Evolution is a pretty impressive thing. It selects organisms with the best characteristics to survive in a given set of circumstances.
So when bubonic plague ripped through Europe in the mid 1700s, it’s not surprising that people with the best ability to survive had an advantage and passed that onto their children.
What is surprising is that those genes are still present in the population and may be causing autoimmune diseases such as Crohn’s disease and rheumatoid arthritis – 700 years later.
Peaking in the middle of the 1300s, the Black Death is estimated to have claimed 200 million lives across Europe, Asia, and Africa. It was caused by the Yersinia pestis bacterium and transmitted to humans via fleas (everyone blames the rats but they were just the carrier).
Early symptoms of the disease included sweats and vomiting, but this soon gave way to uncontrollable spasms as the body lost its ability to control muscle function. Black bruising under the skin and black pus-filled buboes (large swellings) developed in the groin or under the arms, giving rise to the name.
According to Historic UK:
At the time, it was thought that should the buboes burst on the fourth day, you may have a slim chance of survival, but historians now believe that 70% of victims died within five days. As the disease developed into another strain called pneumatic plague and became airborne, the survival rate evaporated: now 100% of those contracting the pneumatic plague died. In total 30-40% of the English population perished and in some villages, the death toll reached 80-90%. It is estimated that London’s population reduced from 100,000 to 20,000 in a single generation.
Mutations for survival
Researchers analysed teeth from 206 skeletons from London and Denmark. They focused on a 100-year window looking at those who died before the plague (retrieved from a London mass grave), those who died during, and those who survived and died sometime later.
By comparing the genomes, the researchers were able to identify four genes that were associated with the Black Death. They were selected with a speed never seen either before or since in human history – there was a 10 per cent shift over three generations.
One gene, ERAP2, meant that you were 40 per cent more likely to survive if you had two identical copies.
While ERAP2 and its ilk were the difference between life and death 700 years ago, today they have been linked to autoimmune diseases and cancer.
So while this isn’t good news for those of us with these diseases, it is interesting to consider how pandemics can modify genomes but go undetected in modern populations. And that there was once a time when an overactive immune system wasn’t a bad thing.