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Other than photosynthesis being an issue, if the Earth's surface became inhospitable and humans were forced to live underground, what biological changes would they be expected to go through to adapt to the new environment?

For example, let's say the makeup of the atmosphere changes and the environment aboveground becomes too harsh to exist on, forcing the population to live underground. How would generations of no sunlight effect skin colour? Or the ability to absorb needed vitamins? Would eyes develop to adjust for less light or would new senses replace the less useful ones?

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    $\begingroup$ Highly related: worldbuilding.stackexchange.com/q/394/28 $\endgroup$ – Monica Cellio Dec 18 '14 at 2:09
  • $\begingroup$ "let's say the makeup of the atmosphere changes"... so which is it? Has the atmosphere changed, or has sunlight gone away? If the atmosphere has changed you have bigger and more immediate problems than lack of sunlight. $\endgroup$ – Sam Axe Jan 27 '17 at 5:03
  • $\begingroup$ It wouldn't affect IT people :o) $\endgroup$ – Alexander von Wernherr Jan 27 '17 at 6:33
  • $\begingroup$ people denied sunlight often show severe depression, there is a reason several far northern countries create artificial sunlight. $\endgroup$ – John Dec 12 '17 at 21:15
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You've touched on the major points already: vitamins and skin colour. They are linked to an extent.

Vitamin D is the major vitamin we get from sunlight. It's partly a "happy vitamin" in that it provokes serotonin neurotic responses, which makes you happier (have a look over at Biology.SE for more details on this response). So, one big problem you'd have is higher depression rates (and at risk of being a bit more morbid, higher suicide rates). You would, however, find that light-skinned people suffer less: they are capable of producing vitamin D through UV-B exposure faster. On that note, bring the tanning beds underground with you.

Melanin is a pigment found in hair and skin. It is the chemical that determines your skin colour. It appears to be an adaptation to sunlight. This is why, generally, people who live by the equator have darker skin. When underground, this chemical would be needed far far less, so you would find that over time and several tens of cycles of macroevolution, most people are fair-skinned.

Depending on the conditions underground, you may also find people's senses altering. Moles, for example, live their entire life underground and have almost no sense of vision, despite having binocular eyes. If the underground homes that humans now live in are dark, you will find that through both adaptation and evolution, they de-evolve having such good vision. You can find better definitions for micro and macro evolution on Biology.SE, but in general terms:

  • Adaptation is small changes that can take place over an individual's lifetime. For example, you might find that the colour-sensitive cones in a human retina degrade in favour of the light-sensitive rods.

  • Evolution is the Darwinian theory: individuals with characteristics that make them more likely to survive are more likely to reproduce and pass these characteristics on (natural selection). This means over time, cones may be eliminated completely.

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    $\begingroup$ Your definitions of micro and macroevolution are a bit off. Microevolution is evolution that happens within a species, not within an individual. For example, if dark skinned people have trouble making vitamin D and suffer an increase in morbidity as a result, there will be more light skinned people. This would be an example of microevolution. Macroevolution generally refers to species level change. In mammals, this is usually just lots of microevolution over a long period of time, but faster changes are common in plants. Both are fundamentally the same mechanism. $\endgroup$ – ckersch Dec 17 '14 at 18:43
  • $\begingroup$ Aye, @ckersch, but there is more than one definition of microevolution. Macroevolution I agree, and (I hope) that definition is what's conveyed here. $\endgroup$ – ArtOfCode Dec 17 '14 at 18:46
  • $\begingroup$ The specific definition that I've seen everywhere is "a change in gene frequency within a population." (evolution.berkeley.edu/evolibrary/article/evoscales_02), or some variation on that. What other definitions have you come across? $\endgroup$ – ckersch Dec 17 '14 at 18:51
  • $\begingroup$ @ckersch The definition I was taught - the one above. Someone thinks it's right, if they're still teaching it... $\endgroup$ – ArtOfCode Dec 17 '14 at 18:54
  • $\begingroup$ Microevolution - new child born underground will start with same number of of cones as adult which was born aboveground, and in time would adapt (any adaptation of adults is NOT inherited). Macroevolution would be when the changes are inherited. Do I have it right? $\endgroup$ – Peter M. - stands for Monica Dec 17 '14 at 20:09
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Keep in mind that essentially all energy in the world comes, directly or indirectly, from sunlight. While caves may have some life most of it either regularly travels above ground for nourishment or hunts those that do. If the earth above was rendered inhospitable to all creatures the real issues would be starvation due to an inability to get food/energy.

The only two sources of energy, and thus food, that would be viable are geothermic, which is very limited, and sunlight. This means figuring out a way to grow/capture/use food from above ground to sustain the below population. Perhaps plants still grow reliably above ground and those living below can manage limited trips to fetch it? Still, if they are not farming that means far less food available, and thus a much smaller population, in any given area.

This is a follow up to the below comment, but is a point I feel deserves better stressed then my comment could, as I think it's the most important detail. Humanity's evolution, regardless of environment, is first and foremost driven by our technology! We no longer adapt to our environment, we adapt it to us. This doesn't mean we are not evolving, but it's a very different evolution then the type you're used to. We are evolving to be more intelligent, to better work in a complicated social enviroment where working with others is the only way to achieve anything, and evolving to work with our technology. We are not, however, evolving to be better suited to living in the cold, or heat, or any enviroment really because we simply build technology to work around our limitations.

For instance, our leading cause of death in America is heart disease. We would be evolving to not die from the fatty foods we eat. Our next most common cause of death is Cancer, which likely means slowly evolving means to detect and destroy cancerous cells (which our body already does a pretty good job of, only a very very tiny fraction of cancerous cells become cancer). Of course, those are today's causes of death, with as fast as our technology evolves, our leading cause of death in 2 generations may be completely different; which means we won't really have time to evolve to adapt to those particular threats. Instead the long term adaptions will likely all be about working in social groups, use of technology, and general fertility.

This makes questions of human evolution in the underground hard to answer, since first I have to ask what technological approach we would use to adapt the underground. For instance our eyes are a great example. Plenty of cave-dwelling species have either lost their eyes or had then degrade to a fraction of what they were, since being able to detect light is useless in an enviroment where there is no light and eyes are a vulnerable spot we need to protect. Does that mean our eyes would degrade, probably not! Humans would instead set up lights to allow them to see like the did above ground, because that's how we work now and it's allot easier to adapt our enviroment to support our need for light then it is to adapt our eyes to work without light. Over a few generations we may get better about seeing in darker areas perhaps, but our eyesight would not be significantly changed unless we were unable to create lights. The same principle exists everywhere, we will adapt our enviroment to our current needs long before we would evolve to better suit our enviroment!

Some changes may still occur in terms of physical evolution over a short time period, However, these will be simply favoring certain existing genomes over others. Certain types of humans may become more common rapidly, for instance perhaps one skin tone is less prone to vitamin deficency from lack of light then others, or maybe short people may quickly become dominant as it becomes hard to survive in tunnels if you're too tall. These changes could start to have a measurable, though still not overwhelming, effect in a small number of generations if a substantial enough number of humans die out (the more deaths, the more evolutionary pressure) However, this isn't creating new phenotypes, simply prioritizing certain existing ones. Humans would still look human, just with certain types of humans being a bit more/less common. It would take a huge number of generations to have any significant change to phenotype, for humans to start looking different than humans of today. By that time, our technology and culture should have adapted to the difficulties of living underground and we would have long since adapted our enviroment to support us. In short, I don't anticipate any huge change in human structure or format, at most, certain current physical traits become more common. Which become more common, depends on what evolutionary pressures we were unable to address with our technology. IF I knew exactly what enviroment humans would create for themselves below ground, I could anticipate what the most common things to kill us below ground were and anticipate what evolutionary pressures they would give us. However, I would first need to project our technological and cultural adaptations before I could begin to infer our evolutionary ones.

The end result is that I see this question as being one about how our technology and culture will evolve and adapt. The real question is what cool technology will we use to work around whatever problems forced us underground. That technology drives everything else, and is a very interesting question in it's own right. However, it's also a very difficult question to answer without knowing very exact details as the cause of our moving underground, thus the reason I have to ask follow up questions to give any useful advice.

Since sunlight is such a huge source of energy for...everything humanity would therefore do everything they could to find a way to harness that energy. Thus exactly how off-limits the above world is would be a huge question, because we will find a way to use technology to harness the energy and light above ground if at all possible, and that ability impacts everything about later technological, cultural, and physical evolution.

The question of how much warning humanity has before being forced below ground is also critical. How much technology we have to help us adapt depends on how much time we had to develop and build technology before moving below ground, which in turn affects everything about eventual development.

Finally how many humans died during the first 1-2 generations is a huge factor on whether we would notice any phenotype difference due to moving underground. If only 10-20% of humanity died out, then I wouldn't expect a huge difference in phenotype, beyond skin coloration, to occur because we clearly adapted our enviroment fast enough to avoid facing significant evolutionary pressures. if 50% of the population died out, then we would expect to see those with existing phenotypes that are better adapted to life below ground to be the norm. Again this is more about how common or perhaps exaggerated existing traits are, they would still likely look mostly human.

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  • $\begingroup$ So how would human biology change to combat such issues as a reduced or differing food supply? $\endgroup$ – James Hunt Dec 17 '14 at 14:05
  • $\begingroup$ The real question is how would human culture evolve. Human evolution now of days is driven by our technology, not our enviroment. Right now humans are evolving to be able to handle fatty foods without heart attacks and have the sort of reflects that would keep them from having car accidents, because those are the things that kills us today, not our enviroment. To make any realistic estimate of how humans evolve I need to know what technological approaches are taken and analyze what pressures those approaches place on us. I can't do that without knowing better what the disaster is. $\endgroup$ – dsollen Dec 17 '14 at 14:12
  • $\begingroup$ Surely the cause of the disaster is irrelevant, only the result. Above ground is off limits. So assume no access to topside of Earth at all. $\endgroup$ – James Hunt Dec 17 '14 at 14:17
  • $\begingroup$ so..how do we get food? if it's from geothermic energy (the only viable source if no above ground is available) then we would be living in very scattered settlements and the risk of inbreeding is a major evolutionary drive. If we find a source of food above ground we may be less scattered and our driving force is building technology. Are limited resources forcing fighting that drives us? how much did our tech devolve? I need to know our culture and tech to guess at evolution To know that I need to know exact details about the enviroment to guess how we would use technology to adapt to it $\endgroup$ – dsollen Dec 17 '14 at 14:40
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    $\begingroup$ You seem to be forgetting that the Sun has been dumping energy into the Earth for billions of years. We have a lot of stored energy in the form a fossil fuels. Generators can power lights that can be used to grow plants. If the population were severely reduced and had access to oil wells they could survive on that energy source for many generations. $\endgroup$ – Samuel Dec 17 '14 at 18:08
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To change and adapt human biology natural way would take many thousands of years, especially if changes are drastic, like substantial changes in our senses or metabolism. Even natural selection does not make too strong darwinian pressure because our society does not allow to people die because they are less optimally adapted - because if we as society have resources, we help such people survive (i.e. diabetes).

Much will depend how much resources the underground society has (which is not obvious from your question).

  • How do you power and feed such civilization? Would all food have to be grown in hydroponics powered by nuclear power? Where oxygen comes from?
  • Do you have enough people surviving to be able to repair malfunctions and train replacement as people die out? For all complicated technologies? Repair mining machinery?
  • Is it possible to go out for pillage party to bring some resources, metals, oil, books, equipment, maybe harvest food? Or is surface too deadly even for short trips?

We had few questions in recent few weeks about how small remnants of society after disaster can survive, do your research.

Well-functioning underground civilization with enough resources can focus research on developing (bio-engineering) adaptations necessary for such conditions. If you don't have resources, then well, it will take much longer and your civilization might not make it.

Most likely possibility is that even if surface is uninhabitable for few decades, life was not completely wiped out, and after few decades people can start returning to surface, at least for short time (farming, harvesting). So you don't need to adapt to underground life forever, and civilization will survive even if substantially weakened in number and knowledge.

There is way too many moving parts to provide answer better than "it depends", because - it depends.

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Well considering many animals that live with lack of sunlight, humans could possible become like moles or bats. That is if we didn't add man made light. We would have less melanin which would cause lighter skin since the lack of sunlight. But many plants will die if all of Earth lost sunlight, so we would be extinct before we could adapt unless edible plants adapted before us. But this is all off the top of my head so :/

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  • $\begingroup$ How about reading Art of Code’s answer and other detailed answers before just going “off the top of your head”? Plants are discussed more in comments… addressing this issue, not just “we need plants”. $\endgroup$ – JDługosz Jan 27 '17 at 8:44
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I want to take a different approach to answering this question. The person asking has gotten great factual answers, but no fun ones. Just for fun let's remove some variables and do a thought experiment. Let's suppose that right at the time homo sapiens appeared, some of them departed the group and began living underground. At that point there technology was limited to sharp rocks and fire. Now fast forward to the present. Assuming a very high level of evolutionary pressure, What would the descendants of those people look like. Here is my guess.

Average height: 4'5'' Skin tone: Extremely pale white, bordering on translucent. Face: No visible eyes at all, vestigial eyes would be shrunken and hidden behind the skin of the face and the skull which now covers the vestigial sockets. Build: Powerful, well muscled, very little bodyfat Hands: Oversized by 1/3, Nail thickened into blunt wide claws for digging. Proportions: Arms are elongated and hang by the knees Movement: lopes on all fours when running, fully bipedal otherwise. Optional adaptations: -Echolocation, which might com with larger possibly pointed ears. Short sensitive tentacle like appendages hanging like a long moustache from the face, prehensile and loaded with touch receptors. -Bioluminescence, can be controlled with will but naturally reacts to the emotions of the individual, comes with functioning eyes that are very large, similar in shape and size to grey alien but with human like whites, iris and pupil.
Lifestyle: Hunts other underground creatures and farms algae or fungus that can survive in underground climates.

This answer produces a fun sci-fi like subterranean hominid. I have no idea if that is helpful, but I enjoyed writing it.

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I haven't read a single reference to mating. Females (and males for that matter) natural instinct is to seek out the strongest to mate with. In our current societies, that person is extemely intelligent but it's the 'athletic intellegint and not the 'nerd intelligent' as always. In the future, will the most athletic alpha be the person of a height that makes the best of low level tunnels and/or larger than average eyes sockets to enhance available light (and cones and rods). I'm talking of course about the studly males commonly featured in Anime. Evolution depends on natural selection for the most part and natural selection is dependent on mate selection. In totalality, my post is an oversimplification but aren't they all?

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  • $\begingroup$ Anime is clearly preparing us for this apocalypse :) $\endgroup$ – Hohmannfan May 27 '16 at 22:26
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Vitamin D already covered.

Your people would have serious problems with Alcoholism and probably with SAD.

It is not known why people living in Northerly parts, and especially North of the Arctic circle, have high rates of alcohol-related problems. It seems to be universal, in that the problem is shared by Europeans, Orientals, and Native Americans.

The only answer known so far is banning alcoholic beverages, or for individuals to abstain. Absence of sunlight is the probable cause.

Also linked to absence of sunlight is SAD (seasonally affected depression). Some find staring at a bright light for some minutes each morning helps. It is very hard to get artificial light at sunlight intensity and even harder to match it's spectrum. It may be that solar UV on skin is the key, not bright visible light in eyes. We know about vitamin D but is there something else made in our skin?

For some, only the certainty that Spring will arrive keeps them going. For some, even that promise is not enough. Suicide rates are anomalously high, along with alcoholism. And your people are never going to experience sunlit summer days.

Finally there is "cabin fever" or "Yukon fever". After months cooped up indoors with extreme cold and darkness outside, people go.mad. Not sure if this is recognised as a medical Northern winter syndrome or not.

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