If this is a question of what I believe, then, yes the bones formed as a result of a mutation or several mutations that were advantageous to survival. However, I don't believe those mutations "just happened to form". As I have said, I believe that life has been designed so this mutation and changes were orchestrated from the beginning.

As far as what science has told us so far, I am not well enough informed as to the mechanism that precipated the changes to give you a definitive answer. My educated guess based on what I do know about the changes and what I know about the theory of evolution is that the mutation occurred to produce from the multiple-boned double-hinged jaw of the reptile, separated bones that eventually were the hearing bones and a single-bone single-hinge jaw like mammals have. This happened so gradually that it may not be one of those X-men type of mutations at all but simply small modifications over generations that resulted in the same bones being used for an enhancement of an existing function. In a reptile the bones of the jaw are also used in their hearing process.