It's a tricky one. On a similar line, I was watching something regarding narrowing down the planets that have the potential to support life the other day...very interesting.
Anyway...it is believed (watched a documentary), that the very first micro organisms, formed around, well they looked like small warm vents in shallow water. Something about the vents enabled life to form there.
So, clearly...if that were true, then these would have appeared everywhere, but there would have been difference in the surroundings that would have created slightly different organisms.
As the Earth changed, so these organisms would have had to adapt to their changing surroundings, some living on land or mud flats (the transition between land and sea) etc.
And then as was described, the various different organisms, either died out or survived depending if their adaptation was suitable for their environment.
As for the spark, well...I do think that life could have been brought here from another world, perhaps deliberately.
There is also "the missing link". What do we know about that?
Apparently, within any solar system, a planet such as Earth has to exist in a very fine boundary, between too hot to maintain liquid water and too cold to maintain liquid water.
This cuts down the number of potential life supporting stars, in our Galaxy, or local neighbourhood so to speak, to perhaps 10,000 (as seen on said program).
On a different tack. I fail to see, why some should etc, teach children to believe in God and have them believe that he created man and so on and then teach them later on, that man evolved from Apes...seems a waste of time to me. Not that I think having Faith, is a waste of time, but certainly Churchianity has it's issues.
Also, I fail to see how any intelligent, or even average person, can look into the sky at night and see an endless sea of Suns, each potentially with planets around it and be absolutely certain, that we are the only planet to support life. How arrogant is that? People like that are either stupid or scarred.
If you do the math, it is probably a higher chance of their being life on other worlds, that there is of wining the lottery and we know that happens!