I was just about to write all the stuff that Bruce just wrote but he beat me to the punch so I will just point this little bit out.

Your thinking seems to be linear but life is not that way. After a few 10s of generations that "feature" will be in millions of individuals (talking microbes here). This is a parallel process. So if the chances of it happening are 1 in a million and you have several million individuals then it will happen several times. ( Think of how many of the same type of bacteria must be in the oceans! The numbers are so huge that we just can't even comprehend it.) So assuming it gives a reproductive advantage it will rapidly spread to the rest of the population. So *incredibly*, easily becomes inevitable.