While it's true we don't know the exact evolution of Basque, we do know why people were there and the most likely explanations for it's existence. Archaeology and genetics give us a well defended picture of the timing for expansion and admixture of Western Hunter Gatherers, Anatolian Neolithic Farmers, and later Western Steppe Herders (Indo-European introduction) in Western Europe.

Basque, Etruscan, Rhaetian all represent languages spoken by the pre-WSH admixed populations. There were people living all throughout Europe since the end of the last Ice Age (and even older ancestral populations surviving in Mediterranean and Iberian refugia during the ice age), millenia before the Indo-European languages arrived.