Ancient Smallpox DNA Recovered from Chilean Mummies Reveals Variola Evolution and American Arrival

Overview

Original publisher Nature reports on the recovery of damaged variola virus DNA from mummified remains of two Indigenous people in northern Chile, dating roughly 1492 to 1631. Researchers reconstructed two variola genomes in silico, providing direct molecular evidence of ancient smallpox lineages and their relationship to European strains. The work clarifies how smallpox reached the Americas and how the virus evolved toward human-specific transmission, while confirming that no intact virus particles were recovered.

• Ancient-DNA fragments were used to assemble two variola genomes
• Chilean genomes belong to an extinct variola lineage and diverged from medieval European strains around 1296
• No live virus or intact genome was recovered; reconstruction was computational
• Findings support a colonial-era introduction of smallpox to the Americas

Overview

In a Nature-style report, researchers describe fragments of variola virus DNA recovered from naturally mummified remains of two Indigenous individuals buried in northern Chile. The remains date from about 1492 to 1631, spanning the transition from Inca rule to the Spanish colonial era. The team sequenced naturally degraded DNA and used computational methods to assemble the fragments into two complete variola genomes. The study provides direct molecular insight into how smallpox was evolving in the centuries before vaccination and how it became adapted to human hosts, while maintaining the historical context that the Americas were affected following European contact.

Ancient DNA and Genome Reconstruction

Ancient DNA typically breaks into short pieces and bears chemical changes with age. The Chilean fragments did not constitute an intact virus; instead, researchers reconstructed the genetic information on computers, aligning fragments and predicting how they once fit together. This reconstruction yields two genomes that occupy a now-extinct branch on the variola family tree, distinct from later lineages that circulated in Europe and the Americas after 1492.

Evolutionary Insights and Host Specificity

The Chilean variola genomes diverged from known variola lineages around 1296. On a phylogenetic tree, the Chilean samples sit after early medieval European strains and before lineages found in later centuries, suggesting the American introduction was contemporaneous with colonization. The genomes illuminate reductive evolution in variola, where certain host-range genes were inactivated or lost, reflecting adaptation to humans as the sole host. This supports the view that variola’s specialization for human infection progressed over time, with implications for understanding why smallpox was so deadly in immunologically naive populations.

Historical Context and Immune Landscape

The study is set against a broader historical backdrop: smallpox arrived in the Americas after European contact, became a major driver of Indigenous population declines, and was eradicated globally in 1980 after a concerted vaccination campaign. The Chilean genomes bridge medieval European strains to later variola lineages, providing a clearer narrative for how smallpox traveled during the colonial era and how vaccination later reshaped its evolutionary trajectory.

Safety and Limitations

Importantly, the study recovered only degraded DNA fragments and did not recreate any live virus. Earlier synthetic reconstructions, such as horsepox, required deliberate laboratory steps beyond what was used here. The Chilean work demonstrates a computational assembly of historical DNA without creating a functional pathogen, addressing biosafety concerns about reviving ancient viruses.

Implications

By filling gaps between medieval European variola genomes and later lineages, the Chilean samples help scientists trace how smallpox migrated to the Americas and how its genome gradually adapted to human hosts. The findings also illustrate how changes in the immune landscape—such as vaccination introduced in 1796—may have influenced which mutations persisted over time, although the study cannot conclusively prove causation.