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Why does the left hemisphere control the right and the right hemisphere control the left? I googled it but didn't find a good answer regarding this. Could someone explain? Does this adaptation help in the speed of transmission of nerve impulses?

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I'd bet it's non-adaptive. –  Noah Snyder Oct 5 '12 at 15:08
    
You seem to accept that we have two brain hemispheres that each control one half of the body without questionning this fact and then ask why does the left part control the right part and vice-versa. If we do so, as @NoahSnyder, I would tend to think that such thing is not adaptive because I think there were two possible solutions (cross wiring or no same-side wiring) that are equally fit and one was randomly chosen! –  Remi.b Jan 1 at 18:58
    
See also: cogsci.stackexchange.com/q/5771/301 –  Memming Feb 20 at 0:09
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3 Answers

Just to get the ball rolling here. This particular aspect of brain evolution is very old. The cross wiring of the hemispheres of the brain seems to be as old as the right and left hemispheres itself. It predates lizards - i.e. hundreds of millions of years ago.

Digging back deeper, we can see that worms have bilateral brain structure, I would guess that this means that the phenomenon is as old as bilateral symmetry itself - putting the development in the pre-cambrian era, which is the best part of billion years ago. Echinoderms (like starfish) are 'missing links' in bilateral symmetry as their larval stage has bilateral symmetry even though the adult has radial symmetry. Such wiring may be so embedded in the way most animal body plans develop that it hasn't changed in evolutionary history.

Still one might guess that the cross over of neurons in brain control is good for integrating the signals from both sides of the organism somehow. I'm sure someone else can post a better guess than this!

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When I was in school it was discussed as an evolutionary survival advantage... If you are attacked from the right side, the left side of the brain is less likely to be damaged and can use the right sided limbs to fend off the attack as opposed to the right side being damaged and less responsive..

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+1 Interesting Hypothesis. –  Remi.b Jan 1 at 18:52
    
I've also head this repeated at undergraduate level –  Rory M Feb 9 at 16:35
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Being a keen student of game theory and behavioural sciences one reason that comes to my mind is for the opposite wiring of brain hemispheres might be that

  • if right hemisphere controlled the right side and left hemisphere controlled the left side than cross hemisphere links and activity would be hard to achieve so the most efficient way of increasing cross hemisphere communication, links and activity with the least stress on physical resources is the opposite hemisphere wiring.

  • The second view (it might sound like voodoo I know) that also compliments this cross arrangement might be the underlying law of increase in complexity and entropy (second law of thermodynamics), which is embedded in the universe from its births, dictates an increase in complexity hence cross wiring also achieves this underlying goal of increasing complexity because if you think about it straight wiring would have been too simple for such a complex structure as the brain.

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It seems a bit voodoo indeed! I don't think the second law of thermodynamic can bring any insight into the general tendency observed in living things to increase in complexity through evolution. But the discussion might be interesting. But if your answer was an real explanation, you could use it to explain so many different weird phenotypes on earth. That makes me think that it is not really a satisfying explanation. But it's good to try such voodoo-like suggestions ;-) –  Remi.b Jan 1 at 18:52
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