But there's much more to our tenuous existence than just that. Without: the tilt of the earth, the moon in orbit, the rotation of the earth, the atmosphere sheilding off solar radiation, etc., climactic conditions would be so harsh as to prohibit human life. If that series of comet impacts that hit Jupiter a few years back, had instead hit earth, there would be no life on earth of any higher order. The clouds of dust from the impact were larger than the earth. Jupiter
It is an awesome and wonderous site to view the works of the almighty God of the universe. It is humbling to meditate, by comparison, on the frail, yet beautiful aspect of our individual existence.
If those things had not happened, so that life would not have evolved to its present state, the atmosphere would not have so much oxygen. The oxygen is due to our close cousins--the plant kingdom, who are as highly evolved as we are. It would be high in methane and CO2, although it might have water vapor.