The highly contagious and sometimes fatal disease was finally eradicated in the late 1970s through a worldwide vaccination campaign.
Smallpox, one of the most devastating viral diseases ever to strike humankind, had always been thought to have appeared in human populations thousands of years ago in ancient Egypt, India and China, with some historical accounts suggesting that Pharaoh Ramses V - who died in 1145 BC - suffered from smallpox.
The research was published earlier this week, on December 08, in the Current Biology journal.
"The million-dollar question is when did smallpox jump into humans", said Hendrik Poinar, an evolutionary geneticist and a senior researcher on the project.
Historians have long thought that the disease afflicted the populations of ancient Egypt, India and China thousands of years ago. Some historical accounts even suggest the Egyptian pharaoh Ramses V, who passed away in 1145 B.C., had been struck by smallpox. Nevertheless, specialists have conducted several types of tests on the DNA of the mummy to determine the variola virus. The child's death coincided with a series of severe smallpox outbreaks in Europe, making the team hopeful they would find evidence of the disease. There was no evidence of live virus in the sample, meaning there was no chance of the remains being infectious.
And although smallpox now has been eradicated, "we can't become lazy or apathetic about its evolution -and possible re-emergence - until we fully understand its origins".
That's because scientists were able to reconstruct the variola genome from DNA extracted from the naturally mummified remains of the child, allowing them to compare the genetic signature of the 17th-century strain with that of a more contemporary strain dating from 1940 to 1977, when the world's last case of smallpox occurred. Third co-author Edward Holmes of the University of Sydney in Australia suggested that far more research was necessary to discover how the virus ended up spreading amongst the human population in order to get the full picture of its affect on people.
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Holmes states that most believe that such a first virus bearer must have been an animal.
Poinar said the smallpox virus, which infects only humans, evolved into two circulating strains after 1796, when English physician Edward Jenner developed a vaccine to prevent infection. This suggests that the vaccination may have changed the selection pressures acting on the virus and caused it to split into two strains. Seemingly the ancestor of the minor strain corresponded with the Atlantic Slave Trade which likely helped disseminate it across the globe.
According to Ana Duggan, this fact raises relevant questions about a pathogen's evolution. A report from McMaster University in Canada made public Thursday says that the virus may be thousands of years newer than recently thought.
Of course, the date of 1580 is hugely significant.
In the study, the team investigates the remains of a boy to identify the kinds of pathogens dated in the 1600s.
The team behind the new study recovered the smallpox virus by accident.