Black Death etched a mark on our genetics, warping immune responses, study finds

The Black Death massacred up to 50 percent of the European population in the mid 14th century, which appears to have etched an enduring mark on human genetics.

An international team of researchers led by anthropologists and geneticists at McMaster University in Canada and the University of Chicago have published a study in the journal Nature.

The team dug into the genetics of over 200 people who died before and after the Black Death. According to their findings, the Pandemic was an evolutionary pressure that shifted the diversity of genes related to the immune system. The four genes that were found to be protective against the plaguebacteria were found to be protective against other pathogens. Some of the genes have been found to be associated with an increased risk of autoimmune diseases.

"Perhaps this increased risk simply did not matter during the Black Death and the trade-off was inevitable," evolutionary biologist David Enard wrote in an accompanying commentary. The design of the study was praised by Enard.

The findings suggest that even humans with small populations can adapt quickly. It might come at a cost.

Deathly selection

The authors tried to answer the question of whether the Black Death triggered an episode of natural selection by reaching for ancient DNA samples. They had enough data from 206 remains to start with. Of the 206 ancient DNA extracts that made the cut, 67 were from the pre-Black Death period and 97 were from the post- Black Death period. There were 42 samples left from people who died during the Black Death. The plague cemetery in London contains samples from people who died there.

The team looked at variations of immune system related genes. They searched for genes that seemed to have become more common after the plague. Natural selection was suggested by the hundreds they found. They looked for people with different frequencies during and after the Pandemic. If a variant of the genes was protective, it would have a low occurrence among those who died in the Black Death, but a higher occurrence in the years after. There were 35 variant of interest. The number was brought down to four after they looked at variant that appeared to be under strong evolutionary selection.

The four genes identified in the analysis seem to have roles in governing immune defenses. One variant increased the immune system's ability to detect the Y. pestis signatureProtein. antigen presentation is when specific immune cells present key bits of a harmful germ to other immune cells so they can quickly identify and attack the germs when they encounter them. The researchers estimated that people with two copies of the protective variant were 40 percent more likely to survive the plague than people with only one copy.

The problem is that these variant have been linked to diseases. There are hypotheses that ancient epidemics could have a role in the evolution of autoimmune diseases. It will take a lot more work to understand the connection.

The nature of the year 2022. The About DOIs page has more information.