Neolithic Decline
From about 4000-3600 BCE, there was a significant population increase due to the introduction of agriculture. The stable food supply allowed more people to be fed. As did the spread of technologies like the spread of pottery, animal husbandry, and the wheel. Between 3450-3000 BCE, there was a rapid collapse in populations in western Eurasia that's cause is still being debated. During the Neolithic, heavily populated settlements were 'regularly created, abandoned, and resettled', but a 'great number of those settlements were permanently abandoned' around 5400 years ago as well as a decrease in cereal production.
One of the potential causes of the Neolithic decline is the plague, Yersinia pestis, as multiple skeletons from this time period have had ancient genomes of the bacteria. The plague also appeared around 5,000 years ago in Latvia and Sweden, such as an individual from modern day Riņņukalns, Latvia who was infected with 'an early Yersinia pestis strain, shortly after it split from its antecessor Y. pseudotuberculosis c. [about] 7,000 years'. A tomb of 79 people from 4,900 years ago in modern day Frälsegården, Gökhem, Falbygden, Sweden had at least two people who had Yersinia pestis in their teeth with the 'plasminogen activator [catalysts to the breakdown of the proteins in blood clots] gene that is sufficient to cause pneumonic plauge', which is extremely deadly and is airborne, so highly communicable. Gallery graves, megalithic structures that were made of upright stones with capstones over them, of the Wartberg Culture where potentially hundreds of individuals found Y. pestis in only 2 of 133 individuals from about this time, leading researchers to conclude there wasn't a major outbreak. The presence of Yersinia murine toxin (ymt) also indicate that Y. pestis wasn't yet transmitted by fleas yet.
In northeast China, a site known as Hamin Mangha, which is about 5,000 years old, holds a smalle structure with nearly 100 bodies, though exactly why so many people were buried in such a small place is uncertain, whether there was a geological disaster, a plague, or a ritual sacrifice that lead to so many people being buried at once. Two similar sites have been found in Maiozigou and Laijia, also in northeast China, but also with an uncertain cause.
The theory that the plague was the cause of the Neolithic decline is disputed, with the lack of fleas as a transmission vector, meaning the plague could only be transmitted by rat bites, which would have caused septicemic plauge, or cases of pneumonic plague that were only just beginning to be contagious, so would have had 'difficulty spreading across vast distances in a short amount of time'. Other signs of mass-outbreaks are also missing.
