Researchers from the Lawrence Livermore National Laboratory and the Centers for Disease Control have sequenced and analyzed the various strains of smallpox they have stored away, to discover that, apparently, the pathogen is much older than was originally thought.
The researchers created a molecular clock by looking at the rate of random mutations in the smallpox-causing virus collected in 47 locations around the world, from 1946 – 1977. The variation between the strains was compared to sequences from the most similar animal poxes.
The results indicated that a mild and more severe strain diverged either 16,000 or 68,000 years before present, depending on whether accounts from East Asia or Africa are used to calibrate the molecular clock. In either case, this divergence stretches further back in time than previously believed.
The authors compare hypotheses about where and when strains of the virus evolved. No one hypotheses is ruled out, but an ancient origin seems most plausible since the slowly evolving virus now exclusively infects humans, implying that any intermediate link to an animal host has long since died out.