Please read the Tower of Bab El.
All the survivors were deaf and unable to pass on their language to their children and other descendants. Only those who could foresee the approach of the planet and seek shelter would escape death and deafness.
Andorra is a small bishopric, a separate realm from Spain to the south and France to the north. I visited there, after a long and difficult road journey. It lies in a very deep gorge, and is surrounded by cliffs that reach 500 metres above the narrow river valley. This location was a natural massive "cave" that attenuated all sounds, as they would only reach the valley floor after many reflections protecting the inhabitants from the massive sonic compressions. Their language survived as they were able to pass it onto their children. They gained a massive competitive advantage over their human neighbours, able to teach their children all the skills that they possessed unlike their neighbours who were deaf.
What their neighbours came up with, eventually settled into the Indo-European group of languages as the children learned to speak to one another. This is the anomaly, not Basque!