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Four billion years ago, back when life began, our planet was a very different place. Earth spun more rapidly on its axis—a day lasted only four or five hours. The Moon was much closer, and giant meteorites pelted the planet. Under these conditions of constant, chaotic change, there was only one place of constancy—a warm spring in the relative safety of the deep ocean floor. There, protected from ultraviolet radiation, existed a place that never dried out, never got too hot or too cold, never became too acidic or too alkaline. Life began there, inside the membranous froth of minerals that surrounded these hydrothermal springs, and the minerals themselves catalyzed the first chemical reactions that made proteins and nucleic acids—precursors to the earliest living things. Every kind of life, including us, carries the mark of that first catalysis in the form of tiny mineral clusters at the centers of our enzymes, still doing the jobs they began so long ago.