Question

Why does right brain controls left side of the body and why does left side brain controls right side of the body

Answer 1

think the question is about the *advantage* of the crossover. In short, why did this crossover evolve? First, it is not about cognitive thinking ... as this crossover is a feature (as far as I know) of all vertebrates ... all the way to salamanders and fish, which don't have much in the way of cognitive processing (high-order intelligence). So this indicates that this crossover is something that appeared very early in evolution of the vertebrate nervous system. So the question is ... WHY? Here's a scientific paper on the subject: "Why are vertebrate nervous systems crossed?" http://linkinghub.elsevier.com/retrieve/... You'll have to unravel a bit of the neurological jargon to get into it. (And you need to be a subscriber to get the whole article, but you can get the gist of the idea from the abstract.) The term 'contralateral' is the adjective that describes this crossover feature of vertebrate nervous systems. (Or more generally, 'contralateralization'.) 'Bilateral symmetry' (or just bilateralism) refers to the fact that bodies of animals have symmetrical left and right halves. This is hardwired right into the development of embryos ... so the development of eyes is left-right symmetrical. This is not just a characteristic of vertebrates but most invertebrates too (like crustaceans and insects). So bilateralism came before vertebrates. So by the time the vertebrate nervous system was starting to develop, it already had left-right symmetry ... with rudimentary eyes on the left and right. But there's an interesting feature of the developing vertebrate eye. It has a pupil. A small opening through which light comes in and focuses on the retina. This pupillary eye "flips" the world. An object near the top of your head, is imaged on the bottom of your retina. And something on the right side of your body, hits the left side of your retina ... and vice versa. This is called 'inversion.' This inversion (flipping) of the world is important. It is the 'environment' in which the central nervous system in developing. The information from the right side of your body (hitting the left side of your retina) has to go through a centralized brain, get processed as quickly as possible, and react. So it is the 'inversion' of the world presented to the brain by the visual system, that leads to the crossover in the nervous system that develops to *respond* to that information. There is advantage for the development of the motor pathways to follow the same structure. If information from the left side of your body is processed in the right side of your brain, then it is advantageous for the right side of your brain to also control the limbs on the right side of the body. It is also easier for an embryo to wire itself up that way (rather than sensory nerve cells and motor nerve cells having to follow completely different rules).