Yersinia pestis superantigen toxin present 5500 years ago drove lethal outbreaks in Siberian hunter-gatherers before flea transmission evolved

The Macleod-Willerslev study combined shotgun metagenomics on 46 individuals from four East Siberian cemeteries with radiocarbon dating and kinship analysis. Nearly 40 percent of sampled remains yielded plague DNA, exceeding many medieval plague-site detection rates. Multiple contemporaneous burials of related children and adolescents align with rapid, clustered mortality rather than sporadic endemic disease. Archaeological context shows no evidence of rodent commensalism or urban density, indicating direct or alternative transmission routes sufficed for sustained outbreaks.

Earlier genomic work on Bronze Age strains had emphasized missing ymt and other flea-adaptation genes, leading to assumptions of low virulence. The Baikal genomes instead contain a novel superantigen locus absent from all later pandemic lineages. This toxin triggers non-specific T-cell activation and cytokine storms, offering a mechanistic explanation for the observed pediatric mortality spike that had puzzled Baikal archaeologists since the 1990s. The finding reframes pathogen evolution as a stepwise accumulation of virulence modules rather than a single transition to flea-borne efficiency.

These results directly inform contemporary risk assessment by showing that high-lethality phenotypes can persist in low-density reservoirs without modern vectors. Surveillance programs focused solely on rodent-flea cycles may miss ancestral transmission modes that could re-emerge under climate or land-use shifts. Expanded sampling across the Eurasian steppe will be required to determine whether the superantigen was geographically widespread or locally lost.

Future aDNA studies should prioritize functional reconstruction of the ancient superantigen and test its interaction with early human immune genotypes recovered from the same skeletons. Such work would quantify how host-pathogen coevolution has shaped plague severity across five millennia.