You simply have a misunderstanding of how evolution works. It is a common misconception that evolution is a purely biological. It is not.

When primitive man developed the ability to control fire and cook and process food we started to evolve away from the need to digest all our food internally. This was due to a change in our environment. (The development of fire is the change in the environment) That change in environment triggered and allowed an evolutionary change. (Because that is how evolution works)

In fact it is exactly this evolutionary change that led to our increased cranial capacity because we became much more efficient at extracting calories from our food.