One of the most striking findings of the study emerged from the comparison of ancient genomes reconstructed from remains found in Canada and Argentina. Despite the geographic distance of several thousand kilometers between these two regions, the ancient strains of M. lepromatosis were found to be remarkably genetically similar. Both samples dated to approximately 1,000 years ago, yet they appeared to belong to two distinct but closely related branches of the Mycobacterium evolutionary tree. These branches are more closely related to each other than to any other known lineage of the bacteria.
The scientists argue that such genetic proximity across such a vast distance implies a rapid spread of the pathogen across the continent. It is likely that the disease moved through human populations and perhaps animal reservoirs within just a few centuries, covering the span from the northern reaches of North America to the southern tips of South America. This rapid dissemination suggests that the Indigenous societies of the era were more interconnected than often portrayed, with trade, migration, and social contact facilitating the movement of the bacteria across diverse environments and climates.
Furthermore, the genomic analysis identified several previously unknown lineages of the pathogen. Most notably, the team discovered an ancestral branch of M. lepromatosis that diverged from the rest of the species’ known diversity more than 9,000 years ago. Remarkably, this ancient lineage is not an extinct relic; the study found that it continues to infect humans in North America today. This finding points to an incredibly ancient and long-lasting diversification of the pathogen on the continent. It also suggests that there is a wealth of unexplored genetic diversity within pathogenic Mycobacteria that remains to be discovered, potentially hidden in both historical samples and contemporary clinical cases.
The research also provides a surprising twist regarding the presence of M. lepromatosis in Europe. In 2016, the scientific community was surprised to find this specific species infecting red squirrels in the United Kingdom. At the time, the origins of this infection were unclear. However, the new analyses suggest that the strains found in British red squirrels are actually part of an American lineage. The data indicates that this lineage was likely introduced to the British Isles from the Americas during the 19th century, after which it spread within the local wildlife population.
This discovery highlights the ongoing and often bi-directional ability of pathogens to cross continents through human activity. While much of the historical focus has been on diseases moving from the Old World to the New World, this study illustrates a clear instance where a pathogen lineage from the Americas established itself in Europe during the modern era, likely through commercial exchanges or the movement of people and goods. It serves as a reminder that the global movement of infectious diseases is a continuous process that did not begin or end with the initial period of colonization.
