Origins of Plague that Wiped Out Half of Eurasia Discovered: Study

The origins of the Black Death are a mystery no more, according to a new study published in the journal Nature.

The Black Death was a plague caused by the bacteria Yersinia pestis that first entered the Mediterranean via trade ships, and proceeded to rapidly spread across Europe, Northern Africa and the Middle East in the 1300s. Up to 60 percent of the population is thought to have died as a result of the disease in its first wave, which extended into a 500-year-old pandemic called the Second Plague Pandemic, lasting until as recently as the 19th century.

Nobody, until now, had known the true origins of the Black Death, with varying theories circulating in the academic community.

"The two leading hypotheses were China and Central Asia. To solve this mystery, ancient DNA from east of West Eurasia was needed," co-author and historian at the University of Stirling, Philip Slavin, told Newsweek.

The only available archeological findings to date come from Central Asia, found in excavations that took place almost 140 years ago. Close to Lake Issyk Kul, now in Kyrgyzstan, there is evidence that an epidemic devastated a local trading community in the years 1338 and 1339, thanks to tombstones reading that those who perished in those years died of "pestilence."

"There are, in total, 467 headstones with precisely dated inscriptions, ranging from the years 1248 to 1345CE," Slavin said. "118 of these are dated 1338-1339, indicating some sort of elevated mortality crisis. Some inscriptions were longer than others, and on 10 of these 'pestilence' as a cause of death has been inscribed (in Syriac language, the liturgical language of local Christians). For over 100 years, this reference of 'pestilence' has puzzled historians. I suspected it could have been the beginning of the Black Death, but only DNA was able to establish it."

In this study, researchers analyzed the human remains at sites where headstones indicated the individual had died from an epidemic, using ancient DNA (aDNA).

"We uncovered aDNA from the teeth of seven individuals—teeth tend to get preserved the best after one's death and they preserve pathogens in blood streams. During the analysis, plague bacterium, Yersinia pestis was detected in all three individuals," he said.

After many years have passed, DNA samples are prone to environmental contamination and degradation, which presented some challenges to the research team.

"Only two out of three teeth had sufficient genomic coverage, but it was good enough to combine them together for an increased genomic resolution, which yielded a very clear picture. Not only Yersinia pestis was detected in all three samples, but the high coverage of two samples allowed us to determine their evolutionary position on the phylogenetic tree. The analysis found that the 1338-1339 genomes fell on a node immediately preceding the 'Big Bang,' which was when the main Yersinia pestis lineage got split into four new branches, of which branch 1 is associated with Black Death genomes from West Eurasia," he said

They also found that plague strains found today around the Central Asian Tian Shan mountain range (which runs through Kyrgyzstan, Kazakhstan, Xinjiang in China and Uzbekistan) are closely genetically related to the reconstructed ancient genomes from 1338-1339. The researchers have concluded that the most probable explanation is that the ancient strain evolved locally, within the extended Tian Shan mountain region.

"This centuries-old mystery has been solved! But apart from that, there are two additional takeaways. To understand the phenomenon of emerging epidemic diseases, it is essential to have as 'bigger' evolutionary picture as possible. It is important to see how these diseases develop evolutionary and historically, it's always important to not treat different strains as isolated phenomena, but as something that is situated within a much wider evolutionary picture. To understand how they develop and get transmitted, it is important to consider the environmental and socio-economic history contexts, in which these processes happen."

The authors hope that their study will set an example of how essential collaborative multidisciplinary research is to important discoveries like these. They involved colleagues from different fields such as historians, archeologists and geneticists.

"We believe that the future of plague history research should be like that," said Slavin.