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I'm thinking about underground humanoid cvilisation without knowledge about outside world and how could they have eyes?

Known fact is that a lot of animal living in dark don't need eyes. (cave or deep ocean animals)

Maybe we could consider red vision, but how and why should they develop it? What about underground light sources? Hot melting metals? What other light source could be considered?

I see how developed civilisation can survieve underground, but how can it grow and as the question here - how could they develop eyes in the evolution process?

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  • $\begingroup$ Are we talking about a civilization that lives underground and comes out into the light occasionally to hunt or acquire materials? For evolution to develop eyes a species has to be exposed to light at some point in its history. $\endgroup$ – SurpriseDog Sep 26 '19 at 16:41
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    $\begingroup$ Welcome to Worldbuilding.SE Adam, glad you found us. We have a tour and help center you might wish to check out. $\endgroup$ – Cyn says make Monica whole Sep 26 '19 at 16:42
  • $\begingroup$ @Benjamin, as mentioned I'm thinking if it's possible without knowledge about outside world. Maybe some stars energy could be transfered by ground somehow to become light again, but I don't know how it could be possible $\endgroup$ – Adam Sep 26 '19 at 16:46
  • $\begingroup$ Not exactly a duplicate, but there's a lot of overlap on the answers: worldbuilding.stackexchange.com/questions/32318/… $\endgroup$ – Morris The Cat Sep 26 '19 at 16:47
  • $\begingroup$ @MorrisTheCat, thanks for the topic. It is in did connected, but here I'm curious the most about the evolution process $\endgroup$ – Adam Sep 26 '19 at 16:51
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Most blind cave animals evolved from sighted surface dwellers (the only exception I can think of are a few species of worms, about the same level as a planarian, and those might also have evolved from similar creatures with eyespots).

Therefore, you need only an excuse for the eyes not to have been lost to evolution.

The simplest is for infrared vision to have gone much further than it has in most mammals. All humans can see a little into what's commonly called "infrared", at least long enough wavelengths to see the kind of weird vision that one sees in infrared photography (black skies, white leaves, etc.). This isn't long enough to see heat radiation, but you could easily handwave a population underground for many millennia to have evolved that level of IR vision.

They'd have eyeballs -- in fact, their eyes might be a good bit larger than ours (to gather more radiation, or to give good resolution with longer waves), but they'd be debilitated in daylight. Not only because it's too bright -- but because the reflected IR from the sun or other sources would be so unfamiliar they couldn't make sense or what they were seeing, even after their eyes adjusted enough not to be just a blinding pain.

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  • $\begingroup$ So it could be possible that their 'on ground' ancestors had eyes and somehow that eyes were kept in evolution underground by changing the functionality? $\endgroup$ – Adam Sep 26 '19 at 16:49
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    $\begingroup$ Humans don't typically see infrared at all, although it's possible to detect wavelengths slightly longer than the typical visible range under the right experimental setup. Infrared photography registers normally-invisible electromagnetic waves and displays them in the visible spectrum - everything you're seeing falls into the normal visible spectrum, you're not seeing infrared. $\endgroup$ – Nuclear Hoagie Sep 26 '19 at 16:55
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    $\begingroup$ @NuclearWang Well, by definition -- if you can see it, it's visible, not technically infrared. My point is that you can see wavelengths normally treated as infrared, and to which your eye is so insensitive that you can't see them unless you take extraordinary steps to block all light in the "normally visible" range and let your eyes fully adapt. At whatever point there's a survival/reproductive advantage to that visual range, evolution will start to select for it. Since different people see different levels, there's variation -- all that's needed is selection pressure. $\endgroup$ – Zeiss Ikon Sep 26 '19 at 18:29
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    $\begingroup$ @Adam Evolution only makes changes if there's a reproductive advantage -- or an accident occurs that has no disadvantage. Male pattern baldness may have given an advantage in Vitamin D production in cold climates -- or may simply have done no harm over the normal reproductive life of a man. Unless being born without eyes made survival more likely, they wouldn't lose their eyes -- and eyes are less costly, in terms of growth, than our big brains, so we'd tend to reduce the brains first if starvation were the pressure. $\endgroup$ – Zeiss Ikon Sep 26 '19 at 18:50
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Make your own light.

Some creatures devolve away their eyes when living in the dark for generations. However, the Stoplight Loosejaw fish didn't... because it evolved a built-in bioluminescent "flashlight". It has bioluminescent photophores near the eyes, which allow it to illuminate prey.

Your humanoid creatures could simply follow this evolutionary path, developing their own bioluminescence, and thus they would definitely have eyes.

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Minerals can be radioactive.

This radioactivity can be used as source of energy for visualizing the surroundings.

Organisms adapting from an above ground to an underground life would slowly adapt their eyes to the light sources available in the new environment.

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  • $\begingroup$ Is it somehow possible to radioactivity produce light without being harmful? I like the idea of glowing rocks, but not sure how to make that real $\endgroup$ – Adam Sep 26 '19 at 18:11
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    $\begingroup$ A combination of any alpha emitter (which is most radioactive minerals) and zinc sulfide (naturally ocurring sphalerite) will give that green "glow in the dark" emission. $\endgroup$ – Zeiss Ikon Sep 26 '19 at 18:32
  • $\begingroup$ This would create an interesting situation, where the creatures would naturally gravitate toward light sources because it would give them better sight and thus increase their survival/prosperity, but those light sources would cause cancer and harmful mutations, decreasing survival/prosperity. $\endgroup$ – cowlinator Apr 21 '20 at 21:29
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If there are warm-blooded creatures in this environment, and the humanoid is cold-blooded, it is possible that the humanoid could evolve eyes that could see infrared light, which is produced by warm-blooded creatures.

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  • $\begingroup$ The OP asks how an underground humanoid could develop eyes. As written your post doesn't answer that question. Unless you edit this post to answer that question it's likely to be deleted for failing to meet this site's quality standards. $\endgroup$ – sphennings Apr 21 '20 at 21:49
  • $\begingroup$ I disagree. The question is about how underground humanoids could develop/evolve eyes. The OP even asks about light sources. This answer mentions that infrared vision would be needed to see heat sources, such as creatures with a high body temperature. $\endgroup$ – cowlinator Apr 21 '20 at 22:05
  • $\begingroup$ @cowlinator Look at the timestamps on the edit history. My comment was left before the most recent edit. $\endgroup$ – sphennings Apr 21 '20 at 22:06
  • $\begingroup$ I have explained how an underground creature could evolve eyes that sense light from sources found commonly underground, which is what the question asked $\endgroup$ – Ichthys King Apr 21 '20 at 22:12
  • $\begingroup$ @sphennings, the edit was trivial. This was always a valid answer. $\endgroup$ – cowlinator Apr 21 '20 at 22:15

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