Maybe you thought it was started by politicians: you scratch my back and I'll scratch yours - my pork barrel, your boondoggle. Not so. Our ancestors did it before they came down out of the trees - your ticks, my fleas. For man and mandrill, mutual grooming is the glue that holds societies together...and lets you know who's boss. He's that big baboon up front, the one who gets his back scratched without ever scratching anybody's back back.