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Humans are stranded on a world with .75 Earth gravity. How would that affect their physical evolution? Specifically, would they be taller, thinner, and have less bone mass?

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    $\begingroup$ Hello @ScottM, welcome to Worldbuilding. We're delighted you could join us, but it's worth noting that we have some rules about asking questions. One question per post, please. Be as specific as you can and provide conditions, limitations, and expectations. Generally, you're doing OK with that, but it's good to learn early on. I recommend reading the following two Help Center pages to better understand this site's limits and expectations: help center and help center. Thanks! $\endgroup$
    – JBH
    Feb 27, 2023 at 20:02
  • $\begingroup$ ScottM is now @SPM with a new avatar? $\endgroup$
    – JBH
    Feb 27, 2023 at 23:03
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    $\begingroup$ I don't think this is answerable as we have just ONE data point - evolution in 1G. Few decades of 0G research merely tell what happens with adaptation, even for non-human thingies that reproduce way faster. $\endgroup$ Feb 28, 2023 at 9:22
  • $\begingroup$ @IanKemp, Be nice $\endgroup$
    – Len
    Feb 28, 2023 at 18:34
  • $\begingroup$ they would probably evolve as they planned to, since they colonized a world on purpose, being already enough evolved to be interstellar, bringing their own tech and habitats that allow them to chose to evolve or not. $\endgroup$
    – user35577
    Mar 1, 2023 at 0:04

2 Answers 2

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This is similar to the following closed question: How can lower gravity affect evolution?. There would definitely be less bone density, it's a known problem already. But taller and thinner? Not necessarily. In fact, probably not.

At issue is the wondrously wide variety of humanity today. We already have short and tall, thin and thick, athletic and not so much... and every combination of those things. The only real guarantee you'd have is that their leg, hip, and back muscles would be underdeveloped compared to their Earth counterparts. Maybe, maybe, the average would shift to thinner (not taller, though). (See addendum below.) But that's the average, lost within that wondrous variety previously mentioned.

Addendum: Now that I've thought more about it, it's more likely that the people of your low-gravity world would be fatter than their Earth counterparts. Unless their behavior changes with the low gravity, they're arriving with the same predilections for sugar, carbohydrates, fats, and, well... BBQ... that humanity has had since time immemorial. With a lower gravity, they're more likely to become fatter. If they became thinner, it would be due to cultural changes that set them apart more from their Earth counterparts than what any evolutionary changes did ("What? You don't eat BBQ? What kind of alien invader are you? That's so un-Texan! I mean, un-American!... I mean... You're not from Earth, are you?"). Finally, remember that evolutionary changes take a honking long time.

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  • $\begingroup$ Obesity has many ill effects beyond having to lug around more weight. Especially fat around the organs is a cause of many diseases. Besides, lower gravity will not reduce the energy needed to accelerate the body. There may also be beauty standards hard-wired into our biology. While today's skinny ideal isn't historically universal, historical texts about women considered beautiful always mention a certain degree of slimness of waist, and never obesity. For men, athletic bodies are also universally considered beatiful. $\endgroup$ Feb 28, 2023 at 14:24
  • $\begingroup$ @KlausÆ.Mogensen I'd challenge the assertion that beauty is hard-wired biologically, given the range of what is considered attractive throughout the world today, and throughout history. $\endgroup$
    – Darren H
    Feb 28, 2023 at 16:20
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    $\begingroup$ Gentlemen... I sense a discussion brewing that has nothing to do with the question or my answer. I'm begging you, on my hands and knees, to not pursue these trains of thought. $\endgroup$
    – JBH
    Feb 28, 2023 at 16:23
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    $\begingroup$ @KlausÆ.Mogensen The only part of your comment that relates significantly to my answer is the comment about the energy needed to accelerate the mass not changing. That is correct. But the energy needed to overcome the acceleration of gravity does, and that can't be ignored. The equation F=mA where A is the acceleration could be written F=m*f(a,A) where "a" is the target acceleration to move the mass and "A" is the acceleration of gravity. (Why a function? Because I'm not worrying about the specifics of the motion.) $\endgroup$
    – JBH
    Feb 28, 2023 at 16:26
  • $\begingroup$ @JBH I was about to say the same. These people will inevitably spend much less energy to stand up, walk around, etc. $\endgroup$
    – jtb
    Feb 28, 2023 at 19:39
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It Could Possibly Effect Our Developmental Morphology

There is a difference between developmental morphology and evolution. There may be morphological differences between a human who develops in normal vs lower gravity without a single change to thier genetics. However, it is hard to predict exactly what these changes will be since there is no real experimental evidence to suggest what long term slightly reduced gravity will do. We know about all sorts of issues with long term zero G living for adults, but nothing about long term exposure to .75G, or what effect lower gravity has on developing children.

Since we don't know, then any conjecture is equally valid. It is possible your humans will land on this alien world, and the very next generation of humans will be noticeable taller and skinnier just because they developed under different environmental factors than thier parents did. It's also possible that colder temperatures, higher atmospheric oxygen, or the nutritional qualities of the local food sources could cause similar changes in the next generation having nothing to do with gravity at all.

Evolution Is Much Less Probable

The issue with pretty much every "Will X cause humans to evolve Y?" question is that modern humans experience very little natural selection the way that animals do. Evolution does not happen just because some new trait would be nice to have, it also requires than the existing trait be so unfit for genetic survival that people with it are noticably less likely to raise surviving offspring. Because we modern humans have medicine, advanced defensive technology, and don't just let each other die off for being a little bit slower or weaker, then your short humans and tall humans will both have the same basic chance to pass on thier genes.

Even if short people for whatever reason had a distinct physical disadvantage, humans have a way of overcoming physical disadvantages through specialization of labor. This may lead to increased genetic diversity in the future with more people of both exceptionally tall and short frames, but not a general push to evolve in either direction.

As to comments about sexual selection, there is actually a lot of reason to believe that this is not a major evolutionary factor in civilized humans either, because we are a generally monogamous species. The kinds of species where sexual selection overpowers selective fitness are generally Harem Polygamies. In a Harem Polygamous species where you put together 10 males and 10 females, the #1 male gets all 10 females. This emphasis on being #1 means that not being sexually ideal is not an option. But, in a monogamous or polyamorous species, The #1 male generally pairs with the #1 female, the #2 male with the #2 female, etc. In fact, the #1 male will generally refuse to mate with the #10 female, even if given the opportunity. This means that even the least desirable of the males (in this case, the shortest) still has a good chance of securing breeding rights. Normally this means that sexually unfit characteristics get paired with selectively unfit characteristics for removal from the gene pool, but going back to modern humans being less likely to ever be selected against, it means that sexually undesirable traits are much more likely to be passed on than in other species.

So, the most likely reason you would see for humans to Evolve to be taller is if your world introduces something that selectively kills or sterilizes your short people or if civilization on this world collapses for a really long time so that the alien predators or constant warfare get to have enough generations of killing off the shorter slower humans to impact the gene pool, or if major and extraordinarily long lasting cultural changes were to take root.

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  • $\begingroup$ Sexual selection is also possible, isn't it? If being taller makes you more attractive to people that want to make babies with you, and people act on that to an extent that more babies have taller / thin parents than the average, there could be some pressure in that direction. A classic example is the peacock; they didn't get that way for the good of their health. (I don't know how strong that effect has to be to make a difference against the background noise, or how plausible it is for a human society to work that way. In western culture, most people can have children regardless of looks.) $\endgroup$ Feb 28, 2023 at 4:37
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    $\begingroup$ @PeterCordes sexual selection, sure, but I have yet to see certain traits to be linked to the level of gravity. Maybe.. if gravity is lower .. "things.. don't.. hang as much"? (both female and male bits)? and.. then that would make what? $\endgroup$
    – Apfelsaft
    Feb 28, 2023 at 10:16
  • $\begingroup$ @CarlBerger: Yeah, I don't have a reason for that being associated with lower gravity, other than the fact that it's physiologically possible without the downsides of being super tall at 1g (the heart has to work very hard). So if a social preference randomly developed, it could push genetics over time without natural selection pushing back? Maybe people living on low-grav worlds would think they ought to look like popular sci-fi tropes of beanpole low-grav inhabitants! $\endgroup$ Feb 28, 2023 at 10:28
  • $\begingroup$ @PeterCordes Sexual selection is also not a very strong evolutionary factor in humans because we so rarely follow the harem model for mating. I've edited my answer to explain this in more detail $\endgroup$
    – Nosajimiki
    Feb 28, 2023 at 14:59
  • $\begingroup$ Thanks for expanding on that; when I first commented it was mostly as a footnote that natural selection isn't the only possible mechanism for evolution, even though I didn't think it was likely for humans. It would require vast changes to how we breed, e.g. a trend toward single mothers that choose donor sperm based on looks, or some other SF world-building element. $\endgroup$ Feb 28, 2023 at 20:33

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