R Douglas Fields in Quanta Magazine:
Dazzling intricacies of brain structure are revealed every day, but one of the most obvious aspects of brain wiring eludes neuroscientists. The nervous system is cross-wired, so that the left side of the brain controls the right half of the body and vice versa. Every doctor relies upon this fact in performing neurological exams, but when I asked my doctor last week why this should be, all I got was a shrug. So I asked Catherine Carr, a neuroscientist at the University of Maryland, College Park. “No good answer,” she replied. I was surprised — such a fundamental aspect of how our brain and body are wired together, and no one knew why?
Nothing that we know of stops the right side of the brain from connecting with the right side of the body. That wiring scheme would seem much simpler and less prone to errors. In the embryonic brain, the crossing of the wires across the midline — an imaginary line dividing the right and left halves of the body — requires a kind of molecular “traffic cop” to somehow direct the growing nerve fibers to the right spot on the opposite side of the body. Far simpler just to keep things on the same side.
Yet this neural cross wiring is ubiquitous in the animal kingdom — even the neural connections in lowly nematode worms are wired with left-right reversal across the animal’s midline. And many of the traffic cop molecules that direct the growth of neurons in these worms do the same in humans. For evolution to have conserved this arrangement so doggedly, surely there’s some benefit to it, but biologists still aren’t certain what it is. An intriguing answer, however, has come from the world of mathematics.