Great question. I don't know where they started. I've heard about the primordial soup, but I know very little about it.
I know very little about it because I didn't study it in college it do my masters in it. But I trust the scientists who did study it and researched it. I also trust the education board who approved this topic as something real enough to be worthy of bring taught to me in school from a very early stage. This, along with the evidence I've been able to see gives my confidence in it.
How about you? How do you think life on earth started and why you believe it?