How did the autonomic nervous system evolve into the role it now plays in human emotions, attention, and information processing? When and in what form did the autonomic nervous system begin to play a role in orienting and defensive behaviors? These are important but unanswered questions in contemporary psychophysiology. From an evolutionary point of view, it seems likely that these responses emerged within the context of predation and predator avoidance, and that their occurrence increased the probabilities of survival and eventual reproductive success. Early in evolution the "what is it?" response observed by Pavlov (1927) was almost certainly not an expression of idle curiosity. Instead, the question must have been: "Is it danger?" or "Is it food?" Autonomic orienting responses to neutral stimuli, to the extent that they represent a separate entity, probably emerged later in evolution than autonomic responses to lifethreatening or life-sustaining stimuli.