In Germania, Tacitus mentioned that all the known West Germanic tribes believed themselves to have descended from three ancestor figures, Istavenones, Ingaevones, and Heriminoes. Despite being a relatively distant source, from what I understand, most scholars appear to accept that the Germanic Tribes did believed themselves to be descended from those figures. At this time, many different tribes, spread over many thousands of square miles, would have believed themselves to have descended from the same figure if Tacitus is to be believed. Do you know or could you provide any scholarly theories as to what potential archaeological, sociological, or historical events led to so many tribes believed themselves to be descended from a common ancestor figure? Any information would be appreciated.

EDIT: I meant to say Supra-Tribal in the title.