Kindasorta inspired by this thread, though I’ve mulled the idea before. Vertebrates have no more than four limbs. Well, that’s economical, land-dwellers need four and no more to get around; some get by with two or none.
But, what if, at some evolutionary fork, vertebrates had developed six limbs as the norm? Then practically all land-dwelling species would have, as it were, an extra set of forelimbs usable as manipulators while it stands/walks on the other four, like a centaur – an arrangement that stimulates mental development, etc. – and without having to wait for such a truly rare evolutionary event as the development of fully bipedal locomotion to free up the forepaws. The result of this might be that at any epoch of any period in Earth’s history, there might be several sentient and semi-sentient species of different families and orders of six-limbed vertebrates sharing the Earth – rather like Burroughs’ Barsoom, or the planet Mongo in the Flash Gordon universe.
Is this a plausible scenario? What would such a world really be like?
There’s no guarantee that it would stimulate mental development. Some invertebrates like lobsters and scorpions have specialized grasping forelimbs, but I doubt they’re much smarter than invertebrates that only have walking limbs.
In fact the mental requirements is a strike against them. Likely the extra set would have restricted the development of intelligence. Brains are pretty damn metabolically expensive and processing using our hands, for example, uses up a huge chunk of those demands. Having an extra set of limbs would have to delivery some pretty big advantages to be worth the cost in either what else of brain processing was given up in return, or in the extra intake required to feed that processing.
Insects generally do six limbs without much central brain processing, and other than consideration at the swarm or hive level, with little evidence for intelligence. The octopus may be an exception, more than four limbs and with evidence of real intelligence, but its solution is to have a brain for each limb with a small central brain to coordinate. In vertebrates a single prehensile accessory, be it tail or trunk, (or two hands) seems to be as much as is worth the extra costs. (Boy, I’d love see a somatotopic map of the elephant brain and find out how much cortex is devoted to the trunk.)
And in addition to the problems mentioned so far you’d need a bent spine for uprightness. Not impossible, but not ideal, and possibly more problematic.
Not if the middle pair of limbs is used only for locomotion.
I meant that having a set of free manipulatory appendages “stimulates mental development” in the evolutionary-feedback sense, not that having two more limbs would make an organism smarter because of the added neural stuff required to operate it.
No, lobsters, etc., don’t seem to evolve any smarter over the epochs, but vertebrate brains are different.
I don’t know what you mean by a “bent spine.” A six-limbed vertebrate would of course need some kind of limb-hanger bone or bone structure at or around the middle of the spine, analogous to the pelvis at one end and the collar bone/shoulders at the other.
Does “uprightness” mean humanlike bipedal posture? Why? There are obvious (in hindsight) advantages to walking on two legs instead of four, but not to walking on two legs instead of six.
Are you claiming in your case it’s definitely for certain, then?
In order to have the front part of the body upright - facing forward, instead of facing downward - the entire organism needs a bent spine or a LOT of flexibility (which would then require muscle structure to support it, spinal problems, etc.). Yes, it could have it grow an entire pelvis, but then that would be no more easy than a human’s adaptations. Perhaps more difficult, since it’s hard to see how it would have evolved that at all.
Compounding this, you’re putting much more stress on the overall frame. A six-limbed creature won’t neccessarily be any stronger than a four-limbed creature. In fact, each limb may simply wind up correspondingly weaker, or the being may have a pair of vestigial or very weak limbs. As nice as another arm might be to handle babies while working on the like, you have to attach it and that requires space and resources. All in all, it may come to be a net negative in terms of survivability.
“Could”?