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In the project I am working on, the main race in my setting has the body plan similar to a primate, and has somewhere between a moderate, or highly arboreal lifestyle. (Haven't quite settled on this yet) Recently, I stumbled upon the topic of human throwing, and how large of a role it played in human evolution. Essentially, changes in our shoulder structure allowed for more arm mobility during throwing, allowing us to hunt and consume a higher quantity of meat in our diets, which helped in brain development. The problem I ran into with this, is that I want my race to maintain both arboreal-proficiency, and the ability to accurately and powerfully throw, which so far appears to be two contradictory actions.

With an ape-like shoulder structure, where the arm's hinge is positioned upwards from the shoulder, it allows for good swinging and climbing, however, limits the upper arm's ability to rotate during the throw, making powerful throws essentially impossible. With a human-like shoulder structure, where the arm is perpendicular to the shoulder, it allows the upper-arm to greatly rotate, and therefore allows powerful throws, however makes humans somewhat mediocre climbers, at best. I am aware that humans are able to climb, and pretty impressively too, but compared to apes, there is essentially no competition, and physically, humans expend much more energy doing so. Is there a good middle-ground structure that allows proficiency in both actions? Or would a potential shoulder structure like this just lead to mediocrity in both?

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  • $\begingroup$ On the definition of "proficiency": I would like them to -At Least- be able to throw powerfully and consistently enough to hunt and kill animals, as well as be able to climb more efficiently than humans. If they end up being slightly worse than humans at throwing, yet still able to hunt with it, I would see this as a good enough outcome. (Also if you have a solution in mind that doesn't directly involve the shoulder, I would accept that) $\endgroup$ – Foosic17 Aug 28 '20 at 1:46
  • $\begingroup$ Also the gravity on the planet is 0.9 G's. I would imagine this is a very barely important detail, still good to know I guess $\endgroup$ – Foosic17 Aug 28 '20 at 1:54
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    $\begingroup$ Chimpanzees can throw stuff powerfully, but their aim is very poor. This has nothing to do with their shoulders, and everything to do with our much better hand-eye coordination; among the great apes, humans have by far the best level of manual dexterity. (Also, chimps may be better than us at climbing trees, but we are better than them at climbing rocks. Same reason.) Orangutans are much better than us at climbing trees, but their locomotion on the ground is pitiful when compared to chimps and humans; like all engineering problems, there is a tradeoff. $\endgroup$ – AlexP Aug 28 '20 at 3:45
  • $\begingroup$ Have two sets of hands, one of which is better at throwing, but still can be used for climbing. $\endgroup$ – NomadMaker Aug 28 '20 at 4:07
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    $\begingroup$ What is wrong with the human shoulder structure for climbing? Humans are fairly proficient climbers - e.g. rock climbing, kids climbing trees & monkey bars, &c. What restricts the climbing ability of most (if not all) humans is not the structure, but the strength of the arm & shoulder muscles, so most climbing is done primarily with the legs. $\endgroup$ – jamesqf Aug 28 '20 at 4:57
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Tools:

I think you can probably use tools to compensate for this deficiency. Neanderthals had joints that also limited throwing. Their bones have injuries in a pattern also seen in people who perform in rodeos - the kind of injuries you get when in close contact with large, angry animals. But they also used strategy to compensate with cooperative hunting. The suggestion is that much of their hunting was based on closing with animals to finish them off. So it's possible for hominids to evolve as hunters and yet still not be great throwers.

But once you have reasonably evolved tool users, they can make better tools to compensate for their limited range of motion. The Atlatl was essentially a stick to increase the relative length of the arm, amplifying leverage. Sharp stone tools reduced the amount that you needed to hit hard, instead amplifying the effects of cutting. Similarly, slings amplify force but wouldn't require the same huge range of motion.

I'll take a stab and guess your brachiators will hunt from trees, emphasizing the importance of patience and ambush. Early humans hunting in arboreal environments may have concentrated on smaller prey, not requiring the large force of a heavy spear Maybe their range of motion still allows strong throws downward. Their strength might allow them to emphasize using weight (like dropping rocks) or jumping on prey from above with spears. They could hunt with traps, or nooses, or drop nets from above. Even simply digging covered pits and driving animals into them enables hunting. Once the bow is developed, your need to be a great thrower will be largely supplanted by this ultimate leverage amplifier. The advantage of ranged weapons is at least somewhat limited in a forest environment compared to open plains.

If your hominids are smart, their brains will compensate for any apparent limits to mere throwing.

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  • $\begingroup$ The rodeo thing may not be as true as we once thought, it turns out those injuries don't match rodeo riders, they just favor the upper body, which is also true of human skeletons from thew same time. archaeology.wiki/blog/2017/05/03/… $\endgroup$ – John Aug 28 '20 at 12:44
  • $\begingroup$ @John good reference. I think the basic points are good. $\endgroup$ – DWKraus Aug 28 '20 at 22:26
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See this question:

What would a creature with very powerful and accurate throwing ability look like?

Basically the point is made that powerful throwing ability need not require stronger and stronger muscles, it requires fancy brain circuitry to handle the timing and rhythm necessary for an optimal throw.

So design your creature for good climbing ability, and then give it a human-ish brain, including human rhythm.

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Humans already have this. The difference between human and great ape shoulders is less due to climbing behavior and more because gorillas and chimpanzees knuckle-walk. Indeed, it's been suggested that part of the reason why humans became bipedal was to be able to efficiently walk on the ground without sacrificing dexterity in the forearms. Knuckle-walking involves a suite of changes to the arm and shoulder, including an inability to pronate the wrist in a flexed position (which is kind of useful if you want to have dextrous hands). Human shoulders and hands are actually a lot closer to the ancestral ape condition than those of gorillas, orangutans, or chimpanzees.

Look at gibbons, who have a shoulder a lot more similar to a human than any other ape (their shoulder joints are more flexible to enable more range of motion, but they have less dexterity and strength as a result). Other changes in the shoulder and hand, like a reduced thumb, are more because gibbons swing hand-over-hand, which is a mechanically unusual and specialized behavior in its own right. The reason humans are poor at climbing has less to do with our shoulder structure and more to do with the fact that we have very long legs that lack an opposable big toe because we are so terrestrially adapted. Humans have a very specialized skull and body from the hips down, but our arms are perhaps the most unspecialized thing about us and are still really good for climbing by themselves.

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