That the human brain exists to promote the increase and endurance of the human genes means that the human brain is dedicated to speciel survival and progeneration. That process requires rational thought.
The building blocks that constitute "our" ability to express and understand a wide range of phenomena, according to Turner, are conceptual metaphors such as CAUSATION IS PROGENERATION and EVENTS ARE ACTIONS, in which two conceptual domains collide and are then used, for instance, to make.