I don't think that's even necessary.<quoted text>
Not at all. All that is required is that the chemicals involved be mixed in the right proportions. That happened spontaneously and via natural processes, but was caused by those processes. Why you think there had to be an intention involved is beyond me.
IF the chemicals are present and mixed and energy sources (and/or catalysts) as well, they will start to compound, unless there are inhibitors present. Or, at least, the proportional composition has a very wide range to allow for the biochemical reactions to occur.
I don't think there is lot of 'fine tuning' needed here.
For instance: if you mix 20% sodium with 80% chlorine, a chemical reaction will occur. The same applies when you mix 80% sodium with 20% chlorine. Or 50% of each. The only difference will be the amounts of salt formed and the amounts of residue sodium and chlorine.