>> Saturday, July 17, 2010
Shakespeare asked: Why do we only have two arms, instead of 3 or 4, since that would make doing things easier?
Why indeed. Well let me preface my "answer" with a few comments. First, I don't know the answer. I can speculate, but I'm not a biologist and, if I were, my answers would likely be educated speculation anyway. Science has a few hard facts associated with it, but the answer to why questions are rarely among them.
Having said that, I'm going to tell you what I think is the answer to "why." Evolution, the development and adaptation of life over long periods of time to make the species we know today, depends on several things: environment that weeds out traits that are not optimal for survival and the originating species itself. Although the environment can force tremendous changes over time, the underlying species often retains key characteristics.
What struck me, when I began thinking of this, is that, though are untold animals that have more than four limbs (tails, I'll get to them in a minute), but I couldn't think of a single one that had a spine. Invertebrates (exoskeletons like bugs/spiders or squishy stuff like jellyfish/octopi) are far less restricted. However, when I think of vertebrates (birds, mammals, fish, reptiles, and amphibians), I realize there's a consistency: two forelimbs, two rear limbs, perhaps a tail.
On birds, that becomes wings and feet. On mammals, the four limbs and, perhaps, a tail or the legs are melded together into a tail and the arms are flippers (like dolphins). Limbs for reptiles and amphibians (though sometimes the limbs are more pronounced than others and sometimes they disappear like snakes). Fish with vertebrae fairly consistently have two main side fins and a tail - not a stretch to see a comparison to the two sets of limbs. But, even with the fish, you don't see six or seven arms on a fish with a spine.
So, my answer to why is that whatever first animals started out with spines, the ones that seeded our species and the rest of the vertabrae, apparently had four limbs. Perhaps that (those) vertabraed survived because the set of four limbs is so versatile, adapting admirably to water and air and land, even walking upright. But it could be a fluke, too, the luck of the draw. In Avatar, James Cameron played with a notion that the animals had an extra set of limbs between forelimbs and hind limbs. The flying creatures had two sets of wings instead of one (and still had fore/hind limbs). Only the N'avi didn't.
But, would the extra arms really be that useful? Would they be just like the other arms only below them on the ribs? If so, you couldn't move them like your top arms - there's no place to put a shoulder joint, or, in fact, any stable bone structure (like a collar bone) to tie to. The ribs, the only bones through there, attach to the spine in one place. You'd have to have muscles there, too, where there aren't any now and it wouldn't be able to move like a regular arm, probably only restricted to two directions for the joint (like legs attach to the hips). That means you could move them forward and backward, but probably not up and down. Pretty limited.
Any way, I wouldn't take anything I've said here as gospel (ha ha!), but that's my two cents on why we only have two arms instead of three or four.