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What kind of evolutions would humanity have to undergo to be able to live comfortably on the moon or any other sufficently low-mass body? As I am aware that it can be bad for bones and the cardiovascular system, but I am not fully aware of what sorts of evolution would be able to adapt to such an enviroment.

Like, what evolutions to the cardiovascular system would theoretically be required in order to prevent things such as cardiovascular atrophy due to microgravity. I have tried finding an explanation on my own but I am having difficulty trying to find out what evolutions would prevent such atrophy. It is a conundrum. A faster beating heart, maybe?

Related question, as I understand it, the lower bone density would not particularly be a problem unless they want to return to Earth, right? That is how it goes as I understand it, as the bone density would be adapted to lunar microgravity.

if possible, try to find a reasonable timescale for such evolutions to develop, please. I ask this due to the fact that I feel smaller scale evolutions such as slight modifications to heart muscles and such would take a far lesser scale than complete speciation. I don't expect it to take millions of years, but maybe tens of thousands under conditions which encourage evolution, further brought about by heavy interbreeding (interbreeding, not inbreeding) amongst an isolated population, which further accelerates divergence and trait development, as I understand it.

Edit: For clarification as people brought this up, they are living in bases which are provided with breathable air and proper pressure. They are not living outside. And no, the civilization does not have artifical gravity technology. Thus they have to deal with the low gravity.

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    $\begingroup$ Nobody as in really nobody has any experimental evidence of how well or how not well humans cope with a gravitational acceleration which is less than normal but clearly greater than zero. What we know is that people who lose weight do not experience any kind of cardiovascular atrophy. (As a side thought, the greatest part of the effort of the heart goes into pushing the blood through the network of vessels. Pushing the blood up to the head is a very minor component.) $\endgroup$
    – AlexP
    Dec 4, 2023 at 17:56
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    $\begingroup$ P.S. The timescales on which natural evolution operates, especially for a slow-reproducing species such as humans, are measured in tens or hundreds of thousands of years, completely uninteresting for a coherent story. As a practical example, the Dutch ancestors of the Afrikaners moved to South Africa more than 300 years ago. Their descendants are still blond and fair skinned. $\endgroup$
    – AlexP
    Dec 4, 2023 at 17:58
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    $\begingroup$ Relevant question: Can non-intelligent life naturally evolve the ability to space travel and to live in interplanetary space?. Further (a) "Reasonable timescales" for evolution is millions of years. (b) The least of your problems are the difficulties involving bones and heart. Your biggest problem is the ability to breathe. There's no atmosphere on the moon. Then comes the ability to eat and drink... no water or agriculture on the moon. After that you can worry about decalcification of bones. $\endgroup$
    – JBH
    Dec 4, 2023 at 18:17
  • $\begingroup$ @JBH I know, they live in moon bases with breathable interiors. Not outside, where you would be correct about all of those issues. And for AlexP, I know it is on a massive timescale, but my world also takes place over a massive timescale, where human speciation would occur. I was just asking about timescale of more minor evolutions, which I assumed would take less time than full speciation, which is around 4.5 million according to estimates I could find online. $\endgroup$ Dec 4, 2023 at 18:31
  • $\begingroup$ So I don't have to worry about breathing and agriculture, those are possible within the large bases. I thought about terraforming it, but the moon's gravity is too lower to contain a sufficent hydrogen-nitrogen-co2 atmosphere. I edited the post to include my clarification about this. $\endgroup$ Dec 4, 2023 at 18:33

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Low gravity speciation will never occur, leaving these humans effectively the same as modern humans, but there may still be some change in variance of genes that will can lead to their looking like a unique race

I almost didn't write this answer because It seemed like it would be dauntingly long to write. Then I decided to cheat by pointing you to a very similar answer of mine which covers most of the details already.

To clarify when I say speciation won't happen I'm referring to what you presumably mostly think of as evolution, that is the development of novel genes which spread through a gene pool until enough novel genes exists to make a population effectively a different species. I've already dug into this all in more detail in the linked question so here's the quick and dirty version of the reasoning:

  • Evolution takes millions of years for novel adaptations to result in any substantial change in a species
  • Our use of technology to adapt our environment to us will likely slow this process further because anything putting a large strain on human survival will result in our putting effort into finding an technological option for solving this problem removing that evolutionary pressure.
  • Any intermingling of moon-humans with earth - or other heavy world - humans would result in mixed 'race' children and thus gene flow between the two groups further slowing any adaptation.
  • Given the literally exponential rate at which technology grows and changes the need to evolve to handle Low G environments will be rendered moot too quickly for novel adaptations to have time to persist.

If you keep remotely true to how technology has developed in the past in a mere tens of thousands of years - barely a blink of the eye from an evolutionary perspective - there will no longer be a reason to adapt to low G environments. Rather it's moving to another world, genetic screening of newborns filtering out novel gene structures before they can be persisted, creation of artificial gravity - which may be as simple as the sci-fi staple of using centrifugal force to fake gravity, reaching some form of transhumanism, or just managing to wipe ourselves out something will have rendered biological adaptation to this world moot long before any meaningful genetic adaptations could evolve. I know it's a boring answer, but I'm afraid it's the only realistic one.

So what has changed?

Good news, that doesn't mean your low G humans can't look different, if only slightly. That's because development and spread of novel genes isn't the only way adaptation occurs, it can also be adjusting the distribution of existing genes already present in the gene pool. That is to say genes that already existed, but may not have been too common, in the human gene pool may prove useful enough in low-g environment resulting in these genes going from being rare to very common within your low-G human population. This process can take only a few dozen to hundred generations; depending on how significantly the gene increases survival and fitness in a low-G world.

This wouldn't make your low-G humans a different species, they would still look like a human, still able to procreate with earth based humans and live on earth; They are ultimately still human. However, it would mean that low-G humans may tend towards certain traits more then other humans, enough that there are certain expectations for low-G human appearance. In short low g humans would not be a new species but could become a new race of humans, a physical phenotype that allows you to say "that guy looks like he was born on a moon".

As to what these racial differences would be, I admit while being well versed in evolution I'm not versed enough on the biology and physics of low-gravity to feel as well equipped at speculating at which human traits would fair better in low gravity. I'd image a shorter and bulkier build with relatively lower muscle mass would be common. The shorter and bulkier build help tie you down to ground so you don't fly off into space with every step while the lower muscle mass is because there is no reason to waste energy on lots of muscles if you are insufficiently grounded to use that strength without shoving yourself away from whatever you are pushing on. That being said I think a question as to what existing human traits would be more useful in low-g may be worth asking in itself. My main point is that these difference will be minor, a slight redefining of which traits are more common not a completely different species.

I'd also add that if you have regular interactions with higher-g humans and gene flow with them this would slow the development of a standard racial trait with humans. regular gene flow can prevent any racial differences from developing or at least slow that tendency so drastically that low-g humans will barely have time to start developing their own racial traits before technological changes completely alter the pressures they face.

If you really want low-G humans to be different you can always cheat

Genetic engineering is always a great excuse to having humans change more rapidly then could be justified by mere evolution. And lucky for you there is already a question asking exactly what these genetically engineered humans would look like.

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    $\begingroup$ the founder effect can do a lot for this, the founding population will be tiny. if say 2 of the pioneers happen to carry the genes for albinism for instance, you will end up with a huge portion of the population with the genes. $\endgroup$
    – John
    Dec 7, 2023 at 0:06
  • $\begingroup$ "Good news, that doesn't mean your low G humans can't look different". I disagree with that being good news, I would say that is the worst news that there could be for this subject. I also disagree that speciation won't occur even if given millions of years, and object to your claims of using artifical gravity, as I actively want to avoid that as I just find it very boring using the soft sci-fi magical gravity, and I can't think of any way to set up a city that rotates in a way to create artifical gravity and still structurally make sense. It just won't work out. $\endgroup$ Dec 7, 2023 at 2:38
  • $\begingroup$ I know that the minor changes happening near the start (relatively speaking) would not be speciation. But I feel speciation would occur given millions of years. As it wouldn't just be evolutions helping them with their enviroment, but also other aspects that are just random compounding of mutations that survive due to less risk to life, thus mutations are more likely to survive and pass down. So, I disagree that they will never split into their own species. I don't think technological advancements would magically completely halt genetic drift. I apologize if I am rude, not my intent. $\endgroup$ Dec 7, 2023 at 2:41
  • $\begingroup$ You also are making a ton of very large assumptions about the culture and technology of my world that was not stated in the main post, and base your thesis upon these assumptions. Even ignoring my direct statement that they do not use artifical gravity. $\endgroup$ Dec 7, 2023 at 2:57
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I think that by far the biggest factor would be the adverse affects of growing up in a low gravity environment.

Adults can live pretty well even in microgravity for years, and will take all sorts of measures to stay healthy, so something as simple as lunar gravity will create very little evolutionary pressure.

However, being born, and growing up in lunar gravity is very likely to make lots of systems go haywire. Nobody knows which and how exactly, and I can't think of a way you'd measure it. An obvious guess is that you'd get very tall, thin people, and all sorts of developmental defects.

That's not evolution, I can hear you say. No, but it's going to be a source of major evolutionary pressure. Assuming you have a large and diverse starting population, some moonchildren will suffer less in their elongated bodies than others. They will live longer, have a better chance at being fertile, will be more attractive and will be able to claim more prestigious positions in lunar society.

The pressure will be pretty brutal, so you won't need millions of years for differences to becomes apparent. Domestication of animals takes between hundreds and tens of thousands of years, and causes serious changes to the appearance. To be fair, selective breeding likely accelerates the more extreme changes, but there is evidence of substantial evolutionary changes happening within a century under sufficiently strong pressure. For instance, elephants losing their tusks.

The key here is that you will get roughly what you expect in a low-gravity environment: elongated people. However, you wouldn't get it directly from evolutionary pressure. Being long doesn't help you survive on the moon. Rather, growing up on the moon makes you long, and there is a strong selection for people who more or less survive that process. Most likely, the strong survivors are the least tall, but still taller than they would have been on Earth.

The key difference is what would happen if you were to transplant these people back to Earth, and let them grow up there. The early generations would look tall on the moon and normal on Earth. The later generations would be tall on the moon, but less so than the early generations. If these were to then grow up on Earth they would be very short.

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There would be very few, if any evolutionary changes

The main issue as others pointed out is human reproduction. Low gravity environment would make a successful pregnancy almost impossible - you wouldn't get deformed children, you would get still-births. Sure, that would be quite a harsh evolutionary pressure for rapid evolution... if you could get people to play ball. But unless your lunar colonists are prisoners exiled there you wont exactly get many volunteers for the project. Which would be higly immoral in any case. It is kinda frowned upon to force women to give births when expected success rate is less than 1 in 1000. Especially since there is literally no good reason to do so. Low gravity worlds just aren't a good places for human colonization.

So, how to solve that issue? By lunar colonist not actually living on the Moon. It makes no sense to actually live there long term. They would instead live in orbit in rotating habitats, enjoying that sweet 1 g gravity. You get way more control of your environment that way, and much more living area/mass (using whole Moon as a construction material would get us a long way into building a Dyson swarm). Those colonist could still work on the lunar surface if you want them to. But I imagine such work would be similar as work on an oil rig: harsh, unpleasant work done for a few months at a time, then you return to greener pastures. Parallels with oil rigs are deeper: you wouldn't want to live on one full time, nor would you want to bring there your spouse and raise children there. Not that I expect many people would physically work on the lunar surface, when it is easier to remotely control the drones from orbit (well, not control as they would be automated, but being an overseer).

But actually living on the surface long term is almost impossible - it is not guaranteed you would be able to make the evolution path viable even with going immoral route by throwing millions into evolutional meat-grinder. Even genetical engineering would be of little help. Making changes that would make humans thrive in low gravity environment are one of the most difficult modifications. If would be easier to make us an aquatic speecies than a low-gravity one.

TL,DR: There would be no evolutionary changes because successful pregnancy is most likely more or less impossible in low-gravity environment. That means people wouldn't want to live on the Moon full time. Not that it is expected that humanity will do much colonization of other planets/moons, when there is much more confortable option of using space habitats.

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