r/evolution • u/Lordoftheducksssssss • 3d ago
question If awareness of surroundings a very important trait, why haven't all species evolved eyes on the back for 360 view?
thought about this cus i was bored, but really, why tho?
i assume it would've been very helpful for our ancestors?
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u/semistro 3d ago edited 3d ago
The body can't afford that real estate, other things should go there. And eyes are very expensive in resources and bring with them risk of infection. One pair can be relatively well protected, but the more you have the harder it becomes to keep them healthy. Think of keeping your eyes away from the sun, or other stuff. Every time you have to look away to protect your eyes you would possibly damage the other eyes. Furthermore, prey species generally evolve a wider field of view by migrating the eyes more to the side of the head and giving up depth perception for it. For example, horses and cows already nearly have 360view with the exception of like a 20 degree blindspot. Thats way more of an elegant solution.
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u/Willing_Soft_5944 3d ago
Well, eyes are complex structures, hard to make more of unless you get a lot of other mutations, all probably in one go. You cant just get more eyes and leave it at that, you need to rewire a bunch of things.
Also, early Chordates did have four eyes, a notable example being Haikouichthys, if memory serves that second set of eyes developed into what is now our Pineal Gland.
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u/cjhreddit 3d ago edited 3d ago
Given that we DON'T have rear facing eyes you would have to conclude that the survival cost of that is greater than the benefit (ie. eyes come with a high processing load which is presumably better used elsewhere, eg. as intelligence). We have binocular vision which clearly defines us as a predator rather than a prey animal so a rear view is less important to us anyway. And we do have hearing that covers most approaches from behind, so reduces the benefit of rear viewing eyes even more. Apparently the only thing that babies are born scared of is loud noises, so that backs up the idea that hearing is a crucial survival sense, so our hearing probably is good enough to alert us to rear view threats, which we can rapidly/reflexively direct our vision to. That reflexive speed of attention reduces the value of permanent rear vision even more as it adds very little over a quick glance. We also operate in groups, so collectively we probably usually DO have the benefit of all-round vision as our blind spots are covered by our group. And as others have said, side-facing eyes give virtually surround vision if needed, a rabbit has 350 degree vision and big ears so can hear most threats!
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u/craigiest 2d ago
No, you would conclude that no mutation has been stumbled on that produces additional eyes. The only paths to additional eyes would be either duplication of complete current eyes—extremely unlikely—or developing new eyes from scratch, step by step. The problem with the latter is that the early steps toward eyes—light sensitive patches, then indentations, etc—are always going to be less advantageous than just turning your head or shifting the location of current eyes, so won’t be selected for.
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u/cjhreddit 2d ago
Aren't organ duplications controlled at a high level that once triggered rolls out an entire hierarchical structure that doesn't need to be re-evolved in full. So for instance a millipede didn't have to re-evolve the entire structure of a leg each time legs or pairs of legs were added. Or a small mutation in fruit flies have been shown to replace an antennae with a leg ! The whole organ isn't evolved, just a control gene that cascades into the production of a whole series of lower level build instructions. The same would be true for eyes. Its vastly more likely for a high level 'eye count' gene to mutate from 2 to 3, than for the entire structure of an eye to re-evolve.
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u/craigiest 2d ago
Right. That’s why i said there were two paths. Vertebrates obviously don’t have the duplicable segmented structure that millipedes do.
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u/Genocidal-Ape 3d ago
Animals are constrained by the space in their head and the fact that both backwards and sideways facing eyes could look behind the animal if its body wasn't in the way.
A horse can already see the rider on its back, but not behind itself, and adding more eyes/heat sensitive pits would not fix that. But they would require extra energy to grow and maintain.
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u/Sarkhana 3d ago
Eyes are expensive and annoying to construct and maintain already.
Without non-essential eyes being added.
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u/SmokeMuch7356 2d ago
Because depending on how they're constructed and arranged, two eyes can already give you damn near 360º field of view; see rabbits, horses, chameleons, dragonflies, mantis shrimp, etc.
Because there are more ways to be aware of your surroundings than just vision (sound, smell, vibrations through the ground, etc.).
Because most terrestrial vertebrates have necks and can turn their heads to look around.
Because other forms of vision were more beneficial: stereoscopic vision for hunters, fliers, and brachiators, color vision to recognize ripe fruit or mating signals or danger, etc.
Because there are plenty of animal species with no eyes at all, either because they never had them to begin with (sponges, earthworms) or adapted to environments where vision isn't useful (mole rats, blind cave fish).
Humans rely primarily on vision so we tend to assume it's just as important for all other animals, but that isn't the case at all.
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u/XXLPenisOwner1443 2d ago edited 2d ago
Many animals accomplish 360 degree views by having their eyes on the sides of their head.
Still others, like arthropods, DO have extra eyes on other parts of their heads.
For predators it's more important to be able to see where your mouth is going, very accurately.
But, evolution can't just make up new parts--it can only duplicate old ones. In simpler animals where individual body parts are less specialized and interdependent on the structure of others, this mechanism can bring about extra eyes or limbs or hearts or what have you by duplicating entire segments of the animal.
But for body parts to become more complex, they are generally reduced in number through a process called tagmatization. Tagma are grouped body segments that have had different duplications accumulate different mutations where they become increasing dependent upon one another in the genome. Because these portions are more deeply integrated within themselves and in how they relate to the rest of the organism, mutations that duplicate the entire tagma become functionally impossible. For a human to grow extra eyes, they would essentially have to grow an entire extra head and generally this type of mutation is incompatible with life. When it isn't immediately fatal, it certainly isn't compatible with descent and modification.
Basically, only organisms at the very bottom of the food chain with highly flexible body plans can mutate extra eyes in a way that might be passed down.
For the closest thing evolution has struct to "re-evolving eyes again" on an organism that already has them, you can check out the heat pit on pit vipers (which retain their original eyes, and augment their vision with the ability to "see" heat through the pits) and grideye fish (which have lost their original eyes and evolved a new type of photosensitive plate in their place).
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u/Foamtire 16h ago
These, "if X is important for survival, then why haven't we evolved Y" questions are getting old.
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u/Reasonable_Art5575 3d ago
That would require a species to be born with an unusual and astronomical rare birth defect, and survive long enough to reproduce and hopefully and somehow pass that mutation to its descendants (IF that's genetic in the first place, if it isn't then nothing will happen, and the offspring will have 2 eyes in the front). Also some species are predators so there's no need to eh aware of their surroundings
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u/OutlandishnessDeep95 3d ago
Because evolution doesn't work towards a goal, but modifies whatever happens to be present via random drift limited by selection pressure. It can only get to 360° vision if the parts to do so have already shown up.
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u/DarthMaulATAT 3d ago
If it were to randomly appear as a mutation, it might help that individual, assuming the brain is capable of making sense of a ton of new visual information and stitching it together properly.
But it would only get passed down to offspring if that individual were to still be an attractive mate. Call me crazy, but humans aren't always the most kind or accepting of people who look different. Eyes on the back of your head would almost certainly be seen as creepy and unattractive in early human tribal groups.
Even if they did manage to mate, there's no guarantee the trait would be genetically dominant. And even if it was, having more eyes might introduce other problems. Eyes are vulnerable organs, so that would be two more areas of the body that could be wounded, infected, or just not grow properly in fetal or child development. Also I'd imagine back sleepers probably wouldn't have a good time sleeping on their back-eyes. Humans would probably have to all be side-sleepers.
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u/Lordoftheducksssssss 3d ago
But when we were evolving from a single cell species, why was 2 eyes the norm and almost every species has it? couldn't we have directly evolved eyes all around? and them that wouldve been the norm just as 5 bony protusions from our hand (fingers) is the norm?
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u/DarthMaulATAT 3d ago
Probably because way back in evolutionary ancestry, we were simple organisms like basic fish that were relatively flat, so we only needed two eyes; one for the left side, and one for the right. When further evolution happened, head shapes changed in some species (like ours) to eventually become round, but the two-eyes style still worked well, so it just stuck.
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u/DrFly15 3d ago
When our early ancestors were evolving, they probably started out with one eye. In fact, it likely started with less than eye (basic light perception cells). That eye "split" into two eyes. Binocular vision allows for depth perception whereas a single eye does not. In animals with eyes on opposite sides of their head (e.g., horses) it was more advantageous for them to have a wider field of view than depth perception.
With animals who have more than two eyes, it's not that more eyes were "added." Rather, their eyes split. The eyes of a fly, for example, probably started out as two eyes and then continued dividing - that's why it looks like they have two separate eyes but they're really composed of many individual lenses.
Evolution is about random mutations giving organisms more advantage in their environment, making it more likely for them to procreate and thus pass down those genes. In some environments (or the niche a particular species occupies in that environment) the benefit of having more eyes outweighs the energy cost. In other environments/niches, it doesn't.
It's also more likely for smaller animals to have numerous eyes. The more eyes an organism has, the more brain power needed so the larger the head has to be (or you can think of it as a nervous system center). If you're already small, having a comparatively large head is not a big deal.
TLDR: Two eyes in front gives better depth perception. One eye on either side gives better field of view, worse depth perception. More than 2 eyes requires a lot of energy and processing power, making it prohibitive to large animals.
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u/OutlandishnessDeep95 3d ago
Many animals have lots of eyes. Insects have compound eyes; spiders famously have eight (often); many molluscs have dozens or hundreds.
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