Black Death Made Humans Evolve

Charotar Globe Daily

Infectious diseases are one of the strongest selection factors in the evolution of our species. Now, researchers have found genetic evidence that human evolution may have been shaped by the Black Death.

Their study, published in the journal Nature, has found evidence of positive selection for specific variants of certain genes by comparing the frequency of these variants in populations before and after the 14th-century pandemic. However, such rapid natural selection may have come at a great cost.

The Black Death was one of the deadliest pandemics in human history. It began to spread through Europe, the Middle East and North Africa in 1346 and killed up to 30 to 50 percent of the population at the time.

The disease is caused by the bacterium Yersinia pestis, which is carried by rodents and spread by the bite of infected fleas. “This bacterium displays an astonishing ability to grow massively in the different organs it infects,” Javier Pizarro-Cerdá, one of the study’s co-authors, told Newsweek. “This massive bacterial replication will produce multi-organ dysfunction…leading in most cases to death.”

Charotar Globe Daily yersinia perstis bacteria
A stock image shows an artist’s impression of Yersinia pestis, the bacteria that caused the Black Death in the 14th century. People with certain genetic variants would have been more resistant to this bacterium.
Peddalanka Ramesh Babu/Getty

Subsequent waves of plague continued for the next 400 years, although many of these waves were associated with reduced mortality. One hypothesis for this reduction is that humans were able to evolve genetic adaptations to resist the bacterium.

In the latest research, the team analyzed 206 samples of ancient DNA from individuals who had been alive before, during and after the Black Death in London and Denmark.

Four distinct genetic variants were identified in the London and Denmark populations that were common in the post-plague groups but were rarely seen in the pre-plague samples. One of these variants was shown to control Y. pestis in laboratory experiments with white blood cells.

In many cases of evolution, natural selection acts on a newly formed genetic variant, which can take many generations to spread throughout a population. “In the case of selection described in our paper, selection did not act on a newly emergent variant,” Luis Barreiro, another of the study’s co-authors, told Newsweek. “Instead, it acted on a variant that was already present in the population but that became advantageous when Y. pestis appeared.”

Charotar Globe Daily Member of Barriero lab
A member of the laboratory run by Luis Barreiro, one of the researchers who have identified evidence of human evolution after the Black Death.
The University of Chicago Medicine

He continued: “The genetic variant that we show to have been protective is involved in the chopping of small peptides from the bacteria [in order] for them to be presented to other immune cells, notably CD8 [killer] T cells,” Barreiro said.

Given the immunological role of these variants, it seems likely that selection for these genes would have conferred a certain amount of resistance to the bacterium. “We think that being able to present those Yersinia pestis–derived peptides allowed for a more robust immune response against the pathogen,” Barreiro said.

However, this same variant has also been linked to an increased likelihood of developing autoimmune diseases, such as rheumatoid arthritis and Crohn’s disease. In other words, it can lead to an overactive immune system.

“In the context of autoimmune diseases, the immune system erroneously responds against the self,” Barreiro said. “In the case of Crohn’s disease, for example, the host immune system responds against commensal bacteria in the gut—the good bacteria…. Presenting peptides from those good bacteria to CD8 T cells might lead to an exacerbated response that will lead to increased inflammation.”

The study’s findings raise questions about the effects of contemporary and future pandemics on human evolution. Whether a pandemic leaves a lasting impact on the human genome will depend on the age at which the infection has the most severe consequences.

“We do not expect COVID-19 to be “seen” by natural selection because it primarily kills people after reproductive age,” Barreiro said.

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