Scientists have now sequenced ancient herpes DNA from the rotting teeth of human remains

• Researchers sequenced the genome of a strain of ancient herpes from four human remains.
• Before this study, genetic data for herpes only went back to 1925.

Researchers, who for the first time successfully sequenced the genome of an ancient herpes strain, say our modern-day strain of the virus arose around 5,000 years ago.

In a study published Wednesday in the journal Science Advances, researchers looked at the remains of four individuals stretching over a thousand-year period. They extracted herpes DNA from their rotting teeth, as the viral infection often flares up with mouth infections, and sequenced its genome, or the complete set of genetic information. Then researchers compared the ancient DNA to modern-day herpes samples.

The oldest sample dates back to the late Iron Age, around 1,500 years ago, and came from an adult male excavated in Russia's Ural Mountain region. Two other skeletal remains were found in the United Kingdom, a female dating to the sixth or seventh century, and a male from the late 14th century. The final sample came from the skeletal remains of an adult male excavated in Holland, who most likely died during a French attack on his village in 1672.

Herpes simplex virus 1, or HSV-1, is prevalent among modern humans, as it spreads easily and is a lifelong disease once infected. Two-thirds of the global population younger than age 50 carry the strain, according to the World Health Organization. Still, ancient examples of HSV-1 have been hard to find.

Before this study, genetic data for herpes only went back as far as 1925, but scientists knew the virus' history stretched back millennia. Comparing the ancient samples with herpes samples from the 20th century allowed researchers to put a timeline on the virus' evolution.

"The world has watched COVID-19 mutate at a rapid rate over weeks and months. A virus like herpes evolves on a far grander timescale," Charlotte Houldcroft, co-author of the study and genetics researcher at the University of Cambridge, said in a press release. "Facial herpes hides in its host for life and only transmits through oral contact, so mutations occur slowly over centuries and millennia."

Facial herpes detected in the ancient DNA may have coincided with the advent of mouth-to-mouth kissing, which has not always been a common practice, researchers wrote in the new study. Kissing as a sign of affection began during the Bronze Age, and may have spread westward, from South Asia into Europe and Eurasia.

Centuries later, the researchers said, Roman Emperor Tiberius tried to ban kissing at official functions to limit the spread of disease — a decree that may have been related to herpes.

"Every primate species has a form of herpes, so we assume it has been with us since our own species left Africa," Christiana Scheib, co-author of the study and researcher at the University of Cambridge, said in a press release. "However, something happened around 5,000 years ago that allowed one strain of herpes to overtake all others, possibly an increase in transmissions, which could have been linked to kissing."

Now researchers aim to track the virus even further back in time, into early human history. "Neanderthal herpes is my next mountain to climb," Scheib added.