Interesting comment I read from Guy Pratt (Pink Floyd's bassist when Roger Waters is having a sulk)...

"The bassist is the glue between the music and the drummer."

Ok, in a way it's a slight on drummers, but the point he's making is that, like I said, the bass is the instrument that's both rhythm and harmony/melody. Your right hand tracks the drummer's right foot. your left hand tracks the guitarist's left hand (well, sort of. In spirit).