I remember discussing this some time ago with a group of friends, and one of us opined that the reason for the symmetry was that it was the most efficient form for a life form to take up, after hundreds of thousands of years of evolution. I had to leave around then, so I couldn’t follow up on that line of thought, but it came back to me as I was watching the Animal Planet channel today.
I’m going to discount abnormalities caused by illness, etc. for this question, since they aren’t a product of evolution (well, not exactly!).
I can understand the need for two eyes - it’s so that we can see in 3-D, right? Similarly, I can also understand the need for an ear on either side of the head. But how about two nostrils - why do we need two nostrils when they go and join up later anyways? Or why do males have 2 testicles and not one? Why are the female’s breasts of approximately equal dimensions? See where I’m going with this?
I wasn’t sure whether I should have posted in GQ or not, since I wasn’t sure whether there was any documented study on this topic.
Not quite sure exactly what you’re asking, but we were descended from animals with bilateral symmetry, so why bother trying to get rid of it?
Two nostrils–you never have just one get clogged up? Isn’t it annoying? I also read somewhere that they hypothesized that the air flow in each nostril was different, leading to the ability to smell different things in each.
Two testicles–backup in case something goes wrong with one? (Tom Green would be a good, or maybe not-so-good example)
Two breasts–well, humans and primates have 2, but cows have 4 teats, and dogs and cats have multiples of two. I would think just one, or two of vastly different sizes, would tend to throw the balance and biomechanics off, in either human or primate females.
There are some animals (example–jellyfish) that are radially symmeterical, and some (most notably, the echinoderms) that have pentaradial symmetry.
Well. If your left half if just a mirror of your right half then you don’t need to store as much data on how to build a bady in your DNA as you would if every inch of your were unique.
Also now that I think about it I would imagine that symmetry is a trait that is highly desirably in fish and other swimmers having to plow through the rather thick liquid of the oceans. Being unsymmetrical in that medium would require more effort to move slowing unsymmetrical fish as well as tireing them out faster then their streamlined symmetrical brethren.
I would agree with the ‘It’s easier to build’ and that it provides an automatic redundant system. If you poke one eye out you still have another. If a bear bites off one arm you still have one left.
I think you should as why some of our parts are singles.
One mouth?
One pipe for air and food?
One stomach?
One penis?
One heart?
Of course, humans are not symmetric on the inside. I think that symmetry is one of those universally appealing traits, and any external asymmetries are selected out of the gene pool. This doesn’t answer the question, of course, because I don’t know why symmetry is appealing, but it does rephrase it.
. What if it were a pack of lionesses, and they tore out both arms? Sorry! Couldn’t resist that :)…
And well rephrased, Achernar, but still…!!
I mean, even if the airflow in the two nostrils were different thus giving the ability to smell different scents in each, why? I mean, two nostrils means an extra bit of cartilage spearating them, when a modifed smelling technique could probably do with just one (Not asking for a factual answer here, just opinions…).
And sorry easy e, but I find it hard to buy the backup theory for males having two testicles - what if the other testicle fails too?! And why not have one breast on the female smaller than the other?? And so on…
gouda - remember natural selection is not about, “what is the best possible design?” The implicit question is, “Of all the genetic mutations that have occurred to some species in some area in a given time period, which is the best for survival?” The result doesn’t have to be the best possible design - there was an article in Scientific American a few months back about how the human body could have been better “designed to last.” The design that “wins” is simply the one which is the best for survival. It’s definitely possible that there could be a better design with only one nostril - but the mutation never occurred to be “tested”.
Hmm, offhand I can’t think of any animal with more than one mouth.
Or entirely seperate bronchial and esophagial tubing.-It seems to me that the mutations needed to create such a system would be more complex than those needed to link both systems to a pre-existing orifice. IIRC hydras, and certain other microscopic creatures have two layers of cells. The outside layer can get oxygen etc from the surrounding water. Water for the inner layer is provided by a hole leading to the creatures hollow interior. So we're starting from a single hole for respiration, and ingestion in simple muticellular things.
I assume that goes for stomachs as well. Mutations built on a system of a single, roughly central chamber for storing and digesting food. Ruminants (eg cows) and others have more than one stomach for breaking down their food. Homo Sapiens Sapiens is just not adapted to live on grass(Cheech and Chong not whithstanding.)
The amount and type of penises (penice? penii? penisi?) that some species have is amazing. Some reptiles have hemipenises-one base branching into two genitals. In some species of spider, the male’s reproductive tubing(a kind of corkscrew-shaped bit of tubing) is in his mouth. In other spiders, the male keeps it tucked away in a slit on the inside of one off his leg joints.
Hmmm- Why is it some animals have more than one heart and some don’t? I’d think that a second heart might reduce the amount of work each heart needs to perform. They’d have to keep in sync. But, that’s true of the muscles of a single heart. hmmm
Keep in mind that there are two major forms of symmetry among animals: radial and bilateral.
Radial symmetry appears first in the history of life (though there are instances of radial symmetry evolving from bilateral ancestors - e.g., echinoderms). Such symmetry is advantageous for sessile organisms since it allows them to face their surroundings (so to speak) equally from all sides. If you’re rooted in place, that’s generally a good thing since you never know from which direction a threat, or food, will come.
Mobile animals tend to be bilateral (though this is by no means an absolute). Bilateral symmetry could be viewed as radial symmetry taken to its most-reduced form; thus, we have only two “sides” instead of several. However, with bilateral symmetry come numerous changes, like cephalization (the concentration of sensory organs toward the anterior end, since that’s the end most likely to encounter anything).
The asymmetry of the gut can be explained by recognizing that it is essentially a hole through the middle of the organism. An organ such as the small intestine (or large intestine) remains radially symmetrical (since it is just a glorified tube), but the organ as a whole appears asymmetric because of all the material that must be squooshed into the body cavity – try to cram a length of hose into a small space, and it’s not going to look very neat. Other digestive organs are typically just an outcropping (or invagination, if you prefer) from this central tube.
Both sides of a bilateral organism look similar because the same developmental controls govern both sides. Thus, for any organ / tissue that is developed from non-endoderm (the germ layer which forms the gut) cells, you get two: one on each side, essentially mirror images of each other, if all goes well. This is obviously a much more efficient method than having separate developmental controls for each side, and guarantees (again, barring developmental problems) that both sides match - which is important for locomotion, and especially, streamlining.