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This question already has an answer here:

The human body is amazing. But there seems to be some evolutionary changes that appear to not make sense regarding redundancy.

While not necessarily the primary reason for their existence, the human body has a great deal of redundancy.

  • Two eyes.

  • Two arms.

  • Two legs
  • Two ears
  • Two nostrils
  • Two lungs
  • Two kidneys
  • Two sets of teeth
  • A very large liver that can work in part.
  • Two brain hemispheres that can (to some degree) do the work of the other.
  • An appendix that is not critical.

But why do we not have two hearts? It would seem like a good idea to have two smaller hearts, together they work above optimally, but survival would be possible with one.

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marked as duplicate by anongoodnurse, WYSIWYG Oct 13 '15 at 7:21

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Having two of almost all of those body parts is absolutely better than one i.e there is no redundancy. Even if a smaller liver can sustain a human, a bigger one is more tolerant to potentially harmful toxins. The brain can reassign parts of the brain to an incredible degree after a hemispherectomy, but not completely depending on age and type of trauma. It also looks like our appendixes are important for our digestion system too. $\endgroup$ – James Oct 13 '15 at 5:15
  • $\begingroup$ Two atria and Two ventricles.... $\endgroup$ – AMR Oct 13 '15 at 6:37
  • $\begingroup$ One esophagus, one trachea, one stomach, one spleen, one thymus, one small intestine, one large intestine, one thyroid, one spinal cord, one diaphragm, one penis (apologies to those with Diphalia), one tongue, one vagina, one clitoris, one cervix, one uterus, one urethra, one umbilicus, one rectum, one anus... what's your point? $\endgroup$ – AMR Oct 13 '15 at 6:56
  • $\begingroup$ @AMR If you're referring to my comment re: two is better, I should have prefaced it with the ancestral symmetry answer. This question is flawed in my opinion because it assumes symetrical anatomy has absolute redundancy. These symmetrical body parts work together to perform optimally with their partner and are not redundant. The question requires a fairly speculative answer based on false assumptions that we may never be able to scientifically test. $\endgroup$ – James Oct 13 '15 at 9:44
  • $\begingroup$ @GoodGravy oh heaven's no... I was not commenting about your comment... It was an observation about the question... $\endgroup$ – AMR Oct 13 '15 at 17:25
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It is not about redundancy. There are evolutionary and developmental reasons why we have two eyes or two legs and one heart. However just to give you short examples, two eyes are better than one because of our binocular vision while we have two legs because of its locomotor energetics.

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