If one kidney can function just as well as two, why do humans have two kidneys? The cost of growing two kidneys must surely be quite high, especially since one kidney is all that is really needed.

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    $\begingroup$ Perhaps because humans are pretty much bilaterally symmetric, and have two of almost everything: arms, legs, eyes, lungs... even two brain lobes. A better question might be why we don't have two hearts or livers... $\endgroup$
    – jamesqf
    Feb 12 '18 at 6:02
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    $\begingroup$ All vertebrates have two kidneys. There's no selection pressure to reduce to one. $\endgroup$ Feb 12 '18 at 7:09
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    $\begingroup$ @james the answer to that is kinda cool you might want to ask it, it has to do with the fact the ancestral circulatory system is not paired but one way with a single central vessel going in each direction, and the heart evolved from a portion of the vessel. the liver as part of the digestive system has a similar reason. $\endgroup$
    – John
    Feb 12 '18 at 16:09

One of the easiest answers to the question; "Why do we have ____?" is "Because our ancestors did". This is not trivial or flippant, as it is a significantly important answer that is so often overlooked. Humans are members of Bilataria; a deep branch of the animal tree that is characterized by bilateral symmetry. Humans are typical of other vertebrates in having two kidneys. Selective pressure to reduce to one of two organs is typical in snakes, whose body form favors reduction. But even snakes have two kidneys. Natural selection does not reduce features we don't need unless such a reduction would increase fitness.

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    $\begingroup$ So are you saying it would cost more energy-wise to evolve to one kidney and possibly restructure the body than it would save to only have to grow and maintain one? $\endgroup$ Feb 15 '18 at 5:41
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    $\begingroup$ Its about dying before you can produce all the offspring possible. So if the second kidney does not kill you, there would be no need to optimize the numbers. How would a single kidney result in more offspring? "Energy savings" would be pretty slim. $\endgroup$
    – Karl Kjer
    Feb 15 '18 at 10:58

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