Biologist Charles Darwin described the mechanism of natural selection as a process of perfecting the genetic qualities of species to optimize their adaptability and ensure their survival. This process takes hundreds of thousands, if not millions, of years to cause significant changes in a species. However, there are also instances where “rapid natural selection” occurs, causing genetic changes in individuals over a few dozen years. Experts suggest that this is what happened during and after the biggest epidemic on record: the Black Death.
The science and other stuff to know
A multidisciplinary research team from McMaster University in collaboration with the University of Chicago developed a study recently published in Nature, where they claim that the Black Death has bequeathed us an unmistakable genetic mark. It is a defense mechanism against the bacterium Yersinia pestis, which causes plague, but it may also produce uncontrolled immune system responses, eventually leading to autoimmune diseases such as lupus.
David Enard is a biologist at the University of Arizona and—although he was not involved in the collaboration—he praised the well-designed research, suggesting to CNN that “Perhaps this increased risk just didn’t matter during the Black Death; the urgency of the pandemic might have made compensation unavoidable.” In other words, the rapid adaptation that our species had to undergo in order not to become extinct—since the Black Death killed almost half of the European population between 1346 and 1353—had consequences that remain in our genes to this day.
The experts analyzed more than 500 DNA samples extracted from human remains found in common cemeteries where those killed by the Black Death were buried. Samples from England and Denmark, the regions hardest hit by the epidemic, were analyzed, and four different genetic variants were discovered that caused favorable effective immune responses against the bacterium while also demonstrating that they were the result of a rapid natural selection mechanism.
The truth is that it is not known if this genetic heritage still protects us from the Black Death, since the frequency of cases is fortunately rare, so no studies have been carried out in this regard.
In their paper, the researchers note that “selection for pathogen defense in the presence of pathogens such as Y. pestis may be offset by the costs of immune disruption, resulting in a balanced long-term signature of selection”.
These genetic distinctions continue to impact human immune systems today, with genes that once conferred protection against the plague now related to a higher risk of autoimmune illnesses like Crohn’s and rheumatoid arthritis.
In any case, research in this area will continue to yield information on immune system genetic mutations, its response under adaptive pressure (such as a pandemic), and the behavior of autoimmune illnesses, all of which will allow the development of effective medical treatments.
Do we risk paying the same price as our ancestors for acquiring immunity against COVID-19? Luis Barreiro, a co-author of the article, suggests not.
The coronavirus showed a much lower case fatality rate than the Black Death had in the 14th century. In addition, the infection caused by COVID-19 tends to fatally affect people who have already passed their reproductive age, so any “adverse effect” of a possible rapid natural selection would not be successfully inherited by the following generations.