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I have a mental image of a humanoid species with hands possessing three fingers and two thumbs, on opposite sides of the hand:

hand with two opposing thumbs

*note: it could be animalistic or reptilian, it doesn’t need to be exactly humanoid.

-What creatures would this species evolve from?

-Why would that species have evolved this design?

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  • $\begingroup$ You need to restrict yourself to one question at a time or you risk being closed, also perhaps too broad & opinion based as currently presented, check out the sites rules. $\endgroup$ – Pelinore Mar 21 at 16:55
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    $\begingroup$ linked to question worldbuilding.stackexchange.com/questions/21418/… but this question asks about its evolution. $\endgroup$ – A. I. Breveleri Mar 21 at 16:56
  • $\begingroup$ @A.I.Breveleri : Nice find, not a duplicate question as such but isn't there answers to everything Capillary is asking in there? $\endgroup$ – Pelinore Mar 21 at 17:05
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    $\begingroup$ Hi & welcome! Some brilliant ideas in here, but do take note: although we appreciate and encourage creativity, which is why we're all here!, Stack Exchange is all about asking a single focused query and getting viable, on point answers in return. If you check out the tour and help center, you should get a better idea why your query was closed. It's not because it's bad or anything, it just needs to be broken down into multiple queries and edited to conform to the Rules. Once that's done, it'll likely be reopened! $\endgroup$ – elemtilas Mar 21 at 19:25
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    $\begingroup$ After the edit he first answer to the linked question seems to answer the remaining questions. Specifically it answers the why, getting omni- or bidirectional grip for catching fish or climbing trees in odd orientations while feeding. While the first question is not really interesting. Anything can evolve this if it feeds itself in a manner that benefits from it. $\endgroup$ – Ville Niemi Mar 22 at 4:40
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A group of people gets a stable form of functional polydactyly (1). Combined with evolutionary pressures to do more complicated stuff with your hands it would mean this genetic abnormality would more often happen until the people without it are considered the mutants who reverted a part of their evolution.

(1): https://www.google.com/amp/s/www.sciencealert.com/a-sixth-finger-on-our-hands-would-actually-be-super-useful

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    $\begingroup$ The link is about a different hand structure with different number of fingers. 1+5 vs 1+3+1. Polydactyly vs zygodactyly. $\endgroup$ – Ville Niemi Mar 22 at 17:06
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I'd risk saying tree dwellers, maybe some type of theropod before the loss of 2 of its fingers. You see, this design could be considered as zygodactyl as seen in these images:

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Note that the external toe of the owl can swivel to switch between a zygodactyl foot for perching and a "eagle" foot for grabbing prey.

Now, why is this beneficial? The fact that it's common in tree-clinging birds should say enough: it's a great design for climbing and perching. So your hand could perhaps appear in a 5 digit reptilian that went somewhat along the theropod evolutionary path, eventually becorming a feathered creature with traits more found in tree-dwelling primates, maybe backed by a tail for additional support and improved balance. This hand would help it climb trees with ease and wrap its hands around branches, and would likely have sharp claws at the end of each finger. This path wouldn't be too different from ours, as our own opposing digit comes from our tree dwelling ancestors, except yours might also be able to have more controlled descends, going down trees head first due to having backwards facing claws, a trait which would stick around even after they left the trees.

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This hand arrangement could evolve from an intelligent creature whose hands had 2 fingers and a thumb that develop on the ulnar side. Then, a mutation might give the species an advantage over other species, but also causes ulnar dimelia.

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