On Jun. 30, 2025, a new study from an international team of researchers led by the Max Planck Institute for Evolutionary Anthropology has reconstructed two 4000-year-old genomes from the rare pathogen Mycobacterium lepromatosis. Hansen’s Disease, more commonly known as leprosy, is a chronic disease that can lead to physical impairment. Today it exists in over 100 countries, and while the infection is treatable, access to treatment varies widely with socioeconomic conditions. Its mention in historical texts give us a glimpse into its past impact on population health in Europe and Asia.

Prolonged untreated infection can result in characteristic changes in bone, and these have been documented in archaeological skeletons as early as 5000 years ago in Europe, Asia, and Oceania. So far, absence of these characteristic changes in the pre-contact American contexts suggests that leprosy was one of the many diseases introduced to the continent in the colonial period. Thereafter it afflicted humans and curiously also armadillos.

From a genetic perspective, Hansen’s Disease is caused by either the globally dominant Mycobacterium leprae or the newly identified and rare Mycobacterium lepromatosis. The recovery of M. leprae from archaeological bone in Europe suggests the disease originated in Eurasia sometime during the Neolithic transition about 7000 years ago. Similar emergence estimates have been proposed for other notorious diseases such as plague, tuberculosis, and typhoid fever. Ancient genomes for M. lepromatosis have remained elusive, and these may hold important clues on the history of Hansen’s Disease.

The pathogen is related to all known modern forms of M. lepromatosis, but as there are so few genomes available for comparison, there is still much to be learned about it. This work has shown that a pathogen considered rare in a modern context caused disease for thousands of years in the Americas. Rodrigo Nores, professor of Anthropology at the National University of Córdoba, Argentina is convinced that more cases, both ancient and modern, will be identified in the coming years: “this disease was present in Chile as early as 4000 years ago, and now that we know it was there, we can specifically look for it in other contexts”.

Once more genomes surface, the researchers will be able to piece out further details of its history and better understand its global distribution today. The pathogen has recently been discovered in squirrel populations from the United Kingdom and Ireland, but in the Americas it has yet to be found in any species other than humans. With such little data, mystery still surrounds its origin. “It remains to be determined if the disease originated in the Americas, or if it joined some of the first settlers from Eurasia”, adds Bos. “So far the evidence points in the direction of an American origin, but we’ll need more genomes from other time periods and contexts to be sure.”