Also, as far as we know, single-celled life also evolved exactly once. This is one thing I haven't seen a good answer for -- if life came about so early, that

Also, as far as we know, single-celled life also evolved exactly once. This is one thing I haven't seen a good answer for -- if life came about so early, that would indicate that it is easy. But since it occurred only once, that would indicate that it is hard. The common explanation is that once life evolved and began to spread, that it would prevent other life from appearing by out-competing it. But there is plenty of raw materials and sunlight to go around, so I don't buy that explanation. The only thing that seems to fit, is that life is hard to evolve, but has extra-terrestrial origin. If that is the case then life should exist (at least in bacterial form) wherever a suitable habitat is found.