To find the traces of the Black Death in European genes, Romanian immunologist Mihai Netea hit on a very clever scheme. The Roma or gypsy people emigrated from northern India to Romania around a thousand years ago, but never interbred much with the locals. Netea reasoned that any way the Roma are genetically more similar to other Europeans than to the people of northern India must represent selection that happened to them in Europe over the past thousand years.
With the help of evolutionary biologist Jaume Bertranpetit in Barcelona and others, Netea studied 196,000 spots on the genomes of 100 Roma living in Romania, 100 non-Roma Romanians, and 500 people from northern India. They found 20 genes for which the Roma and other Romanian forms are similar to each other but very different from the forms found in India:
Those genes included one for skin pigmentation, one involved in inflammation, and one associated with susceptibility to autoimmune diseases such as rheumatoid arthritis. But the ones Netea and Bertranpetit were most excited about were a cluster of three immune system genes found on chromosome 4. These genes code for toll-like receptors, proteins which latch on to harmful bacteria in the body and launch a defensive response. “We knew they must be important for host defense,” Netea says.A remarkable piece of research, opening up all sorts of interesting avenues.
What events in history might have favored these versions of the genes in gypsies and Romanians, but not in Indians? Netea and his colleagues tested the ability of the toll-like receptors to react to Yersinia pestis, the bacterium that caused the Black Death. They found that the strength of the immune response varied depending on the exact sequence of the toll-like receptor genes.
Netea and Bertranpetit propose that the Roma and European Romanians came to have the same versions of these immune system genes because of the evolutionary pressure exerted by Y. pestis. Other Europeans, whose ancestors also faced and survived the Black Death, carried similar changes in the toll-like receptor genes. But people from China and Africa—two other places the Black Death did not reach—did not have these changes.
What if our susceptibility to Rheumatoid arthritis was part of our defense against the plague?