Abstract
What was the cause of the Black Death (1346-1353)? From the first moment on, scholars tried to give an answer to this question, which leaves a great mystery behind it. The discovery of the causative agent of the third pandemic, the bacterium Yersinia pestis, and the description of the epidemiology of the Indian outbreak, could have settled the debate already in the 19th century. But historians and other scientists were not convinced. Many discrepancies in the description of symptoms, in the way of contagion accounted for several doubts about the origin of the first pandemics, the Justinian plague (5th-6th centuries) and the second epidemic (14th-17th centuries).
A new kind of biological investigation began about 30 years ago. Ancient DNA (aDNA) analysis allows going deeply into the genetic structure of an ancient organism and making inferences about its nature, its identity, its genetic relationships and even about the population it comes from. Several attempts were already made to establish the causative agent of the Black Death by means of aDNA analysis, but they have not been very convincing so far for two reasons. First, the majority of the efforts were not carried out in specialized aDNA laboratories and, second, the similarity of the ancient lineages with the modern ones was too suspicious. Thus the idea that another cause like smallpox or an unknown hemorrhagic fever could have been responsible, entered the world.
Two recent studies demonstrate unequivocally that Y. pestis caused the Black Death by means of aDNA analysis. The first of these was also able to define the genetic lineage of at least two distinct clones. By comparing the results with those obtained from modern Y. pestis strains, the position of one of these ancient strains on a genealogy tree could be defined and even claimed that this clone has probably become extinct. For the other lineages, the position could not be better defined but they are genetically highly distinct from the first one, probably indicating that at least two or more ancient strains of Y. pestis were present in Europe at the time of the Black Death. An additional extensive study on modern DNA genomes suggested that this bacterium came from China, over and again, and tried to describe the possible travel routes of the epidemics.
Some incongruence persists, for instance how is it possible that the Indian model of transmission does not fit perfectly with the description of the pathology in the historical texts and how can it be possible given the hygienic conditions in the Middle Ages that the Plague did not extinguish the European population at the time of the pandemics? To address this last question, we are testing by means of aDNA the possibility of a different susceptibility to plague and similar infections of the bloodstream due to specific variants in the human genome, which may confer an advantage or a disadvantage in the fight of the organism against the pathogens.
Dr. Barbara Bramanti, PhD
Institut für Anthropologie
Johannes Gutenberg-Universität
Germany