Genetic analysis of 1500-year-old teeth gives clues to origins of
24 February 2014
Two of the world’s most devastating plagues – the plague of
Justinian in the sixth century and the Black Death — were
caused by distinct strains of the same pathogen, according to a
study of teeth from a Bavarian burial site.
One plague faded out on its own, the other lead to worldwide
spread and re-emergence in the late 1800s. These findings suggest a
new strain of plague could emerge again in humans in the future.
The findings are dramatic because little has been known about the
origins or cause of the Justinian Plague — which helped bring an end
to the Roman Empire — and its relationship to the Black Death, some
800 years later.
Scientists hope this could lead to a better understanding of the
dynamics of modern infectious disease, including a form of the
plague that still kills thousands every year.
The Plague of Justinian struck in the sixth century and is
estimated to have killed between 30 and 50 million people —
virtually half the world’s population as it spread across Asia,
North Africa, Arabia and Europe. The Black Death would strike some
800 years later with similar force, killing 50 million Europeans
between just 1347 and 1351.
Researchers from many universities including McMaster University,
Northern Arizona University and the University of Sydney, isolated
miniscule DNA fragments from the 1500-year-old teeth of two victims
of the Justinian plague, buried in Bavaria, Germany. These are the
oldest pathogen genomes obtained to date.
Using these short fragments, they reconstructed the genome of the
oldest Yersinia pestis, the bacterium responsible for the
plague, and compared it to a database of genomes of more than a
hundred contemporary strains.
The results are currently published in the online edition of
The Lancet Infectious Disease. They show the strain responsible
for the Justinian outbreak was an evolutionary ‘dead-end’ and
distinct from strains involved later in the Black Death and other
plague pandemics that would follow.
The third pandemic, which spread from Hong Kong across the globe
is likely a descendant of the Black Death strain and thus much more
successful than the one responsible for the Justinian Plague.
“The research is both fascinating and perplexing, it generates
new questions which need to be explored, for example why did this
pandemic, which killed somewhere between 50 and 100 million people
die out?” questions Hendrik Poinar, associate professor and director
of the McMaster Ancient DNA Centre and an investigator with the
Michael G. DeGroote Institute for Infectious Disease Research.
“We know the bacterium Y. pestis has jumped from rodents
into humans throughout history and rodent reservoirs of plague still
exist today in many parts of the world. If the Justinian plague
could erupt in the human population, cause a massive pandemic, and
then die out, it suggests it could happen again. Fortunately we now
have antibiotics that could be used to effectively treat plague,
which lessens the chances of another large scale human pandemic”
says Dave Wagner, an associate professor in the Center for Microbial
Genetics and Genomics at Northern Arizona University.
The samples used in the latest research were taken from two
victims of the Justinian plague, buried in a gravesite in a small
cemetery in the German town of Aschheim. Scientists believe the
victims died in the latter stages of the epidemic when it had
reached southern Bavaria, likely sometime between 541 and 543.
The skeletal remains yielded important clues and raised more
questions. Researchers now believe the Justinian Y. pestis
strain originated in Asia, not in Africa as originally thought. But
they could not establish a ‘molecular clock’ so its evolutionary
time-scale remains elusive. This suggests that earlier epidemics,
such as the Plague of Athens (430 BC) and the Antonine Plague (165
-180 AD), could also be separate, independent emergences of related
Y. pestis strains into humans.
“The tick of the plague bacteria molecular clock is highly
erratic. Determining why is an important goal for future research”
says Edward Holmes, an NHMRC Australia Fellow at the University of
Our response to modern infectious diseases is a direct outcome of
lessons learned from ancestral pandemics, say the researchers.
“This study raises intriguing questions about why a pathogen that
was both so successful and so deadly died out. One testable
possibility is that human populations evolved to become less
susceptible,” says Holmes.
“Another possibility is that changes in the climate became less
suitable for the plague bacterium to survive in the wild,” says