Philosophy Discussion Forums | A Humans-Only Club for Open-Minded Discussion & Debate

[Trollinginger]

This is a question I have been pondering and puzzling over for years now, but no one that I ask seems to have a fully justifiable and suitable answer. Research yields several different explanations, none of which are conclusive, so I ask you all.

Why are nearly all organisms on earth, mostly plants and animals, more or less bilaterally symmetrical? The left and right sides of each organism are basically the same, with very few exceptions. It seems that, with the process of evolution and the potential for asymmetrical advantages that lead to a large population of whatever organism, there would be far, far less unanimity amongst life forms. Why is this so?

[Sy Borg]

I've wondered this too. While the forum doesn't recommend Wikipedia, the article "Symmetry in Biology" is a useful overview, covering the main basic types of body morphology with examples and diagrams. The main advantages of bilateral symmetry are said to be cephalisation - the concentration of sense organs at the front end of the body, to form a head and brain - and greater mobility.

[Grunth]

Maybe symmetry is merely apparent because all we really see are reflections.

[Trollinginger]

What do you mean by that?

[Grunth]

What do you mean by that?

Part of the illusion. Good and bad, up and down......always the suggestion that we could take a wrong path when presented with only two options. Maybe we are always both options or two things and so when we look at one it is really only the the flip-side of the whole (of which we really are), so each thing in isolation is actually a mirror of the other which also lends itself to a symmetry therefore. Symmetry, maybe, merely gives an illusion of order when it is really just chaos and sheer randomness (within the whole......because 'within' is our general experience). After all, it seems to me that our general experience, save for the occasional release, is tension and contraction. We get lost in illusions of order, which symmetry is an effect of, or lost in the impulse to seek order.

[Sy Borg]

The options:

1. Amorphous
2. Radial symmetry
3. Bilateral symmetry.

The first organisms where probably spherical, with their outer lipid membrane acting like a bubble in water. The trouble with amorphous and uneven forms is unbalanced locomotion, which reduces an organism's capacity for flowing movement* and makes them slower than their more symmetrical peers.

* Aside from the flowing movements of amoebas and The Blob, although neither is noted for speed.

[Jutfrank]

Everything points to the Sun?

[John Bruce Leonard]

The trouble with amorphous and uneven forms is unbalanced locomotion, which reduces an organism's capacity for flowing movement* and makes them slower than their more symmetrical peers.

This is an extremely clear and persuasive explanation, Greta. Thank you for that. I am not altogether certain, however, how this account could be extended also to the symmetry that one finds certain aspects of the vegetal world (for example, in certain bilateral leaves and flowers), or to the symmetry that one finds in the inanimate world (for example in crystalline structures or in the movement of heavenly bodies or subatomic particles). Do you have any ideas on this?

With Greta's account in mind, we might in the end be harder pressed to explain the jests of asymmetry in the natural world - to select a few examples, singular organs of the body, like the liver or the heart, that grow on one side of the body, rather than in the perfect center; radical differences between the two halves of the brain; the irregular arrangement of branches in most plants; right- or left-handedness in human beings; the lopsided claws of hermit crabs; the utter lack of symmetry in geography; or the disquieting phenomenon of situs invertus.

John Bruce Leonard

[Sy Borg]

I am not altogether certain, however, how this account could be extended also to the symmetry that one finds certain aspects of the vegetal world (for example, in certain bilateral leaves and flowers), or to the symmetry that one finds in the inanimate world (for example in crystalline structures or in the movement of heavenly bodies or subatomic particles). Do you have any ideas on this?

With Greta's account in mind, we might in the end be harder pressed to explain the jests of asymmetry in the natural world - to select a few examples, singular organs of the body, like the liver or the heart, that grow on one side of the body, rather than in the perfect center; radical differences between the two halves of the brain; the irregular arrangement of branches in most plants; right- or left-handedness in human beings; the lopsided claws of hermit crabs; the utter lack of symmetry in geography; or the disquieting phenomenon of situs invertus.

Very good questions.

It would seem that the sphere is fundamental, being the most compacted possible shape, symmetrical in all directions. If we continue to think of entities forming under outside pressure, then we can readily deduce angular shapes such as triangles, squares, pentagons and hexagons as "attempts" to form spheres with particles too large to create smooth arcs, eg. if you attempt to make a circle with lego blocks you will end up with some kind of polygon. Everything forms as a combination of those basic shapes. So molecules with pentangular shapes form crystals based on the basic atomic/molecular shape.

Many plants are radially symmetrical in the cross section, morphologically akin to a worm, which is the basic prototype shape of most animals, including us (in essence we are are omnivorous cephalised worms with limbs). However, flowers and leaves have both radial and bilateral symmetry. It seems that the existence of different kinds of symmetries and asymmetry in plants is far less clear cut than in animals due to the mobility issue. botanyprofessor.blogspot.com.au/2012/07 ... lants.html

Re: asymmetry in organs and the brain, I can only imagine that these are the results of efficacious mutations. Since genetic copies are not always perfect, an advantageous slight shift from radial symmetry would be naturally selected.

[Steve3007]

Nice subject. It looks like the consideration of bilateral symmetry in biology has quickly expanded to a consideration of symmetry in general. That's philosophy as I like it. Going from the specific to the very, very general and, in so doing, looking for the deep principles.

When you talk about symmetry in the most general possible terms what you're really talking about is repetition and pattern. Physical symmetry is a particular example of a kind of pattern; a repetition. Bilateral symmetry = take one half, repeat it and flip it. And one of the most basic observed properties of nature - the property that allows us to do science - is that it is repetitive. Which is another way of saying that it is not entirely chaotic and random. Why? The easiest answers are all reflexive/anthropic/Cartesian ones. i.e. a universe capable of contained beings which can ponder such questions must, by necessity, contain order and therefore symmetry. Only a universe containing symmetry could contain beings capable of wondering why the universe contains symmetry.

Incidentally, somebody earlier said that geography contains no symmetry. But it does contain striking examples of a particular kind of symmetry: symmetry across scale. A.K.A. fractals.

[John Bruce Leonard]

Greta, once more you have set us on a promising trail by bringing the sphere into our discussion - it being, as you say, “the most compacted possible shape, symmetrical in all directions.” If I am not mistaken, Steve3007 is thinking along similar lines here:

Physical symmetry is a particular example of a kind of pattern; a repetition.

Correct me if I am stepping awry, but I think we might state this perspective as follows: symmetry is simpler than asymmetry, insofar as it involves less variation.

It may be useful to pause a moment on this point. I say this because the issuance of complexity from simplicity is, to my mind, one of the great mysteries of things, and one of the principle stumbling blocks to the final comprehensiveness of scientific investigations. (Though it may well be that I am simply ignorant of the relative scientific theories.) For our present inquiry, then, I propose the following questions as worthy of consideration: can symmetry be regarded as a principle of economy on the part of the world? Or is symmetry, as a species of order, actually more complex than chaos?

John Bruce Leonard

[Steve3007]

symmetry is simpler than asymmetry, insofar as it involves less variation.

Well put. As I said, one of the explanations that some people in the past have proposed as to why the universe contains such patterns is that only a universe that contains such patterns could give rise to organized collections of matter (a.k.a. human beings) that are capable of wondering why the universe contains patterns.

But this anthropic principle, and its variants, has its opponents. Some say it's an "explanation" that explains nothing.

[Sy Borg]

Incidentally, somebody earlier said that geography contains no symmetry. But it does contain striking examples of a particular kind of symmetry: symmetry across scale. A.K.A. fractals.

Hi Steve. Bin a while:)

I couldn't find the quote you meant but I agree. The Earth's radial symmetry and the various crystalline forms come to mind.

-- Updated 16 Feb 2016, 17:42 to add the following --

Correct me if I am stepping awry, but I think we might state this perspective as follows: symmetry is simpler than asymmetry, insofar as it involves less variation.

It may be useful to pause a moment on this point. I say this because the issuance of complexity from simplicity is, to my mind, one of the great mysteries of things, and one of the principle stumbling blocks to the final comprehensiveness of scientific investigations. (Though it may well be that I am simply ignorant of the relative scientific theories.) For our present inquiry, then, I propose the following questions as worthy of consideration: can symmetry be regarded as a principle of economy on the part of the world? Or is symmetry, as a species of order, actually more complex than chaos?

The order of symmetry is borrowed from chaos. Living systems have a quality where they take energy and information from the environment to slow their own entropy and in doing so, not only increase the entropy of the affected environment but also increase the total net entropy. This includes living systems that are not technically "life", such as stars, planets, viruses, prions and other free proteins. All signs or order/symmetry reveal an entity's relationship with entropy because at some stage energy and information were "borrowed" from the environment to create that order.

Self ordering can easily come from chaos simply through probabilities. Once you have ordered entities like atoms, ordered structures building on the atoms' structures would seem inevitable. What of the state of the early universe, initially too hot for particles to form? Could order may have existed that gave rise to the order of the first heavy particles or was that the first?

[Lucylu]

Why are nearly all organisms on earth, mostly plants and animals, more or less bilaterally symmetrical? The left and right sides of each organism are basically the same, with very few exceptions. It seems that, with the process of evolution and the potential for asymmetrical advantages that lead to a large population of whatever organism, there would be far, far less unanimity amongst life forms. Why is this so?

I agree with earlier posts re simplicity being of a higher order than a more disordered state, but I'm not sure why left and right should be the dominant question? It makes sense to duplicate in case one side is damaged or attacked, and it seems logical to have an eye on each side, to be aware of our surroundings etc, but to have eyes and legs all around would cause confusion. I'm sure this niche was filled at some point by some poor soul, but they just didn't last very long-- if they saw an enemy, they wouldn't have known which way to run!

However, clearly there is asymmetry..if we lose our heads we die, but we can 'stand' to lose a leg or two.

I've always imagined that early life forms joined together very gradually to form a unified 'body', hence why each organ has very different life spans after death, with the 'LIVEr' lasting the longest (twice as long as the brain) and being able to regenerate, much like traits from our reptile ancestors. Not sure if that's accurate, its just what I've imagined.

[Sy Borg]

I agree with earlier posts re simplicity being of a higher order than a more disordered state, but I'm not sure why left and right should be the dominant question? It makes sense to duplicate in case one side is damaged or attacked, and it seems logical to have an eye on each side, to be aware of our surroundings etc, but to have eyes and legs all around would cause confusion. I'm sure this niche was filled at some point by some poor soul, but they just didn't last very long-- if they saw an enemy, they wouldn't have known which way to run!

As a matter of interest, a species of brittle star, Ophiocoma wendtii, has an outer skeleton of high quality crystalline lenses, each attached to an optic nerve, making the animal effectively a complex eye with a digestive system. Try to imagine how it might perceive the world.