Ancient smallpox genomes reveal how European colonization brought the disease to the Americas

Ancient smallpox genomes reveal how European colonization brought the disease to the Americas Researchers from Trinity College Dublin have uncovered and analyzed the first ancient smallpox virus genomes ever identified in the Americas, offering the strongest direct evidence to date that the disease was introduced through European colonization. The study, published in Science, examined viral DNA retrieved from two individuals buried in northern Chile between the late 15th and early 17th centuries. These samples revealed that the viruses belonged to a now-extinct lineage of the variola virus, which lies genetically between medieval European strains and more recent ones. This finding confirms that smallpox arrived in the Americas during the colonial era, aligning with historical accounts of its devastating impact on Indigenous populations. The arrival of smallpox coincided with the rapid expansion of European settlements in the Americas, marking a turning point in the region’s demographic and cultural history. Smallpox, caused by the variola virus, contributed significantly to the dramatic population decline among Indigenous peoples following contact with Europeans. Historical estimates suggest that these declines may have resulted in the deaths of hundreds of millions of people, fundamentally altering the social and political landscapes of the continent. Until now, there had been no direct genetic evidence linking smallpox outbreaks in the Americas to strains originating in the Old World. The study’s analysis of the ancient genomes showed that the virus underwent substantial genetic changes up until the 16th century. However, its evolution slowed considerably during the 17th and 18th centuries, a period marked by intense colonial activity and frequent smallpox epidemics. According to Dr. Lara Cassidy of Trinity’s School of Genetics and Microbiology, this suggests the virus may have reached an evolutionary equilibrium, enabling it to spread efficiently without requiring further adaptation. She noted that this stability might have been reinforced by the low levels of immunity in newly exposed populations. The researchers also observed that this evolutionary slowdown appeared to end in the 19th century, coinciding with the rise of vaccination programs. As immunization efforts expanded, the rate of viral evolution resumed, indicating that public health interventions can influence the trajectory of pathogen evolution. Dr. Bruno Romero González, a doctoral student at Trinity’s School of Medicine, emphasized that the study demonstrates how major historical events, including colonization, population shifts, and medical advancements, have shaped the evolution of one of humanity’s most deadly diseases. Beyond its implications for understanding the past, the research has broader relevance for contemporary public health. It highlights how factors such as population movement, immunity levels, and healthcare policies can affect the evolution of infectious diseases. Questions raised by the study are particularly pertinent to modern pathogens like influenza, SARS-CoV-2, and monkeypox, all of which exhibit complex patterns of mutation and transmission. The findings underscore the importance of studying ancient pathogens to better understand and combat current and future disease threats. For archaeologists and historians, the study represents a significant advancement in the field of ancient pathogen genomics. By analyzing ancient DNA, scientists can uncover aspects of disease history that are often absent from written records. This approach offers new insights into how past epidemics affected human populations and how they interacted with societal structures. The work also sets a precedent for using genetic data to explore the interplay between human behavior and microbial evolution, opening avenues for further interdisciplinary research. The discovery adds another layer to the story of smallpox’s global journey, reinforcing the idea that human activities have played a crucial role in shaping the course of infectious diseases. As scientists continue to explore the genetic histories of pathogens, they gain valuable tools for addressing both historical mysteries and pressing modern challenges. The study’s findings serve as a foundation for future investigations into the dynamic relationship between humans and the microbes that have shaped our world.