About Proto-Germanic

[Jaska]

Rodoorn:

“So I guess that gross we could say that before the SGC influx the language of TRB prevailed, but with the influx of clearly IE speaking SGC and BB it became a substrate.”

This seems possible, although we cannot say anything for certain. Languages replace other languages much slower than humans and items move. It takes usually some generations before earlier population becomes bilingual, and yet more generations before they desert their original language forever and language shift is completed. And there could still remain pockets of people speaking the earlier language for many centuries or even for many millennia.

Moreover, there very probably existed also ancient Indo-European dialects which did not survive. Therefore, an assumed Indo-European-related migration to a region where we much, much later find Proto-Germanic, does not necessarily represent the linguistic predecessor of Proto-Germanic – it might represent some other ancient Indo-European dialect. In that case the Germanic lineage was introduced to the region only later, and it would have been carried there by some other migration. There is no shortage of migrations in prehistoric Europe, and there are many possible migrations which could in theory be associated to the Germanic lineage.

Again, there is no method with which we could see the linguistic processes from DNA or material culture. If we have linguistic results helping to anchor the process in certain time and place, then we can try to find matches for those results in genetic or archaeological data; but if we do not have conclusive linguistic results (which is unfortunately the case with Germanic), it is a mere lottery game. 

Rodoorn:

“Ok, and with regard to the content, when we can say it talks like a duck, it walks like a duck it has feathers like a duck, it is a duck? And then with regard to pre-Germanic? (ok the soundshift is clear but that's Proto Germanic)”

I am not sure if I understand your meaning here correctly. If you mean when we can say for certain that the language was Proto-Germanic, then I would say: the uniform Late Proto-Germanic disintegrated little before the Common Era, and the changes related to and leading to Late Proto-Germanic can be dated roughly in the centuries around the mid-first millennium BCE.