I have a species that is humanoid but has two sets of arms and I was wondering what reasons there could be for why they evolved that way?

  • 2
    $\begingroup$ This would be helped with more details. Are the arms an example of quadrilateral symmetry? Are they all the same size? Are two located in a sort of false pelvis somewhere between the real pelvis and the shoulders? If you don't know those details, what about having four arms is important to your story? $\endgroup$
    – Brythan
    Aug 12 '18 at 3:16
  • 1
    $\begingroup$ Considering most land animals and fish have 4 limbs, you could just have your first fish evolve with six limbs and everything else just evolved from that. $\endgroup$
    – Starpilot
    Aug 12 '18 at 3:37
  • $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. To be fair, the complexity of a double-shoulder bone and its inherent weakness might make this a difficult sell. A benefit after the fact (like fighting or manufacturing) doesn't justify evolution. Hmmm..... $\endgroup$ Aug 12 '18 at 4:18
  • 1
    $\begingroup$ Your question has inaccurate terminology. As far as I can tell "humanoid" is defined as having the basic body plan of humans - 2 legs, upright body, 2 arms with hands, and a head on top. Chimps and gorillas are roughly humanoid. If an alien species has some major difference from the basic humanoid body form it is not humanoid but something else, like maybe "Hindu-god-oid" for example. $\endgroup$ Aug 13 '18 at 5:27

The evolutionary reason is that your humanoids evolved from an early ancestor that had a body plan with six major appendages from the central trunk (not including the head). No other reason is needed.


This humanoid is the descendant of some kind of Sarcopterygian with six fins

This is a Sarcopterygian, or lobe-finned fish:

enter image description here

As you can see, it has four fins aside from the tail. In the Devonian period, one lobe-finned fish started to go on land a bit, and eventually produced the first tetrapods.

The word tetrapod means "four feet", which is the characteristic shared by all tetrapods (Except for snakes, caecilians, legless lizards and cetaceans). They all have four feet because the lobe-finned fish that colonized the land had four fins. But say if, in your world, there was a creature that looked like a Sarcopterygian, but had six fins, and it colonized the land, the six fins would develop into six limbs.

Much later on in the world's evolutionary history, one hexapod achieves orthostasis, walking on two legs, with the back upright and the other four off the ground.


You start with mammals that have four limbs, and then you evolve a humanoid with six limbs.

I'd suggest it might start with a founder effect. A small group of survivors carries the gene as a recessive, say. A quarter of the second generation have the trait, but since there are so few of them, it accidentally turns out to be half or more. And then by unlucky accident all of the individuals that have normal arms die.

At first it would be entirely a disadvantage. People have a second smaller set of arms with a second shoulder girdle complete with scapula on top of the regular arms. They get in the way, but not as much as they would if they were below the regular arms, or inside them, or somehow mixed with them.

The group of humanoids starts out with low genetic variation because they are descended from a few founders. The population expands fast and collects more variation, and starts to select for improved architecture for that second shoulder girdle etc. People come up with ways to use the new limbs, and that makes functional limbs more valuable. And it goes from there.

It's easier to get small, nonfunctional extra arms than functional ones. So I propose it might happen in steps. First they get stuck with something by random chance, and then they get selection for improved function.

extra limbs


I'd say more arms means more punches, methods of making tools, and all the things that are only able to do by arm. Since people are more proficient of using their arms than any other source of energy, it may be that humans will grow an extra, or several arms.

Then there is another reason why not. REmember the peacock's feathers, well, they're there just so that they mate. Any peacock without colorful feathers will not be able to give that DNA to it's offspring, same thing will happen to people. If people don't want to marry someone with four arms, no matter how good that person is at typing he will not pass the four arm DNA to other humans.

  • $\begingroup$ Considering that land animals mostly evolved from fishes, it's hard to figure out why a fish would come out of the water for punching. And don't forget the integration of the shoulder-arm articulation in the middle of the torso.... $\endgroup$
    – L.Dutch
    Aug 12 '18 at 3:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.