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Here is what I've got on Reddit about how long human limbs are in proportion to the lengths of their bodies:

With regard to humans, in ideal proportions, an adult is 7.5-8 heads tall. The legs (including the ankle of the foot, but not the rest) comprise 3.5-4 of those heads, or 46.7% to 50% of the total body length. The arms (not including the hands) are about 2.5 heads long, or 31.25% to a third of the total body length. A hand from its base to the tip of the middle finger is 1 head long, so if you include the hand the arms become 43.75% to 46.7% of the total body length.

Those are ideal body proportions, which is what character artists use to draw people who are visually appealing and proportionally "right." In reality, because of genetic variety, human body proportions can vary from ideal proportions quite a bit. For example, really short people have wildly different proportions.

In an alternate Earth that I've been working on, humanity is still a primate, but under a different species under a different genus under a different family. One of the ideas I've had in differentiating our humans from their humans is to change the proportions of their limbs from human to gibbon. If our limbs are identical in proportion to the length of the rest of our bodies, how long--and how short, considering gibbons--would they be?

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    $\begingroup$ I'm having a hard time not downvoting this question. Why can't you apply basic algebra? If the ratio of the average adult gibbon's arm length to body length is A/B and the ratio of the average adult's arm length to body length is X/Y, then why can't you just solve for X (X = YA/B)? You might be over-thinking this. $\endgroup$ Aug 25 '20 at 19:21
  • $\begingroup$ @JBH Because I don't know what the ratio of the average adult gibbon's arm length to body length is, and Google wasn't being helpful. $\endgroup$ Aug 25 '20 at 21:26
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    $\begingroup$ Problem solved in less than 2 minutes. I get it that Google can be problematic sometimes.... but when a Q is this trivial.... I'm just sayin'. A better question is whether or not a 1:1 ratio with humans would result in a creature that's too fragile to survive. Six feet of arm is a lot of bone to break and 200# average male weight is a lot of leverage against those bones. $\endgroup$ Aug 25 '20 at 21:37
  • $\begingroup$ @JBH How did you find the link? $\endgroup$ Aug 25 '20 at 21:42
  • $\begingroup$ "I don't know the ratio": Here is a nice comparative drawing showing the skeletons of the extant genera of apes -- gibbons, orangutans, gorillas, chimpanzees and humans. $\endgroup$
    – AlexP
    Aug 25 '20 at 21:43
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[According to Primate keeper Carly Hornberger of the National Zoo in Washington D.C.] (https://nationalzoo.si.edu/animals/news/how-long-are-gibbons-arms-and-more-gibbon-facts)

...if humans had similar proportions, a 5-foot-4-inch woman would have an arm span of about 10 feet, 7 inches!

That's presumably fingertip to fingertip with arms outstretched, including the width of the shoulders. This public facing article doesn't address how long (or short) the lady's legs would be, however.

If you look at the [Intermembral Index of different primates] (http://abacus.bates.edu/acad/depts/biobook/IMindex.png) - which is a ratio of leg to arm lengths - you'll see that bipedal primates like humans have an index well below 100, whereas gibbons (like the hylobates in the linked chart) and other brachiating (tree swingin) primates are well above 100.

So, if you want to create your alternate humanoid with gibbon-proportions, you could try and working the IMI formula in reverse.

Now, if your alternate reality humanoids have these proportions, they will likely have a [lot of fractures from falls] (http://www.inquiriesjournal.com/articles/799/2/ape-fracture-patterns-show-higher-incidence-in-more-arboreal-species), as they're likely to spend much of their time above the surface of their world. What's on the surface that's so bad? Is there a toxic gas that's heavy enough that it usually settles in the forest floor? Predators? Other humanoids who are happy enough to leave them alone as long as they stay up in the canopy? Do these humanoids adapt to live in places without trees? If so, do they adapt their own behavior or do they adapt their environment, maybe building "sidewalks" like we'd make above ground power lines? It's an intriguing change to make.

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