This is a pretty much a general speaking, because for example our vision is only partially “crossed”, and our sense of smell not at all. But yes, the “phenomenon” of the right side of the brain having control over the left side of the body has a place. It is mainly because brain hemispheres (left and right part of the brain) are being connected by a very special structure which is called Corpus Callosum (CC). This structure enable them to communicate between each others. But why during the evolution this way of communication was preferred? It looks like it was more efficient, but why? I’m not sure. I never came across a full explanation. Although, here is some good website, which is maybe not explaining the phenomena fully, but is describing some other interesting evolutionary changes which happened: did you know that insects have their nervous system in their belly, not in their back like we do? I think it’ll be interesting for you to know it 😉
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Ava O’C commented on :
Thank you! I read through the website and they have so many different points of view! And I never that about insects!