Would there be any major disadvantages for a species to have six legs instead of four?
I want a world in which animals roam the wilderness on four legs, yet at least some of them are able to do the kind of carrying and fine handling of objects done by humans. The planet is superficially similar to Earth, but it is not Earth, and there are no humans around (at least not yet; who knows what humans might do once they figure out interstellar spaceflight...)
There seem to be basically two ways to go about reaching that goal; either give the ancestor of land life on that planet six legs, allowing for two to evolve into arms and hands in a manner similar to how those of humans evolved; or give the ancestor four legs, and have their forelimbs serve both purposes, not unlike gorillas on Earth.
For the alien feel, as well as the additional options it gives, I'm currently leaning toward six legs, the foremost two of which could evolve (or not, depending on the species) into hand-like extremities.
This is intended to be a realistic world, so the normal issues of evolutionary selection pressure apply.
Suppose that an intelligent designer is faced with the choice of, for an ancestor species, ticking the "four legs" or the "six legs" checkbox on the requisition form. What, if any, would be good reasons for them to select four legs rather than six? Or, in other words, what disadvantages would be confered to the creature by having six legs as opposed to four?
For simplicity's sake, you may assume a single ancestor species for all relevant land life on the planet, so for example "some other species would be more energy-efficient in not needing to grow an extra two limbs" does not apply. Other species on the planet may have a different number of legs (compare to how there's also spiders and caterpillars on Earth), but can be ignored for the purposes of this question.
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