"scientists still exclude the possibility of Mind and Choice occurring at the molecular level before sentient life existed."

That's not how science works. Describe the experiment or observation that would test this notion and then it can become a scientific issue.

Not all scientists are atheists, so it's an irrelevant ad hominem.  No one has ever proposed that it "just happened". The entire point of this line of research is to determine how life began. There is no assumption that it "just happened". 

"Do jigsaw puzzles EVER assemble themselves?"

It hasn't been observed, but in an infinite universe the odds that it happened at least once could be more favourable than you think. In any case, this is the problem of induction that all science faces. No matter how many observations you make, a black swan event is always possible. So scientists don't believe anything with 100% conviction. The just support a given explanation with varying levels of confidence.