Unlocking the Mysteries of the Black Death: Historians and Geneticists disagree
Red rashes, blackened fingers, fevers, chills, and nausea were the symptoms of the 14th-century European pandemic known as the Black Death. This pandemic was one of the most devastating in human history, with death counts ranging from 75 million to 200 million people. The Black Death indelibly impacted medieval European society, influencing cultural, economic, and religious ideas for ages.
Considering these impacts, geneticists and historians alike have believed that the Black Death significantly impacted the human genome, especially in areas concerning immunity and health. This is highly plausible based on what is already known about population genetics. Usually, when there is a sharp reduction in the size of a population, also known as the bottleneck effect, one expects a decrease in genetic diversity, especially when disease is the cause. Genetic diversity is the variation resulting from humans having distinct combinations of inherited alleles (one of two or more alternative forms of a single gene). Hence, when the Black Death wiped out half of Europe, one would expect that the aftermath produced a population lacking genetic diversity where rare gene variants are no more, a wholly genetically different demographic. However, a recent study shows no evidence of changes in immune and health-related genes in populations before and after the Black Death. Furthermore, they found that after analyzing genetic ancestry, there is no evidence of long-distance migration after the plague, which would be expected because of labor shortages. Both of these things contradict what historians have believed and dramatically shift the narrative surrounding what we know about medieval society pre- and Black Death.
The researchers set out to determine whether or not evidence from ancient DNA (aDNA) could aid in reconstructing what happened during the Black Death and how it affected the human genome. To do this, they collected skeletal remains from medieval Cambridgeshire, an East England county. For a comprehensive study group, they collected data from many different social groups like urban and rural parishes, inmates, friars, and patrons. In total, they extracted DNA from 250 medieval skeletons and 25 post-medieval skeletons to report 275 ancient genomes (genomes isolated from ancient specimens). The age of the skeletons was determined via radiocarbon dating. This method utilizes the rate at which a radioactive isotope of carbon decays to determine the age of organic materials. The researchers took teeth and skull bone samples, purified them, and transferred them to PCR amplification tubes. PCR amplification can make billions of copies of DNA, allowing scientists to experiment with what would initially have been an impossibly small amount.
Next, researchers verified that they were actually working with aDNA, which can be distinguished by certain features such as short fragments and a high frequency of cytosine to thymine substitutions. Substitution is a mutation in a strand of DNA when one nucleotide is replaced by another nucleotide; in this case, it is cytosine being replaced by thymine. The researchers used a program to measure the frequency of these substitutions to verify the authenticity of their aDNA.
Then, the researchers focused on a specific section of DNA called the HLA region. The HLA region is the most gene-dense region of the human genome, and it contains several crucial immune response genes, so it is the perfect place for scientists to measure change, especially when it comes to disease. They measured change in this region by many metrics; however, two stand out: Heterozygosity and Nucleotide Diversity. Both are metrics used in population genetics to estimate genetic diversity or the degree to which more than one allele is observed for a single gene. After running several of these metrics, researchers found no change in the HLA locus between medieval and post-medieval groups during the Black Death.
This is confounding news for the scientific community as it contradicts established research on population genetics. Another confusing finding the researchers discovered in their genetic ancestry analysis is a lack of evidence for long-distance migration after the plague. Historians have believed and constructed the historical record around this, as other evidence has shown that people migrated far away after the plague to find jobs and a new life. This study causes us to question and look back at something we might have missed. It has also shown that whole genome sequencing, when combined with historical context and archaeological evidence, can aid in understanding the mysterious past.
