Somewhere on DeviantArt, there is an insightful tutorial by someone named "Uzlo" on how to create anatomically believable angels. Among the instructed details is the possibility of developing a second pair of pectoral muscles for the wings.

But in an alternate Earth, humans have evolved this second pair without the need for flight. Is this feature within the realm of primate evolutionary possibility? And if yes, then what would they be used for?

  • $\begingroup$ No, but you could grow the already existing ones accordingly $\endgroup$ – Adrian Colomitchi Mar 13 at 1:58
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    $\begingroup$ Pectoral muscles. Surprise. $\endgroup$ – AlexP Mar 13 at 7:48

Yes. We did that already.

pectoralis major and minor

BEHOLD true believers: the mighty pectoralis major and its little friend the pectoralis minor! 2 sets of pectoralis muscles, as requested, and in our bodies evolved as they are to throw things, beat drums, pet cats and so on.

Pec major does most of the work, with minor responsible for fine tuning scapular angles.

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    $\begingroup$ Some people even have a third smaller string pectoral muscle. $\endgroup$ – user75552 May 18 at 23:04

Form follows function.

If there's no need for an extra pair when the (maybe grown bigger) existing ones suffice, then the second pair will not evolve.

If it will, by a genetic accident, very likely they'll disappear; over-complexity is a liability, more chances for something to go wrong, extra energetic cost, etc.

So, without a specific need to justify it, humans will not evolve an extra pair.

Things may be different if other changes don't "cost" too much but offers advantages. E.g. the arm sockets evolve to allow extra degree of freedom for the arms and the environment selects for agility. Then it will be likely to see a co-evolution of a second pair of pectorals or whatever else needed to actually take advantage of, well, the other advantage.

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