Aridas
Crazy Little Green Dragon
Leaving insects out of the discussion, you don't think a couple billion years of mammal evolution is enough to conclude if 6 legged mammals were going to evolve they would have?
It's closer to 200 million years, on a quick check. Still, I'm willing to go with a "probably" to answer this question. Yet, that question is also notably different than whether 4 limbs is actually "ideal." There are other factors that would play notable roles, only a couple of which were referenced before.
As for the complexity of growing an extra pair of legs, it's not that much different from growing six fingers on both hands. The genetic control for how a finger develops is different from the genetic control of how many fingers develop. Therefore when a mutation occurs in numbers of fingers, you don't get extra fingers that are only bones with no skin or muscles.
The placement of the extra two limbs, effects on balance, effects on the more general body structure that's pointedly adapted to make the most of the usual 4 limbs, and how well they can be controlled are still points of concern, even if you deal with the basic genetic controls. An extra finger doesn't actually change much in any of those regards compared to extra arms or legs, given that there really aren't any other notable changes needed overall.
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