The most mysterious and compelling unanswered question about ourselves is how we came to be. What caused us to evolve our marvelous intellects and our unsurpassedly complex social life? Why are we so different from our closest relatives? What happened to all of our extinct ancestors? Why is it that during the several million years since hominids diverged from the ancestors of modern apes, while other rapidly evolving forms of life were speciating prolifically, no part of the evolving human line has survived as - or possibly even became, contemporaneously with other incipiently human species - a different species? Why are we all alone at the pinnacle of the particular direction of rapid evolutionary change that led to the combining of such traits as a huge brain, complex intellect, upright posture, concealed ovulation, menopause, virtual hairlessness, a physically helpless but mentally precocial baby, and above all our tendency and ability to cooperate and compete in social and political groups of millions? What, precisely, was happening to the evolving human line in different parts of the world 50,000, 500,000, or (among our prehuman ancestors) 5 million years ago? [...]

Given the woefully incomplete knowledge of our long and distant past, the public has had to satisfy its thirst for self-understanding however it could. [...] Biologists and biologically-minded anthropologists, in particular, who take it as given that all forms of life have come about through an organic evolution guided primarily by natural selection, are not satisfied with either vague or supernatural arguments. Even if their theories do not easily yield convincing or comprehensive answers, they keep probing and questioning-trying, it might be said, to construct a "rough draft" of a scenario or theory of how humans evolved. Such a rough draft-even based on no more than speculations not easily dismissed-has been difficult to complete. Natural selection implies reproductive advantage. But there are whole suites of human activities that seem to have nothing to do with reproduction, and that no one has been willing to tackle in such terms. How does one explain art, music, opera, literature, humor, politics, science, or religion, using arguments from biological evolution? Conversely, why should we take evolution seriously, in trying to understand ourselves, if such important activities seem immune to its probings?

These problems can be discussed in three parts:
(1) what general selective forces drove the evolution of hominids, while for the most part keeping them a single species, (2) what combinations of selective forces caused the appearance of the various unique or distinctive features of humans and their social life, and (3) how can the answers to these two sets of questions be combined to yield an overall synthesis?

[From author's Introduction]