I'm just an armchair wikipedia browser, but it's an interesting read: https://en.wikipedia.org/wiki/Proto-Basque_language

This article talks about the Basque language from before contact with the Romans, 5-1 centuries BCE. It also references a "pre-proto-basque" language, that would have been the one before the Celtic invasion of Iberia (see https://en.wikipedia.org/wiki/Celtiberians).

The rabbit hole kind of ends there, as not much linguistic artifacts or history remain from BC unfortunately. But one can imagine the Basque society would live in relative isolation for a long time before that.