The plague-causing bacterium Yersinia pestis has been identified by researchers within the 5,000-year-old skeletal remains of a hunter-fisher-gatherer from Latvia, and represents the oldest plague strain ever discovered.
The find sheds news light on the early origins and evolution of plague, which was responsible for three historic pandemics including the Black Death, a catastrophic event that wiped out around half of Europe’s population in the 14th-century.
At the Stone Age shell-midden site of Riņņukalns, situated on the banks of the Salaca River in Latvia, the remains of four individuals interred in single graves were discovered. Analysis of teeth and petrous bone samples from all four specimens yielded a radiocarbon date of c. 3,300-3,050 BC and genomic sequencing identified an ancestry specific to hunter-fisher-gatherers in Eastern Europe.
The team conducted ancient DNA analysis and pathogenic screening, the results of which have been published in Cell Reports, and were surprised to identify the genome of Yersinia pestis in the remains of an adult male, aged 20-30 years old.
Comparisons between the genome and other ancient strains determined that the individual is the oldest-identified plague victim. Further phylogenetic analysis revealed that this now-extinct strain, along with a slightly younger Y. pestis genome from a Swedish farmer dated to c. 3,040-2,867 BC, are distinct from later Neolithic and Bronze Age strains.
The study reports that the strain emerged around 7,000 years ago, after splitting from its antecessor, Yersinia pseudotuberculosis, around 7,400 years ago; however, it still lacked the crucial gene that allows Y. pestis to spread via the bite of a flea. Dark, gangrenous pustules called ‘buboes’ are a common symptom of flea-borne transmission, and gave the Black Death its name.
It is unclear as to what extent the individual experienced plague symptoms. Yet, the high bacterial load of Y. pestis in his bloodstream indicates it was likely transmitted through the bite of an infected rodent, resulting in septicemia.
As the others buried contemporaneously with the individual were not infected, and its means of transmission were limited, it appears the strain was less virulent than later lineages, and leapt sporadically from animal populations.
This significant discovery contradicts the popular hypotheses that early strains of Y. pestis were highly transmissible, and led to a dramatic population decline in Western Europe at the close of the Neolithic period.