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I've got a fairly small community of people that, after a genocide, some survivors moved into a cave system. They've been living there for about 5 or 6 generations now and rarely leave the caves except occasionally at night when they need to get food. These caves are pitch-black except for a faintly luminescent material in some areas, that they have mixed into tattoo ink for some minor light to assist in communication and finding other members of their group. I'm wondering how this lifestyle- living in little to no light- would have affected their overall appearance by this point, particularly how their eyesight might have changed in response to this level of light. I'd also imagine they might be rather pale due to lack of sunlight, but I could be wrong.

*Edit- Assume the vitamin D issue does not apply and that they have a consistent source of it. My primary focus is more eye changes/sight, though other changes are welcome!

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    $\begingroup$ I think that this particular question might help you find the answer to your question: worldbuilding.stackexchange.com/questions/42427/… $\endgroup$ Feb 14, 2022 at 0:02
  • $\begingroup$ The main answer to that question was that essentially that if a dark-skinned population were to live underground for a long period of time, natural selection would slowly select for lighter skin tones, but the process for natural selection itself is slow. $\endgroup$ Feb 14, 2022 at 0:05
  • $\begingroup$ The OP did not specifically say if natural selection was at work in their population. If modern humans today moved into dark caves (not pretending to know why), we would have no natural selection influencing our gene pool. $\endgroup$
    – Vogon Poet
    Feb 14, 2022 at 2:06
  • $\begingroup$ "Say, Frank, did it ever strike you as weird that the mountain with all those caves often has cooking-smoke coming out of it? And the river emerging from under the mountain sometimes has trash in it?" $\endgroup$
    – user535733
    Feb 14, 2022 at 2:36
  • $\begingroup$ Something to look into are actual subterranean cities, like Derinkuyu in Cappadocia. People have been excavating and living, sometimes for long periods, underground for thousands of years. $\endgroup$
    – elemtilas
    Feb 14, 2022 at 17:33

3 Answers 3

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Welcome to the forum!

The short answer to your question is that genetic changes your society will experience in 5 or 6 generations living in a cave, are identical to the changes you and I have experienced from our 5-generation ancestry. Real-world evolution has no goal and does not "respond" in any way to changes in conditions.

99% of all species which have ever lived on Earth are extinct because their environment changed. Extinction is a very real and normal answer to dramatic changes in the environment. So I will start by saying this world you are creating has unrealistic genetics, and can only exist if you have designed some form of alternate evolution into your world. That's fine, you can invent some alien manipulation or divine guidance that is actively protecting your people from extinction. In that case, whatever eyesight enhancements you want them to have will be designed in by your genetic designers.

Normally I would assume the question wants an answer within the scope of real-world genetic, but I don't make that assumption here because of your one comment:

"I'm wondering how this lifestyle- living in little to no light- would have affected their overall appearance by this point, particularly how their eyesight might have changed in response to this level of light."

That said, there is small room for epigenetics to make superficial physical changes to each individual living in the caves, but these will not be transgenic. Future generations may benefit from epigenetics only if those changes create a live-or-die scenario, and competing gene pools die off without them. This deserves an example:

Humans have invented artificial groups called "races" based on melanin content in their skin. There are no biological 'races' of homo sapiens based on genetics. But, there seems to be more melanin in aboriginal people on the African and Australian continents than in Northern continents, and melanin serves to protect us from the sun's UV radiation. Genetics did not do this "in response" to more or less sunlight. What happened is, people with less melanin could not survive as well in the full African sun, so they either migrated North, or died off. If your ancestor happened to arbitrarily have the "switch" for more melanin, they probably decided the sun wasn't a big deal and stayed. But his or her brother may not have been so lucky, and they couldn't take the heat. The gene pool in the Southern continents eventually had very few people who can't produce more melanin, and at some point there would be a mutation in the people that left them naturally darker at birth. They have an obvious advantage in Africa, and either killed off or chased off people who couldn't take the scorching sun. In other words, the better quality was a pure accident but survived by natural selection. It is important to note that the genetic change that produced more melanin can happen in Africa just as easily as it could have up North, but in Africa and Australia it provided a survival advantage and became dominant in the population. Thus, living under a scorching sun had absolutely nothing at all to do with the actual mutation that makes people darker in Africa and Australia; it only made that mutation 'win' in natural selection. So your people in the cave will have the exact same genetic changes you and I will have, randomly.

For your society to have permanent genetic changes under real-world genetics, they must be exposed to natural selection. For example, if you want them to have better night vision, then you need to have something remove people with poor eyesight from the breeding pool (a stealthy predator, a selective disease, or make it illegal for them to breed). Even this won't likely make a noticable change in only 5 generations however.

Now, even though your scenario isn't realistic, it is very common in science fiction. The original Planet of The Apes had humans who developed huge brains and telepathy while they were living underground. But they had radioactive fallout speeding up genetic mutations through hundreds of generation, so that was their in-world excuse. Please understand there is no Hard Science answer to your question.

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  • $\begingroup$ 'Illegal to breed' is no longer natural selection. Just nitpicking a bit here. Overall I think it is a good answer, upvoted. $\endgroup$
    – Otkin
    Feb 14, 2022 at 18:30
  • $\begingroup$ As a side note, guided breeding aided by radiation is why we have pink grapefruit. Radium gardens (which are more or less what they sound like, a radiation source with plants around it) resulted in increased genetic mutation and when they got bits switched that they liked, they tried to breed for that trait. Easier, of course, with plants that stay a fixed distance from the radiation source, are fairly simple, and have a short lifecycle. $\endgroup$ Feb 23, 2022 at 19:09
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A group of people living in an underground civilization is undoubtedly going to suffer from serious vitamin d deficiency, leading to serious health problems such as osteoporosis or cancer.

Here are a few links regarding vitamin d deficiency: https://www.healthline.com/nutrition/vitamin-d-from-sun#overview https://www.vitacost.com/blog/vitamin-d-deficiency/

Considering these side effects, I am not sure how long an underground civilization like this one would even be able to survive let alone propagate for long enough to actually flourish unless they have a source of vitamin d in food that is readily available or technology I am unaware of.

The development of human skin pigmentation in nature is linked to the amount of UV radiation that people receive. Areas with high amounts of sunlight and UV radiation developed darker skin tones and areas with lower amounts of sunlight were naturally selected for people with lighter skin tones, so living in an underground area with no ultraviolet light should most likely select for people with little to no pigmentation in their skin.

Here is a link that helps get across my point: https://www.nasw.org/article/vitamin-d-levels-determined-how-human-skin-color-evolved

With time and enough generations, people living underground would undoubtedly get paler.

However, they would most likely suffer from so many health issues due to vitamin d deficiency that their existences would be unpleasant at best and nightmarish at worst.

Edit:

Assuming that Vitamin D deficiency is not an issue, then the development of eyesight would probably be similar to that of skin tone. People who are better at seeing in the dark would be more likely to survive, and natural selection would, over successive generations, filter out in such a way that people with better dark vision would become the norm.

On top of being paler, they might also have white hair and violet/red eyes which are common of people who are born without pigmentation in their skin. In the book series Gregor the Overlander, by Suzanne Collins, the person who is most known for writing the Hunger Games Trilogy, there is an underground civilization of a similar nature to the one that you are describing. They might serve as an inspiration to the people you are writing about.

Also, even with a steady supply of vitamin d, there are a lot of other complications that come about with keeping people underground. For example, without a day and night cycle, it would initially throw off the biological clock of the people in this society. Then, after many generations. They would develop their day and night cycles differently, and I'd imagine that would greatly change their sleep cycles, which might affect the mental health of the people living in this society.

One more thing to consider. When a person is in the dark for a long time, their eyes become naturally adjusted to the dark. Living in the dark full-time would probably lead to these people being extremely sensitive to bright light sources, so being suddenly brought into the sun would not be pleasant. These underground people would probably get sunburns easily as well.

In conclusion, I believe that people living underground would eventually, after several generations, select for traits of being extremely pale, with likely pale white hair and light eyes that might be pale violet. Their eyesight might be good in the dark, but it would be extremely sensitive to light sources. The lack of a day and night cycle would throw off their biological clock, leading to many unforeseen difficulties that may affect their mental health in the long run.

Edit2: Vogon Poet is most likely correct in saying that such changes would probably take place over far more than 5 or 6 generations, so the changes depend on how quickly natural selection and/or genetic mutations would take place within this population.

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    $\begingroup$ I am curious how "living in an underground area with no ultraviolet light should most likely select for people with little to no pigmentation in their skin." What survival advantage does lower pigmentation provide these people, or, what would prevent darker people from propagating their genes in the dark? Natural selection works by attrition - removing disadvantageous adaptations. UV doesn't cause a pigmentation gene. $\endgroup$
    – Vogon Poet
    Feb 14, 2022 at 6:13
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    $\begingroup$ @VogonPoet, producing pigments uses energy, producing no pigments saves that energy. That is the advantage. $\endgroup$
    – L.Dutch
    Feb 14, 2022 at 12:23
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    $\begingroup$ Unlikely that will make or break a species in life and death, agree? There is more energy difference between individuals due to their weight than the immeasurable difference here, so if the population is splitting that line, all fat people are already dead. $\endgroup$
    – Vogon Poet
    Feb 14, 2022 at 13:54
  • $\begingroup$ @vogonpoet the lesser production of pigment happens even with sun exposure IRL- why not here? $\endgroup$
    – PipperChip
    Feb 14, 2022 at 16:23
  • $\begingroup$ It's not a matter of weather it happens or not, only attrition causes selection of the gene. Why would people who still have pigment stop breeding when they live in a cave? What is preventing the pigment gene from continuing on, while people with lower pigment survive? Is pigment making them sterile in the dark? $\endgroup$
    – Vogon Poet
    Feb 14, 2022 at 16:54
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Perhaps you should also consider bodily changes that happen to the individuals during their lifetime. Apart from vitamin D deficiency (and probably others depending on their diet) already mentioned elsewhere, perhaps there are other changes.

Perhaps there is atrophy in cone cells in the eye if light levels are sufficiently low.

With no light cycles circadian rhythm will be affected.

Perhaps they develop better hearing. Or new fears.

Do they do enough physical exercise?

How are newborns affected during their first 2-3 years? Can they see their care giver's mouth and expressions?

How large is their community? Are there in-breeding effects?

And so on. :-)

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  • $\begingroup$ They do exercise, they’re fairly nomadic they just live in a large cave system for safety reasons. They’ve dwindled over time but have managed to avoid inbreeding. Newborns can somewhat see their parents’ facial expressions due to faintly bioluminescent markings around the mouth and forehead areas. $\endgroup$
    – None
    Feb 24, 2022 at 18:49

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