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In my country, we are split down the center by an intensely large mountain range. Us desert people live on the eastern half of the country, while on the other side of the mountain range the forest people live. Although we are separate for most of the year, every winter we are forced by brutal winter storms into the mountain caves.

200 years ago, a third group of people, the ones who built out the caves and made the central trade tunnel from east to west, suddenly disappeared. Just as they started to fade from memory, there has been an increase in reports of attacks on caravans journeying through the tunnels. It has been suggested that the attacks have been more brutal then the standard highway pirate fare, only a single survivor had managed to escape but without water he died in the tunnels.

It is becoming clear that the attacks were carried out by the lost people, but we have little to no idea of what they even look like. How would 10 generations of living underground with no light, sourcing all foods from things grown and found underground, and a societal focus on digging tunnels change the human body?

Edit: To specify I am not expecting much evolutionary change. What I am expecting is that living in those conditions will result in a body that does not look like a surface dweller. Most obvious would be the pale skin. Not an evolutionary trait, but a product of the environment that changes the population's body. The lost people have developed massive fungi that reproduce quickly and have a high caloric value, so starvation is not an issue for the lost people still associated with the main pack. And technology is equal to that of 14th century Europe, with significantly more metallurgy knowledge. The best answer will focus on physical changes based on living in a cave and less on the evolution of the species.

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    $\begingroup$ The Dutch went to South Africa at the beginning of the 17th century, some 400 years or 12 to 16 generations ago. Their descendants look just like any other Dutch people, maybe just a little bit more massive. $\endgroup$ – AlexP Apr 26 at 16:10
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    $\begingroup$ I'm not sure 10 generation is enough to have the people change drastically enough to make them unrecognizable, but that depends highly on how strong the evolutionary pressures were, in other words, how many died horrible due to living in caves. If your implying they chose to live in the caves of their own free will presumably they didn't have their population decimated (or worse) after they moved to the caves? I need to know how/why they entered the caves and what technology/magic they used to make the caves liveable to suggest the evolutionary affects. $\endgroup$ – dsollen Apr 26 at 16:10
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    $\begingroup$ I would say realistically the population would be cut down to a fraction of it's original numbers if forced to live in caves exclusively, mostly due to starvation because the human body needs more calories then can easily be gotten in a comparatively barren cave system. If your going to be realistic and show the kind of mass starvation and significant deaths having to survive without access to the outside world would offer then that could have enough of an affect to change them in 10 generations; but it would beg the question why they would ever choose to live in caves to start with. $\endgroup$ – dsollen Apr 26 at 16:14
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    $\begingroup$ @dsollen the Lost People were drawn into the depths because as they dig deeper the song of their main god gets louder. The evolutionary effects are close to null, it's more of the nurture side effects then the nature that would create the most visible change I think $\endgroup$ – Alex Apr 26 at 16:51
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    $\begingroup$ @AlexP Underground is more different from deserts/forests than South Africa is from Holland. $\endgroup$ – kingledion Apr 26 at 17:57
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You want a fast change on the evolutionary timescale? Here is one:

Think of blind people. Those who were born normal, and lost their eyesight later in life. Soon, their other senses take over to make-up for the loss. Some blind people may even develop echolocation abilities as shown here:

https://abcnews.go.com/Health/MindMoodNews/blind-man-echolocation/story?id=13684073

Now, imagine how developed is that echolocation compared to bats. Well, not much, but an evolutionary starting point does exist. If the talent becomes integral part of mate selection, evolution will do its part. Other senses, such as smell and vibrations through the tunnel walls (Infrasonic hearing included) are helpful. Something like a cross between a man, a bat and a mole.

Food: That's where your plot may fail. Use a cave echology. Chemosynthesis or other non-light energy sources found in caves may give you a good thing to hold on. You have a few:

Chemosynthesis -- the cave dwellers may have found a cave that is fed by some sulfides from sulfurous springs. The movile cave in Romania, cut-off from the outside world for millions of years has its own energy source from sulfurous springs that feed a complete ecosystem. The cave men must find a way to harvest what grows there without being poisoned. They may be more resistants as a result, though not necessarily being able to dwell the spring on a permanent basis. http://www.bbc.com/earth/story/20150904-the-bizarre-beasts-living-in-romanias-poison-cave

Radioactivity like near a Uranium ore deposit, your fungus absorbs radioactive elements to use instead of photosynthesis. It can be a mushroom, if you want, since this fungus seems to do that: https://en.m.wikipedia.org/wiki/Radiotrophic_fungus

Water Ah! Most important. Aquifers!

Other changes -- The need to dig-out and enhance the tunnels may favor the short and stocky physique.

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Nothing they did not see after year 1, except some eyesight issues you will see after generation 1 that will disappear in generation 11.

The only changes you will see is the same ones people saw a year after living underground, pale skin from lack of sunlight and vitamin D deficiency for the same reason. Malnutrition and tiny population because isolated caves have abysmal production (surviving 10 generations is unrealistic). Eyesight will be the big issue, generation 2-10 will never use their sight, they will not even know what it is, and will have to learn to see when they return to the surface. They will also not have any depth perception, that develops early and only with sight, but as someone without it that will not have much effect.

10 generations is not enough time for any noticeable physical evolutionary changes no matter how strong the pressure, especially in something social like humans. The bottleneck caused by who was trapped will have a much bigger effect.

Cultural changes will be large, they will not know many things, and there will likely be strong shifts in language due to lack of sight, basically you have a blind culture that can suddenly see (poorly at first).

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  • $\begingroup$ I agree mostly with you. However, I'd argue living in a cave would kill so many in the first few generation if one was realistic that it would put a far more drastic evolutionary pressure on these people then is normally seen, enough to drive more rapid adaptation, most noticeably shrinking physical size to cut back on caloric intake. The problem is a realistic depiction of the difficulty of surviving in caves would have everyone dead, so depicting a lethal enough lifestyle to drive rapid adaption but have the people still survive and choose to live in the caves so long is hard... $\endgroup$ – dsollen Apr 26 at 16:32
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    $\begingroup$ generation 2-10 will never use their sight That's silly. They'll use artificial light, basic fire is need be. After all, their brains won't stop working. Just because they've no natural light doesn't mean they're going to mope around in the dark forever. $\endgroup$ – StephenG Apr 26 at 16:33
  • $\begingroup$ @dsollen the purpose for going underground was based on what the lost people consider to be the pull of their main god. $\endgroup$ – Alex Apr 26 at 16:41
  • $\begingroup$ @StephenG the OP specifies no light sources. which is completely believable, there is not much to burn in caves. $\endgroup$ – John Apr 26 at 19:58
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    $\begingroup$ @John If there's nothing to burn, there's nothing to eat either. Presumably anything edible enough to sustain life for that long is also flammable. Unless we're going to accept magic in this universe, they'd either have both a source of food and light, or neither of them, in which case they'd all starve in the first generation. $\endgroup$ – Darrel Hoffman Apr 26 at 20:04
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10 generations is a long time... I think they would have started to fade into memory about 8 generations ago lol but anyway:

Species that only live inside caves are known as troglobites. Troglobites have a few distinct adaptations. They may have developed an ability to use food more efficiently by slowing their metabolism. They maybe have developed anophthalmia or a loss of eyesight or an efficiency in low-light. They may also have devloped depigmentation of the skin. Some creatures may develop extra chemical and tactile sensory organs to make up for loss of visual stimuli. Keep in mind they may not actually be troglobites they could just be troglophile's. Troglophile's have the ability to live both in and outside of caves.

So for your cave dwelling humans I am imagining a semi-loss of hearing, eyesight and skin pigmentation. They are most likely getting shorter and more stocky. They could be developing hunched backs and hardened skin. They are perhaps even developing extra sensory organs such as antennae.

EDIT: Based on others insights I would have to agree that 10 generations is not going to be long enough a period of time to see any significant evolutionary changes such as antennae.

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    $\begingroup$ They manage to stay relevant by the success of their expansive civil engineering projects which is an essential part our country. Some sections of the tunnels have chiseled art pieces of the lost people working, and of their gods. The maps we still use were all drawn up by the people as well, and their lessons of how to most effectively use the tunnels are taught to all minors in our country. The newest writers stopped attributing this knowledge to them, that's why they had started to fade. $\endgroup$ – Alex Apr 26 at 16:04
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    $\begingroup$ 10 generations is not long enough for serous physical changes due to evolution. $\endgroup$ – John Apr 26 at 16:13
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    $\begingroup$ @John yes, but I am not necessarily talking about evolution. The need to crouch might result in the population developing major spine dis-morphia. The use of hands for digging might overtime result in a population with top heavy muscular mass. All these traits would effect the human body, per the question. $\endgroup$ – Alex Apr 26 at 16:33
  • $\begingroup$ if these changes are effecting subsequent generations that is evolution. Also why would they loose hearing sound works perfectly fine in a cave.heck considering humans can use primitive echolocation their hearing might get better. $\endgroup$ – John Apr 26 at 19:57
  • $\begingroup$ @John Oh lol yeah I probably didn't think that through. When you said they were diggers I had moles in mind and was thinking about mole people. Moles have very small ears. $\endgroup$ – Rob Apr 26 at 20:35
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Everyone so far has focused in on the idea that evolutionary changes can't happen in the span of 10 generations, but there is sufficient scientific evidence to suggest the epigenetic traits can change within one individual's lifetime. https://www.whatisepigenetics.com/exposure-to-cold-temperatures-can-change-our-gene-expression-and-fat-cells/

So I'd say you could have just about any changes you wanted - they naturally produce more Vitamin A and have sharp low-light vision - but maybe get easily stunned by bright lights.

They grow to shorter heights, their skin is tougher - the nails of their hands and feet have grown thicker and stronger.

The ability to sense temperature, pressure, and moisture has been enhanced.

You name it, you can probably justify it.

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  • $\begingroup$ Hi, welcome to Worldbuilding.SE! Please take the time to read through our tour if you haven’t yet and visit our help center if you need more information. I would encourage you to visit the Sandbox on Worldbuilding Meta if you are unsure if a question is suitable for our site. I also encourage you to visit our list of worldbuilding resources for inspiration and help with general questions: worldbuilding.stackexchange.com/questions/143606/… $\endgroup$ – Liam Morris Apr 26 at 23:41
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You could look for studies describing developmental differences for human populations living at high altitudes. Some of those characteristics are likely to be present here due to the lower oxygen levels.

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Jews have been cutting their foreskins off for thousands of years, and each new generation stubbornly grows some again.

Your settlers will only be changed by a lifetime of lack of sunlight and whatever environmental impacts there are. The genetics won't change.

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  • $\begingroup$ wouldn’t there be some physical change though? Like people who are smaller and can move around caves and cave systems faster would be more likely to reproduce because they’ll be more productive to the societies goal. $\endgroup$ – Alex Apr 28 at 13:49
  • $\begingroup$ Evolution doesn't work like that on short timescales of a few generations. Ultimately, maybe, but being able to navigate in the caves is one swrvival strategy among many.. pretty people. People good at trading. People good with plants and farming. Engineering skills. These contribute and compete with the short stature. You also have to unevolve millions of years of evolution saying fit, muscular physique is attractive. Just look at eskimos, do they have thick fur instead of skin? Do arabs have humps like camels or different hydration needs? These have been exposed for hundreds of generations. $\endgroup$ – Innovine Apr 29 at 6:55
  • $\begingroup$ The average person of Eskimo decent is 5 ft 4, while the average person from Dutch decent is 6ft tall. People of Arab decent on average have more skin pigmentation then that of a Dutch person, as well as having darker eyes and hair. (Better suited for sunny hot environment) The people who are Eskimo need less food and water on average then a person of Dutch heritage, just based on size. So maybe not a hump but some definite physical differences do exist in humans based on location. $\endgroup$ – Alex Apr 29 at 12:15
  • $\begingroup$ Yes, but nothing extreme, and those changes over a hell of a lot of generations. Chinese people are short, and in a very different environment so that is inconclusive. Average height may be completely epigenetic. Pigmentation is about the only change we can see in thousands of years of evolution that directly related to environment. $\endgroup$ – Innovine Apr 29 at 13:31

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