New genetic evidence seems to indicate that in Central Europe, at least, cultural innovations were associated with population replacement. Scientists looked at the DNA of successive occupants of a region of Central Europe over a span of 4000 years, from 7500 ya to 3500 ya.
What they found was that the shift in the frequency of DNA lineages closely matched the changes and appearances of new Central European cultures across time. In other words, the people who lived in Central Europe 7,000 years ago had different DNA lineages than those that lived there 5,000 years ago, and again different to those that lived 3,500 years ago. Central Europe was dynamic place during the Bronze age, and the genetic composition of the people that lived there demonstrates that there was nothing static about European prehistory.
Needless to say, this suggests (but does not gaurantee) the worst about our stone age ancestors - that cultural change was associated with genocide, either by wiping out the previous inhabitants or driving them away. It also aims a pretty big truck at the notion that there might have been a widespread superculture that spoke Proto-Indo-European over a wide range of Europe and Asia 10,000 or more years ago - a favorite fantasy of those who doubt an Indo-European expansion.