The politically incorrect answer would be that like the rest of R1b, they were descended from the neolithic farmers who came to Europe from anatolia speaking various related languages. The Bell Beaker people were a development from these farming societies. When the R1a came conquering from the steppes, some mixed races speaking predominantly Indo-European languages were created, such as the Celts. But the languages of the ancient farmers continue to exist well into Roman times till they were eventually subsumed into the majority. Languages such as old Iberian, Etruscan and Basque. Eventually only Basque held out. Its pretty much the the Han colonisation of South China. All the Yue were Sinicized except the Vietnamese. Yet their DNA is still closer to Vietnamese rather than Han. No one would identify O2a as DNA of ancient Han, but its a third of the southern Han. The only thing which stopped West Europeans from identifying R1b as neolithic farmer as against being steppe nomads are their self-identification as Aryan conquerors rather than members of a conquered race.