A recent study has detected a previously unknown ancient clade of the variola virus (VARV) – the causative agent of smallpox – which appears to have been widespread in Britain and Scandinavia during the early medieval period. (A clade is a group sharing an ancestor.) Previous research indicated a most-recent common ancestor from approximately 440 years ago, but the new data push back the earliest definitive smallpox infections by over 1,000 years.
A team led by researchers from the Universities of Cambridge and Copenhagen screened for VARV DNA in 1,867 human remains located across Eurasia and the Americas and ranging in date from 31,630 to 150 years ago. In all, they discovered VARV DNA in 26 individuals and had sufficient sample material to investigate 13 individuals further. All those identified were from northern European contexts, including one from Oxford, and all but two dated to AD 600-1050, suggesting a concentration of the virus across the Viking world.
Four of the VARV genomes were almost completely sequenced. This showed that these Viking-era infections were part of an ancient – now extinct – VARV clade. The most-recent common ancestor between this newly identified clade and the modern one we are more familiar with appears to date to 1,700 years ago. Written records and Egyptian mummies suggest smallpox infected humans much earlier, however, so future ancient DNA studies may help further close the gap between definitive infections and the often-ambiguous descriptions of possible earlier ones.
Helena Wilhelmson from Lund University, an archaeologist on the project, said: ‘The verification of smallpox in these remains offers an opportunity to start a new, and very exciting, discussion on the social aspects of the spread of infectious disease at the individual level. Intriguingly, a significant proportion of these cases were clearly deviant burials, despite the very large number of sampled, mainly normative, burials. For example, there were two mass burials (one in Oxford, one in Öland, Sweden) and also a prone [face down] burial at Ljungbacka in southern Sweden, placed superimposed in the opposite orientation over a second individual in a coffin [BELOW].’
The full results of this project were recently published in the journal Science (https://science.sciencemag.org/content/369/6502/eaaw8977).