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In my setting humanity is setting out to colonize space with very advance terraforming technology. A group of colonizers has terraformed a planet to be habitable for humans. But an accident happens and they lose all their advance technology. Ten thousand years later another group of human arrived at this planet and found out that all humans there are crawling instead of walking. What sort of environment cause them to have this adaptation? About the crawling humans:

  • They crawl as a main method of locomotion. They can still stand on 2 legs but only for short amount of time like bears.
  • Their technology is only at tribal level and only lives in a small area of the planet with a population less than 50,000.
  • Any additional adaptation is fine.

The planet itself is terraformed and contains breathable atmosphere, liquid water, and is populated with plants and animals for food. Other than that it can be any way.

The enviromental factors needed to be specified:

  • What type of planet is it. The composition, temperature, atmosphere, gravity, moons, or type of star it orbits etc. Anything that would have an effect in causing humans to adapt to crawling.
  • The biome that they live in. Would the crawling people be living in plains, jungles, mountains, islands, etc.
  • Other enviromental factors. A predator or natural disasters they have to avoid or a food and resources they have to collect that might favor them to crawl instead of walk.
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    $\begingroup$ Humans are terrible at crawling; they will die before making any reasonable progress using crawling to locomote. Do you mean walking on all fours? Because humans are a bit less terrible at walking on all fours. $\endgroup$ – AlexP Oct 29 '20 at 15:58
  • $\begingroup$ @AlexP, citation needed; I wouldn't say I can "walk on all fours" in any meaningful way. That said, I agree with everything but your last sentence, and I had the same question. As for being able to walk on all fours — the main issue with present humans is the relative length of legs to arms — I would assume we're talking about humans that have adapted to do so more readily than I can. $\endgroup$ – Matthew Oct 29 '20 at 16:07
  • $\begingroup$ @ AlexP I suppose a Bear crawl could work better. I'm fine with any quadruped locomotion. Should I edit the question? $\endgroup$ – Win2win Oct 29 '20 at 16:08
  • $\begingroup$ As to the actual question, if we are allowed to adapt these human descendants for better all-fours locomotion, there are actually various advantages to that over bipedal locomotion, especially if they can still stand on their hind legs at need. The main disadvantages — i.e. reasons modern humans don't walk on all fours — are that it makes it much harder to carry things, and reduces visibility. You'll need to neutralize these somehow, as well as provide a reason why there would be selection pressure against walking bipedally. $\endgroup$ – Matthew Oct 29 '20 at 16:10
  • $\begingroup$ @Matthew: I wouldn't say that I'm good at walking on all fours, but I can definitely make forward progress easier and faster than when actually crawling. (And anyway, I cannot actually crawl; at best, use a sort of quadrupedal gait with my belly brusing the ground. Crawl like a reptile, no way.) $\endgroup$ – AlexP Oct 29 '20 at 16:13
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Low ceilings.

On this planet, it's not safe to spend long periods on the surface. That could be for any number of reasons: large predators, a lack of radiation screening in the upper atmosphere, some kind of toxic substance that exists in the topsoil. But for whatever reason, it's much safer to live in caves underground. And while there may be some spaces that are larger, most of the cave system is made up of passages that are only 1-1.5 meters high. Therefore, humans who are anatomically better suited to crawling through these passages are more likely to survive and flourish than those who aren't.

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  • $\begingroup$ How do you not just end up shorter? People walk upright so adeptly and crawl so terribly that dwarfism would be selected before crawling traits would be. I suppose one explanation is that children grow up only ever crawling and never learn to walk due to lack of opportunity and since they outnumbers the dwarves to begin with that might be a sufficient advantage. The problem is that children start out short enough to walk upright in those 1-1.5m passages and would only be forced to convert to crawling as they grew which puts them at a serious disadvantage compared to those with dwarfism. $\endgroup$ – DKNguyen Oct 29 '20 at 22:00
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Religious penance, or to avoid being higher than their leader.

The human body is terrible at crawling and walking on all 4s relative to bipedal motion, doing this would slow us down, stress parts of the body in new ways, and throw our senses out of calibration - we're not going to evolve like this unless something really strange occurs (like a predator that kills anything over 1m high that we can't control), and even then, 10,000 years doesn't seem like enough time for such an evolution to happen.

What I can see however

  • A Jesus like saviour is born, told all humanity that we're sinners going to hell, but if we give penance now by suffering a little bit, we'll get to heaven.
  • A cruel leader insisting that no human be above him (literally). Everyone crouches or walks on all 4s to avoid offending the leader.
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Gravity

This planet is twice the mass of earth and bone strength has not yet evolved to match the increase in gravity, so humans brace themselves with 4 limbs as much as they can.

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