4 ms·

Neither 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

by avar 7y ago

Neither 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 ago
From 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 ago
  By "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.
  - perl4ever 7y ago
    "As an example, think of what it would take to add a third leg to a human being" Apparently the way radial symmetry actually develops is that one side of the body grows and the other withers away, and the remaining side produces the radial parts.

by avar 7y ago

lucideer 7y ago

From 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 ago
By "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.
- perl4ever 7y ago
  "As an example, think of what it would take to add a third leg to a human being" Apparently the way radial symmetry actually develops is that one side of the body grows and the other withers away, and the remaining side produces the radial parts.