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Is there any explanation why we have two brain hemispheres? Is there any advantage of having two in contrast to having one (or three / four)?

From what I've read so far (not too much; I'm not a biologist) each half can take the job of the other half when it comes to injuries. Is that correct?

Are there animals which have more than two hemispheres?

edit: I have seen What was the evolutionary reason for cross lateralization of the brain?, but that question seems to be about the question why the left hemisphere controls right body parts and the right hemisphere controls left body parts. My question is why there are two hemispheres at all.

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    $\begingroup$ Possible duplicate of What was the evolutionary reason for cross lateralization of the brain? $\endgroup$ – Remi.b Jan 14 '16 at 17:36
  • $\begingroup$ You will probably want to read Do the Left Brain and Right Brain have different functions? too. $\endgroup$ – Remi.b Jan 14 '16 at 17:37
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    $\begingroup$ Bilateral symmetry evolved to be the predominant body plan. According to Wikipedia it accounts for about 99% of all animals. And while this is pure opinion, it kind of makes sense when you consider that almost everything starts from a single cell and we divide in two. And as the genes that control body plan are highly conserved in most organisms, then the simplest way to form a multicellular body that starts from a single cell and grows by dividing in half many times is to have two sides. $\endgroup$ – AMR Jan 14 '16 at 18:17
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    $\begingroup$ "From what I've read so far (not too much; I'm not a biologist) each half can take the job of the other half when it comes to injuries. Is that correct?" The brain does have plasticity, but there are limits, and if you have passed certain developmental targets, then it becomes very difficult to compensate. Over long periods of time and a lot of occupational therapy, some people have recovered some of the functionality, but it depends a great deal on where in the brain the damage was done. $\endgroup$ – AMR Jan 14 '16 at 20:24

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