"What should we have for dinner?" To one degree or another this simple question assails any creature faced with a wide choice of things to eat. Anthropologists call it the omnivore's dilemma. Choosing from among the countless potential foods nature offers, humans have had to learn what is safe, and what isn't-which mushrooms should be avoided, for example, and which berries we can enjoy. Today, as America confronts what can only be described as a national eating disorder, the omnivore's dilemma has returned with an atavistic vengeance.