I see what you mean; we know the ingredients (necessary elements and energy) for earth-like life are there. However, we don't know the probability of all the s

I see what you mean; we know the ingredients (necessary elements and energy) for earth-like life are there. However, we don't know the probability of all the steps leading to life actually taking place in the given time frame. The only reference value we have is earth, so it's just one data point and hardly possible to generalize. And mathematical estimations are difficult due to way too many factors to be considered (I've never seen one actually). At the least we can say that formation of complex structures like DNA is far more complicated and thus less likely to form than methane, by orders of magnitude. So one could argue that it is generally far more likely for methane to be encountered than any (earth-like) life form.