20 October 2022

Traces of the plague have been preserved in the genome

The Black Death has rewarded modern Europeans with a tendency to autoimmune diseases

Alexander Berezin, Naked Science

According to modern ideas, the black death in the XIV century killed from 30 to 50 percent of the population of Africa and Eurasia for a few decades. The total number of deaths as a result of that pandemic is at least as high as the number of victims of the Second World War, and by many estimates exceeds them. Considering that the Earth's population was much smaller then, this is the most serious test in the history of our species over the last ten thousand years.

A new work by an international team of researchers published in Nature (Klunk et al., Evolution of immune genes is associated with the Black Death) has revealed an unexpected side of the epidemic: it left a noticeable mark in the genes of those who survived it — at least if we are talking about Europe. Moreover, this trace was useful in the Middle Ages and Modern times, but it is definitely harmful today.

The authors studied the DNA of 206 people from European populations who lived before, during and after the plague pandemic of the XIV century. As a result, they found that one of the genes that became more common in the DNA of populations after the black death should make it easier for the immune system to fight the bacterium that causes the plague. Tellingly, this gene has become more common not only in the populations of medieval England, but also in modern Denmark.

That is, we are talking about the fact that the plague spurred a kind of convergent evolution of two different, spatially remote populations. Perhaps it had a similar effect on the inhabitants of Ancient Russia of that period or China with Africa: the authors of the work did not cover the ancient DNA from there.

In addition to one particular gene, at least dozens of other genes have become more common, but it is quite difficult to say with certainty what the functions of many of them are in terms of immunity. A significant part of mutations in the human genome in general can be neutral and still spread in the population if it was accidentally present in a line of individuals with increased resistance to plague.

One thing is for sure: when the authors of the work grew cell cultures (from human cells) that had those genes that began to occur more often after the epidemic, their defeat by plague bacteria turned out to be difficult. The plague bacteria affected the cell culture much more easily without such features.

It turns out that the plague significantly modified the human gene pool in just a few generations. Although it did not bring them completely new genes, it dramatically increased the proportion in the population of those whose genes allowed them to resist it better.

Although up to the XVII-XVIII centuries these features were useful, after the advent of full-fledged quarantines, the plague ceased to be a mass disease. That is, the benefits of such mutations have been lost for today.

However, the same genes increase the risk of autoimmune diseases, the authors note. Consequently, the "plague" effect on modern people is not just that the disease changed the history of human societies in the XIV-XVIII centuries. It turns out that the plague has a negative effect on the health of our species so far, 700 years after its strongest outbreak — even if this influence is indirect.

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