Well I believe that an unknown supreme being started the big bang. As to where the supreme being came from I can't explain, nobody can.

The micro organisms were formed by the methane ammonium and water vapor that were on earth, which have the right ingredients for life(some guy actually did an expirement and created organisms with these three things.)

The micro organisms respired, putting o2 out, which was broken down into o1s by UV rays. The o1s grabbed onto either other o1s(creating o2s or oxygen) or onto o2s(creating o3 or ozone) which formed the ozone layer, allowing the organisms to be protected from the rays, over time minor mutations (very minor things, like DNA stuff switching around) created separate species.