Positive selection shown to have selected those with certain immune system genes.
The Black Death was one of the most devastating pandemics in human history, resulting in the deaths of an estimated 75 to 200 million people in Europe during the fourteenth century. The plague didn’t just wipe out millions of people—it also left a mark on the human genome. According to a new study, the Black Death facilitated survival of individuals who carried certain immune system genes, which helps to explain why Europeans respond differently from other people to certain diseases.
The Black Death originated in Asia, and was brought to Europe along silk trade routes in the 1340s. The disease was caused by the plague, brought about by the bacteria Yersinia pestis, and was transmitted through animals that came in close contact with humans, such as rats. Nowadays, the plague can be easily controlled with antibiotics, but in the fourteenth century, there was no knowledge of any of these concepts.
Human populations evolve in the face of disease. Certain genes that we possess help us fight infections better than others, and people who carry those genes tend to have more children than those who don’t. This is positive selection—the beneficial genetic versions persist, while other versions die off.
Information from populations that have reproduced in an isolated environment has helped in identifying beneficial genes. Several hundred years before the Black Death, a group of Roma migrated from Northern India to Romania. They did not often intermarry or reproduce with Europeans, making them genetically isolated. This group was also exposed to all of the same environmental factors of the Black Death. From this, it was assumed that Roma and European Romanians would have a higher concentration of beneficial genes than those from Northern India, where the plague did not spread.
Mihai Netea, an immunologist at Radboud University in the Netherlands, and colleagues looked for differences at more than 196,000 places in the genomes of one hundred Romanians of European descent and one hundred Roma. The researchers also catalogued these differences in 500 individuals who lived in northwestern India, where the Roma came from. These groups were then analyzed to see which genes had changed the most, and to see which were favoured by selection.
Researchers discovered that many of the variations were on a cluster of genes that code for toll-like receptors (toll-like receptors help the body eliminate bacterial contaminants). After hundreds of years of plague outbreaks, Roma and European Romanians were strongly selected for their increased immune response variants.
The same genes that helped their ancestors during the Black Death may also influence their interaction with modern disease. Those with European ancestry have higher instances of inflammatory disorders, as they are caused by an overactive immune system, that those who were not affected by the plague. The researchers next step is in investigating how these genes respond to other disease-causing bacteria in order to see if the selective pressure of the Black Death was really the cause for the genetic convergence.