  Probability of Life To have life in the universe, you need a rare conjunction of many coincidences. Life, which depends on a variety of complex biochemical reactions, can easily be rendered impossible if we change some of the constants of chemistry and physics by a small amount. For example, if the constants that govern nuclear physics were changed even slightly, then nucleosynthesis and the creation of the heavy elements in the stars and supernovae might become impossible. Then atoms might become unstable or impossible to create in supernovae. Life depends on the heavy elements (beyond iron) for the creation of DNA and protein molecules. Thus the smallest change in nuclear physics would make the heavy elements of the universe impossible to manufacture in the stars. We are children of the stars; however, if the laws of nuclear physics change in the slightest, then out "parents" are incapable of having us.
As another example, it is safe to say that the creation of life in the early oceans probably tool 1 to 2 billion years. However, if we could somehow shrink the lifetime of the proton to several million years, then life would be impossible. There would not be enough time to create life out of random collisions of molecules. In other words, the very fact that we exist in the universe to ask these questions about it means that a complex sequence of events must necessarily have happened.
It means that the physical constants of nature must have a certain range of values, so that the stars lived long enough to create the heavy elements in our bodies, so that the protons don't decay too rapidly before life has a chance to germinate, and so on. The existence of humans who can ask questions about the universe places a huge number of rigid constraints on the physics of the universe - for example, its age, its chemical composition, its temperature, its size, and its physical processes.
The Anthropic argument... If Earth had been placed too close to the sun, it would be too hot to support life. If too far, it would be too cold. The fallacy of this argument is that millions of planets in the galaxy probably are sitting at the incorrect distance from their sun, and therefore life on them is impossible. However, some planets will, by pure accident, be at the right distance from their sun. Our planet is one of them, and hence we are here to discuss the question. -Michio Kaku, "Hyperspace", Pg 258-259 
