Plate tectonics plays at least three crucial roles in maintaining animal life: It promotes biological productivity; it promotes diversity (the hedge against mass extinction); and it helps maintain equable temperatures, a necessary requirement for animal life. It may be that plate tectonics is the central requirement for life on a planet and that it is necessary for keeping a world supplied with water. How rare is plate tectonics? We know that of all the planets and moons in our solar system, plate tectonics is found only on Earth. But might it not be even rarer than that? One possibility is that Earth has plate tectonics because of another uncommon attribute of our planet: the presence of a large companion moon, the subject of the next chapter.
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