Mar 14 2013

Bilateral Symmetry

I recently received the following question:

Why do we, and animals generally, have single or twin body parts, but not not triple or quadruple ones? Surely three eyes are better than one. (Four legged animals tend to have two front legs the same and two back legs the same) . And why is one side of the body a mirror image of the other anyway?. Why dont we just have two lungs or two eyes that look exactly the same?

This is an interesting question. First, however, I have to address the premise that animals in general display bilateral symmetry. This is mostly true for vertebrates, but not for many invertebrates. Sea stars, for example show radial symmetry, and have 5 or more radially symmetrical arms. Spiders have 8 legs and more than two eyes.

Most animals (the vast majority) do have some form of symmetry. Sponges are a notable exception.

Bilateral symmetry if found in several phyla of animals – Platyhelminthes, Arthropoda, Annelida, Mollusca and Chordata. Bilateral symmetry occurs in most phyla in just one plane, which is called the sagittal plane. If you imagine a human, there is only one plane that can bisect a person and create two halves that are essentially mirror images of each other.

Internal organs are more complex. Some come in bilateral pairs – lungs, kidneys, eyes, ears; while others are roughly midline and single, like the heart, brain, and bladder. Still others are one one side and single, like the liver or spleen.

Bilateral symmetry has mostly to do with developmental biology. As cells divide chemical gradients determine how cells specialize, migrate, further reproduce, and die. By necessity certain geometric and mathematical principles will apply.

Remember – development does not follow a blueprint. Cells are not placed like bricks in a building. Rather genetic instructions are a list of processes. Structure emerges from those processes. Repetitive processes, gradients, feedback loops, and geometrical relationships necessarily follow mathematical principles.

Bilateral symmetry is simply how our phylum evolved. It is so deeply embedded in our genetic instructions, in fact, that biologists find it very difficult to break this symmetry – to mess with the genes in order for one side of a bilaterally symmetrical creature to look different from the other side. Related to this is the fact that bilateral symmetry emerges very early on in development – matched pairs of cells called somites form in the very early embryo. This is the origin of much of the vertebrate bilateral symmetry.

Also keep in mind that the bilateral symmetry mainly applies to external morphology. Internal organs, as I noted above, show significant asymmetry. This is because organ development involves processes that break symmetry. This may involve, for example, the action of cilia in tissue migration.

In short, symmetry vs asymmetry derive from underlying developmental processes. These are, of course, complex and not fully understood, but developmental biologists have identified a number of developmental processes from which some form of symmetry spontaneously emerges.

29 responses so far

29 thoughts on “Bilateral Symmetry”

  1. haggholm says:

    Dawkins, in The Ancestor’s Tale, suggested that it may be a matter of simple utalitarianism that has bilaterality so common. Briefly summarising my recollection of what he says,

    * It makes sense to differentiate front from back. Movement chiefly in one direction suffices; it makes sense to have the sensory apparatus (and associated nerve clusters) at the front s.t. an animal can analyse what it encounters as early as possible. It makes sense to have the mouth close to the sensory organs, and to have excretory organs far away from the mouth.

    * It makes sense to differentiate up from down, as gravity ensures that the environment is different. Down may be the sea floor, up where the predators swim. Or in a free-swimming animals, maybe different predators threaten from above and below, and certainly cryptic colouration/countershading needs to differ.

    * There’s no strong need to differentiate anything else. Symmetry allows for efficient movement (imagine having legs of different length on the left and right side), and whether you travel vertically or horizontally, there’s rarely a need to suppose significant differences in the environment on different sides.

  2. Cornelioid says:


    Mario Livio makes the same point in the early pages of The Equation That Couldn’t Be Solved and cites a few earlier sources, the first being Hermann Weyl’s Symmetry, which i wasn’t aware until just now is freely available online. 🙂

  3. Haggholm, I think you hit upon the big picture point: “symmetry allows for efficient movement.”
    We’d be running in circles all day long with different leg sizes/lengths. Our backs would hurt with one huge heavy arm and one skinny wimpy arm. Etc. Etc. Evolution working its magic yet again.

  4. I won’t argue that bilateral symmetry does not result in a fairly efficient design for moving about, but I think it’s unwise to argue for the existence of bilateral symmetry on the basis of efficiency. There are two reasons why.

    The first is that, if we did have one leg bigger than the other, and we evolved that way, we wouldn’t continually move in circles. Walking may seem like a natural thing, but there are specific parts of the brain that make sure we do it unconsciously. If those parts are damaged in some way, we stumble around like idiots. So if we evolved without the aforementioned symmetry, our brains would no doubt evolve along with that, and we’d have no trouble walking — as long as walking were an activity we regularly engaged in.

    And that’s the other part. While evolution crafts magnificent biological machines, it’s not exactly efficient. There are a whole range of inefficiencies in the human design, and they crop up when humans try to do something they weren’t “designed” to do. That seems to be part of the reason why sitting in a desk chair all day can lead to back problems, or why reading a computer screen all day can lead to vision problems.

  5. Andyo says:

    Just the other day I was wondering about this. I seem to recall Dawkins having written about it in another of his books (haven’t read Ancestor’s Tale). Maybe The Blind Watchmaker?

    or why reading a computer screen all day can lead to vision problems.

    This seems like a great theme for a blog post *wink wink*. Is there evidence to suggest that the fatigue is due to computer screens (i.e. light-emitting), or does the fatigue manifest also when reading paper or e-paper? It seems to me that most people spend far more time in front of a computer screen or TV than they would reading anything on paper, so how do they isolate the light-emitting variable?

  6. SARA says:

    “By necessity certain geometric and mathematical principles will apply.”

    Why by necessity?

    This whole post reminded me of the flat fish whose eyes start off bilaterally and then move across it’s head as it develops so it can lie on it’s side on sea floor and see with both eyes. So it end up without bilateral eyes.

    You have to admit that is a cool evolutionary design.

  7. haggholm says:

    A torturous evolutionary development constrained by bilateral embryology, I’d say.

    Embryology may also be part of the answer. A symmetric embryology is presumably simpler to evolve — a set of (directionally) linear gradients rather than the more complicated instructions necessary to set up anything so complex and weird as a flounder? But this is pure layman speculation.

  8. elmer mccurdy says:

    I thought it had something to do with artificial farts.

  9. Jared Olsen says:

    Given your fondness for pendantry Steve, I must ask this question:
    Isn’t the ‘bi’ in bilateral redundant? Wouldn’t just ‘lateral symmetry’ do? Or is there such a thing as
    trilateral (etc) symmetry? I realise that ‘bilateral symmetry’ is the standard nomenclature, but doesn’t the word symmetry imply 2 sides only?

  10. Jared Olsen says:

    Andyo, a little off topic, but I always assumed the fatigue from looking at computer screens was a result of the refresh rate. That is, the image ‘flickers’ at 60 (or more) times per second and that seems to be tough on the eyes and brain. The appeal of e-ink is it doesnt do this, the text is static.

  11. BillyJoe7 says:


    “… but doesn’t the word symmetry imply 2 sides only?”

    What about radial symmetry.
    But I agree that “lateral symmetry” describes the situation adequately. And it fits with terms such a “horizontal symmetry”, and “vertical symmetry”.
    Multi-radial symmetry anyone?

  12. BillyJoe7 says:


    ” I always assumed the fatigue from looking at computer screens was a result of the refresh rate. That is, the image ‘flickers’ at 60 (or more) times per second and that seems to be tough on the eyes and brain”

    I don’t think this could be the explanation.
    The brain cannot distinguish anything greater than 25 frames per second. In other words, anything greater than 25 frames per second looks like smooth action.
    The fatigue is more likely due to sitting around on your ass all day.

  13. Sara – if you build something by repetitively following rules, you will end up with a structure that can be described with certain mathematical formulas that relate to those rules.

    Here’s a good example – the reaction-diffusion model and equiangular spirals:

    Jared – lateral means one side, bilateral means both sides. Lateral symmetry would be an oxymoron.

  14. BillyJoe7 says:

    …sorry, damn iPad keeps incorrectly correcting the spelling of your name.

  15. BillyJoe7 says:

    The following letters exhibit vertical symmetry: A H I M O T U V W X Y
    The following letters exhibit horizontal symmetry: B C D E H I K O X

    So why not lateral symmetry?

  16. “or why reading a computer screen all day can lead to vision problems.”

    This is because our vision is typically corrected (by glasses, contacts) so that our lenses in our eyeballs are relaxed, and not under tension, when looking at things at a far distance. So in order to bring things near to us into focus on the retinas, the muscles attached to the lenses tense up, bending the lens, and thus focusing near objects on the retinas.

    So when reading newspapers, looking at computer screens, basically doing any near work at a fixed distance for extended periods of time, this puts significant eyestrain on the lens muscles (causing eye fatigue, headache, muscle strain, etc.; these symptoms tend to fall under the name “asthenopia”). Your mom was serious when she said to sit back, away from the TV.

    Once you hit about 40 years old, you start losing the ability to flex your lens (presbyopia). So you can’t focus on anything near, and this nearpoint keeps steadily drifting away, requiring the use of bifocals in middle age. The human visual system is not really adapted for extended near-vision work, especially in “older” age. I guess it’s another example of the “good-enough” engineering of evolution you have all been mentioning.

  17. Oh and I should add that similar considerations involve eye alignment (vergence). You have to cross your eyes to look at things close, but your eyes are *generally* more relaxed when pointing straight ahead and parallel. Forcing your eyes to cross at a fixed distance for hours at a time can also contribute to eyestrain, headaches, etc.

    Check out the “Near Triad” or “Near Complex” of vision, in which eye alignment, lens focusing, and pupillary diameter all have to coordinate and change in synchrony using some physiological effort in order to look at things that are close, as opposed to far:,%20p.%2085-100.pdf

    Interested parties can also check VDT syndrome (video display terminal), this was popular for a few years (trying to figure out why people’s heads hurt so bad from working on computers).

    SOLUTION: get up, walk around every few minutes of an hour, let your eyes flex, relax, focus on things at a distance (20 ft or farther), “stretch” your eyes, get some cheap reading glasses from Rite-Aid for near work (this will let your ciliary lens muscles relax). You’ll feel a lot better.

  18. BillyJoe7 said, in part:

    “The brain cannot distinguish anything greater than 25 frames per second. In other words, anything greater than 25 frames per second looks like smooth action.”

    I think either you or your source are confused on this point, BillyJoe7. As a video and animation professional I can assure you that the human eye can distinguish differences in frame rates higher than 25 fps — in fact I could do it with one eye tied behind my back.

    I’d be surprised if in real world conditions casual viewers could distinguish any difference beyond 60 fps, but I thought I recalled reading somewhere once (nice citation, eh?) that 120 fps was the frame rate beyond which no human could hope to distinguish differences.

    Certainly, the difference between even 25 and 30 fps is obvious to anyone who has converted footage from PAL to NTSC or vice versa. The difference between 30 and 60 seems pretty obvious to me, too, but I’ve been in the business for many years so maybe I’ve learned a knack.


  19. ccbowers says:

    “So why not lateral symmetry?”

    Jared and BJ-
    Lateral means ‘to the side,’ and when used alone refers to one side or the other. The “bi” in bilateral specifies that something applies to “both” sides. Bilateral symmetry applies to your horizontal and vertical symmetry letter examples. The term ‘lateral symmetry’ therefore does not make sense, because if it is to one side only (whether it is left, right, up, down, etc) then it is lacking in symmetry along that axis.

  20. ccbowers says:

    On second thought for letters or characters “reflective” or “reflection symmetry” may be a better general term for symmetry that is inclusive of horizontal and vertical symmetry as bilteral refers to “sides,” and this is more consistent with your vertical symmetry examples.

  21. ConspicuousCarl says:

    Confucius say, “man who like lateral symmetry have hidden side.”

  22. sonic says:

    Here is a paper that describes what looks to be a very interesting theory as to the development of bilateral symmetry —
    Even explains external skeleton and internal skeleton using the same hypothesis regarding the orientation of the initial daughter cells and symmetry breaking…
    I haven’t read the entire paper– but it seems excellent thus far–

    The Origin, Evolution and Development of Bilateral
    Symmetry in Multicellular Organisms
    Eric Werner

  23. BillyJoe7 says:

    Okay, I’m going to accept your arguments for the moment.
    Interestingly there is a valid use of the term “lateral symmetry”:

    (I expect everyone here except sonic will understand this article)

  24. Davdoodles says:

    I wonder what starfish, sea urchins and trees make of our bilateral symmetry.

  25. Jared Olsen says:

    BJ, I think the brain can discern frame-rates higher than 25 FPS. The higher FR of “The Hobbit”
    is a recent example. I haven’t seen it yet but from what I’ve heard, it looked ‘funny’ or ‘not cinematic’,
    compared to the standard FR…

  26. BillyJoe7 says:


    (Ah, gotcha iPad)

    In fact, you are correct.
    The frame rate for cinema projectors is 24 frames per second, but each frame is illuminated three times, which gives a refresh rate of 72 per second. So I’m guessing that the brain cannot distinguish refresh rates greater than that because the action in movies certainly appears to be non-flickering*, if not continuous. I’m guessing that for action to be continuous, the frame rate would have to be 72 frames per second.
    This also provides a rationale for the higher frame rates for The Hobbit. I wondered if, in fact, the frame rate for The Hobbit was 72 frames per second, but it is apparently it is only 48 frames per second


    *In contrast to old movies which had a frame rate of 16 frames per second and which had an obvious flicker.

  27. ccbowers says:

    “BJ, I think the brain can discern frame-rates higher than 25 FPS.”

    This issue is a bit more complicated than one number. The perception of video at given frame rate is dependant on many things, including the quality of the video the nature of the objects and the amount of movement. It also depends on what you mean: the ability of the brain to distinguish flicker, versus the perception of fluid movement, versus at what frame rate does increasing the frame rate no longer affect perception of the video in any way? There is also the effect of the blurriness of the video’s movement. The blurrier the movement is the lower the frame rate can be without problems. For very crisp high definition videos (like a video game) the same frame rate as one with blur may seem choppy.

  28. ccbowers, you are right that perception of flicker is a complicated issue, and to point out the difference between perception of fluid motion versus threshold detection of flickering, which aren’t quite the same thing.

    many, if not most, people can detect flicker that is much faster than 25 FPS (some citations I’ve seen up to 60 Hz standard, 80 Hz, even in the hundreds). This all depends on individual factors, whether its in a dark room, brightness of display, viewed peripherally or straight on, etc.

    But *most* object motion looks pretty smooth with standard 60 Hz, for *most* people. Exceptions can be sports and movies with lots of fast visual movement (like action movies), hence the new trend to double-flash the imagery with 120 Hz refreshes, and the nice-looking motion offered by progressive as opposed to interlaced video (720i versus 720p, or 1080i versus 1080p). But adding smoothness to movies can sometimes make them look “unmovie-like” which can be perceptually strange (hence reports about the Hobbit looking weird). I find newer televisions with fancy image processing and 120 or 240Hz refresh rates have this same weird effect when I watch movies on them.

  29. BillyJoe7 says:

    I had a feeling that it would turn out to be just a bit more complicated than that.
    Now I’ll have to see The Hobbit. I’m curious to see if it turns out to be un-movie like. I don’t even know what that could mean.

Leave a Reply