Good exposition Quetzal A question: During the time of prebiotic evolution, Earth was under an intense bombardment of spatial debris. Some of the impacts were so big as to evaporate all the water and eradicate the incipient life. Perhaps the possibility of life surviving (or even surging) in subterranean niches, should be added to the list.
I have read somewhere that big sterilizing impacts could have lasted as late as 3,8 billions years ago. Any way, if by 3,5 billion years forms of life as complex as cyanobacteria (stromatolites) were already present, the time for life to evolve seems a little scarce. Here is where the possibility of life surviving underground fits nicely.