Ancient smallpox DNA in Chilean mummies confirms European origin of devastating pandemic

Two mummified bodies buried in Chile centuries ago carry within their preserved tissue the genetic fingerprints of smallpox—fragments of viral DNA that have now confirmed what historians long suspected: the disease arrived with European colonists and devastated Indigenous populations across the Americas.

The remains date to somewhere between 1492 and 1631, a span that encompasses the collapse of Inca rule and the establishment of Spanish colonial authority. When researchers sequenced the viral material recovered from these individuals, they found something striking: the Chilean smallpox strains belonged to a lineage that had branched away from other known variants around 1296, and on the evolutionary family tree, they positioned themselves between early medieval European viruses and the strains that would circulate in later centuries. The timing and genetic relationships point unmistakably toward a single conclusion—smallpox reached this South American community through the same route that brought Spanish conquistadors and settlers across the Atlantic.

Historical records had already documented the catastrophe. The first recorded outbreak in the Americas occurred on Hispaniola in 1518, just 26 years after Columbus. Throughout the 16th century, successive waves of smallpox swept through Indigenous populations with apocalyptic force, becoming one of the primary engines of the demographic collapse that accompanied European conquest. The virus killed far more people than any sword or musket. Now, for the first time, scientists have recovered ancient smallpox genomes from the Americas themselves—direct molecular evidence that corroborates the historical record.

The mummified remains also bore small skin lesions, which earlier research had attributed to chronic arsenic poisoning. The region's rivers naturally carried arsenic from surrounding rock formations, contaminating food and water supplies. But the discovery of smallpox infection adds another possible explanation for those marks. The genetic evidence confirms both individuals carried the virus, though it cannot determine whether smallpox caused their deaths or whether it alone produced the lesions they bore. Both explanations remain plausible.

One question has circulated since the genomes were announced: could ancient smallpox return? The answer is no, and the distinction matters. The researchers recovered only fragments of viral DNA, degraded and broken into short pieces by centuries of decay. They used computers to reconstruct what the complete genomes might have looked like, assembling the genetic sequences like a puzzle. But assembly on a computer is not the same as creating a living virus. A functional poxvirus particle contains not just genetic material but also the proteins and enzymes necessary to invade a cell and begin reproducing. The ancient fragments lack this machinery entirely. They cannot spontaneously cause infection. Even deliberate laboratory reconstruction would require highly specialized procedures—as demonstrated in 2018 when researchers synthesized horsepox virus from synthetic DNA, a process that demanded helper viruses and equipment far beyond standard molecular biology.

What the genomes do reveal is how smallpox became a human-only pathogen. Related poxviruses can infect multiple animal species, carrying accessory genes that help them adapt to different hosts and evade various immune systems. Variola virus, by contrast, infects only humans. Over centuries of evolution, many of these accessory genes became unnecessary for human-to-human transmission. Some remain as non-functioning copies called pseudogenes; others vanished entirely. Scientists call this process reductive evolution. By comparing which genes are intact, disabled, or absent in ancient and modern smallpox genomes, researchers can trace how the virus became increasingly specialized for human infection alone.

The Chilean sequences also reveal something unexpected about the virus's evolutionary pace. Gene inactivation proceeded at a steady rate until the late 16th century, then slowed, then accelerated again. The authors propose an explanation: during the massive colonial-era epidemics, smallpox was already perfectly suited to spreading among populations with no acquired immunity. Few new variants gained an advantage, so genetic change accumulated more slowly. Later, when vaccination was introduced in 1796, it altered the immune environment, potentially changing which mutations persisted. The timing suggests a connection, though the ancient genomes cannot prove causation.

These Chilean sequences fill a crucial gap in the viral family tree—the missing link between medieval European strains and the later lineages that circulated globally. They are helping researchers understand not just how smallpox reached the Americas, but how it evolved as it spread across a world of human populations it had never encountered before. The virus that killed millions across the Indigenous Americas left its mark in the bodies of the dead, and that mark has now been read.