It doesn't cause the genetic change, the genetic change is random. But if you were, simply by chance, born with a bigger brain that required more calories to su

It doesn't cause the genetic change, the genetic change is random. But if you were, simply by chance, born with a bigger brain that required more calories to support, you would be at a disadvantage to everyone else, and your mutation would quickly die out unless you were in a situation where you had excess calories available, such as by eating meat.