For centuries, the specter of the "Great Mortality" hung over the world, a recurring nightmare that reshaped civilizations, toppled empires, and left an indelible mark on the human genome. The bacterium responsible, Yersinia pestis, has long been studied as the ultimate biological predator, yet a fundamental mystery remained: how did this pathogen manage to persist in human populations for hundreds of years, appearing in devastating waves before certain lineages simply vanished?
A groundbreaking study published today in the journal Science provides a definitive look into the evolutionary engine of the plague. Conducted by an international team of researchers from McMaster University in Canada and France’s Institut Pasteur, the research identifies a specific genetic mechanism—a "volume knob" of sorts—that allowed the bacterium to fine-tune its lethality. By adjusting the copy number of a single gene, the plague could transition from a rapid, high-mortality killer into a slower, more persistent infection, a strategy that ensured its survival across generations even as it eventually led some strains toward extinction.
The study addresses the core mechanics of pandemics: how they jump into human populations, the ways they adapt to cause widespread illness, and the evolutionary trade-offs they make to persist within a host population. To understand these dynamics, the researchers looked back at the three great plague pandemics that have defined recorded history.
The first, the Plague of Justinian, erupted in the mid-500s and ravaged the Byzantine Empire and Mediterranean world for three centuries before disappearing. The second pandemic, famously known as the Black Death, emerged in the 14th century and remains the deadliest event in human history. It is estimated to have killed between 30 and 50 percent of the populations of Europe, Western Asia, and Africa. Far from a single event, the Black Death re-emerged in successive waves for more than 500 years, haunting the globe until the mid-19th century. The third pandemic began in China in 1855 and spread globally; while its impact is now mitigated by modern antibiotics, it remains a persistent public health threat in regions such as Madagascar and the Democratic Republic of Congo.
"This is one of the first research studies to directly examine changes in an ancient pathogen, one we still see today, in an attempt to understand what drives the virulence, persistence and/or eventual extinction of pandemics," explains Hendrik Poinar, the study’s co-senior author and director of the McMaster Ancient DNA Centre. Poinar, who holds the Michael G. DeGroote Chair in Genetic Anthropology, has spent decades decoding the secrets of ancient diseases, but this study marks a significant leap forward in connecting ancient genetic shifts to modern bacterial behavior.
The researchers focused their investigation on a gene known as pla. This gene is a high-copy component of the Y. pestis genome and plays a critical role in the bacterium’s ability to colonize a host. Specifically, pla acts as a key that allows the bacteria to bypass the human immune system undetected. Once inside, it facilitates the migration of the pathogen to the lymph nodes, where it multiplies and eventually spreads throughout the rest of the body, leading to the characteristic "buboes" of the bubonic plague.
By analyzing hundreds of samples from both ancient victims of the first two pandemics and modern victims of the third, the team performed an extensive genetic screening. They discovered that the copy number—the total number of pla genes present in a single bacterium—was not static. Instead, it fluctuated significantly over the course of a pandemic’s lifespan.
