Virtually all animals on earth ranging from insects through to swimmers, fliers, mammals and reptiles, have a symmetrical body plan. In other words legs and wings come in pairs, as do most sensory organs. The mouth is centrally placed, etc. http://en.wikipedia.org/wiki/Symmetry_in_biology#Bilateral_symmetry

Would there be any reason why a creature might evolve a non-symmetrical body plan?

Really I'd be interested in a world where the dominant and/or most common body plan was asymmetrical in some way, and a way in which that could arise.

To help inform the debate there is a list of example animals on earth which are asymmetrical here: http://en.wikipedia.org/wiki/List_of_animals_featuring_external_asymmetry (Thanks to Pavel Janicek for the link).

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    $\begingroup$ Define Symmetrical Body Plan. $\endgroup$ – JDSweetBeat Mar 10 '15 at 13:03
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    $\begingroup$ related link: en.wikipedia.org/wiki/… $\endgroup$ – Pavel Janicek Mar 10 '15 at 13:08
  • $\begingroup$ @PavelJanicek: I believe transforming that into a full answer would be best. $\endgroup$ – SF. Mar 10 '15 at 13:14
  • $\begingroup$ @SF. Problem is, that I can only provide "link only answer (and not spam)". Thats why I posted it only as comment $\endgroup$ – Pavel Janicek Mar 10 '15 at 13:16
  • $\begingroup$ I've added the link into the question, thanks :) I've also tried to expand on the question a little. $\endgroup$ – Tim B Mar 10 '15 at 13:20

11 Answers 11


Directional Consistency

An animal might be more likely to develop an asymmetric body plan if it evolved in a place where events consistently occur from the same direction.


I'll explain more, but first an obvious example:

Feet. Humans are bilaterally symmetric, but ignoring which way a human is supposed to be oriented we can see that humans have special adaptation for gravity. Gravity, for our evolution, was always one direction. Because of that, it was useful for us to develop some motility appendages in that direction. That asymmetrical adaptation is so obvious we usually don't see it as asymmetry.

Where this can be found

Ok, so back to gravity being down, what about horizontal directionality? This is less common, and with only a few exceptions, is likely the reason most animals on Earth are symmetric in this plane.

However, it's not hard to imagine a world, or at least a place with specific conditions on that world were horizontal symmetry is not as advantageous.

For example, consider a tidally locked planet, on this planet there is a thin terminator where life exists and evolved. There are a few natural conditions with consistent direction: gravity is always down, in one direction it's always light, the other is always dark, the winds would also likely have a constant direction depending on latitude. A sessile creature evolving there may only look for food in the direction of the oncoming wind, defecate in the opposite direction, absorb energy from the sun side, and dissipate heat on the dark side.

This isn't too far fetched an example. It could be something as simple as a primary predator or competitor that always attacks from the left in face to face battle. A consistent ocean current on a reef or wind pattern in a valley. Anything where the direction of a natural force is consistent for enough time to evolve an adaptation for it.


I think the key subject to cover when discussing symmetry is motility. Motility is simply the ability to move of your own ability. So while a rock may be mobile it is not motile. I like to think of a peg legged pirate. When you are not symmetrical your mobility and agility are decreased, this, if it is the natural state for a creature will put it at an evolutionary disadvantage compared to symmetrical counterparts.

There are of course other items to consider.

Gravity as it exists on earth combined with creatures being able to move effectively have resulted in the selection of a symmetrical body type as the best functioning biology.

Self defense is another aspect worth considering. An asymmetrical body type would have more obvious weak spots which could be exploited by predators. Even in an environment where gravity does not play a role there would be an advantage to being symmetrical.

Senses this sort of plays into the other topics but symmetry also allows us to effectively observe the world around us. If we had one ear on our head and one ear on our left knee sure...we'd be able to hear, but we would not have the ability to ascertain direction and we get similar problems when we consider asymmetrical vision.

A world without symmetry... is not going to happen. You could hypothetically create a world where symmetry is less dominant however.

I think the most likely scenario is a world dominated by water, maybe a world entirely covered with oceans. Gravity reduction would go a long way in reducing the drive toward symmetry. If the world is somehow less predatory that would help as well.

In the end this will be an uphill battle as even cells have a preference for symmetry. As on earth, niche players are likely your main asymmetrical creatures...so create more niches.

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    $\begingroup$ A generally excellent answer, but I'd say that, looking at typical lifeforms in Earth's oceans, a water-covered world doesn't seem very likely to promote asymmetry. To put it simply, an aquatic environment favors swimming as a method of locomotion, and it's very hard to swim straight if you're not (at least bilaterally) symmetric. $\endgroup$ – Ilmari Karonen Mar 10 '15 at 16:24
  • $\begingroup$ About your comment on ears, I recall some animal, perhaps a species of owl, actually has asymmetrical earholes so that it can better determine where sound is coming from. $\endgroup$ – DaaaahWhoosh Mar 10 '15 at 16:33
  • $\begingroup$ That is true @DaaaahWhoosh the some (if not all) owl species have one ear lower than the other and it improves their hearing. In the more extreme case I mentioned (intentionally) it would not function as well. Also, the owl still has one on each side so it has some symmetry. $\endgroup$ – James Mar 10 '15 at 17:50
  • $\begingroup$ @IlmariKaronen True, but my answer was considering a planet where complex life was able to exist AND allow for non-symmetric creatures, so while your comment is absolutely true I felt it was the best possible scenario. $\endgroup$ – James Mar 10 '15 at 17:52
  • $\begingroup$ I think that you're right to say that it is motility which requires symmetry. it's easy to imagine life evolving in an environment where all life had to stay anchored to the surface: look at plants for example, which generally are non symmetrical. Perhaps the surface conditions are not suited to motility - it could be extremely windy for example. $\endgroup$ – Max Williams Apr 5 '16 at 11:53

The wiki article in the comments to your question lead to some great reading about symmetry. The section on gravity and movement leading to a top, bottom, front, back, tends to push toward symmetry.

I would think that if your creature evolved in an environment that did not provide these up/down forces. Say in zero gravity, there could be some leniency in the need for symmetry. Sponges made me think along these lines. However most things 'floating' in water tend to spherical symmetry and space would I think lead to the same tendency.

It might help to understand the 'need' for non-symmetry in your original question. Perhaps it could be self-inflicted, even without providing an advantage. (I can take you with one arm tied behind my back.)

EDIT: When looking up stronger bones, I came across this article. (http://www.ncbi.nlm.nih.gov/pubmed/18769962) Which says that the dominate arm and leg of a fencer becomes stronger. Thus a forced asymmetry based on preference and usage is another avenue. Not sure how to make this work on a something like the 'head' unless I am a pirate and use a single ocular telescope all the time. ;-)


Another possibility would be exploring non-bilateral symmetry. Creatures evolving in low or no gravity might be more inclined to evolve with trilateral symmetry, pentilateral (not sure if that's a real word) symmetry or something stranger.

Many plants exhibit symmetry based on the Fibonacci sequence, so it's not that much of a stretch to imagine animals that aren't particularly mobile (or weren't early in their evolution) having a similar symmetry. This might be you key if you are looking for something internally consistent that doesn't look completely symmetrical as it's not always obvious in plants, see below:

enter image description here

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    $\begingroup$ Good point, there are other forms of symmetry. $\endgroup$ – superluminary Mar 11 '15 at 17:15

Star fish are not really bi-laterally symmetrical, they have at least 5 limbs and some have many more than that, though I think they are generally considered radially symmetrical.

My best one off the top of my head would be flounders. both eyes are on one side of their head and the rest of their body is strangely twisted.

Some crustaceans have differing levels of asymmetry, most commonly having one claw much larger than the other.

Generally these come about from unique living arrangements or specialized actions, niche living, either where they live or some special food they eat etc. The animals have adapted to specific things in their environment.


You need to figure out what environmental reason there would be to not be symmetrical.
The asymmetric earth creatures I can think of are crabs and lobsters, where one claw is for crushing and the other for cutting. Lobsters for instance don't have much of a mouth, and no teeth, so they use their crushing claw to break open an oyster, and their cutting claw to snip it into tiny pieces that they can throw at their mouth.

A fictional example of asymmetry are the Moties in "The Mote in Gods Eye" by Larry Niven and Jerry Pournelle.
These creatures had a caste system where their body plan would give them an advantage for their jobs. Workers had two arms on one side with delicate manipulating hands and a massive gripping hand on the other side.
On a side note, this led to asymmetric points of view "On one hand you have A, on the other hand you have B, on the gripping hand you have C".

All you need for a world where symmetry isn't the norm is a reason why this would give an advantage at an early stage of development, since any further adaptations would follow the same pattern.


Based on some of the other answers here, I came to the realization that the best way to make an animal asymmetric is simply to turn it ninety degrees. Bowlturner mentioned flounders and crabs (well, he mentioned crustaceans, crabs come more readily to my mind), both of which are animals that seem to have turned their lives ninety degrees: the flounders seem to be swimming on their sides, while the crabs seem to be walking sideways.

My best explanation for how this would occur in evolution would be the introduction of a species to an environment with a lot of tight spaces. For instance, if you've ever been spelunking (or walking through a crowd), you'll know that there are many occasions when you'll need to turn sideways to get through a narrow gap. Just like how people are left or right-handed, people will naturally choose to turn to a specific side when navigating a cave. If the decision is one-sided enough, a mutation favoring the dominant side may provide enough of a benefit to be passed on meaningfully and develop more over time.

Of course, this is just one path evolution may take. Perhaps the animal would leave the caves for a more suitable habitat (or simply die out), or evolve to be smaller or have thinner shoulders. Also, even if the asymmetric path is chosen, eventually I think the animal would still tend towards symmetry. That said, due to the immense amount of time it takes to evolve, there would definitely be millions of years before both sides matched up again.


Crabs are not bilaterally symmetrical

Crab with the left pincer larger than the right


My best guess on why animals would develop asymmetrical body plans would have to be sexual selection.

To be a little more clear...

Most animals, even humans, favor symmetry when it comes to selecting sexual partners. Symmetry tends to be a sign of good reproductive health. Poor nutrition, disease, and/or traumatic injury are often the causes of asymmetry in many species.

"Hey, baby, I see that you aren't missing any parts, and that your parts appear to be in the correct locations... " You get what I'm saying

To make a species, or several species, asymmetrical all you really need is for that asymmetry to offer a slight advantage. Chances are pretty good that sexual selection will take that asymmetry and exaggerate it.

The lop-sided claws you see in many species of crab are a good example of this tendency. Having one large claw for defeating rivals and one small claw for feeding offered a crab a slight practical advantage. Over generations the claw used to defeat rivals will grow larger because those with a larger claw are getting more chances to mate. If female crabs are also more attracted to fellas with one large claw, those with a larger claw will be doing even more mating.

So... Bottom line if you want asymmetry just make it sexy...

  • $\begingroup$ Ah, sexual selection. I knew there had to be a reason for that big claw. Strange how for humans, the one with the extra-strong arm is usually the one that doesn't get to mate... $\endgroup$ – DaaaahWhoosh Mar 12 '15 at 2:10
  • $\begingroup$ @DaaaahWhoosh discovery.com/tv-shows/other-shows/videos/… $\endgroup$ – apaul Mar 12 '15 at 2:20

I recently added an asymmetrical blob to my world. It is a mass of cells that evolved to survive in a cave system and be resistant to radiation. By definition it asymmetrical on the macro level as it can take any shape due to its jelly-like structure and ability to mold into just about every crevice in the cave. It evolved this way because surface life is impossible and even underground it can be exposed to higher levels of radiation.

It evolved to have every cell have all functions (e.g. Any cell in the organism could function as a part of the respiratory system, a vascular system, a liver, etc.). I am not cure how plausible this scenario is, but I have been working on a way to prevent mutated cells (cancer cells) from killing an organism and this seemed like a good idea. Therefore an asymmetrical organism of the macro level might be a blob of cells evolved to cope with radiation levels (although the more plausible solution is to get an armored exo-skeleton).

An ocean world might evolve asymmetric organisms. An octopus comes to mind. While it is symmetric it can adjust its body to fit into nearly any nook or cranny. As James said no matter how the world you design comes to be, you will always have some level of symmetry.


Consider the Halibut

enter image description here

This is a normal fish, that has evolved to swim on its side, rather than vertically.

Notice how the eyes have crept over the bridge of the 'nose' so that they're on the same side.

What we have here is a fish that has rotated its orientation 90 degrees.

Article on Flatfish Evolution


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