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We have two eyes, but we don't have two hearts. Why do humans have two of some organs, but not all?

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    $\begingroup$ I think the question is answerable, so long as OP doesn't want to know about EVERY organ! $\endgroup$
    – James
    Dec 5, 2014 at 3:38
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    $\begingroup$ Several processes come into play here, e.g. 1) functional redundancy, 2) evolutionary contingency, 3) energetic costs of redundant organs and 4) developmental constraints (related to evolutionary contingencies). $\endgroup$ Dec 5, 2014 at 9:36
  • $\begingroup$ Also the fact that you can easily use some of the already developed genes . $\endgroup$
    – Probably
    Apr 26, 2016 at 15:44
  • $\begingroup$ We don't have that many double organs (en.wikipedia.org/wiki/List_of_organs_of_the_human_body): Most of musculoskeletal system. Digestive: salivary glands. Respiratory: bronchi/lungs. Urinary: kidneys&ureters. Reproductive: ovaries/testes + tubes. Endocrine: parathyroids (four even), adrenals. Circulatory: Mostly two for minor vessels, lymph nodes, bone marrow. Nervous system: Mostly two in PNS, two eyes & ears. Plus mammary glands. Everything else usually exists only once (though possibly symmetrical). $\endgroup$
    – Armatus
    May 3, 2016 at 13:15
  • $\begingroup$ The body appears so symmetrical mainly due to the musculoskeletal system and a handful of large organs. $\endgroup$
    – Armatus
    May 3, 2016 at 13:18

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Here is my overly succinct answer.

I doubt we will ever know this for sure. But, it basically comes down to ancestral bilateral symmetry in development; this defaults to two organs and is broadly symmetrical except where the organs are central. See this article for a more thorough answer regarding bilateral symmetry.

enter image description here

Further exceptions to symmetry occur where evolution pressured the body into not bothering to grow the second of an organ due to a waste of resources for the body, or functional advantages emerged from asymmetrical evolution. I always like the ears of an owl for an excellent example of two asymmetrical features that provided a discrete advantage when it comes to locating the source of a sound.

skull of an owl showing one ear hole is higher on the skull than the other.

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    $\begingroup$ @DavidBlomstrom The function reasoning doesn't explain how or why two organs are generally prevalent. Using function as an explanation for evolution is the wrong way around and arguably is retro-causation. You could say "symmetry resulted in two eyes and natural selection kept them for functional advantage" but not "We got two eyes for functional advantage and hence symmetry emerged." $\endgroup$
    – James
    May 3, 2016 at 9:52
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    $\begingroup$ The heart is centrally located in fish. And when you look at how the heart develops... it is symmetrical until it starts twisting upon itself and growing partitions in its middle. <google.com/…> $\endgroup$
    – JayCkat
    Jan 13, 2017 at 7:26
  • $\begingroup$ How about octopuses? Octopuses have 3 hearts $\endgroup$
    – Pablo
    Nov 8, 2018 at 20:07
  • $\begingroup$ @Pablo People dedicate their lives to studying the evolution & function of the octopus anatomy! You could ask a new question on some specific piece about that. Let's not forget that the OP is aiming to find out about why human anatomy seems to have one or two of each organ. $\endgroup$
    – James
    Nov 13, 2018 at 12:03
  • $\begingroup$ I didnt mean to make the question about octopuses. What I meant was, if humans have one heart because they only have duplication when the organs arent centrals, shouldnt octopuses have one heart also? The comment is meant to question if humans have only one heart because the heart is in the middle , considering that rule doesnt seem to work in other species $\endgroup$
    – Pablo
    Nov 13, 2018 at 12:59
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For some organs, the function is really dependent on having two: two eyes offer the possibility to appreciate distances, two ears to appreciate directionality of sound and to balance oneself.

Bilateral symmetry is both a good way to obtain these twinned organs, and may also be physically easier to obtain in the course of evolution (see the work by Vincent Fleury, University Paris Diderot, e.g. http://www.ncbi.nlm.nih.gov/pubmed/21784360). Perhaps this is mechanically quite advantageous to have symmetry for balance reasons. You'll note that the vehicles designed by the human are mostly symmetrical too.

This may explain why there are two lungs, e.g. Two brain hemispheres are justified with a symmetric body plan as many anatomical features are mapped directly in the brain geometry (see Fig 5.6 in http://www.bem.fi/book/05/05.htm, of course sensory and motor are in both hemisphere). But evolution has also been able to depart from this symmetrical plan when having twin organs would be more of a coordination nightmare: e.g., two hearts would make it difficult to have a sensible circulatory system (at least some dyssymetrisation would be needed so that they would direct the flow in the same direction! [no reference on that, you can draw a symmetric circuit with two pumps and see for yourself])

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  • $\begingroup$ Edited. As the post covers a number of subject, it'd be good to know where the moderator feels more references are needed. $\endgroup$
    – Joce
    Oct 13, 2015 at 15:56
  • $\begingroup$ @DavidBlomstrom The function reasoning doesn't explain how or why two organs are generally prevalent. Using function as an explanation for evolution is the wrong way around and arguably is retro-causation. You could say "symmetry resulted in two eyes and natural selection kept them for functional advantage" but not "We got two eyes for functional advantage and hence symmetry emerged." $\endgroup$
    – James
    May 3, 2016 at 8:10

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