Each side of the brain primarily controls the opposite side of the body.

Nerve fibers carrying motor commands from the brain’s left hemisphere cross over to the body’s right side in the lower brainstem, and fibers from the right hemisphere cross to the left, in a structure called the pyramidal decussation. As a result, damage to one side of the brain — from a stroke, for instance — typically produces weakness or paralysis on the opposite side of the body.

The same crossed wiring applies to much of the sensory system: touch and pain signals from the right hand travel up and are processed primarily in the left hemisphere, and vice versa. Why the nervous system evolved this crossed arrangement, seen across most vertebrates, is still debated among neuroscientists, though it likely has deep evolutionary roots tied to how the early nervous system and sensory organs developed.