None of us ever contracted it, but it was the Black Death that made our immune systems what it is today. And Covid will do the same for future generations. To show how “pandemics are one of the strongest engines of human evolution” are researchers from McMaster University in Hamilton, Canada, and the University of Chicago Medicine. They analyzed samples of medieval DNA of the victims of the black death and identified over two hundred genetic variants conferred by the plague, highlighting how “infectious diseases can rapidly evolve the genes involved in immune responses, becoming one of the engines of natural selection”.
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The massacre of the black plague
The black plague, caused by the bacterium Yersinia pestis, spread to Europe, the Middle East and North Africa from 1346 to 1350 and caused the death of between 30 and 50% of the population of the time. The high mortality rate suggests that populations had little or no prior immunological adaptation to that pathogen, and it is no coincidence that death rates have dramatically decreased in subsequent plague outbreaks.
‘This may be the result of a change in cultural and health practices or the evolution of pathogens, but it represents first and foremost human genetic adaptation,’ the researchers write. The discovery has just been published in the journal Nature, providing evidence that infectious pandemics have shaped our susceptibility to disease and suggesting they may continue to do so in the future.
To explore the evolution of variation in immune-related genes, the geneticist Luis Barreiro and colleagues analyzed 516 ancient DNA samples extracted from individuals who died before, during or shortly after the London and Denmark Black Death outbreaks. The samples were dated using historical records and radiocarbon. Most were collected from an English plague cemetery used between 1348 and 1349.
The authors found evidence of “positive selection” and identified “highly differentiated” genetic variants, due to whether or not they were exposed to the bacterium, highlighting the role that past pandemics have played in shaping the risk of disease today.
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Genetic diversity of descendants shaped
“Our results provide evidence that the Black Death had an important selective force and shaped the genetic diversity of offspring. We identified four loci – that is, the location of a gene within a chromosome – strongly differentiated before and after. the black plague, in addition to other 245 variations due to natural selection “, explains Barreiro, who also found the DNA” more advantageous, able to give a lower cytokine response (the same cytokine storm that causes Covid to degenerate in our organism) and limit bacterial growth “.
“During the Industrial Revolution, the pigmentation of moths changed over decades, favoring the darker ones that best camouflaged with the soot. This is a classic example of what evolutionary biologists have known for a long time: natural selection can happen quickly. under the right conditions – comments Professor David Enardis, evolutionary biologist at the University of Arizona, in his editorial in Nature -. The bubonic plague caused the deadliest pandemic ever recorded in the history of Europe. And it is natural to think that survivors possessed characteristics that made them more likely to survive than others. Medieval DNA sequencing has now made it possible to trace those “protective” genetic variants which in turn allowed their descendants to survive. “
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The cost of natural selection
All this, however, has come at a cost. With the natural selection of individuals who best tolerated the bacterium, many genetic variants were lost. And this speed of adaptation has prevailed in the selection of DNA variants certainly more resistant to the plague, but on the contrary more predisposed to chronic inflammatory and autoimmune diseases such as rheumatism and arthritis, in fact handing down these pathologies to this day.
“Even now we are paying the price of that pandemic – concludes Enardis -. Perhaps this increase in risk was simply not significant during the Black Death and the speed of the pandemic could have triggered an inevitable exchange”.