The First Black Death Pandemic
Farming in the early Neolithic created surpluses, and a little wealth. It coincided with the development of the wheel and animal traction (103), which, in turn, increased trading, improved roads, and enabled greater human interaction.
It also facilitated the growth of large scale monuments (not least having the time and energy required to build them), and regular mass assemblies at these monuments for religious, commercial and social reasons, but, in the end, it also caused the decimation of Neolithic peoples across Britain and Europe due to the spread of plague.
It's long been known (not least through pollen counts) that there was a significant decline in agricultural activity for the last 500 years or more of the Neolithic era (beginning about 5,350 years ago) (104). The obvious assumption is that a combination of climate change and comparatively poor quality seeds, over use of land, and limited husbandry knowledge caused this decline, but while those factors do come into play, the killer factor was not merely disease, but plague, the same plague that killed perhaps over 50% in Britain and Europe in the 14th century.
2019 integrated phylogenetic, molecular clock, and genotyping (phew!) 'analyses revealed that multiple and independent lineages of Yersinia pestis branched and expanded across Eurasia during the Neolithic decline... [resulting in] a prehistoric plague pandemic that likely contributed to the decay of Neolithic populations in Europe.' (105) This was facilitated by Neolithic peoples living in communities close to domesticated animals, so, for example, hunter-gatherers living in the same Swedish region had no Y. pestis.
While this research makes no reference to Britain, it seems highly unlikely that a pandemic in south Sweden and Denmark that spread thousands of miles eastwards to (what is now) Ukraine and Iraq didn't spread a few hundred miles westward. While Neolithic Britons didn't live in large communities, they did regularly assemble in large groups, and it would have quickly become obvious that there was a correlation between such gatherings and the spread of plague among those returning home. So, the monuments which define Neolithic culture and religion, and hosted large scale Neolithic gatherings later became the cause of their decline. The decline of farming, the decline of monument culture, the decline in usage of those monuments, and the decline of the Neolithic population, all had the same cause: plague.
When the steppe-originating Bronze Age 'Yamnaya arrived a few centuries later, they were entering a Europe with a small and weakened indigenous population that could offer little resistance' (see note below) primarily due to plague. By contrast, the incomers were 'probably in better shape to fight too. Ancient DNA suggests they were unusually tall for the time. And they had a highly nutritious diet...They were healthier and probably physically quite strong.” What's more they had a huge advantage as warriors and farmers: 'the Yamnaya were accomplished horse riders.' While Neolithic animal traction was almost exclusively by oxen and aurochs, these incomers used horses which were stronger, more agile, and faster.
A new Age had begun.
NB The Yamnaya references in this New Scientist article are overly sensationalist, and heavily biased towards violence in order to justify putting the story on their front cover. The rest of it seems well researched and credible.