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Would a creature that's 180 - 210 cm tall walk and run comfortably upright for long distances if they have shorter legs than humans, no lordosis and arms (though shorter than a gibbon's) whose wrists reach their knees? Their feet are also different, having long palms, fingers, and very long thumbs, resulting all fingers being about the same length. Gibbons are mostly arboreal, but on ground are bipedal. They are however very short and light, and their walking style is quite awkward. I thought that making the creatures' legs straighter (like humans') would help... I'm not really trying to make them walk and stand like humans do, just run and walk bipedally without being super slow or jumping from place to place despite their size. They're semi arboreal too, but have to travel long distances on foot and am not sure how a creature of this size and with such long arms could pull that off bipedally.

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    $\begingroup$ (1) The ratio between the length of the legs and the length of the torso is highly variable in humans, to the effect that the meaning of "shorter legs than humans" is quite fuzzy. (That's why a good tailor is so important.) (And the leg to torso ratio of gibbons is quite similar to that of humans.) (2) There exist humans with very long arms; this is considered of great advantage in boxing, for example. (4) Comparative anatomy says that a bipedal, gressorial, cursorial creature will not have feet with "long palms, fingers, and very long thumbs". $\endgroup$
    – AlexP
    Jan 7 at 1:33
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    $\begingroup$ Anyway, the only possible answer is "yes, of course": one can always imagine a set of biomechanical adaptations allowing for all the conditions in the question. For a direct example, consider chickens: their forelimbs are longer than their hindlimbs, they have no lordosis and yet they spend most of their lives walking and running bipedally. So that it is clearly possible. (Yes, the skeleton of chickens and of birds in general is very different; they have stiff spines, the legs are attached differently etc. But the question does not exclude such mechanisms.) $\endgroup$
    – AlexP
    Jan 7 at 1:36
  • $\begingroup$ I know that the title says bipedal, but what it's referring to is not obligate bipedalism (they're semi arboreal), just the ability to walk that way for long distances. Long thumbs aside, gibbons also have long soles and fingers on their feet. I can't use chickens as a reference, since I'm trying to use a gibbon skeletal structure as a basis and while I can make a few changes, a chicken (or a bird skeleton in general) is just way too different. $\endgroup$
    – Tanya
    Jan 7 at 1:57
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    $\begingroup$ Yes, gibbons have four hands. They are also utterly incapable of walking or running over any reasonably long distance. If the creature is adapted to walk or run over long distances on its hind legs, then its feet will be constructed for this purpose. But, as I said, one can always imagine a set of adaptations allowing for the small set of requirements in the question; but the creature won't look very much like a gibbon at all. Gibbons look the way they look because they are brachiation specialists. $\endgroup$
    – AlexP
    Jan 7 at 1:57
  • $\begingroup$ I see, thank you. $\endgroup$
    – Tanya
    Jan 7 at 2:06
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I would think yes, But in addition to modifying the legs you'll need to change the hip structure.

As one of your commenters pointed out - there is also a very real difference in the feet. These changes are also going to impact their social structure.

No evolutionary change happens in isolation - there are always ripples. Some big, some small.

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  • $\begingroup$ What differences would there be at the hips? If their babies are small, wouldn't that make large hips unnecessary? $\endgroup$
    – Tanya
    Jan 9 at 16:52
  • $\begingroup$ The hips need to be positioned to support enormous gluteal muscles to support long distance walking, as gibbons, like humans, do not have tails to balance their weight on the ground. $\endgroup$
    – Globin347
    Jan 9 at 22:33
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As you note, gibbons already walk bipedally when not in trees and don't have a huge problem. Bipedalism is thought to be an ancestral ape locomotory pattern when not in trees, gorillas and chimps merely became specialized for knuckle-walking independently and humans became obligate bipeds.

The long arms won't be a problem, human arms are incredibly short relative to our height (they only reach the crotch) and you could make them a lot longer before you run into problems).

Lack of lordosis is going to be your biggest problem. The entire point of lordosis is to redistribute the weight of the spine in a manner that allows the animal to stand bipedally. The lack of lordosis is one reason why other apes are such poor bipeds. Indeed, looking at the cited paper in the link, non-bipedal apes even develop a lordosis when they are taught to walk bipedally for long periods of time, so it might not just be evolutionary but reinforced by developmental. loading patterns.

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