Evidence of Black Death Still Seen in Human Genome

The Black Death swept through Europe during the 14th century, killing millions. This extreme natural selection left a lasting imprint on the human genome, with the survivors passing on genetic differences that may explain why modern-day Europeans react differently to some diseases and are susceptible to certain autoimmune disorders, according to a new study.

Mihai Netea, an immunologist at Radboud University Nijmegen Medical Center in the Netherlands, is a native Romanian. Before fleas brought the Black Death to Romania, the Roma people - also known as Gypsies - immigrated to Europe from north India. The Roma people intermarried little with European Romanians, and so the two groups experienced the Black Death with distinct genetic backgrounds.

The study, published in Proceedings of the National Academy of Sciences, compared the genome of 100 European Romanians with 100 Roma to look for convergences that could have been positively selected for by the Black Death. The convergences were then tested for in 500 individuals from northwestern India, the genetic birthplace of the Roma.

While the Roma and northwestern Indians share much of the same genetic material, Netea and his team found 20 genes that had changes seen in the Roma and Romanians, but not the Indians.

The genes "were positively selected for in the Romanians and in the gypsies but not in the Indians," Netea explained in an article published by Science Now.

According to the article, of the 20 genes examined, three were of great interest to Netea and his team. The three immune system genes were found on chromosome 4 and code for toll-like receptors, which help launch a defensive response to harmful bacteria in the body.

"We knew they must be important for host defense," Netea said in Science Now.

To verify that the toll-like receptors' evolution was caused by the Black Death, Netea tested the receptors ability to react to Yersinia pestis, the bacterium that caused the outbreak. They found the immune system response varied based on the exact sequence of the toll-like receptor genes.

The study's conclusion was further supported by evidence that, while other Europeans carried similar changes to the toll-like receptor genes, people of Chinese or African ancestry do not carry it. The Black Death, the deadliest plague in history, was not experienced in China or Africa, the researchers said.

According to Science Now, skeptics wonder if the convergence of the toll-like receptor genes could be explained by interbreeding between the Roma and Romanians. Lluis Quintana-Murci, a human population geneticist at the Pasteur Institute in Paris who was not involved in the study, thinks further genetic sequences could answer these questions. Quintana-Murci also wants to verify if another disease couldn't have exerted the genetic influence, saying, "This will inspire other labs to see if other bacterial infections could also explain the [selection]."