By Jonny Lupsha, Wondrium Staff Writer
Bubonic plague survivors’ genes still influence us today. A study of how germs affect human history shows that our ancestors’ plague immunity is involved in modern disease susceptibility. The Black Death’s strength owed to a mutation 100 years earlier.
Humans who survived contact with the Black Death in medieval times are still teaching us new lessons. A DNA study of 200 people who died in the period surrounding the bubonic plague gave startling insight into their genetic makeup. The study identified four genes that either protected against or made the subjects more susceptible to the bacteria that causes bubonic plague.
Unfortunately, what helped them survive the Black Death made them more susceptible to other autoimmune diseases such as Crohn’s disease.
Genetics are at the heart of the Black Death itself. In her video series The Black Death: New Lessons from Old Research, Dr. Dorsey Armstrong, Professor of English and Medieval Literature at Purdue Unviersity, pinpoints the mutation that made the plague so deadly.
Hiding in Plain Sight
“It seems clear that what precipitated the disaster that was the Black Death in medieval Europe in the 14th century was an event that occurred at least a century earlier—an event known as a polytomy and referred to by scientists working on the subject as the ‘big bang’ of Yersinia pestis,” Dr. Armstrong said. “This occurred sometime around the 1250s or so, or perhaps even earlier.”
Scientists believe that the Yersinia pestis bacteria has existed for 6,000 years or so. It is an animal disease but is zoonotic, meaning occasionally it can jump from animals to humans, like rabies. Much like all other branches of life, the bacteria has evolved into different branches or strains over the millennia. According to Dr. Armstrong, this evolution happens with an event called a “single nucleotide polymorphism” (SNP).
“Now, what happens is that, for whatever reason, there is a substitution of a single nucleotide at a particular position in the genome—as far as we can tell, it’s a spontaneous mutation,” she said. “Once that change happens, a new strain of plague is born, and it, too, may undergo some variation at some point in the future.”
This leads to multiple descendants, or “cousin” strains, of the plague. These may exist at the same time as the parent or ancestor strain, or the parent strain may die out—leaving one or more of the descendants. Alternatively, one or more of the descendants may die out, leaving the ancestor strain.
Severe Decline in Lethality after Mutation
“The changing of just one nucleotide in the Yersinia pestis bacterium seems to potentially have a dramatic effect on the bacterium’s lethality, transmissibility, and more, and this change creates a new lineage,” Dr. Armstrong said. “In 2013, a group of scientists published an article that sometime in the mid-1200s, Yersinia pestis appears to have undergone what’s called a polytomy—meaning that for whatever reason, it suddenly diverged into three and subsequently four divergent lineages.”
The Black Death strain of Yersinia pestis went extinct, presumably when it ran out of hosts. However, the strain that caused a pandemic in the 19th century only differs from its ancestor by two or three SNPs. Despite this close similarity, the lethality of the more recent strain is far lower than its ancestor.
“Science doesn’t yet have an answer to this question—how just a difference of a couple of SNPs could have caused this death and destruction, and then, with a slight tweak, could have evolved into something still definitely plague but much less lethal 600 years or so later,” Dr. Armstrong said.
It is widely accepted, though, that the strains of plague responsible for the Black Death had descendants that survived for several centuries in isolated populations, mutating and gaining lethality. This eventually led to the Great Plague of Marseilles in 1720.
The Black Death: New Lessons from Old Research is now available to stream on Wondrium.