Most people are aware that their brains are (for the most part) split into two sides to create a right and left hemisphere. Furthermore, it is fairly common knowledge that (once again, for the most part) the right part of your body is controlled by the left side of your brain, and the left side of the body is controlled by the right side of the brain. This is more succinctly known as contralateral control. When signals to and from the brain do not cross over to opposite sides, it is instead known as ipsilateral control. What I have never heard a good explanation for, however, is why the human nervous system is primarily set up utilizing contralateral control. It seems to simply add an unnecessary complication to an already ridiculously complicated system. There is, of course, the possibility that contralateral and ipsilateral control offer virtually no selective difference to an organism, and it is therefore a 50-50 random chance sort of thing that just happened to end up favouring contralateral. However, that is a rather boring answer, and, like I said, leaves out the fact that contralateral is at least a small bit more complicated than ipsilateral, meaning there just might be something that made that complication actually worth while.
When my neuroscience professor mentioned Mauthner cells in one of my lectures, I realised that perhaps I had an answer. There is a reflex in fish and tadpoles called the Mauthner Cell C-Reflex. The Mauthner cell is the largest vertebrate neuron and it is activated by tactile stimulus to a creature's head. The signal from the Mauthner cell then innervates muscles on the opposite side of the body, quickly turning the head away from the stimulus for the fastest possible escape. Thus, here is a system in which there is a tremendous difference between contralateral and ipsilateral control. An ipsilaterally wired Mauthner cell would be a very bad thing, since it would turn its creature towards rather than away from the stimulus (which might very well be a predator).
Therefore, I wonder if the Mauthner Cell C-Reflex (or whatever reflex system was the precursor to it) was the first contralateral system in the vertebrate nervous system. Once it was wired up contralaterally, the whole somatosensory system followed, which of course led to other things until virtually the entire organism's nervous system was set up with contralateral control. By the time we crawled out of the oceans and lost our Mauthner cells it was too late to straighten things out.
Of course, this is just idle wondering into the murky evolutionary past, but I would love at some point to see if there actually is something to it. Of course, like most of my idle ideas, I will likely never actually get around to looking into it, but now that I've written it down, maybe it will stick with me for a rainy day when I'm in a researching sort of mood.