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Assuming that a person can survive on a world that's super-habitable, with it's 1,5 atm atmosphere consisting in 38,4% of oxygen, with increased concentration of (~0,5%) hydrogen and (1,2%) carbon dioxide, and it's surface gravity of 1,24g, how would a human body evolve and adapt over generations to this environment?

We're taking a baselane human, colony and we're supposed to see how would human body change over generations, to adapt to those new conditions and use them for own advantage.

Maybe let's try to predict changes after a century, half a millenia and five millenias.

Everything below is just my guess, if you know the answer, you can stop reading now and answer.

After a century, we might be seeing slight alteration of the hemoglobin, to try and adapt to those high concentrations of oxygen, which would cause the cells to live and multiply much more rapidly, possibly increasing a chance of cancer and a shorter lifespan, but also a lifespan "much more lively" - higher and more efficient metabolism releasing much more energy for our disposal. We'd probably also see more stoutier people, with bone and muscle mass growing, to hold off against the gravity, and with the height decreased, to not waste so much energy pumping blood high up.

After half a millenia, I think we could start observing changes inside the ribcage, with lungs slowly shrinking with generations (there's already plenty of oxygen, no need for lungs that big), to make place for a stronger and bigger heart, that could allow the blood to be pumped higher up the body, possibly allowing the humans to preserve their "default" height and opening up room for further growth.

After 5 millenias, and that's going to be probably the least accurate guess, an average person would have a heart size of their head, making it an easy job to not only be over 2 metres in height, but to also maintain the muscle and bone structure needed to withstand the higher gravity.

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    $\begingroup$ There's a lot of weird assumptions here. Why would cell life cycles change just because of additional oxygen? Millions of years ago, oxygen levels on Earth were higher, and nothing we've ever found suggests that metabolic rates were substantially different between modern plants and animals and their prehistoric counterparts. $\endgroup$
    – jdunlop
    Apr 13 at 8:01
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    $\begingroup$ (Also, the assumption that humans somehow have a "default" height is faulty.) $\endgroup$
    – jdunlop
    Apr 13 at 9:24
  • $\begingroup$ the word is millennia, and it's plural. the singular is millennium. $\endgroup$
    – ths
    Apr 13 at 11:40
  • $\begingroup$ That's the first time I've ever seen someone using the decimal comma. You may want to consider using a decimal period in the future when posting on international English forums, it's less likely to cause confusion as it's what's used primarily in English. $\endgroup$
    – dsollen
    Apr 13 at 15:46
  • $\begingroup$ Alright, thank you two for the corrections $\endgroup$
    – Yulian
    Apr 13 at 19:09

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Setting apart the issue with how they manage to survive long enough to reproduce in an environment where they would be poisoned by 0.576 effective bar of O2 (as stated in this answer): for us humans a century is barely 3 generations. 5 centuries is about 15.

No significant changes will happen. Caucasian, fair skinned people have been living in regions where local natives have higher levels of skin pigmentations, and dark skinned people have been living at higher latitudes for that much, and they haven't changed their skin pigmentation, unless they have mated with locals.

5000 years might start bringing some changes like higher resistance to oxygen poisoning, changes to hemoglobin, lungs, bone structure and so on.

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    $\begingroup$ +1 Very much "might". The Inuit probably developed their layer of subcutaneous fat over the time from the last ice age to the present - about eleven millennia. Actual dramatic changes like cardiac gigantism and bone structure would probably take even longer to manifest. $\endgroup$
    – jdunlop
    Apr 13 at 8:53
  • $\begingroup$ Covered the points I was going to make more succinctly then I would have. This is a blink in the eye from an evolutionary stand point, too short to have any significant measurable changes. $\endgroup$
    – dsollen
    Apr 13 at 15:49

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