Ever wonder why the left side of your brain controls the right side of your body? So have I. And so have a few neuroscientists whom I’ve asked. None have an answer, much less a good one. The internet has proven to be equally uninformative on the topic: many sources are quick to tell you about the crisscrossing of sensorimotor information flow, but none address its evolutionary advantage. I mean, really, why does it make any sense (har) for sensorimotor tracts to be crossed?
Perhaps this decussation stems from the lateralization of visual processing, which DOES make sense. Light entering the eye passes through the lens and then hits the opposite side of the retina. Spatially, it’s logical for such visual stimuli to get relayed to the brain hemisphere closest to the stimulus receipt site, which occurs contralateral to the stimulus. Evolutionarily, it would be advantageous for motor and sensory information related to a particular field of vision to coincide anatomically with the processing of that field.
This could be bollocks, of course. Take a species without any visual organs (not including species with ancestral visual organs, like cave fish who have vestigial eyes) and demonstrate contralateral connectivity of sensorimotor information and brain hemisphere, and you’ve got yourself a negation on this theory. Until then, folks.