There's barely any life in the Sahara. It looks a lot like spores to me. I want a measure of life that includes speed. Some kind of energy use or maybe cell divisions. I expect the probability of life developing in a place to be proportional to amount of life there after it arrives. Maybe that's silly; there certainly are exponential effects of molecules arriving the same place at the same time that aren't relevant to the continuation of life. But if you can rule out this claim, I think your model of the origin of life is too detailed.