First of all, I think we can rate this book "G". It took me about 30 or 40 seconds to figure out this one from p.71: "With the evolution of the mammals occurred an elaboration of the limbic system (the 'smell-brain' of reptiles), a group of structures lying just above the brain stem that mostly function in the control of basic bodily processes and of emotional responses related to survival and reproduction (often known as the four F's: feeding, fighting, fleeing, and sex). Otherwise, this book is so chock full of extremely interesting information about how we evolved -- read it!