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Why are there no animals with three legs?
- perl4ever 7y ago"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." What about starfish and other echinoderms? They may not have exactly three limbs, but they aren't bilaterally symmetrical, and they are animals.
- mishac 7y agoWeirdly, echinoderms are considered part of the Bilateria clade. Apparently they're bilateral as juveniles and only become radially symmetric as adults (https://en.wikipedia.org/wiki/Bilateria https://en.wikipedia.org/wiki/Bilateria)
- elliekelly 7y agoWhoa, it does actually look like a little biped flubber: http://science-at-home.org/wp-content/uploads/2009/12/starfish_larva.jpg http://science-at-home.org/wp-content/uploads/2009/12/starfi...
- cjensen 7y agoYep. If you look carefully at a most starfish, you can see that they are bilaterally symmetric. For example, when there are five legs, if you mentally halve the animal along a leg, only one of the five legs results in both sides matching.
- gowld 7y agoThat still undermines the articles thesis and that an odd leg can't be bilateral.
- kittiepryde 7y agoI have one head and four appendages (two for each side, but only one head bilateral down the middle). I don't see why they couldn't have a leg, bilateral down the middle ( like a head?)
- fla 7y agoOr a tail
- smackay 7y agoAnyone know if there is "handedness" in tails, particularly in monkeys where they use their tails while climbing. Do individuals always curl their tails clockwise or anticlockwise around a branch?
- e12e 7y agoWell, technically you have a tail too. I suppose a better description would be a spine, stunted in one end (tailbone), extended at the top (head), with two set of limb pairs (arms, legs).
- matt-attack 7y agoWell said and excellent point.
- eternalny1 7y agoHence the "almost all".
- perl4ever 7y agoWell, if I were a starfish, I would take issue with that dismissive phrase. They are members of a large, and, I assume, proud phylum. "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." https://en.wikipedia.org/wiki/Echinoderm https://en.wikipedia.org/wiki/Echinoderm
- perl4ever 7y agoWell, that's exactly what proves the point that you could evolve three legged animals from bilaterally symmetric ones.
- robbrown451 7y agoBut he's right in that echinoderms split off before much in the way of legs, fins and flippers evolved. (despite technically being members of Bilateria as someone else points out)
- elif 7y agoThis line confused me as a place to end the answer to this question. 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.
- Aardwolf 7y agoIt would need toes of the same size on each side and a symmetric knee plus muscles to control it, I think it would need a leg and foot or hoove design completely independent from the outer ones, it couldn't copy or reuse the existing design, so less likely to evolve. But if the kangooroo tail counts as a leg, it's indeed very different, but symmetric.
- kijin 7y agoYeah, it would seem that we have plenty of three-legged animals already. A stout tail is what you get when you try to make a third leg that is perfectly symmetric and optimized for being positioned in the middle.
- coldtea 7y ago>A full third leg would still be bilaterally symmeteric. 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.
- perl4ever 7y agoYes, things like starfish start out as bilaterally symmetric, both developmentally and evolutionarily. 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.
- avar 7y agoYou'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.
- JoshTko 7y agoDifference in environment. And odd numbered appendage on land is costly and does not provide a speed advantage (Enough to offset cost). In sea, locomotion is completely different and an odd number appendage could linearly provide benefit commensurate with cost.
- jedberg 7y agoI wonder if anyone has tried an evolutionary locomotion algorithm that has bi-pedal and tri-pedal animals compete. See if it can even come up with a reasonable try-pedal model.
- Bootwizard 7y agoHave you never played Spore? :) It was pretty good at this. Great fun for experimenting with locomotion.
- monkeydreams 7y agoEvolutionary pressure was never baked into the final game - it was all intelligent design. This was a missed opportunity, in my opinion.
- toxik 7y agoSpore share a surprisingly similar trajectory to No Man's Sky. Overpromised, underdelivered. Hey, I guess that's just like AI!
- Bootwizard 7y agoThat's true, but it did figure out the locomotion for you no matter what way you designed it. I ran into very few creatures that couldn't move. 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.
- Baeocystin 7y agoI saw an early (think mid-90's) attempt at this when I was in college. The only tripod that was successful was something along the lines of what kangaroos do, hopping with two while using a robust tail as a spring.
- NohatCoder 7y agoMost quadroped mammals can run by moving their legs in pairs, in this mode one of the pairs could be replaced by a single leg. Would make normal walking awkward though.
- java-man 7y agoThere were attempts to evolve, during Ediacaran period. [0] https://en.wikipedia.org/wiki/Tribrachidium https://en.wikipedia.org/wiki/Tribrachidium [1] https://en.wikipedia.org/wiki/Trilobozoa https://en.wikipedia.org/wiki/Trilobozoa
- locusofself 7y agofascinating .. so there we go. there was an attempt!
- 3leggedcatman 7y agoMy cat has three legs.
- saagarjha 7y agoBut cats, in general, don’t.
- reportgunner 7y agoI don't understand the downvotes, the title of the article is pretty poor indeed. 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.
- joe_the_user 7y agoThe Kangaroo moves with two legs and a tail. How close is that to three legs? Kangaroos hop with two feet but use their tail for slower movement.
- Kaibeezy 7y agoPlenty of fictional tripeds. Watch this Idiran go... https://m.youtube.com/watch?v=iC6sJ-Ysb2Y https://m.youtube.com/watch?v=iC6sJ-Ysb2Y
- ganitarashid 7y agoBecause you can’t walk on three legs
- no1youknowz 7y agoJake the Peg, did just that [0]. [0]: https://www.youtube.com/watch?v=KJleJbn9G6Y https://www.youtube.com/watch?v=KJleJbn9G6Y
- aurbano 7y agoI actually dislike this style of article writing, or title writing. 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?
- _nalply 7y agoI think that this is a trick to make people stay with the website. This works similar to a cliffhanger.
- meggar 7y agoTechnically a centipede does have 3 legs.
- PeterStuer 7y agoSome do consider the Kangaroe to be a 3 legged species https://www.discoverwildlife.com/animal-facts/mammals/how-many-legs-does-a-kangaroo-have/ https://www.discoverwildlife.com/animal-facts/mammals/how-ma...
- anotheryou 7y agoWell I'd say they walk 5-legged https://youtu.be/HWiLyIqcK24?t=12 https://youtu.be/HWiLyIqcK24?t=12 and "run" 2 legged https://youtu.be/ftgY63SlmKY?t=67 https://youtu.be/ftgY63SlmKY?t=67 https://youtu.be/S-V2MqNLgdg?t=14 https://youtu.be/S-V2MqNLgdg?t=14
- RenRav 7y agoIf "aquatic locomotion" counts... seals, walruses, dugongs, manatees. Look at their skeletons if you have time, it's pretty interesting.
- snissn 7y agoWhales and dolphins have fused legs and if you count their two flippers they have 3 legs.
- deleted 7y ago[deleted]
- jjtheblunt 7y agoTarantulas have 3 legs, with 5 spares, i thought, having just seen one this morning out with our dog, though i realize that's a technicality wrt the posed question.
- cellular 7y ago"The code for having two sides to everything seems to have got embedded in our DNA "... Heart, liver, gallbladder
- cellular 7y agoI've always wondered how birds evolved to fly. It seems like wings are useless until they can fly.
- ken 7y agohttps://en.wikipedia.org/wiki/Origin_of_avian_flight https://en.wikipedia.org/wiki/Origin_of_avian_flight
- m463 7y agoI thought the octopus has 3 brains, but it might not. looking it up, it might have 9 brains and 3 hearts. In any case the octopus is a pretty different kind of creature.