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Earthquakes are feared for their destructive, deadly force. But they are part of a geological process - plate tectonics - that some scientists say is vital for the existence of life itself. Without the ever-changing land surfaces that plate tectonics produces, or the high continental masses raised above sea level by earthquake activity, planet Earth would atrophy into a lifeless mass, like our neighbour Mars. But why is Earth the only planet with plate tectonics? And, when did they start? The clues are so faint, the traces so ephemeral, that researchers are only now beginning to find tentative answers. Extraordinarily, some say that life itself has changed the forces in plate tectonics, and helped to shape the world.