Adam's and Eve's ?

[Subduction Zone]

What a strange question.

Natural selection automatically occurs when you have organisms that reproduce with variation and are in competition over limited resources in an ever-changing environment.

So it started the second there were organisms that reproduced with variation.

I would say that it was a little bit later than that. Though it is hard to say. It started once populations were high enough that there was competition.

And there may have been multiple species of "first life". It did not have to happen only once. It is just that once life existed one form definitely outperformed all of the others, if they existed, and that is what led to us. That part of our past is likely to be forever obscured from us.