I'm always confused by that. So there's only one form of life possible? Otherwise life may have arisen before, been wiped out when Earth changed, and life arose again. We may be just the latest in a long series of life forms.
Good point. It might be that the kind of life that arises on a planet depends on what it is bombarded with and which of those various candidate life-forms ‘takes’ on the specific environment.
theoretically possible but there is no evidence to suggest this. Weighing the various hypotheses, most people would suggest the "simpler" (fewer wipeout/arise events) hypothesis, given the assumption that life arising is considered very rare.
Because the earth remade itself several times. Any previous life would have surely been erased. The assertion was that life arose just once. That's hard to prove.
