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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 '14 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). $\endgroup$ – fileunderwater Dec 5 '14 at 9:36
  • $\begingroup$ Also the fact that you can easily use some of the already developed genes . $\endgroup$ – Probably Apr 26 '16 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 '16 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 '16 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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    $\begingroup$ Sorry, but I don't agree. The brain is centrally located, but not the heart. I think symmetry is part of the answer, but function is also an important consideration. $\endgroup$ – David Blomstrom Apr 26 '16 at 3:11
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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 '16 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 '17 at 7:26
  • $\begingroup$ How about octopuses? Octopuses have 3 hearts $\endgroup$ – Pablo Nov 8 '18 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 '18 at 12:03
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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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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

  • $\begingroup$ 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 '15 at 15:56
  • $\begingroup$ I think Joce nailed it. Though symmetry may be part of the answer, one can't escape functionality. We wouldn't have depth perception if we had just one eye. On the other hand, two brains would be an obvious example of overkill. $\endgroup$ – David Blomstrom Apr 26 '16 at 3:08
  • $\begingroup$ @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 '16 at 8:10
  • $\begingroup$ All vertebrates possess bilateral symmetry. Yet various major organs occur in both pairs and alone. For example, no vertebrate has more than one tail, and rhinos don't have paired horns. But a whale with a single flipper on just one side of its body would obviously be ridiculous - not just from a standpoint of symmetry but (and probably most important) from a functional perspective as well. $\endgroup$ – David Blomstrom May 7 '16 at 2:08

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