User blog comment:Withersoul/How would man have fought against the grim before using dust?/@comment-10390252-20130902144144/@comment-10390252-20130902151839

This is one of the reasons why I favour the 'cyclic' theory of Grimm activity where the Grimm become more and more active on a cycle (every 100-150 years, based on Jaune's comments about his great-great grandfather). This means that the first struggle against the Grimm during which Dust was discovered could have been centuries ago and there has been semi-regular returns ever since.

The alternate explanation that still enables you to stretch out the timeline to longer than 150 years is the 'episodic return' theory where levels of Grimm activity are closely connected to the moral character of human society. Only when humans with dark hearts rise in number are the Grimm powerful enough to strike out. The advantage of 'episodic' over 'cyclic' is that the return of the Grimm becomes harder to predict and the unwillingness of the leadership to admit to a moral decline in society makes it even harder to convince them to allow preparations to be made.