The engine of the brain is energy. It is older than the Sun. It is as old as the universe itself. The Sun's energy is only equal to 4.5 trillion atomic bombs per second but the universe is far more energetic.
Some life has learned to use the energy of the sun directly but we belong to a different class of life. How we have managed our hunger for energy is a magnificent complex story yet it is a very old story in which our next meal is never guaranteed. Our brain has sought to create more guarantees. We spend energy to protect our energy. Our defenses have now become clever to a fault.