Smallpox DNA is recovered from Spanish conquest-era Chilean mummies

When Spanish conquistadors arrived in the New World some five centuries ago, they brought muskets, cannons, armor, horses and attack dogs as they set about colonizing the Americas. But the biggest blow they delivered came from something else entirely — smallpox and other infectious diseases that ravaged indigenous populations. Researchers now have obtained the first direct molecular evidence of the smallpox unwittingly carried from the Old World to the New World that wiped out untold millions of people in Mesoamerica and South America. They recovered ancient viral DNA from the mummies of two people who died during the early Spanish colonial period in an Inca community in northern Chile. Smallpox is caused by a pathogen called the variola virus. An analysis of the genomes of the variola viruses present in the bodies of these two people showed they belonged to a now-extinct lineage of the pathogen situated evolutionarily between strains known from medieval Europe and those that circulated in later centuries. Europeans arrived in the Americas in 1492. The indigenous people had no previous exposure to smallpox and, therefore, no natural immunity. The virus was transmitted by person-to-person contact, triggering a cataclysmic population collapse. “The earliest documented smallpox outbreak in the Americas occurred on the island of Hispaniola in 1518, while it was being settled by the Spanish,” said Bruno Romero González, a geneticist and a doctoral student at Trinity College Dublin, and lead author of the study published on Thursday in the journal Science. Hispaniola is now shared by Haiti and the Dominican Republic. “Smallpox was one of the most devastating infectious diseases in human history,” Trinity College Dublin geneticist and study co-author Shigeki Nakagome said. “It is thought to have affected human populations for at least 1,500 to 2,000 years — possibly longer — before it was eradicated through a global vaccination campaign in 1980. Before eradication, it caused repeated epidemics around the world, and is estimated to have killed hundreds of millions of people.” In the Americas, smallpox and other viral diseases killed, according to some estimates, roughly 90% of the indigenous people. This helped the Spanish vanquish the Aztec Empire in modern-day Mexico, the Maya city-states in Central America and the Inca Empire that stretched from modern-day Ecuador through Peru and into Chile. NATURALLY MUMMIFIED The two people whose bodies bore the smallpox pathogen lived in the southern reaches of the former Inca Empire, at a site close to the Pacific coast near Camarones Cove in Chile. One was a woman 30 to 35 years old. The other was a man 18 to 20 years old. “The individuals were naturally mummified and buried in bundles, wrapped in camelid wool blankets and accompanied by various grave goods,” said study co-author Constanza de la Fuente Castro, a Universidad de Chile biological anthropologist specializing in ancient DNA. “It is important to distinguish between artificial and natural mummification. While their bodies were wrapped in textiles, they were not chemically or physically altered like Egyptian mummies. This practice of bundle burial predates the Inca and was common across earlier periods,” she added. The researchers were unable to determine precisely when the two were infected, but narrowed it down to a range centering on the 16th century. The Spanish, led by Francisco Pizarro, toppled the Inca emperor in the 1530s. In addition to killing millions, smallpox delivered a psychological blow. Its rapid and deadly spread — causing high fevers, bleeding pustules and severe disfigurement — led many indigenous people to feel that their traditional gods had forsaken them or were impotent against whatever spiritual force assisted the Spanish invaders. The viral genomes recovered from the two people provided insight into the pathogen’s evolution. The genomes were almost identical, suggesting that the two victims were infected during the same outbreak or by the same circulating viral strain. The researchers found that the pathogen had undergone a long period of genetic evolution up to the 16th century, but that this decelerated during the major smallpox epidemics accompanying the Spanish colonization, picking up steam again only after the first vaccine was introduced in 1796. This, they said, suggests the virus had achieved an evolutionary optimum that let it spread with scant need for further adaptation as it burned through populations lacking immunity. The last naturally occurring smallpox case was recorded in Somalia in 1977. Three years later, the World Health Organization declared smallpox the first — and still only — human infectious disease eradicated worldwide.