Although its origins are unknown, smallpox was formerly one of humanity’s most destructive diseases. Historical records and scientific theories on the origins of the smallpox virus have diverged for many years.
Currently, a recent study shows that the virus is 2,000 years older than scientists had previously suggested. This confirms historical records and establishes for the first time that the sickness has afflicted human cultures from antiquity.
The paper appears in the journal Microbial Genomics, published by the Microbiology Society.
The variola virus that causes smallpox is likely most known for being the only contagious human illness that has been eradicated globally. However, the illness was a significant cause of death until recently, taking the lives of at least 300 million people over the 20th century. This is roughly the equivalent of the population of the United States.
Until relatively recently, the earliest genetic evidence for smallpox was only from the 1600s. The virus was then discovered to have existed at least another 1,000 years earlier in 2020 according to a study that examined the bones and dental remains of Viking-age skeletons.
However, some historians believe that smallpox has been around since long before the Vikings. Suspicious scarring on ancient Egyptian mummies (including the Pharoah Ramses V who died in 1157 BC) leads some to believe that the history of smallpox stretches back at least 3,000 years. So far, the missing piece of scientific evidence to support this theory has remained hidden.
Variola virus may be much, much older than we thought. This is important because it confirms the historical hypothesis than smallpox existed in ancient societies. It is also important to consider that there are some aspects in the evolution of viruses that should be accounted for when doing this type of work.
Dr. Diego Forni
By comparing the genomes of modern and historic strains of variola virus, researchers at the Scientific Institute Eugenio Medea and University of Milan in Italy have traced the evolution of the virus back in time. They discovered that distinct smallpox strains all descended from a single common ancestor and that just a small portion of the genetic elements discovered in genomes from the Viking era had survived until the 18th century.
They also worked out an estimate for when the virus originated. In their estimate, the researchers accounted for something called the ‘time-dependent rate phenomenon’. As a result, viruses appear to change more quickly over brief periods of time and more slowly over longer periods of time, indicating that the rate of evolution is dependent on the length of time over which it is being observed. The phenomenon has been well-documented in DNA viruses like variola.
To more precisely date evolutionary events like the emergence of a new virus, scientists can account for the time-dependent rate phenomena using a mathematical equation. This gave the team a new estimate for the first emergence of smallpox: more than 3,800 years ago. Just as historians have long suspected.
The researchers anticipate that these results will put to rest a protracted debate and shed new light on the origins of one of humanity’s deadliest diseases.
“Variola virus may be much, much older than we thought,” said Dr. Diego Forni, first author of the study. “This is important because it confirms the historical hypothesis than smallpox existed in ancient societies. It is also important to consider that there are some aspects in the evolution of viruses that should be accounted for when doing this type of work.”
