What are the major life-supporting factors on Earth?

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First, of all the planets around the Sun, the Earth has, through the laws of physics, maintained an atmosphere.  This atmosphere serves two specific purposes in sustaining life.  First, it protects living things on the surface from such forces as radiation and space debris (meteors, asteroids, etc.).  Without it, meteors would be hitting the ground with devastating effects on life.  Secondly, the atmosphere provides a cyclic environment, one that sustains life by cycling elements and compounds; the obvious example is the exchange of oxygen, carbon, and carbon dioxide.  Because it is a closed environment that does not allow substantive escape of essential ingredients, the atmosphere gives life the ability to regenerate itself, like a furnace that supplies energy to a drive system the delivers more fuel to itself.   In fact, part of our definition of Life includes the ability to replicate and reproduce itself. 

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