Why are there no three limbed animals?

Discussion in 'Biology & Genetics' started by esbo, Sep 16, 2011.

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  1. esbo

    esbo Registered Senior Member

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    Strange there are none.
     
    esbo, Sep 16, 2011
    #1

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  3. Orleander

    Orleander OH JOY!!!! Valued Senior Member

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    are there any odd numbered limbed creatures? insects have 6, octopus have 8...
     
    Orleander, Sep 16, 2011
    #2

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  5. Hercules Rockefeller

    Hercules Rockefeller Beatings will continue until morale improves. Moderator

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    No, it’s not really strange when you consider that from about starfish/sponges onwards, animal evolution adopted a bilateral symmetry in its embryonic development.
     
    Hercules Rockefeller, Sep 16, 2011
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  7. esbo

    esbo Registered Senior Member

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    That's a circular argument, evolution did not evolve 3 limbed animals because in the past evolution did not evolve any 3 legged animals!!

    It's not really an argument or explanation at all, it's basically repeating the question but omitting the 'Why' at the start!!

    The question why, saying that is how is it is not any answer!!
     
    esbo, Sep 16, 2011
    #4
  8. Hercules Rockefeller

    Hercules Rockefeller Beatings will continue until morale improves. Moderator

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    That’s not an accurate summation of my post.


    Okay, calm down. What’s with all the exclamation marks?

    I don’t see why it’s a circular argument. Bilateral development occurred very early in the evolution of the animal kingdom. Thus, the vast majority of the extinct and extant animal species are bilaterally symmetrical with, as a consequence, an even number of appendages. To me this seems like an answer to your question as to why there aren’t three-limbed animals.

    If you want to shift the goal posts and ask why bilateral development evolved, then fine. Do so. (Hopefully without getting unnecessarily aggressive in the process.)
     
    Hercules Rockefeller, Sep 16, 2011
    #5
  9. Trippy

    Trippy ALEA IACTA EST Staff Member

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    "Because Odd number plus (same) Odd number equals even number" is circular? :head scratch:
     
    Trippy, Sep 16, 2011
    #6
  10. esbo

    esbo Registered Senior Member

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    It is a circular argument.
    If I said why are their no palm trees in Antarctica saying "Well there have never been any there", is a pretty poor answer.
    I am not shifting the goal posts, the exclamation marks are for the lameness of the answer.
    You can answer any question with such flippant answers.
    You might as well say because there aren't any!!

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    esbo, Sep 16, 2011
    #7
  11. Trippy

    Trippy ALEA IACTA EST Staff Member

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    No it isn't.

    Oh, and incidentally...

    Bathypterois grallator:

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    Note that it is balancing itself on modified fins.

    Also note, that we could consider a prehensile tail in the same way.

    The point being that odd numbers of limbs (like a starfish) requires radial symmetry, with the body segments arranged around a central axis. All the higher animals that we know of use bilateral symmetry - essentially the body segments are arranged along a central axis.

    The only way to get an additional (odd) limb would be as an extension of the central axis - IE a prehensile tail.
     
    Trippy, Sep 16, 2011
    #8
  12. Cifo

    Cifo Day destroys the night, Registered Senior Member

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    Lots of starfish.
     
    Cifo, Sep 16, 2011
    #9
  13. MacGyver1968

    MacGyver1968 Fixin' Shit that Ain't Broke Valued Senior Member

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    I have a third leg.
     
    MacGyver1968, Sep 16, 2011
    #10
  14. phlogistician

    phlogistician Banned Banned

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    Starfish.
     
    phlogistician, Sep 16, 2011
    #11
  15. phlogistician

    phlogistician Banned Banned

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    Esbo got PWNED!
     
    phlogistician, Sep 16, 2011
    #12
  16. cosmictraveler

    cosmictraveler Be kind to yourself always. Valued Senior Member

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    cosmictraveler, Sep 16, 2011
    #13
  17. billvon

    billvon Valued Senior Member

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    There are three "legged" snakes. (Two legs and a big tail.) Many animals have five limbs (two arms, two legs, tail.)
     
    billvon, Sep 16, 2011
    #14
  18. esbo

    esbo Registered Senior Member

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    You seem to assumed bilateral symmetry is a God given thing but you have failed to explain why.

    I am surprised at how many people think this kind of answer is a valid one.
     
    esbo, Sep 16, 2011
    #15
  19. Trippy

    Trippy ALEA IACTA EST Staff Member

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    And you seem to have avoided addressing actual responses - for example, I gave you an example of a species that is for all intents and purposes 'three limbed'.

    But, once again, your assertions seem to be founded in ignorance.

    Let me ask you a question. Why should there be three limbed animals? What makes you think they should have evolved in the first place.

    Bilateral Symmetry
    In other words, bi-lateral symmetry occurs because breaking it requires further information than achieving it - yet more proof supporting my 'Lazy Designer' theory.
     
    Trippy, Sep 16, 2011
    #16
  20. Hercules Rockefeller

    Hercules Rockefeller Beatings will continue until morale improves. Moderator

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    From a developmental biology point of view a tail is not a limb. A tail is merely an extension of the existing spine formed during somitogenesis, whereas a limb requires limb bud formation and additional skeletal elements.
     
    Hercules Rockefeller, Sep 16, 2011
    #17
  21. Orleander

    Orleander OH JOY!!!! Valued Senior Member

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    Yeah, I was watching favourite show and realized.....5

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    Orleander, Sep 17, 2011
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  22. visceral_instinct

    visceral_instinct Monkey see, monkey denigrate Valued Senior Member

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    What others said - non-bilateral symmetry simply didn't happen to emerge in higher animals.

    Also, odd numbered limbs even if they happened as part of evolution would be very awkward and pretty much a disadvantage. Good luck running from a predator on 3 legs...
     
    visceral_instinct, Sep 17, 2011
    #19
  23. Orleander

    Orleander OH JOY!!!! Valued Senior Member

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    we do have 5 fingers and toes...
     
    Orleander, Sep 17, 2011
    #20
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