Symmetry yrtemmyS
December 7, 2009 12:25 PM Subscribe
Evolutionary Biology Filter: Why are we symmetrical?
IANAEB, so just wondering why seemingly almost all creatures on earth are generally symmetrical. Two sides - left / right - two arms, two feelers, 3 sets of two legs (insects), two eyes, two flippers etc. I'm sure there are exceptions (plants certainly), but what is it in our collective DNA that precludes three legged, three eyed, one-sided, 7 armed creatures?
IANAEB, so just wondering why seemingly almost all creatures on earth are generally symmetrical. Two sides - left / right - two arms, two feelers, 3 sets of two legs (insects), two eyes, two flippers etc. I'm sure there are exceptions (plants certainly), but what is it in our collective DNA that precludes three legged, three eyed, one-sided, 7 armed creatures?
Best answer: The bilaterian animals are bilaterally symmetrical. These animals all share a common ancestor which was also probably symmetrical. There are non-bilaterian animals which do not share this common ancestor—jellyfish and sponges among others. (Of course, jellyfish have radial symmetry.) But even bilaterians are not totally symmetrical—look at humans with our spleen on the left side and the liver on the right.
posted by grouse at 12:36 PM on December 7, 2009 [1 favorite]
posted by grouse at 12:36 PM on December 7, 2009 [1 favorite]
Symmetry also buys you a lot of redundancy - we have two mirrored arms, so if we lose one, we haven't lost any "unique" functionality we might if, say, we had one huge clawed arm for crude work and one more elegant arm.
posted by Tomorrowful at 12:40 PM on December 7, 2009
posted by Tomorrowful at 12:40 PM on December 7, 2009
I only half remember my undergrad biology classes but I do remember that bilateral symmetry is something that evolved later. The first organisms didn't need to be symmetrical, then they developed radial symmetry (increased complexity with guts, sex organs, etc.), then bilateral (for movement) (then some lost bilateral symmetry and became radially symmetrical again).
Apparently there are a couple genes that are responsible for this layout which probably restricts the body plans available:
"The data summarized here suggest that bilateral symmetry evolved before the split between Cnidaria and Bilateria. Both taxa exhibit bilateral symmetry. Both taxa exhibit staggered Hox expression domains along the primary body axis and asymmetric dpp expression along the secondary body axis. Homology is the most parsimonious explanation for the shared possession of these morphological and molecular traits."
Borrowed from pharyngula who borrowed from Finnerty JR, Pang K, Burton P, Paulson D, Martindale MQ (2004) Origins of bilateral symmetry: Hox and dpp expression in a sea anemone. Science 304:1335-1337.
IAAB, IANAG/EB (geneticist or evolutionary biologist)
posted by hydrobatidae at 12:57 PM on December 7, 2009
Apparently there are a couple genes that are responsible for this layout which probably restricts the body plans available:
"The data summarized here suggest that bilateral symmetry evolved before the split between Cnidaria and Bilateria. Both taxa exhibit bilateral symmetry. Both taxa exhibit staggered Hox expression domains along the primary body axis and asymmetric dpp expression along the secondary body axis. Homology is the most parsimonious explanation for the shared possession of these morphological and molecular traits."
Borrowed from pharyngula who borrowed from Finnerty JR, Pang K, Burton P, Paulson D, Martindale MQ (2004) Origins of bilateral symmetry: Hox and dpp expression in a sea anemone. Science 304:1335-1337.
IAAB, IANAG/EB (geneticist or evolutionary biologist)
posted by hydrobatidae at 12:57 PM on December 7, 2009
Symmetry also buys you a lot of redundancy
Just think if we were radially symmetrical! We'd have all kind of arms to lose (like sea stars).
I'm not so sure redundancy is important. American Lobsters have exactly one huge clawed arm for [fighting] and one more [puny] arm so even though they're symmetrical, they don't have any redundancy.
posted by hydrobatidae at 1:03 PM on December 7, 2009
Just think if we were radially symmetrical! We'd have all kind of arms to lose (like sea stars).
I'm not so sure redundancy is important. American Lobsters have exactly one huge clawed arm for [fighting] and one more [puny] arm so even though they're symmetrical, they don't have any redundancy.
posted by hydrobatidae at 1:03 PM on December 7, 2009
what is it in our collective DNA that precludes three legged, three eyed, one-sided, 7 armed creatures
Fitness cost. Creatures with asymmetries require more complex biochemical coordination of developmental processes, more complex metabolism, more complex biomechanics for getting around, more complex brains, etc.
It's not impossible to evolve three legs, three eyes, etc. but there can be costs for all the intermediate mutations required to get there. Those costs appear to have been too high for such intermediate organisms to reproduce.
When surviving is hard enough as it is, mutations that add non-beneficial complexity may be "wasteful" and therefore difficult to propagate through generations.
posted by Blazecock Pileon at 1:05 PM on December 7, 2009
Fitness cost. Creatures with asymmetries require more complex biochemical coordination of developmental processes, more complex metabolism, more complex biomechanics for getting around, more complex brains, etc.
It's not impossible to evolve three legs, three eyes, etc. but there can be costs for all the intermediate mutations required to get there. Those costs appear to have been too high for such intermediate organisms to reproduce.
When surviving is hard enough as it is, mutations that add non-beneficial complexity may be "wasteful" and therefore difficult to propagate through generations.
posted by Blazecock Pileon at 1:05 PM on December 7, 2009
Best answer: Well we all started out as sea animals and as sea animals having a flipper on only one side would make locomotion pretty difficult. I mean, unless they like swimming in circles.
posted by magnetsphere at 1:05 PM on December 7, 2009
posted by magnetsphere at 1:05 PM on December 7, 2009
I wish I could remember the fossil they found that exhibited trilateral symmetry. I found that fascinating.
posted by swimming naked when the tide goes out at 1:10 PM on December 7, 2009
posted by swimming naked when the tide goes out at 1:10 PM on December 7, 2009
What causes someone to be imperfectly symmetrical? Aren't we aware that "beauty" is largely a factor of symmetry? Would viruses, poor implementation of DNA? What?
posted by jefficator at 1:14 PM on December 7, 2009
posted by jefficator at 1:14 PM on December 7, 2009
I wish I could remember the fossil they found that exhibited trilateral symmetry.
Tribrachidium?
posted by zippy at 1:22 PM on December 7, 2009
Tribrachidium?
posted by zippy at 1:22 PM on December 7, 2009
Best answer: Stephen Pinker (again) mentions this on p. 308 of The Language Instinct: "There must be something in an animal's lifestyle that makes a symmetrical design worth its price. The crucial lifestyle feature is mobility: the species with bilaterally symmetrical body plans are the ones that are designed to move in straight lines. The reasons are obvious. A creature with an asymmetrical body would veer off in circles, and a creature with asymmetrical sense organs would eccentrically monitor one side of its body even though equally interesting things can happen on either side." He notes that locomoting organisms are typically symmetrical perpendicular to the direction they move, but almost never symmetrical front-to-back or up-and-down.
In contrast to Blazecock Pileon, he says that "making a body bilaterally symmetrical is difficult and expensive" since symmetry is so improbable - it would be easier just to grow organs and limbs wherever, but then then there would be costs in functionality.
posted by Dr. Send at 1:34 PM on December 7, 2009
In contrast to Blazecock Pileon, he says that "making a body bilaterally symmetrical is difficult and expensive" since symmetry is so improbable - it would be easier just to grow organs and limbs wherever, but then then there would be costs in functionality.
posted by Dr. Send at 1:34 PM on December 7, 2009
Our faces aren't generally symmetrical (at least not completely), and even in folks who exhibit a high degree of facial symmetry, facial expressions often involve asymmetrical muscle contractions.
posted by solipsophistocracy at 1:35 PM on December 7, 2009
posted by solipsophistocracy at 1:35 PM on December 7, 2009
"Humans are members of the phylum Chordata . All of the chordates have elongated bilaterally symmetrical bodies."
..like grouse says, it's because a common, successful ancestor passed that trait onwards.
posted by bonobothegreat at 1:42 PM on December 7, 2009
..like grouse says, it's because a common, successful ancestor passed that trait onwards.
posted by bonobothegreat at 1:42 PM on December 7, 2009
Here are a couple of interesting articles -- one about what happens when symmetry breaks, and one on how it's not as important for attractiveness as we once thought. Neither address your direct question, but they're both related & interesting read.
posted by brainmouse at 1:43 PM on December 7, 2009
posted by brainmouse at 1:43 PM on December 7, 2009
I remember reading about the Burgess Shale (care of Stephen Jay Gould), and the most extraordinary discoveries such as Hallucigenia. Things could have been very very different.
As a side note, don't a lot of studies of beauty reinforce that symmetry is rewarded in terms of fitness?
posted by wilful at 2:41 PM on December 7, 2009
As a side note, don't a lot of studies of beauty reinforce that symmetry is rewarded in terms of fitness?
posted by wilful at 2:41 PM on December 7, 2009
it would be easier just to grow organs and limbs wherever
Stephen Pinker is a smart person, but I think any developmental biology textbook would seem to contradict this assertion, in that factors get distributed throughout a developing embryo that induce polarities in gene expression. Once those polarities have evolved, it seems hard to mess with them (to allow organs and limbs to grow "wherever") without potentially introducing reproductive fitness costs.
posted by Blazecock Pileon at 4:44 PM on December 7, 2009
Stephen Pinker is a smart person, but I think any developmental biology textbook would seem to contradict this assertion, in that factors get distributed throughout a developing embryo that induce polarities in gene expression. Once those polarities have evolved, it seems hard to mess with them (to allow organs and limbs to grow "wherever") without potentially introducing reproductive fitness costs.
posted by Blazecock Pileon at 4:44 PM on December 7, 2009
Right, that may have just been my bastardized paraphrase; I don't think he was saying anything about gene expression, just that the locomotive benefits of symmetry increase fitness compared to amorphous blobs. But I'm not Stephen Pinker, nor am I a biologist, so you probably know better than I do what the costs of symmetry are.
posted by Dr. Send at 8:45 PM on December 7, 2009
posted by Dr. Send at 8:45 PM on December 7, 2009
Response by poster: Great answers! Making an uninformed attempt at summing up...
1. Seems that bilateral symmetry proved its advantage VERY early in the evolution of life. Look at all the waaaay-back sea critters that are b.s. Magnetsphere has it... symmetry makes for good swimming. As mentioned above, mobility is a key survival advantage. Thanks Dr. Send via Pinker: "The crucial lifestyle feature is mobility: the species with bilaterally symmetrical body plans are the ones that are designed to move in straight lines."
2. Since we evolved from these critters, it would seem likely that we too have b.s. -- unless it proved disadvantageous as some point, which it clearly has not.
3. External b.s. seems more important than internal. Thus our innards are not so symmetrical (though redundancy is important internally, eh?).
4. I would wager a guess that the % of land animals that are b.s. is higher than that for sea creatures (99.99% vs. 95%?). In fact, I cannot think of a land animal this is not generally b.s. - can anyone? In water, it seems that there is more viable ways to get about.
Reasonable?
posted by ecorrocio at 9:19 AM on December 8, 2009
1. Seems that bilateral symmetry proved its advantage VERY early in the evolution of life. Look at all the waaaay-back sea critters that are b.s. Magnetsphere has it... symmetry makes for good swimming. As mentioned above, mobility is a key survival advantage. Thanks Dr. Send via Pinker: "The crucial lifestyle feature is mobility: the species with bilaterally symmetrical body plans are the ones that are designed to move in straight lines."
2. Since we evolved from these critters, it would seem likely that we too have b.s. -- unless it proved disadvantageous as some point, which it clearly has not.
3. External b.s. seems more important than internal. Thus our innards are not so symmetrical (though redundancy is important internally, eh?).
4. I would wager a guess that the % of land animals that are b.s. is higher than that for sea creatures (99.99% vs. 95%?). In fact, I cannot think of a land animal this is not generally b.s. - can anyone? In water, it seems that there is more viable ways to get about.
Reasonable?
posted by ecorrocio at 9:19 AM on December 8, 2009
This thread is closed to new comments.
posted by ludwig_van at 12:30 PM on December 7, 2009