Women are extraordinary in lacking the estrus, or period of heat. The females of most other primate species become sexually active, to the point of aggression, only at the time of ovulation. Why has sexual responsiveness become nearly continuous? Unusually frequent sexual activity between males and females served as the principle device for cementing the pair bond.

Evolution by natural selection is not an idle hypothesis. The genetic variation on which selection acts is well understood in principle all the way down to the molecular level.

To the extent that philosophical positions both confuse us and close doors to further inquiry, they are likely to be wrong.

The price of these failures has been a loss of moral consensus, a greater sense of helplessness about the human condition. ... The intellectual solution to the first dilemma can be achieved by a deeper and more courageous examination of human nature that combines the findings of biology with those of the social sciences.

The genius of human society is in fact the ease with which alliances are formed, broken, and reconstituted, always with strong emotional appeals to rules believed to be absolute.

In the process of natural selection, then, any device that can insert a higher proportion of certain genes into subsequent generations will come to characterize the species.

Most people are surprised when they hear my somber figures: we know of 2 million species of plants, animals and microorganisms, and we can give them each a scientific name and a diagnostic description. We know, perhaps generously, more than just a little bit of the anatomy in no more than 10 percent. We have done thorough studies in fewer than one-tenth of 1 percent. And the total number of species on Earth is unknown to the nearest order of magnitude.