User blog comment:Zathronas/Analysis by an author: Volume 2 episode 9: Search and Destroy/@comment-10390252-20141003134556/@comment-10390252-20141003142538

Actually, I interpret that line from the prologue in the opposite way as yours - It could be used to prove that Grimm appeared after humans.

I also re-iterate, Grimm are a product of aura (specifically the darker aspect of it); they do not have a soul or aura of their own. When a human focuses on his darker impulses, it generates a darker aura; Grimm home in on this but I think it is possible that they are also created by it, especially in the concentrations created by large-scale dark and selfish behaviour.