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Many designs for sapient aliens and sapient fantasy races alike - including one of my own - resemble four-armed humanoids; As covered in other questions on this site, a six-limbed vertebrate-analog is not impossible, and actually has a reasonable chance of evolving on other planets (although six-limbed and four-limbed vertebrates on the same planet are apparently unlikely), but this seems to me to raise another question: Does a six-limbed vertebrate have any realistic chance of becoming a four-armed humanoid when becoming centaur-like is a possibility?

My reason for asking is that it seems to me that the benefits of four hands would, in most cases, be less than those of the added stability of having four legs, especially when you consider that the elongated torso needed to put four arms on a humanoid would make the resultant organism quite top-heavy.

Is a my reasoning four doubting that four-armed humanoids would be competitive with centaurs sound, or are there situations where four-armed humanoids would be better?

If four-armed humanoids are competitive but only under certain conditions, an explanation of these conditions would be appreciated.

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  • $\begingroup$ You may find the question Anatomically correct multiarmed humanoids and its answers helpful, though obviously not a dupe as what you're asking is if it would be as or less likely to evolve from 6 limbed vertebrates than a 4 legged body plan. $\endgroup$
    – Pelinore
    May 14 at 11:28
  • $\begingroup$ Not sure how unlikely six- and four- limbed vertebrates sharing an ecosystem is. We've got plenty of vertebrates with no limbs to speak of here on earth, and some animals are approaching only having two or three (Kiwi birds, whales, manatees, seals, Carnotaurs...) $\endgroup$
    – Nolimon
    May 14 at 15:15
  • $\begingroup$ The quite well known sapient (and telepathic!) sphinxian treecats of the Honorverse have six limbs; the front-most two are called true-hands and work as hands only; the middle two can be used either as hands or as feet, depending on the current situation; and the hind-most two are true-feet and are only used for locomotion. $\endgroup$
    – AlexP
    May 14 at 16:25
  • $\begingroup$ we are a two armed humanoid who evolved from a four legged ancestor so yes, utility is a bigger advantage than stability. $\endgroup$
    – John
    May 14 at 19:50

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If we assume that hands developed to climb trees, and having more grapplers helps with that (looking at other apes, they generally have feet capable of grabbing too). So when human feet, evolved to be better at walking/running compared to a chimpanzee, at the expense of worse climbing ability. It was because the early humans gained more from an increase in walking ability, than they lost through loss of climbing ability.

So in a similar fashion, I believe it would make sense to start of with an animal that has evolved to live predominantly in trees, but with a significant amount of time spent on the ground (think a chimpanzee or gorilla just with 4 arms).

If that species is then introduced into an environment where they are incentivized to run/walk more. Let's say a savanna-like grassland, with copses of trees spread out with several hundred meters between them. They now still primarily live in the trees, but they still need to move between the different copses of trees regularly. It is in their interest to move in the open as quickly as possible, and therefore evolve feet more like humans, while still maintaining 4 limbs for climbing.

This would be a situation where going from 6 grapplers to 4, is less of a loss, compared to the gain of going from 0 to 2 actual running legs/feet. But going to 2 grapplers and 4 feet, would sacrifice too much climbing ability compared to the extra gain in running ability.

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  • $\begingroup$ Another way to strike at the numeric difference is that when climbing, a four-armed individual would have each arm carry 50% of the weight of a leg; but a four-legged animal would need its arms to carry 200% of the weight of a leg. That's quadruple the effort, and I haven't even added that the torso weight per arm has also doubled when you halve the amount of arms you have. Overall, you're looking at about feeling 3 times heavier by being two-armed and four-legged, as opposed to being four-armed and two-legged. That's going to be really hard (if not impossible) to adapt to organically. $\endgroup$
    – Flater
    May 14 at 23:31
  • $\begingroup$ @Flater That assumes that the feet of the quadruped are not well suited to climbing. It could be small and climb like a gecko. It could be large, but have ape-like feet with opposable digits that actually are usable for climbing. The fact that centauroid body arrangements are almost always depicted as either ungulates or less commonly canids or felids does not mean that other limb structures are not possible for such a creature. $\endgroup$ May 15 at 1:31
  • $\begingroup$ @AustinHemmelgarn: That's a different answer than this one, and it should be posted as such. There's many things you can nitpick to death but the distinction made by the question, this answer and my comment is that a hand is not a foot, and an inbetween kind of appendage, while biologically viable, isn't directly being asked about, nor does this answer focus on it. I suggest redirecting your additional information to an answer in and of itself because it's trying to answer the question rather than provide feedback on this answer. $\endgroup$
    – Flater
    May 15 at 2:06
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Absolutely both can evolve.

We are the descendants of 4-legged ancestors, so arms are a solid benefit and 2 walking limbs is actually fairly common. The weird thing about humans is the upright stance which is not great for a spinal column. We have animals with more than 2 grasping appendages now, many animals have evolved grasping tails, and elephants have trunks. So the advantages of more grasping appendages are significant.

For climbers, any additional grasping limbs are a huge advantage, so a climber that has to walk around may well only have two walking limbs the way we do; there is not much benefit to extra legs once you already have advanced balance, which long-limbed climbers need. The stability gain is pretty small. You could even see a weird hybrid knucklewalking: two dedicated arms, two dedicated legs, and two knucklewalking arms. You may even see specialization of the arms: two big arms for power, two small ones for delicate stuff.

For elephants, a grasping appendage was a big advantage, so centaur-like structure works fine as well. Arms have evolved many times.

Really the question of can it evolve is three simple questions: does it function biomechanically, does it grant an advantage, is there a path to get there in which they are still useful for something. As long as the answer to each is yes, then it can evolve on an alien planet.

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Is a four-armed body plan as realistic as a centaur-like body plan?

Sure! We know nothing about evolution or genetics other that what we've observed here on Earth. Speculation isn't worth the paper it's printed on and both solutions are as realistic as Superman flying around the Earth so fast as to turn back time. Since both are equally fantastic, they're equally realistic. But that's not a useful observation.

"Realistic" is what you get when you rationalize something fantastic well

If your goal is scientific perfection you are, in my opinion, barking up the wrong tree. No one will judge you right or wrong scientifically because no one can prove either of these body plans as being scientifically supportable. Proudly beating your chest and declaring that your design is as realistic as possible isn't defensible since nothing anyone can say here can prove you right or wrong. Remember, science demands repeatable experimentation and/or observation. Without at least a second world's worth of evolution to work with, there's nothing here to experiment on or observe that can tell us whether or not something will or won't evolve.

Which is why the goal of worldbuilding is rationalization. And here's the problem: I can rationalize either of those body plans being superior to the other.

Simplifying horrifically, what we understand about evolution is that it tends to favor simplicity over complexity and functionality over redundancy. Rather than starting with something like an ant or spider, let's start with a creature that looks a lot like a terrestrial millipede.

  • Simplicity over complexity tells us that if our trusty millipede survives the eons, it will grow in size and lose most of its legs, reducing complexity and increasing adaptability. The result is a roughly human-sized creature with six appendages. So far so good.

  • Let's assume your planet has a higher gravity than Earth. In this case a large upper-body held up by two legs is not favored. The stability of four legs and just two arms is a simpler, more functional combination in a high gravity environment. Should this creature meet a four-armed/two-legged creature in this high gravity environment, it would have superior leverage to compete with the two-legged creature and take its territory and hear the lamentation of their women.

  • Let's assume your planet has lower gravity than Earth. Now the four-armed/two-legged solution makes more sense as the weight-bearing issue doesn't exist and the ability to better manipulate the creature's surroundings takes precedence. Should this creature meet its four-legged counterpart, the more adaptable and functional upper-body would win out over the other in the lower gravity environment, having the ability to easily throw twice as many rocks as the other, who cannot move as adroitly to avoid them. Queue the lamentation of the women.

Which solution "makes the most sense?" The one that's best suited to the environment of your world — which you have not explained — and that meets the needs of your story, which you have also not explained.

Evolution, insofar as we understand it (which is precious little), pits the slow recombination of "features" (leaves, fins, teeth, energy absorption...) against the problems caused by the environment, which is constantly (in an evolutionary context) changing. Life may find itself developing on a sea shore surrounded by cliffs.

  • One evolutionary possibility is that life dies out because it can't expand to find new sources of food.

  • Another is that it re-develops features suitable for the sea and it returns to the water.

  • Another is that it develops arms to climb the cliffs.

  • Another is that it develops feathers and flight.

And here I have ignored things like competing with other creatures for limited resources, competing with creatures in the same species for mating privileges, etc.

There is no guarantee that any of those is the "right" or "most realistic" answer. In fact, I've oversimplified the issue to much that from the point of view of science, none of the circumstances are plausible. But they do rationalize why one might have birds on a particular island.

Conclusion... what are you trying to achieve?

Many worldbuilders are trying to create the most realistic world possible. That's great! I'm not particularly a fan of it, but it's a perfectly legitimate way to build worlds. But a worldbuilder who asks which of two options is more realistic is forgetting a critical component of real life.

They're not rolling dice and living with the results.

Again, we're oversimplifying something ferocious! But evolution includes random chance. What came first of the four options I just mentioned? Flight? Did it succeed in finding new sources of energy and opportunities for procreation? Don't know? Roll the dice. Maybe some aspects of evolution are deterministic and will always result in the same developmental path. We don't know. All we do know is that the slow march of mutation has a distinctly random component to it — at least it looks that way based on what we know today.

But that might not be your proverbial cup of tea. Random chance includes the planet being hit by a meteor and whole branches of the evolutionary tree dying. So much for all that hard work! Perhaps the more productive method of worldbuilding is to decide what you want to have. Four-armed or four-legged? You pick the outcome you want, then you develop the rationalization that justifies that as the "most realistic" path to rationalize that conclusion.

So... pick one. It doesn't matter which as they're equally rational, equally viable, and equally supported by the hypothetical musings of scientists who have yet to find even a second world worth of evolution to observe and test. Then build the nature of your world to support the end result you've selected.

Or build your world first and use dice to decide whether or not one path survives over another. Choosing not to use the dice means you've opted for the former solution and simply haven't admitted that to yourself yet.

Personally, I like a little insight into Einstein:

Einstein described his "private opinion" of quantum physics in one of the 1945 letters by referencing a phrase that he had already made famous: "God does not play dice with the universe." In the letter, he wrote: "God tirelessly plays dice under laws which he has himself prescribed." This variation clarified his argument that quantum particles must adhere to certain rules that don't change randomly, and that the quantum world required better explanations for particle behavior. (Source)

You're the god tirelessly playing dice under laws that you have prescribed for your world. Embrace the madness, my friend! And when you have, come back and ask again, because "which is more realistic?" is a meaningless question here. "I want my for-armed creatures! How do I rationalize that?" is much more what we do. We help you rationalize your choices. We don't make the choices for you.

To reiterate the answer to your question: both options are equally realistic given the information you've provided about the nature of your world and needs of your story.

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    $\begingroup$ "Speculation isn't worth the paper it's printed on" gets my upvote for sure. Evolution is far too complex a process for us to have a prayer of guessing what could realistically happen unless we've seen it (or something very similar) happen before. $\endgroup$ May 14 at 19:26
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    $\begingroup$ "both .. are as realistic as Superman flying around the Earth so fast as to turn back time" // sorry, but no, they are both relatively & realistically quite possible, while the superman thing is pure unadulterated magic, maybe you were using hyperbole to illustrate a point, but it's a flat out untruth of a rather large order, so gonna have to [-] for that ;) $\endgroup$
    – Pelinore
    May 14 at 19:47
  • $\begingroup$ @Pelinore Prove to me using the Scientific Method that either evolutionary outcome is "relatively & realistically quite possible." $\endgroup$
    – JBH
    May 14 at 22:08
  • $\begingroup$ @JBH Don't be silly, four limbs are not inevitable (the only reason we ended up with four limbs as standard is pure chance that that was how the chordate all vertebrates are descended from happened to be), turning back time by flying really fast on the other hand is simple impossible, not to mention that he can fly at all is also pure magic .. to compare a thing that was pure chance and happenstance with a .thing that is actually impossible according to physics as equally improbable is just being outrageously silly for the sake of being silly, and I know you know better than that. $\endgroup$
    – Pelinore
    May 15 at 0:05
  • $\begingroup$ @Pelinore And what's wrong with silly? The OP is asking if one of two things that never happened could be "realistic." Since they never happened, they're unrealistic. Neither hypothetical nor theoretical are "realistic," nor are could-have-beens or maybes. "Realistic" requires some kind of supporting evidence suggesting the possibility is at all plausible. Unless you can show me a four-armed humanoid or a four-legged humanoid, they are equally impossible quite literally to the degree of Superman spinning the Earth backward to reverse time, notably because they're all just story fiction. $\endgroup$
    – JBH
    May 17 at 8:10
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Evolutions isn’t always as straightforwards as “this gives an advantage”. We have for example a nerve that runs down to our necks before going back up to the head simply because evolution caused our necks to become longer but the original nerve never evolved a separate line to circumvent the increasingly long neck (compared to what it was).

Maybe the genes that add the second pair of arms never had a chance to disappear. Maybe the other pair of arms could but were much more useful so they stayed. Maybe you have two “strains”, with one having the bottom arms disappearing and the other the top arms, but if they mate all four arms could still appear when the right DNA mixes.

I would look more to how and where they are placed. Unless you place them at the hips, all four arms will share muscles. For example if the arms are simply one above the other then if you want to raise the bottom arms a muscle will pull the top arms down, requiring the muscles of the top arms to counterpull to keep them in place (this is no different than how many other muscles do it, like one of your Quadriceps of the legs). The most logical configuration would however put the second pair of arms on the shoulderblades but bending around the body facing forwards. Especially a 4 armed Centaur would be able to do this as you can attach some big and powerful muscles from the Horse part to the back-mounted arms to lift them (yes lift them despite pulling down). Additionally since they are on the shoulderblades it makes it easier to support the regular arm movement without losing too much stability.

And since this is a Centaur with Horse strength, it would make sense if it had stronger arms, in this case four arms, to carry and manipulate loads. The loss of the ability to squad or bend far would also allow for more arms to have a function if they have that reach. Arms on the back that can rotate to face backwards could also be used to steady loads on the back as well as fend off predators that would likely try to attack from behind as well.

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  • $\begingroup$ Unless I misread or it's been edited this isn't a four armed centaur, he's asking if (with a 6 limb body plan) four arm bipeds are as, more or less likely (evolutionarily speaking) to evolve as two armed four legged 'centaurs', I could be wrong but I get the impression from some of your answer that you're thinking he wants 8 limbed (4 armed 4 legged) centaurs? $\endgroup$
    – Pelinore
    May 15 at 17:38
  • $\begingroup$ @Pelinore re-reading it, you are right. $\endgroup$
    – Demigan
    May 17 at 16:58
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Four armed humanoid is arguably MORE plausible than a centaur.

  1. Adding extra arms to the shoulder blades or chest is more bio mechanically plausible that a creature with 4 legs and two arms, which would require 2 pelvises to work, and essentially two spines (or one spine bent into L-shape at all times).

  2. 4 armed bipedal creature is not particularly disadvantaged compared to a centaur. A 4-armed biped can run and sprint, arguably FASTER in the initial seconds than a 4-legged centaur (the way human sprinter can outrun a horse on a short distance), but also, a biped can outrun a centaur at very long distance (bipedal humanoid can jog for miles after a horse-like creature keeled over from exhaustion). 4-legged creatures are only advantaged at mid-distance. 9/10, a 4-armed biped is going to hunt down horse-like centaur to extinction, due to this two features alone, and the fact that it can use tools and weapons better than a centaur (and in fact, better than a human).

  3. 4-armed biped is a very universal design: it can sprint, climb, fight effectively, use tools, swim and dive, pounce and grapple with other animals, but also persistence hunt them. if the 2 extra arms are swept back when running, it can also maintain balance better.

  4. Four arms means the creature needs to consume more and better food, because it needs to feed extra muscle mass and extra brain mass required to control the arms. It is likely the 4-arm humanoid would evolve to be more aggressive than humans, as well as more prone to migrate, evolve culturally and invent more, simply due to greater needs + greater capability.

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    $\begingroup$ "bipedal humanoid can jog for miles after a horse-like creature keeled over from exhaustion" true for humans in respect of a great many prey animal (four quadrupeds and otherwise) but this is not a function of our bipedalism it's a function of our cardiovascular system and a few other adaptions that allow better cooling etc. $\endgroup$
    – Pelinore
    May 15 at 17:45
  • $\begingroup$ "it needs to feed .. extra brain mass required to control the arms" Quick! somebody tell the centipedes they need bigger brains ;) some perhaps but it's really not going to be that substantial as a matter of necessity really, octopi seem to manage just fine with somewhat less brain to body mass ratio (even including all their little 'sub brains') than humans do. $\endgroup$
    – Pelinore
    May 15 at 17:50
  • $\begingroup$ the extreme endurance of humans has little to do with bipedalism, and more to do with cardiovasuclar system, skin, and muscle composition. 4 legged animals are more common for a reason, bipedalism requires a great deal of evolutionary investment to do well. Also 4 legs allows for a much larger digestive tract, a huge benefit to herbivores. $\endgroup$
    – John
    May 15 at 23:14

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