Perhaps you may change your mind if you read up on how easily bacteria swap DNA and just how many of them there are and all the places they live and all the things they eat.

Or to put it another way, what happened since bacteria have been around? Humans. Humans started out as single celled creatures like bacteria. And if we can get from that to humans, is it not reasonable to assume that bacteria have had the time to figure out how to multiply as fast as possible?