During the successive days of battling with the zombies, the soldiers learned one crucial fact: the most fatal blows were dealt to the zombie's head. Basic Level 0 and Level 1 zombies could be eliminated by simply blowing up their heads.
The brain of an ordinary zombie is identical to the human brain. However, a unique difference arises once they evolve to Level 1: their brains solidify into an Evolution Crystal. Once the Evolution Crystal is formed, the brain loses its function, shrinks, and all bodily functions and nervous systems are taken over by the Evolution Crystal.