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What is the evolutionary advantage with two lungs (kidneys)? Most living beings only have one heart, one stomach. Most internal organs are not doubled and if one lung fails it is not exactly quite transparent due to having an extra lung. Before modern health care one failed lung would kill the individual.

The same can probably be said for kidneys.

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    $\begingroup$ Evolution does not have a purpose. As for why things evolved that way, most animals are essentially tubes. It would seem to take very little evolutionary change - basically a few mutations to the HOX genes - to create bilateral symmetry. $\endgroup$
    – jamesqf
    Mar 6 at 3:59
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    $\begingroup$ The heart is two-sided, each side doing pretty much the same thing, much like the lungs and kidneys. Could easily be separate, though wouldn't work as well together. Also, you should cite sources for your claims. $\endgroup$ Mar 6 at 5:47
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    $\begingroup$ Does this answer your question? Why do some bad traits evolve, and good ones don't? $\endgroup$
    – kmm
    Mar 6 at 14:02
  • $\begingroup$ most organs come in pairs in vertebrates, (the heart starts as two separate organs embryonically) the digestive tract organs are the exceptions, you may want to reframe your question about that. $\endgroup$
    – John
    May 29 at 4:47
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Humans and most organisms have bilateral symmetry. Anything that is on the left side of the body will be on the right also. Not everything can be in the middle, so ... you end up with two. This is evolutionarily labile - for example, fish generally have one dorsal swim bladder, though it is derived from the lungs. In a similar manner, the heart is formed by the fusion of primordia on either side during development - see UNSW for a cute video.

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  • $\begingroup$ -1. Your thesis does not stand. The heart and other organs like spleen does not have bilateral symmetry. $\endgroup$
    – Hans
    Mar 7 at 9:20
  • $\begingroup$ Bilateral symmetry is of course an approximation - biology doesn't know theory. But it is a good one. For example, the spleen begins from the dorsal mesogastrium (the mesentery connected to the greater curvature of the stomach), making it, conceptually, an unpaired midline organ that merely reacts to surrounding circumstances. In some humans the situation is more bilateral $\endgroup$ Mar 7 at 14:11
  • $\begingroup$ Bilateral symmetry goes much further that the internal organs, of course, so bilateralism must be deeply embedded in the genes. (Even structures like vertebrae are symmetric.) So the question really should be why some few organs AREN'T paired or symmetric. $\endgroup$
    – jamesqf
    Mar 7 at 17:45
  • $\begingroup$ As I said, "other organs" in plural, with spleen being but one example, which do not have bilateral symmetry. What is the mechanism of all the other non-bilateral symmetric organs? With respect to the spleen in particular, it just begs the question: what makes the stomach non-bilateral symmetric? You can say they just compete for the same spot and randomly eking out a space and deforming each other. Why do all of them not have two or more which can all be arranged bilaterally? Another question is why we do not have more equal distribution of people having their hearts and spleen situated $\endgroup$
    – Hans
    Mar 7 at 18:36
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    $\begingroup$ @Hans the heart starts as two unconnected blood vessels. this may help open.oregonstate.education/aandp/chapter/… I have a whole answer in the unclosed question about this. biology.stackexchange.com/questions/100928/… Your questions about bilateral symmetry are great questions I suggest asking them as a stack questions (look for identical questions first) but in short is comes down to shoving 30ft of digestive tract in 2 feet of torso and the fact we are no longer laterally narrow fish. $\endgroup$
    – John
    Jun 1 at 0:01
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Two of the duplicated organs work better (e.g.: also kidney) or even enable something new (e.g.: depth perception for eyes). Source

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  • $\begingroup$ Two hearts? Two brains? $\endgroup$
    – d-b
    Mar 6 at 11:00
  • $\begingroup$ Same applies to animals with multiple hearts or brains. mentalfloss.com/article/52337/3-creatures-more-one-heart And medium.com/illumination/… $\endgroup$
    – tsttst
    Mar 6 at 13:23
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    $\begingroup$ @d-b: Vertebrates do basically have two brains, separate left and right hemispheres with interconnections between them. $\endgroup$
    – jamesqf
    Mar 6 at 17:32
  • $\begingroup$ -1 for the falsehood. 1. Why is 2 the optimal rather than 3 or 4 or more like the number of teeth? 2. It is not true for other organs like the heart and spleen. $\endgroup$
    – Hans
    Mar 7 at 9:24
  • $\begingroup$ My answer addresses OP’s question (what are advantages of multiple organs). The question is not, why different animals all are not the same / why they occupy a given niche in the evolutionary landscape (and hence experience different tradeoffs). Additionally Mike Serfas great answer well explains the developmental possibility of two organs. $\endgroup$
    – tsttst
    Mar 7 at 14:31
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Eels have two lungs and so do lungfish and all vertebrates, so the design is at least 375 million years old, although vertebrate lungs have developed different air flow systems, i.e. birds and iguanas. No vertebrates have a very small and big lung as far as I know.

  • Rib punctures can affect one lung which fills with fluid.
  • Stick and tooth punctures can actually cause air to escape the lung through the chest, a condition that can sometimes be survived in th wild, but less likely with only one lung.
  • Pulmonary infections can cause a lung to fill with water, so having two lungs gives some extra protection.
  • Breathed in objects like twigs can sit inside a lung for many years and cause weakness, for example a man coughed up a twig from his lung 14 years after having breathed it in while playing in a hedge with friends.
  • Two lungs may also bring higher surface area and higher performance than one lung.
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  • $\begingroup$ Most of your points are deadly in a non-civilization (i.e., for all animals except humans living in a society that offers some surplus food etc) context, even if you have a spare lung. Re the surface are, that is why lungs are "wrinkled", and that works fine with a single lung. $\endgroup$
    – d-b
    Mar 7 at 9:07
  • $\begingroup$ -1. None of your points answer the question. Regarding the last point, the fractal-like structure of each lung already increases the surface. It is not necessary to have two to increase the surface. $\endgroup$
    – Hans
    Mar 7 at 9:28
  • $\begingroup$ Actually, if your lung fills with water from a stick/rib/pneumonia, you will die irrespective of the fractal structure, which is not part of the question, but having two lungs will stop the liquid spilling into the second lung and choking the vertebrate in question, and two hemispherical spheres have a surface area of 3.14+1.4, i.e. 30% bigger. $\endgroup$ Mar 7 at 16:17
  • $\begingroup$ @d-b Humans have only lived as a civilization for 10,000 years, previous to that they were hunter gatherers with very low populations because the surplus-giving crops did not exist, and they inherited their lungs from other primates and the primates from basal placental animals.... Two hemispheres have 30% more surface area to place the wrinkles on in the first place, so a 5 liter double-hemisphere lung is equivalent to a 7.5 liter single spherical lung, approximately. $\endgroup$ Mar 7 at 16:25
  • $\begingroup$ Given the total volume and a geometric shape the more isolated domain of the same size one makes the more surface area one gets in the form of $n^{1/3}$ where $n$ denotes the number of isolated geometry. So for $n=2$ there should be $2^{1/3}-1~26%$ more area. It is not clear what you are computing and where 3.14+1.4 comes from. If you split a sphere into two hemispheres, the area increases by 50%, not 30%. More importantly, it is not the outer shape and area we are looking at. It is the ultimate (inner) surface shapes, i.e, alveola sacs we are talking about, $\endgroup$
    – Hans
    Mar 7 at 18:28

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