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question. “How is it that fruit taste good?” A physiologist might answer “because it stimulates the sweet receptors on the tongue.” This is a proximate answer. Evolutionary biologists are also concerned with ultimate causes—the final cause or end purpose of a structure or function—the “why does it exist?” type of question. “Why does fruit taste good?” An evolutionary biologist might answer “because there was a natural selection for taste bud receptors and brain modules to produce a pleasurable sensation with certain food substances that are both scarce and healthy.” We can go even deeper and ask what drives the selection process, and postulate that those organisms for whom healthy foods tasted good ate more of them, had less disease, lived longer, and thus left behind more offspring. Since differential reproductive success is the ultimate result of natural selection, and natural selection is the primary driving force behind evolution, we have reached the deepest level of causality in answering our question.

If questions about anatomy and physiology can be answered at the deeper evolutionary level, what about behavior? In the late 1970s the field of ethology—the study of animal behavior from an evolutionary perspective—came of age. John Alcock’s Animal Behavior: An Evolutionary Approach and Irenäus Eibl-Eibesfeldt’s classic text, Ethology: The Biology of Behavior, demonstrated that behaviors are not just the result of reinforced learning in response to changing environments but also the product of millions of years of evolution. A herring gull chick, for example, pecks at a red dot on its mother’s beak. Its mother then regurgitates her food for the chick to eat. The chick did not “learn” this behavior by trial and error in its own lifetime but inherited it from the evolutionary history of the species. The chick was born “knowing” that when it sees a red dot it should peck at it. The mother, in turn, was born “knowing” that when a chick pecks at its beak, it should regurgitate its food.

Of course, compared to herring gulls and other simple organisms, human behavior is vastly more complex and influenced by learning and the environment. Nevertheless we are animals, and no less than any other organism on earth we are the product of evolution. In order to fully understand human behavior we must also address our own ultimate “why” questions from an evolutionary perspective. What humans have done in the past 13,000-year history of civilization is nothing short of miraculous, but we must not discount the orders of magnitude of deeper time that preceded the age of civilization, when the human animal was shaped over the course of hundreds of thousands of years as Pleistocene hunter-gatherers, and over the course of millions of years as primates, and tens of millions of years as mammals.

In the past decade the field of ethology has been joined by the emerging discipline of evolutionary psychology. Where evolutionary biologists focus on the effects of the physical environment, evolutionary psychologists concentrate on the influence of the social environment. Primates are extremely social mammals, and the human primate is, arguably, the most social of all. In their introduction to the field’s most influential text, anthropologists Jerome Barkow and John Tooby, and psychologist Leda Cosmides explain: “The central premise of The Adapted Mind is that there is a universal human nature, but that this universality exists primarily at the level of evolved psychological mechanisms, not of expressed cultural behaviors. In this view, cultural variability is not a challenge to claims of universality, but rather data that can give one insight into the structure of the psychological mechanisms that helped generate it.”

The Harvard evolutionary biologist Edward O. Wilson has done just this in such works as Sociobiology, On Human Nature, and most recently in Consilience: The Unity of Knowledge. Wilson argues “that the etiology of culture wends its way tortuously from the genes through the brain and senses to learning and social behavior. What we