The various proto-Indo-European groups which entered European in the 2nd millennium BCE replaced local languages very thoroughly. Basque and Sami might be remainders of pre-PIE people's languages. Their technology (e.g., horses, bronzeworking) also became very widespread. But what about the people?
On the one extreme, it's possible that the PIEs killed off almost all of the locals, leaving isolated populations (e.g., the aforementioned Basques) and a small fraction of survivors who were genetically dwarfed. I.e., similar to the near-extinction events in America and Australia when smallpox-carrying Europeans arrived.
On the other extreme, it's possible that a relatively small number of well-armed chariot riders put themselves in power but contributed relatively little to the gene pool, like the Mongols in China or the Spanish in Mexico.
Do we have enough information to make a guess as to what place on this spectrum is most likely?