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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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  • $\begingroup$ I'd bet it's non-adaptive. $\endgroup$ – Noah Snyder Oct 5 '12 at 15:08
  • $\begingroup$ 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! $\endgroup$ – Remi.b Jan 1 '14 at 18:58
  • $\begingroup$ See also: cogsci.stackexchange.com/q/5771/301 $\endgroup$ – Memming Feb 20 '14 at 0:09
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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. It possibly predates right/left dominance and the organ asymmetry which puts the heart on the left side of the body.

Digging back deeper, we can see that worms have bilateral brain structure, but as noted in the comments below, the nervous systems do not cross. I would guess that this means that the phenomenon appears sometime before lizards - putting the development having been done by the early Carboniferous period when the first animals came to the land and the first reptiles appears, which is the perhaps 350 million 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. If that is related to other animals trait, it could be a billion years ago. Some argue this is not the case.

One might guess that the cross over of neurons in brain control is good for integrating the signals from both sides of the organism. There are quite a few theories.

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    $\begingroup$ layperson, here... just a theory, but if it goes far back enough, I could imagine that a small enough organism had some advantage to having fewer kinks/bends in the nerve fibers when going through a "neck", so the left half of the body being connected to the right half of the nerve... clump(?) allowed a straighter path... just a thought, unless we can find something REALLY small that seems to demonstrate this... $\endgroup$ – Code Jockey Oct 28 '16 at 12:39
  • $\begingroup$ this is how the evolutionary biology game is played. we try to understand the dynamics of selection by looking for examples that illustrate a point about the relative advantage of some configuration in living things. If we don't see what we're looking for, then we have to re-think our logic. $\endgroup$ – shigeta Dec 22 '16 at 0:01
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    $\begingroup$ Question, if there is an answer (?): how do the nerves go from one side of the brain to the other side of the body? Are they intermingled/intermeshed, or do the left hemisphere nerves typically go on front of the right hemisphere nerves? Or typically behind? If there's a significant correlation in dorsal/ventral or anterior/posterior based on hemisphere of origin, that could indicate some amount of "twisting" that occurred at some point way back in evolutionary time, where the head/brain bucket used to "point" the other way .. $\endgroup$ – Code Jockey Jan 12 '17 at 13:18
  • $\begingroup$ symmetry in biology is inherited v. early in and is the result of genes that maintain the bilateral symmetry in our class of animals. (radial symmetry and assymmetry exist too). Those same factors that generate these symmetry are probably playing a part in the crossing from one side to another if they are symmetric crossings... just a guess. $\endgroup$ – shigeta Jan 14 '17 at 2:59
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    $\begingroup$ This answer is very old, but I just came across it...Contralateral motor control is, as far as I am aware, a vertebrate phenomenon. Yes, worms have a lateralized nervous system, but they do not have contralateral control, so I think it is quite misleading to mention them in this context. There is not evidence that contralateral control is as old as bilateral symmetry. Mollusks and arthropods also have fairly advanced nervous systems but do not show a primary contralateral control. $\endgroup$ – Bryan Krause Jul 9 '18 at 15:50
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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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    $\begingroup$ I've also head this repeated at undergraduate level $\endgroup$ – Rory M Feb 9 '14 at 16:35
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There's a good answer to this on https://www.quora.com/Why-does-our-left-hemisphere-of-brain-control-our-right-side-of-our-body-and-the-right-our-left.

In summary, a biopsychologist states that vertebrates and invertebrates have the digestive system and the nervous system in reverse locations. He also points out that the hearts are in the dorsal area for invertebrates and in the ventral area of an invertebrates. The evolutionary theory of these observations is that the body of an early vertebrate must have been turned upside down, and the easiest explanation is that a vertebrate ancestor swiveled its head 180°. In conclusion, it's believed that one vertebrate had its head turned around and that contralateral arrangement was conserved because it decreased chance of error in nervous system wiring (compared vs simpler same-sided wiring schemes).

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  • $\begingroup$ Certainly an interesting idea. $\endgroup$ – JayCkat Jul 9 '18 at 16:24
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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 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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    $\begingroup$ 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 ;-) $\endgroup$ – Remi.b Jan 1 '14 at 18:52
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It is simple. The brain through evolution split into two hemispheres when binocular vision was evolving. If you follow the neurons from the eye, the lateral neurons travel to the ipsilateral hemisphere. The medial neurons cross to the contralateral hemisphere. I would bet the protopathic pathways were around in evolutionary terms before binocular vision and the epicritic pathways developed after binary vision. Vision drove the hemispheres into two compartments. I could draw the diagram but unable to do so on this response

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It's for control, coordination and balance. From a mathematical point of view, parallel lines won't intersect and thus line A won't affect line B unless they intersect and thus would hardly influence the other unless and an external force is applied to any of the lines.

This is also why corpus callosum connects the left and the right side of the brain.

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    $\begingroup$ Could you please add references to this response! $\endgroup$ – Bez Aug 19 '14 at 0:28

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