Originally Posted by Adaptation
I know that once the Celts come in, slavery becomes the norm in farming and mining
That is a Non sequitur if I ever heard one. Is there any record of Celts having or having been slaves before they encountered the Romans?
To get back to a time before the peoples who were to be called "Celtic" existed, you would have to go before the La Tene, Halstatt and urnfield cultures.
In fact, if you accept the Kurgan hypothesis of about Indo-European origins, which most people do, you would be way before then.
I consider them the indigenous people of Europe, for this epoch.