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Thread: Why two?

  1. #1 Why two? 
    Forum Freshman Fozzie's Avatar
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    I'm interested in why evolution has finished up giving us two lungs and two kidneys. After all, we have only one heart, one pancreas and one liver. If the single kidney which had evolved was not working efficiently, surely the inefficient ones would have died out and the efficient one gone on to reproduce by improving on what they had, not grow another one? Under what conditions would a second one be necessary and what mechanism would be responsible to grow two in the first place?


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  3. #2  
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    Embryologically there is only one kidney or lungs/ many animals only have 1 lung. However they separate and migrate. In humans having two lungs is beneficial as if one lung collapses or contracts pneumonia this wont necessary happen to the other.

    As for the liver, there is only one as it the most efficient way of processing digestive product blood i.e. throught the hepatic portal vein.

    The pancreas interestingly has two origins embryologcically which join forming the single pancreas; deformities of the pancreas are usually usually as a dysfunction of this process.


    As for the argument for the reason for this, some is haphazard evolution. However kidneys and lungs require extra capacity for oxygenation, blood pH and pressure control and electrolyte balance.
    I would argue that the kidneys may be required in 2 as to supply 25% of the cardiac output to the kidneys would not be feasable in only one renal artery and would not allow good regulation of glomerular filtration rates or of blood pressure.

    Someone more read up on evolution may be able to give a more definitive answer as to how it happened.


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    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    It has to do with embryology, I don't remember the details though.

    Birds however have since loss one ovary/testy and one kidney.

    The kidneys and the reproductive organs are connected developmentally. I'm not a devo bio person so I don't really remember the details very much.

    The benefits of having two of these organs is obvious though, if you lose one somehow you don't die. For birds having only one allows them to be lighter and facilitate quick flight.

    Edit: Robbie beat me
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  5. #4  
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    The lung's a branching structure so conceptually you want as many lungs (alveoli) as can be packed into a given cavity... in a way. The cavity just happens to have a heart in the middle, so the lung straddles that.

    It's nice that one lung can be redundant. But I think the two is fine in good health precisely because there's little wasteful redundancy in multiple organs that branch anyway.

    The kidneys I dont' understand. They should be able to fit in there as a single organ. But their purposeful shape suggests an optimal size...?
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  6. #5  
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    Two is the minimum amount allowed (energetically cheapest to make and maintain) while still having a function fallback.
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  7. #6  
    Forum Masters Degree samcdkey's Avatar
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    Earthworms have five heart like structures. Octopus have three, and no lungs.

    I'd say we evolved to have one heart and two lungs because thats what survived.
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  8. #7  
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    There is no "reason" for any of this. reason assumes a rationality the preexists - that life moves toward. Unless one accepts a reasoning "creator" reason here does not exist.
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  9. #8 Re: Why two? 
    Forum Cosmic Wizard spuriousmonkey's Avatar
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    Quote Originally Posted by Fozzie
    surely the inefficient ones would have died out and the efficient one gone on to reproduce by improving on what they had, not grow another one?
    And what would be the reproductive advantage of the individual in dying before it can reproduce?
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    Quote Originally Posted by Jorge1907
    There is no "reason" for any of this. reason assumes a rationality the preexists - that life moves toward. Unless one accepts a reasoning "creator" reason here does not exist.
    uhh...how about survival and species propegation? That seems a rational reason enough to want to progress things, doesn't it?

    Also, and correct me if I'm mistaken here, but are you honestly saying that there is no rationality behind a favorable gene or structure being passed on from generation to generation?

    Speaking of things coming in twos and also kidneys, my girlfriend actually has 3 kidneys. I've also had one of the lizards I used to breed give birth (yes, live birth- placental too!) to a two headed baby wherein the split between heads appears to be in the thoracics and not in the cervicals.
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  11. #10  
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    Quote Originally Posted by mormoopid
    Quote Originally Posted by Jorge1907
    There is no "reason" for any of this. reason assumes a rationality the preexists - that life moves toward. Unless one accepts a reasoning "creator" reason here does not exist.
    uhh...how about survival and species propegation? That seems a rational reason enough to want to progress things, doesn't it?
    Reason is a construction of minds. Evolution has no reason or purpose because it has no mind. It's a process of nature. That which survives, survives. If a thing does not propagate (or is not stable despite that), then it is out competed by things that will propagate or will remain stable.

    Quote Originally Posted by mormoopid
    Also, and correct me if I'm mistaken here, but are you honestly saying that there is no rationality behind a favorable gene or structure being passed on from generation to generation?
    Of course there isn't. It just happens. Is there a rationality behind the persistence of stones in my garden? Is their hardness a trait they display for a reason? Are they striving towards some goal? Life is more complex but no different in terms of reason, rationality or goal. Persistence of the persistent is the rule, and survival of the fittest is merely one element of that.

    The upshot of this lack of foresight, reason or planning is that organisms often display survival characteristics which are "good enough" whilst being rather suboptimal. Often these take the form of rather convoluted exapted structures- and sometimes this means that we have backups of some organs (and genes) but not of others (and their relative importances do not seem to be considered). These features work just fine, but a designer could do better.
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  12. #11 Re: Why two? 
    Forum Cosmic Wizard spuriousmonkey's Avatar
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    Quote Originally Posted by Fozzie
    I'm interested in why evolution has finished up giving us two kidneys.
    Is it because we 'received' two ureteric buds?
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  13. #12  
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    Quote Originally Posted by i_feel_tiredsleepy
    embryology
    Circular explanation, isn't it?


    Regarding mormoopid's "reason", terms which could suggest intelligent design shouldn't be an issue among people who agree ID isn't happening. Therefore you can say having two heads is "a bad idea" and no morality or sentience is implied.
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    Oh okay well I always just assume the worst of people I guess
    except you poing 8)
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  15. #14  
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    Embryologically there is only one kidney or lungs/ many animals only have 1 lung. However they separate and migrate. In humans having two lungs is beneficial as if one lung collapses or contracts pneumonia this wont necessary happen to the other.
    This makes sense. Maybe two evolved in order to help people be better able to survive disease. Maybe it has to do with function..people are obviously better off having two lungs than one...
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  16. #15  
    Forum Masters Degree samcdkey's Avatar
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    You need two lungs to provide the surface area for optimal oxygen perfusion of the blood from normal cardiac output as well as under exertion. Are there cases of people living healthy lives with one lung?
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  17. #16  
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    Quote Originally Posted by samcdkey
    You need two lungs to provide the surface area for optimal oxygen perfusion of the blood from normal cardiac output as well as under exertion. Are there cases of people living healthy lives with one lung?

    Or 3?
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  18. #17  
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    One lung doesn't fit well anyway given a central heart. That's some explanation.

    How about one big nostril? Even elephants have two, with two tubes.
    A pong by any other name is still a pong. -williampinn
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  19. #18  
    Forum Cosmic Wizard spuriousmonkey's Avatar
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    Quote Originally Posted by gottspieler
    Embryologically there is only one kidney or lungs/ many animals only have 1 lung. However they separate and migrate. In humans having two lungs is beneficial as if one lung collapses or contracts pneumonia this wont necessary happen to the other.
    This makes sense. Maybe two evolved in order to help people be better able to survive disease. Maybe it has to do with function..people are obviously better off having two lungs than one...
    I am so happy this made sense.

    However, "embryologically" we have two kidneys.

    So don't rely on sense to make sense of other people's replies.
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