Ancient mummy DNA recovered in Chile has yielded the firmest evidence yet that European colonisation brought smallpox to the Americas.
Smallpox was one of the deadliest diseases in human history before it was eradicated in 1980, largely thanks to vaccination. For centuries, the virus had killed about one-third of the people it infected and left most survivors with deep, disfiguring scars.
Outbreaks struck Asia and Europe and eventually spread to the Americas, where colonial writings, administrative records, and missionary reports documented how it killed millions and devastated Indigenous populations who had never been exposed to the virus.
Now, there is scientific evidence for this spread. An analysis of Chilean mummies provides the strongest genetic link yet to the virus as it existed outside the Americas, offering a better picture of how it reached remote populations through colonisation, said geneticist Shigeki Nakagome of Trinity College Dublin.
In the new study, Nakagome and his colleagues examined 13 bone samples previously collected from an archaeological site at Camarones, in northern Chile.
Researchers found evidence of two smallpox infections in ancient DNA extracted from the leg bone fragments of an adult man and woman.
By dating the remains and examining the genes, scientists concluded the two likely died between 1492 and 1631. Scientists also compared the genes in this version of smallpox to other versions and placed it neatly between European strains from medieval times and later variants. This provided crucial molecular evidence suggesting that the strains of smallpox infecting people in the Americas evolved first in Europe.
"Nobody would ever question that smallpox came to this hemisphere from colonisation, but this proves it," said Patricia Foster, a biologist at Indiana University who was not involved in the research published in the journal Science.
Exactly which population brought smallpox to the Americas from the Old World remains unclear, said the study authors. European settlers arrived in different waves following the explorations of Christopher Columbus in the Caribbean in 1492 and Amerigo Vespucci along the South American coast about a decade later. The disease could also have passed through Africa as part of the transatlantic slave trade.
These smallpox infections were identified in an area with no previous record of the disease, raising new questions about how the virus traveled so far south—possibly spreading along Indigenous trading networks that existed long before the arrival of Europeans.
Studying various strains of the virus also allowed scientists to pinpoint how it evolved over time, deactivating certain genes to become more lethal.
In addition to confirming the history of smallpox, the research shines a light on the human cost. The two individuals studied were between 18 and 35 years old, and the disease is likely what killed them.
The findings point to how devastating the disease was across the Americas, wiping out entire communities and leaving them vulnerable to European conquest, said study coauthor Constanza de la Fuente Castro, a biological anthropologist at the University of Chile.
"It was not just cultural or demographic. It is literally written into biological evidence that we can still recover and read today," de la Fuente Castro said.