Jonah Lehrer went in to WALL-E (an enormously entertaining movie) and came out thinking of Darwin's Expression of Emotion in Man and Animals:
The emotional brain is actually the most ancient part of our cortical machinery, a piece of hardware that's been refined by evolution over the last several hundred million years. That's why, as Darwin pointed out, animals that are utterly lacking in self-awareness - he called them "creatures of pure instinct" - tend to express their emotions in the same manner as humans. Even more radically, Darwin suggested that these expressions were evidence that the animals were also experiencing emotion, even though they were just obeying some ancient biological drives.
Lehrer's recent book is Proust Was a Neuroscientist.