The Goldilocks Theory
Just as Goldilocks found the porridge that was just right, the Earth seems to be just right for living creatures. The Earth seems to be the perfect distance from the sun for lots of water.
Venus is too close to the sun, and too hot for flowing water on its surface. In fact, it is so hot that, like a sauna, all the water has been evaporated into the atmosphere, and Venus has a thick and heavy atmosphere.
Mars is too far from the sun, and too cold for flowing water on its surface. Mars also has no continental drift, so particles of the atmosphere which become trapped within the ground stay trapped within the ground. Thus over time the atmosphere of Mars has become thin, and all the water is frozen into the ground.
The temperature of Earth is just right for flowing water on the surface, and for the rock which allows for continental drift. With continental drift, particles of the atmosphere which become trapped within the ground are brought back to the atmosphere
through eruptions of volcanoes. These conditions cause refreshment of the planet's atmosphere. A medium sized atmosphere helps keep temperatures just right for flowing water (through the greenhouse effect). These conditions are just right for abundant life
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