PART II: Our Solar System
As I'm sure you realize, our solar system consists of four small, "rocky" planets orbiting relatively close to the sun, and four "gas giant" planets orbiting at a much greater distance away (Pluto is no longer considered a planet). Of all of the planets, there is only one where we are certain life exists, and that, of course, is Earth. But why is that? Both Venus and Mars are not all that different in size from Earth, and when compared to the orbital paths of the "gas giants", are at a relatively similar distance from the sun. So why did life evolve and flourish on our planet, but not on our neighbors? Well, consider the fact that the animals and plants on Earth are all primarily composed of water, and, in fact, wouldn't be able to live at all without it. It is a miraculous substance, and life thrives in it.
As everyone knows, water exists in three states; ice, liquid water, and water vapor. On Earth, all exist, although liquid water clearly predominates. And it is liquid water that is conspicuously missing from from Venus and Mars. While Venus is too hot for liquid water to exist (sometimes reaching 900F), Mars is too cold, with it's water trapped in polar ice caps. For life to exist on a planet in our solar system then, it would have to orbit somewhere between Venus and Mars for the temperature to allow the formation of liquid water. The question to be pondered, I think, is whether Earth is orbiting in just about the only path where life could have evolved, or whether Earth's potential orbit could have been anywhere in a large band somewhere between Venus and Mars. Of these two planets, Venus seems to be the most inhospitable, with it's crushing, corrosive atmosphere and 900F temperature, and yet, according to scientists, the vast majority of this incredibly high temperature can be attributed to a run away green house effect, and not solely to the fact that it is closer to the sun. As far as Mars is concerned, telescopic evidence clearly shows that liquid water once flowed on it's surface. And so, I think it likely that a wide band, of unknown width, exists in out solar system where, should a planet be lucky enough to orbit within it, life could potentially evolve. Earth just happened to be orbiting within that band.