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The eye is a lens that focuses rays of light from a limited spectrum, after which the the image is turned over to the brain receptors that interpret it (beginning with turning it the right side up first, I think).

It is a complex and fragile instrument whose sole purpose is to pre-process the image "mechanically" (rather than "electronically") before turning it over to the "CPU." Please bear with me.

Most creatures who have one eye usually have another one as well. This second equally complex piece of hardware has, it would seem, two purposes: one, it serves as a spare; two, it allows its owner to see depth a bit better. (You can still see depth if you cover up one eye: your brain has to make a few adjustments, and a few more assumptions, but it's not like depth perception is completely gone, right?)

Now think of the original camera obscura (the one that had no lens, just a hole). By adjusting the size of the hole (aperture) one could make the resulting upside-down image on the opposite wall a bit sharper. The image is still a bit pale, but the subtle differences in color values would still be enough for a powerful computer (such as our brain) to work out the nuances.

Why would then God (or Evolution, for those who still believe in it) opt for making an infinitely complex piece of hardware (the eye) and then furnishing a backup as well, instead of just adding a few lines of code to the operating system?

Please don't take any of this too seriously.

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    $\begingroup$ Note that in addition to having two eyes, vertebrates have two of almost everything, down to brain lobes. So it might be more informative to ask why a) they're bilaterally symmetric; and b) why there are a few organs, like the heart and liver, that don't come in pairs. $\endgroup$
    – jamesqf
    Mar 14, 2016 at 5:54
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    $\begingroup$ Nothing an organism has is "infinitely complex"... $\endgroup$
    – YviDe
    Mar 14, 2016 at 10:14
  • $\begingroup$ to start with @Ricky you could define "infinitely complex"... but it may be better to offer your definition of evolution, I imagine that's pretty flawed and would explain your stance. $\endgroup$
    – rg255
    Mar 14, 2016 at 18:59
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    $\begingroup$ @Ricky If you care about getting an answer to your questions (whether they regard evolutionary processes or not) you should try to clarify it in response to what I found unclear (see my answer/comment). $\endgroup$
    – Remi.b
    Mar 14, 2016 at 20:02
  • $\begingroup$ @Ricky: Actually, I was giving you what I thought was an obvious answer, which is that vertebrates have two eyes because they're bilaterally symmetric, and so almost every organ comes in pairs. The mechanism for that is probably buried deep in the HOX genes (but I am not an expert on this). Also, not every creature has two eyes: most spiders have 8, starfish have 5 (at the end of their arms!), scallops can have 100 or so... $\endgroup$
    – jamesqf
    Mar 15, 2016 at 5:06

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Welcome to Biology.SE. Here are just a few comments rather than an answer...

Clarification of the question

Can you please clarify how "just adding a few lines of code" could replace a second eye? The question is unclear for the moment.

Perception of depth

A simple amusing exercise you can do to explore how much depth perception you lose by having one eye is to try to catch object with both eyes open and then with one eye open only.

A single eye has much poorer ability to perceive depth than two eyes. However, it can still do so. You will learn much more on depth perception on wikipedia > depth perception.

Believe in evolution

You say [..] Evolution, for those who still believe in it [..]. Evolution is not a thing one believe or does not believe in, it is a thing one understand or does not. You might want to have a look at a very introductory course to evolutionary biology such as Understanding Evolution for example. That will help you to understand what evolution is about.

Mythology and Science

You mention "God". Please, mythical creatures are not welcome on science websites except eventually to make a metaphor. Note, on a slightly philosophical matter (which is not my domain of expertise), there is no contradiction between the existence of a God and Evolution. There is contradiction with the many of the false believes of creationism as often described in the Christian culture

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  • $\begingroup$ I will probably not react to further comments and am just inviting you again to learn about what the science of evolutionary biology is about through introductory source of information such as Understanding Evolution by UC Berkeley for example. $\endgroup$
    – Remi.b
    Mar 14, 2016 at 19:58
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    $\begingroup$ Stack thrusts this closed question into my reading list. therefore I offer Richard Dawkins VIDEO demonstration of the evolution of the eye which is great and useful for the guy who asked the confused query: youtube.com/watch?v=eA1rR6ZCxMY $\endgroup$ Sep 2, 2017 at 17:27

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