In using the Drake equation to discuss and estimate the probability of non-Earth life in our universe, one factor we estimated was the probability of life developing on a given planet in the habitable zone. The problem with making such an estimate as students in an Astronomy 201 course, of course, is that we all know next to nothing about how life comes to be, even when all of the right chemicals are present.
The thing is, nobody knows exactly how abiogenesis happens. But scientists who study it naturally have a lot of ideas. We learned in class about the Miller-Urey experiment, in which organic compounds were made from common atmospheric compounds and energy. While there is no standard model for the beginning of life, most leading models draw from the same ideas that Miller and Urey did, and rely on the results from their experiment and similar experiments.
But without a truly accepted model of abiogenesis, it is difficult to predict what fraction of habitable planets develop life, and to me this is the weakest part of our Drake equation. If any part of the equation is guesswork, then the result is just guesswork—and there is nothing concrete about our guesses on the probability of the development of life.