From these information sections it seems to me that the building blocks needed for beginning the creation of life were plentiful, not just on Earth but in space in general and from the earliest of times. Probably they have been around since long before even the Earth formed from the cosmic debris left behind by the life and death cycle of previous stars and planets, back to the beginning of time. These "seeds of life" no doubt extend through the far reaches of the universe as well as the depths of time (cue Crosby, Stills, Nash and Young ... "We are star dust ...").
It also seems to me that the natural processes for forming more complex structures from those basic building blocks were likely prevalent on the earth 4.5 billion years ago in a variety of forms, levels of completion and locations. We end with a scenario that has a random combination of plentiful and multitudinous organic molecules forming amino acids all over the earth, with a membranous system to contain and concentrate those molecules and their interactions within a protocell type capsule. We also see that random combination of plentiful and multitudinous amino acids into peptides and proteins is feasible, and that concentration and recombination within the membranous protocells enhances the probability that random combinations of them into the first "replicators" (the predecessors to RNA and DNA) is not as far fetched as it seemed at first. A simple building block process where the probability of a successful combination is almost inevitable: it is no longer a matter of "if" but of "when" it will occur under these conditions ... and once self replication occurs the frequency of replication will necessarily outpace the random action, and replicators that are faster and stronger will outpace their competition ... life seems inevitable when given the conditions for life.
That is my take on the probability of life on earth.
That was written in 2005 and more has been learned since then, information that makes it more feasible now, as we see more and more planets that could fall in the range of habitability in other regions of space, and information that provides greater detail about other planets and moons in our solar system.