Neolithic decline

A tomb in modern-day Frälsegården in Gökhem parish, Falbygden, Sweden, contained 79 corpses buried within a short time of one another about 4,900 years ago.[1] This discovery uncovered fragments of a unique strain of the plague pathogen Yersinia pestis found in two individual's teeth.[1][2][4] The strain contained the "plasminogen activator gene that is sufficient to cause pneumonic plague", an extremely deadly form of the plague which is airborne and directly communicable between humans.[4] This strain of plague, researchers claim, alongside high demands of resources whilst living in close proximity to each other, would have allowed a pneumonic plague to quickly spread amongst inhabitants and wipe them out.[1]

Neolithic-era human teeth from Eurasia have also shown evidence of some of the oldest strains of Yersinia pestis.[5] The ages of the skeletons identified between 2,800 and 5,000 years old, with seven of the one hundred and one individuals carrying similar sequences of the bacterium.[5] Additionally, studies of the ancient strains discovered show these ancient strains lack the Yersinia murine toxin (ymt), which would have prevented the strains from using fleas as a vector.[5]

A similar site was found in China in 2011; the site Hamin Mangha in northeast China dates back to approximately 5000 years ago and features a small structure filled with almost 100 bodies.[6] Whilst there are several theories as what the reasons are for so many bodies in one location, such as a geological disaster or a ritual sacrifice, a plague is also considered as a hypothesis.[6] In the case of the plague, despite being the weakest of the hypothesis, the placement of the bodies suggesting others carrying them in, alongside being intact before being burned, and the lack of artifacts alongside the bodies.[6] Two other sites like these have been found in Northeast China: Miaozigou and Laijia,[3][6] but archaeologists did not speculate as to the causal agent.[7]

Some studies have contested the hypothesis that the plague was responsible for the Neolithic decline. Analysis of the plague bacteria that infected a hunter-gatherer in Latvia during this period indicates that, unlike modern plague strains, the strain which afflicted this man was incapable of causing flea-spread bubonic plague and could only cause septicemic plague via a rodent bite or a largely non-contagious case of pneumonic plague, implying that the disease would have had difficulty spreading across vast distances in a short amount of time.[8] The man identified in this particular case, after being studied on, does not have a clear indicator on how much he was actually affected by the bacteria as well.[8]

Studies of the ancient variations of the bacteria have tried to show connections to the specific strain they studied and the more modern strands, such as one's during the Black Death.[1][5] Studies in Sweden, on the Gok2 Neolithic Yersinia pestis strain, was discovered to be the basal to all known Y. pestis strains with the use of genome reconstruction, as well as containing plasminogen activators genes that would have allowed it to start a pneumonic plague.[1]  Other cases revealed a lack of ability to be able to use fleas as a vector of transmission; the case in Sweden contained Yersinia murine toxin which prevented the use of fleas, alongside a separate case studying late bronze-age bodies revealing the use of fleas in transmission would have occurred around the time after the collapse, being a few hundred years off.[1][5]