Why are there no animals with three legs?
phys.org
phys.org
What about starfish and other echinoderms? They may not have exactly three limbs, but they aren't bilaterally symmetrical, and they are animals.
Apparently they're bilateral as juveniles and only become radially symmetric as adults (https://en.wikipedia.org/wiki/Bilateria)
"Echinodermata [is the] second-largest grouping of deuterostomes (a superphylum), after the chordates...Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives."
For instance, a creature with a tail is still bilaterally symmeteric. Us human males are still bilaterally symmetric. A full third leg would still be bilaterally symmeteric.
But if the kangooroo tail counts as a leg, it's indeed very different, but symmetric.
Not necessarily. You could have 2 legs on each side, and a symmetrical looking leg (e.g. hooves, or human like but with 2 toes on each side) on the center.
But according to what I was just reading, the way a five limbed radially symmetric creature develops is not by growing two halves where each half is two and a half limbs.
What echinoderms supposedly do is the left half of their body grows disproportionately and the right half gets absorbed, and all the limbs grow from the left half.
In any case, if you do think it possible to also grow a third leg by halves, then presumably you are even more in agreement that bilateral symmetry does not prevent odd numbers of legs.
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.
"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.
> 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.
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.
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.
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.)
It was pretty good at this. Great fun for experimenting with locomotion.
It's pretty primitive if you go back and play it, but the locomotion system is something I haven't seen in any other game.
The title leads me to believe that I will find an answer to that question in the article, but it just talks about the subject for a while until they decide to end it.
The article was good though, so perhaps just a different title?
https://www.discoverwildlife.com/animal-facts/mammals/how-ma...
and "run" 2 legged https://youtu.be/ftgY63SlmKY?t=67
After reading the article I see that it even does not explain what it asks in the title apart from mentioning what it "seems" to be caused by.
The code for having two sides to everything --seems-- to have got embedded in our DNA very early in the evolution of life
I'm also pretty sure I've seen some three-legged animals in my lifetime, albeit it was because they lost a limb or because it didn't grow properly.
In any case the octopus is a pretty different kind of creature.