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we shouldn’t be surprised at this shift of function for concealed ovulation. Such shifts are very common in evolutionary biology. That’s because natural selection doesn’t proceed consciously and in a straight line toward a distant perceived goal, in the way that an engineer consciously designs a new product. Instead, a feature that serves one function in an animal begins to serve another function as well, becomes modified as a result, and may even lose the original function. The consequence is frequent reinventions of similar adaptations, and frequent losses, shifts, or even reversals of function, as living things evolve.

One of the most familiar examples involves vertebrate limbs. The fins of ancestral fishes, used for swimming, evolved into the legs of ancestral reptiles, birds, and mammals, which used them for running or hopping on land. The front legs of certain ancestral mammals and reptilebirds subsequently evolved into the wings, used for flying, of bats and modern birds, respectively. Bird wings and mammal legs then evolved independently into the flippers of penguins and whales, respectively, thereby reverting to a swimming function and effectively reinventing the fins of fish. At least three groups of fish descendants independently lost their limbs to become snakes, legless lizards, and the legless amphibians known as cecilians. In essentially the same way, features of reproductive biology—such as concealed ovulation, boldly advertised ovulation, monogamy, harems, and promiscuity—have repeatedly changed function and been transmuted into each other, reinvented, or lost.

The implications of these evolutionary shifts can lend zest to our love lives. For example, in the last novel by the great German writer Thomas Mann, Confessions of Felix Krull, Confidence Man, Felix shares a compartment on a train journey with a paleontologist, who regales him with an account of vertebrate limb evolution. Felix, an accomplished and imaginative ladies’ man, is delighted by the implications. “Human arms and legs retain the bones of the most primitive land animals! . . . It’s thrilling! . . . A woman’s shapely charming arm, which embraces us if we find favor . . . it’s no different from the primordial bird’s clawed wing, and the fish’s pectoral fin. . . . I’ll think of that, next time.... Dream of that shapely arm, with its ancient scaffolding of bones!”

Now that Sillén-Tullberg and Møller have unraveled the evolution of concealed ovulation, you can nourish your own fantasy with its implications, just as Felix Krull nourished his fantasy with the implications of vertebrate limb evolution. Wait until the next time that you are having sex for fun, at a nonfertile time of the ovulatory cycle, while enjoying the security of a lasting monogamous relationship. At such a time, reflect on how your bliss is made paradoxically possible by precisely those features of your physiology that distinguished your remote ancestors as they languished in harems, or as they rotated among promiscuously shared sex partners. Ironically, those wretched ancestors had sex only on rare days of ovulation, when they perfunctorily discharged the biological imperative to fertilize, robbed of your leisurely pleasure by their desperate need for swift results.

The Science Masters Series is a global publishing venture consisting of original science books written by leading scientists and published by a worldwide team of twenty-six publishers assembled by John Brockman. The series was conceived by Anthony Cheetham of Orion Publishers and John Brockman of Brockman Inc., a New York literary agency, and developed in coordination with Basic Books.

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Copyright © 1997 by Jared Diamond. Published by Basic Books,

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