The genomic identity of early smallpox in South America

Abstract

Smallpox was a major driver of population collapse in the Americas after European contact, yet the genetic identity of the causal strains remains unknown. Here, we report the first ancient smallpox genomes in the Americas, dating to approximately 1492–1631 common era (CE), recovered from two Inca-Colonial individuals in northern Chile. These genomes form a now-extinct lineage that diverged around 1296 CE, after the splitting of early medieval European strains but before the emergence of modern variola lineages, providing direct molecular evidence for smallpox introduction through European colonization. We further identify a constant tempo of gene inactivation until the late 16th century, followed by a phase of constraint and a subsequent rebound in substitution rates, linking variola virus evolution to major shifts in human demography and epidemiology.