A little over five thousand years ago, something bad happened in central and western Europe. Farming had arrived earlier, and flourished for several hundred years, accompanied by population increase (see the graph above, showing estimated population density over time in that region, from a recent paper in Nature Communications.). Then there was a population collapse, on the order of that caused by the Black Death, but longer lasting. It doesn’t look as if this was caused by climate changes.
Ways of life changed. In Britain and Ireland, cereal farming seems to have stopped completely for hundreds of years (except in the Orkney Islands). Instead people were raising cattle and pigs.
But were they the same people? We know that there were big genetic changes in northern Europe somewhere around this time, in which the Sardinian-like farmers occupying almost all of Europe were replaced by another population with substantial amounts of WHG-like ancestry (western hunter-gatherer) and ANE (ancestral north Eurasian: Sibermen) ancestry.
Something similar seems to have happened somewhat earlier in the Balkans: there had been many agricultural settlements, some large, exhibiting advanced metallurgy- but everything stopped. The tell settlements ended. No permanent settlements in the Balkan uplands can be dated between 3900 and 3300 BC.
We’re going to have look again at the archaeological record in the light of what we know and are learning from ancient and modern DNA analysis. Gimbutas and Gordon Childe are looking better and better: most contemporary archaeologists are not.