Well, methane and ammonia and water vapor were in the atmosphere, methane is CH4 I believe, water vapor is H20, some how these things made some kind of premordial soup and formed life, 2 guys made an experiment like this in the 50s I believe, where they made the same kind of conditions in a lab as there were back then, with just water vapor, methane, and ammonia, and there were signs of living organisms after a few weeks, so that question is easily answered.

The harder question, where did the methane, ammonia, and water vapor come from? Probably formed with the earth after the big bang, but something must have triggered the big bang, which leads me to believe that there is a supreme being.