Actually the only thing we know for sure is that life on Earth happened at least once. It is entirely conceivable that several alternatives to RNA-based life (precursor to DNA-based life) popped up in several different places but then were hopelessly out-competed by the RNA-based lifeforms. I am a biologist by training, and the more I studied biology and evolution, the more life appeared to me as a powerful (somewhat scary even) and inevitable process, rather than a "unique snowflake" that many of us commonly think it to be.

Life, given right conditions and enough time, seems to be about as inevitable as the eventual appearance of viral cat pictures on online forums, in other words.

I think there is little wrong with assuming that the probability of emergence of life given right conditions and enough time is one; however we need to be very careful with estimating how common (and how stable!) those conditions are in our galaxy in the first place. A good place to start would be to look at how many places similar to Earth are out there (which we are already doing).