If you just look at the Y DNA map of Europe, the most obvious answer to this, which are rejected by Indo-Europeanists, is that R1b split from R1a long before the rise of the language itself, and that the R1b folks moved into western Europe in neolithic times, developing many stone age cultures including the Funnel Beaker culture, and speaking many related languages including Vasconic. The R1a Yamnaya people then arrived later from the steppes, with their ability to travel far with their domestication of the horse. They took over central Europe and mixed with the R1b locals to create the hybrid Celtic culture, which eventually spread all over western Europe, becoming the lingua franca. The Basques are just a residual people who did not make the language shift. In fact during Roman times, many R1b folks still spoke non-Aryan languages, particularly in southwestern France, Iberia and northern Italy. Most of them made the language switch only in medieval times. But the Indo-Europeanists are so fixated on being the conqueror race, they cannot accept the idea that R1b Celts and Latin peoples were in some ways subjugated by the R1a Slavs and Balts some time in the past. Even the base y DNA for the Germans was I1. They were first assimilated into the Beaker culture by the R1b folks, then by the R1a Yamnaya folks, creating the unique German branch of Indo-European.