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You've misunderstood what bilateral means, it doesn't mean "two of each", it roughly means that there exists a point at which you could bisect the animal with a
by avar 7y ago
You've misunderstood what bilateral means, it doesn't mean "two of each", it roughly means that there exists a point at which you could bisect the animal with a mirror and "see" the hidden side.
So e.g. humans are bilateral even though we only have one nose, mouth, anus, penis etc, because if you mirror half of a human bisected down the middle you "see" the other side.
The same can apply for animals that have an odd number of appendages such as starfish and other echinoderms.
- deleted 7y ago[deleted]
- ahartmetz 7y agoOur internal organs aren't fully symmetric, though. The heart is more on the left, the spleen is on the left, and the liver is on the right.
- klodolph 7y agoThis is a misreading of the parent comment—“there exists a point”. In general you can dig up problems with using morphology for your taxonomy, but you haven’t actually discovered a problem here. The bilateral animals go through a development phase where they are bilaterally symmetric.
- ahartmetz 7y agoOh, a point in time. I was thinking space. By the way, that part really needs clarification, because it can be trivially true - given indistinguishable cells, one to three cells or four cells on a plane are always mirror symmetric...
- lucideer 7y agoIf that's the definition of bilateral, the article title makes no sense as this property wouldn't preclude animals having three legs. So the definition intended in the article must be narrower.
- avar 7y agoNeither the article nor the paper makes that claim about bilateral symmetry. From the abstract of the paper: "As is reviewed here, the strongest constraint on the evolution of a triped is phylogenetic: namely, the early genetic adoption of a bilaterally symmetrical body plan occurring before the advent of limbs. Presumably, this would greatly constrain any three‐legged animal from ever evolving." In other words, an animal can have an odd number of legs and be bilateral, but due to how animals grow and develop it's less likely to evolve than an even number of legs.
- lucideer 7y agoFrom the article: > why are there no animals with three legs? That might go back a long, long way, Thomson said. > "Almost all animals are bilateral," he said. The code for having two sides to everything seems to have got embedded in our DNA very early in the evolution of life—perhaps before appendages like legs, fins or flippers even evolved. Once that trait for bilateral symmetry was baked in, it was hard to change. (emphases mine) Thomson suggests bilateral symmetry as the reason for not having three legs. It's strongly implied that they're mutually exclusive. Not sure how else you could read this excerpt.
- avar 7y agoBy "that trait" I believe he's referring to that particular way of getting to bilateral symmetry, not that an organism that doesn't do it like that can't be bilaterally symmetric. As an example, think of what it would take to add a third leg to a human being. Now we have two and one leg is a mirror image of the other. Those would presumably be the outer legs, how's the central leg going to look like? Is it going to be a copy of either the right or left leg, or some combination with 9 toes (4 outer, big toe in the middle) etc.? Carrying that sort of information in the DNA is going to be harder than following the default template of "make this here and mirror it", which is how most structures are encoded in DNA. Furthermore I think the reason the paper's abstract mentions phylogenetics (but I don't know, I can't access the full thing) is to say something to the effect that if an organism doesn't have any hands or legs, adding them in pairs of two is inherently simpler than adding three, since the rest of the body structure is already mostly following that template. Once you start going down that road it's hard to "go back", which is why modern animals carry a lot of structures that are only the way they are today because they evolved from some initial past state.
- klodolph 7y agoThe funny thing here is that the particular example is relevant, because sea stars and other echinoderms belong to the taxon Bilateria (look at their larvae!). And it illustrates the difficulty you run into when you try to use morphology or other features to derive your taxonomy. Cnidaria are an interesting comparison, because they are radially symmetric but not members of Bilateria. They are among the closest relatives to Bilateria. (People may recall the article about Ctenophores that was posted recently, which are also not in Bilateria, and are actually much more distant, even from Cnidarians.)
- sysrpl 7y agoOkay, well then what about flounder?
- duskwuff 7y agoBilaterally symmetric during the larval stage; the asymmetry develops during their life cycle.