In terms of adaptability, humans are pretty amazingly useless. Like most animals, we don't much like really hot places, but because we sweat so freely and easily stroke, we are especially vulnerable. In the worst circumstances——on foot without water in a hot desert一most people will grow delirious and keel over, possibly never to rise again, in no more than six or seven hours. We are no less helpless in the face of cold. Like all mammals, humans are good at generating heat but—because we are so nearly hairless—not good at keeping it. Even in quite mild weather half the calories you burn go to keep your body warm. Of course, we can counter these frailties to a large extent by employing clothing and shelter, but even so the portions of Earth on which we are prepared or able to live are modest indeed: just 12 percent of the total land area, and only 4 percent of the whole surface if you include the seas.

Yet when you consider conditions elsewhere in the known universe, the wonder is not that we use so little of our planet but that we have managed to find a planet that we can use even a bit of. You have only to look at our own solar system—or, come to that, Earth at certain periods in its own history—to appreciate that most places are much harsher and much less amenable to life than our mild, blue watery globe.