Another way to think about it is that all sorts of mutations occur and have no effect on Dariwinian fitness of the species until the environment changes and the mutation then confers an evolutionary advantage on its holders. A good example of a mutation which is just there and has yet to confer an evolutionary advantage (or disadvantage) is the ability to roll one's tongue. Some people have it, other don't and it doesn't seem confer a any kind of evolutionary advantage either way. Until an environmental change happens that favors (or disfavors) individuals with that ability, it just continues along in the genome.
By the way, the loosed jaw mutation wouldn't necessarily confer an evolutionary disadvantage on individuals with it in an environment with abundent "soft" food. If there was already a genetic pre-disposition in primates for larger brain growth, the occurrence of the jaw loosening could have been the key that unlocked the door to bigger brains in a relatively short timespan. That, in turn, would have conferred a significant advantage on the proto-humans.