We have already discussed the earlier collapse of the farming communities in (what is now) Eastern Europe. There was a second collapse that effected Europe a couple thousand years later.
What is most important about this collapse is that it was in the vacuum of this collapse that the Indo-European groups moved into Europe. These groups would latter bring us every major European group with only the Basques, Finish, and Hungarians coming from a different language group. Starting in an area roughly analogous to the Ukraine-Crimean these groups moved west, and also Southeast into Asia. Earlier it was thought that they came in as conquering tribes; but it is likely that at least some of their effectiveness was due to the depleted conditions of the existing
Hard Times Followed Booms for Europe's Ancient Farmers
Dan Vergano, National Geographic, 1 October 2013
A later, smaller boom happened around 2800 B.C. Neither the busts nor the booms appear tied to climate conditions, which surprised the researchers. A March study in the journal Science, for example, had pointed to drought playing a large role in the collapse of the classic Maya civilization around 800 A.D.
"I believe their results will be the origin of numerous new studies," says population modeling expert Neus Isern of Spain's Universitat de Girona."Why did the Neolithic economy crash if there was no natural disaster behind it? Was the Neolithic economy not as sustainable as we assume?"
Downey speculates that early farmers may have hastened soil degradation through deforestation and overuse of soils, while also raising the possibility of disease triggering population declines. Another possibility is that migration may have played a role in booms and busts, says archaeologist Ron Pinhasi of University College Dublin.Link to paper (pdf)
As with many of the other societal collapses we have discussed previously, the remnants of these groups were noted, but little has been puzzled out about the details of their civilization. They were the builders of the large monuments (Stonehenge), and did not have large cities, or possibly not even large political organizations, but there was some consistency to their lifestyle. Until the introduction of genetic research, it was not even clear who heavily replaced this native stock was by the incoming groups. Genetically, traces remain, but it is obvious that their culture was submerged by the newcomers.