owls, this could be flying reptiles, or this could even be some sort of angel-looking humanoid, up to you.

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There is one way this could happen, in a manner that's similar to what has probably already happened in vertebrate evolution.

As you may know, for some reason, nerves in higher vertebrates are flipped left/right. What I mean is that, the left hemisphere of the brain controls the right side of the body, and vice-versa.

enter image description here

How did this crazy situation evolve?

First, note that for non-vertebrate animals such as insects are wired "correctly": the left hemisphere of their brain controls the left half, and likewise for the right.

What one scientist proposed is that, during vertebrate evolution, there was a creature that evolved to have its head rotated 90 degrees, so it was looking sideways. Then, there was selective pressure to have it looking "forward" again. Instead of evolving to undo the twist, it kept twisting, to go a full 180 degrees!

What was this animal? Something like the flounder, whose eyes, during development, migrate so that two of them are on one side of its head. It lives its life on the ocean floor, lying and swimming on its side:

enter image description here

So, suppose your humans evolved to have eyes on both sides of their heads, like many prey animals. All it needs to do is evolve to rotate its head 90 degrees, and boom! eyes in the front and back.

As Stephen Pinker explains in The Language Instinct:

No biologist has explained why the left brain controls right space and vice versa. It took a psycholinguist, Marcel Kinsbourne, to come up with the only speculation that is even remotely plausible. All bilaterally symmetrical invertebrates (worms, insects, and so on) have the more straightforward arrangement in which the left side of the central nervous system controls the left side of the body and the right side controls the right side. Most likely, the invertebrate that was the ancestor of the chordates (animals with a stiffening rod around their spinal cords, including fish, amphibians, birds, reptiles, and mammals) had this arrangement as well. But all the chordates have "contralateral" control: right brain controls left body and left brain controls right body. What could have led to the rewiring? Here is Kinsbourne's idea. Imagine that you are a creature with the left-brain-leftbody arrangement. Now turn your head around to look behind you, a full 180 degrees back, like an owl. (Stop at 180 degrees; don't go around and around like the girl in The Exorcist.) Now imagine that your head is stuck in that position. Your nerve cables have been given a half-twist, so the left brain would control your right body and vice versa.

Now, Kinsbourne is not suggesting that some primordial rubbernecker literally got its head stuck, but that changes in the genetic instructions for building the creature resulted in the half-twist during embryonic development—a torsion that one can actually see happening during the development of snails and some flies. This may sound like a perverse way to build an organism, but evolution does it all the time, because it never works from a fresh drawing board but has to tinker with what is already around. For example, our sadistically designed S-shaped spines are the product of bending and straightening the arched backbones of our quadrupedal forebears. The Picassoesque face of the flounder was the product of warping the head of a kind of fish that had opted to cling sideways to the ocean floor, bringing around the eye that had been staring uselessly into the sand. Since Kinsbourne's hypothetical creature left no fossils and has been extinct for over half a billion years, no one knows why it would have undergone the rotation. (Perhaps one of its ancestors had changed its posture, like the flounder, and subsequently righted itself. Evolution, which has no foresight, may have put its head back into alignment with its body by giving the head another quarter-twist in the same direction, rather than by the more sensible route of undoing the original quarter-twist.) But it does not really matter; Kinsbourne is only proposing that such a rotation must have taken place; he is not claiming he can reconstruct why it happened. (In the case of the snail, where the rotation is accompanied by a bending, like one of the arms of a pretzel, scientists are more knowledgeable. As my old biology textbook explains, "While the head and foot remain stationary, the visceral mass is rotated through an angle of 180°, so that the anus . . . is carried upward and finally comes to lie [above] the head. . . . The advantages of this arrangement are clear enough in an animal that lives in a shell with only one opening.")

In support of the theory, Kinsbourne notes that invertebrates have their main neural cables laid along their bellies and their hearts in their backs, whereas chordates have their neural cables laid along their backs and their hearts in their chests. This is exactly what one would expect from a 180-degree head-to-body turn in the transition from one group to the other, and Kinsbourne could not find any reports of an animal that has only one or two out of the three reversals that his theory says must have happened together. Major changes in body architecture affect the entire design of the animal and can be very difficult to undo. We are the descendants of that twisted creature, and half a billion years later, a stroke in the left hemisphere leaves the right arm tingly.

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Humanoids will not be able to develop two sets of eyes from a humanoid form based on evolution theory.

The development of the eye has long been used by creationists in trying to disprove evolution theory on the basis that very slight alteration to the eye would make it not function anymore and thus have no evolutionary basis. It was later proven with a computer model that fish eye could be developed from light sensitive patches to a fully developed eye with each evolutionary step in between beneficial to the creature. I cannot see how you would be able to create such a same track from a normal human to a four eyed human. The whole needed restructuring of the brain would simply not allow it.

Maybe more interestingly why did eyes develop in the front of our head. This traces back to our fish ancestors (I think). They actually don't really have a back of their head since that is where there body is. From there we just evolved with eyes at the front and side.

If you would like eyes in the back of your head I would start with the evolution from those fish with eyes on eyestalks, or maybe from snails. From that tract you could devise an evolutionary path with a third or fourth eye in the back of their head, since via the stalks you can connect them to the same part of the brain. The stalk part can later be developed away as being to dangerous as they are sticking out. It would entail rewriting the complete evolution track for all animals.

Keeping and developing the multiple eyestalks is most likely with encircling predators and predators that like to attack from the back. Later getting eyestalks to retract into the head while keeping the eyes would most probably involve predators specifically targeting protruding eyes as a means to incapacitate an animal.

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Eye location is driven by the direction of the most efficient locomotion and tracking food. Your creature would need to be equally as fast and as agile in two directions and would also need to be equal in food tracking. This forward/reverse motion would also need to be more efficient than just turning around.

A good starter point might be some sort of creature that evolved similar to a pilot fish which follows another animal. Your creature would have evolved in some way to only be able to follow a trail left by another animal in a straight line without breaking off to the side except in extreme conditions (like the line was broken by a fissure or crossed another line close enough to let the creature "switch tracks".

For the Food source requirement, the tracks would need to spontaneously generate food in random locations either in front of or behind your creature that lasted a very short time, making the sight in both directions a food selective and therefore reproductive trait.

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As others have said, it is highly unlikely that your world's version of primates would develop three or four eyes if most of your world's version of mammals, or vertebrates as a whole, had only two eyes.

Four-eyed humanoid beings could only be plausible or likely to occur if most or all animals in their world had four eyes. The evolution of four eyes should happen hundreds of millions of years in the past and four eyes be a trait of all animals on the planet or else a very big group of them - maybe half the different classes and phyla of animals.

So the four-eyed humanoids on your world should be surrounded by thousands of other species of four-eyed animals and every critter should should have eyes in the back of his head.

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A couple of other answers have mentioned spiders and spines, but I don't think anyone has taken this to the logical conclusion yet: Vertebrates on your world can't independently turn their heads. Either their necks don't twist like that (lots of reasons--either the joints are a completely different type of joint, or there is a restriction in the soft tissue--like us compared to owls/praying mantises) or they don't even have necks (like frogs and spiders).

This happened early on (from an evolutionary standpoint) when eyes were simple light-sensing patches, and a random mutation lead to more eyes spread apart to provide a broader field of vision, instead of any of the other solutions common on Earth (rotating neck, or eyes super wide apart). Eventually the common pattern was to have eyes in pairs for binocular vision, with some eyes facing forward and some facing backward. Once the multiple eye solution was going, it was a lot easier for all 4+ eyes to become more complicated simultaneously, and brains naturally kept pace. Only a few odd ball vertebrate animals have fewer than 4 light sensing organs and anything that can rotate it's head is viewed as super flexible and weird.

Your aliens would probably marvel at how fragile and complicated our necks are, one bad fall and you can be completely paralyzed...how did our ancestors ever survive...

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My answer would be that it would happen by chance.

Under similar conditions as Earth, its quite possible that a different kind of sentient species evolved as the dominant species.

Its quite possible that on this planet a long time ago a common ancestor of many current species mutated the trait to have extra light sensitive organs, which would eventually land on the back of the head of a successor which evolved a "head", and these sensory organs became eyes either prior to or after this division.

In order for these extra eyes to continue to exist it is important that they do not hinder reproductive success of individuals across the millennia it would take for a humanoid species to evolve.

A natural selective pressure would greatly assist in the further development of such eyes in some species. Such as a predator prey relationship between two species with a common ancestor. They both would have the extra set of eyes, but its the prey that would benefit more if it had better eyes to avoid death. But that does not stop the predator to further develop its rear view eyes for other reasons.

Its also quite possible that social barriers exist in species that would increase reproductive success in individuals with certain types of eyes. Not necessarily for the better.

Either way, the selective trait must exist before the selective pressure, or then it would just become a race against time for a random mutation to produce a trait that would allow the individuals to survive the pressure prior to extinction. The more complex the organism the less likely this becomes.

So chance it is. The die is being continuously cast since long time ago, and you've got extra eyes.

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As many others have said, I'm sure, there would need to be selective pressure for the use of them. In my opinion, I don't believe it's possible. Having a second pair of eyes similar to our already binocular front would actually be harmful rather than helpful. The head must be enlarged to allow a second network of tissue, and the spinal cord would have to be altered or moved. This also creates the issue of having more physical vulnerabilities. Finally, the cranium would also have to be enlarged because the brain would have to have more connections to process two separate, unconnected fields simultaneously. I feel like the actually selection process has solved this issue. Owls and cats have so many vertebrae they can twist their necks to see a full rotation, with only one field to process. The best I'd say is go the route similar to the Andalites of Animorphs and have them have stalk eyes as well. Or have a different sense that is only concentrated behind the humanoid, such as infrared sensing, some sort of echolocation etc.

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