The foundation of humanity's power was the Phantoms, specifically their Evolution Tree and the force of their genes. However, a human body without an Initial Gene, there was no way it could become as strong as a K0-Phantom, it was simply physically impossible.
No matter how experienced a military man was, no matter how dense and large his muscles were, it was simply not enough, as the difference in strength was too great.
It was like a man trying to defeat a bear with his bare hands. The bear was superior to the man in everything - speed, strength, defense, and arsenal. The man had only fists and muscles, while the bear had sharp claws and an incredibly powerful jaw.
However... Everything changed dramatically if a man had a weapon, for example: a gun. One shot was enough to defeat a beast several times bigger.
