Researchers from Trinity College Dublin have discovered the first direct molecular evidence that European colonizers introduced smallpox to the Americas. By analyzing the genomes of two naturally mummified individuals from northern Chile who lived between 1492 and 1631, the team identified a now-extinct variola virus lineage linked to European strains.
For centuries, the devastating impact of smallpox on Indigenous populations was documented primarily through the written accounts of European colonizers. While historians long suspected these pathogens were the primary drivers of population collapse, the biological proof remained elusive until now.
The Camarones 9 Mummies and Unexpected DNA
The discovery was an accidental byproduct of a broader anthropological effort. Researchers were not hunting for pathogens; instead, they were examining bone samples collected in 1990 from the Camarones 9 archaeological site in northern Chile to trace human ancestry and population history. While the human DNA was badly degraded, the viral DNA was remarkably well-preserved.
Both were naturally mummified and buried in bundles wrapped in camelid wool blankets.
The viral genomes recovered from the two individuals were almost identical. This suggests they were likely infected during the same outbreak or by the same circulating strain. This biological evidence also recontextualizes physical findings at the site; skin lesions on the mummies, which had been previously attributed to chronic arsenic exposure, now appear to be the hallmark pus-filled sores of a smallpox infection.
Mapping the Evolution of the Variola Virus
By comparing these ancient genomes with other known samples, the researchers reconstructed a viral family tree. The strain found in the Chilean mummies emerged around 1296 C.E. and belongs to an extinct lineage that sits evolutionarily between medieval European strains and those that circulated in later centuries. This placement provides the first molecular confirmation that the disease was carried to the Americas during the colonial period.
According to study co-author Lara Cassidy, this deceleration occurred because the virus had reached an evolutionary optimum
. Because Indigenous populations had no previous exposure and thus no natural immunity, the virus could spread successfully with little further adaptation.
The Demographic Collapse of the Americas
The molecular evidence corroborates a historical catastrophe. While the first documented outbreak in the Americas occurred on Hispaniola in 1518, the virus rapidly swept through the continent.
| Timeline Event | Historical/Genetic Detail |
|---|---|
| 1296 C.E. | Emergence of the lineage found in the Chilean mummies |
| 1492 | European arrival in the Americas |
| 1518 | First documented American outbreak (Hispaniola) |
| 1530s | Spanish topple the Inca emperor |
| 1796 | Edward Jenner introduces the first vaccine |
| 1980 | WHO declares smallpox officially eradicated |
Implications for Modern Pathogen Research
Though smallpox was eradicated in 1980, studying its ancient DNA remains relevant for modern public health.
Despite this breakthrough, several questions remain. The authors of the study note that it is still uncertain which specific group transported the virus—whether it was the initial waves of settlers starting with Columbus in 1492 or if it traveled via Africa during the transatlantic slave trade.
