The preservation of DNA in the teeth of humans dating as far back as 1,500 years have revealed the ancient origins of the pervasive herpes virus strain that causes cold sores.
The genomic data suggests that herpes simplex virus type 1 (HSV-1), which today infects around 3.7 billion people globally, emerged and proliferated around 5,000 years ago. And may it have coincided with a new cultural phenomenon that arose and spread at the same time: the smooching of one's boo (or boos).
"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," said geneticist Charlotte Houldcroft from the University of Cambridge in the UK.
"Facial herpes hides in its host for life and only transmits through oral contact, so mutations occur slowly over centuries and millennia. We need to do deep time investigations to understand how DNA viruses like this evolve. Previously, genetic data for herpes only went back to 1925."
The herpes family has a broad and long history, spanning multiple species, and going back millions of years. Of the 115 herpesviruses that we currently know of, only eight infect humans. Herpes simplex virus 1 (HSV-1) is the most common of these, associated with cold sores. The strain associated with genital herpes, HSV-2, affects around half a billion people.
Infection with either of these strains is for life; there is currently no known cure, although outbreaks can be treated and managed.
How HSV-1 emerged as the dominant human strain has been something of a mystery, and surprisingly difficult to trace. So a team of researchers decided to look more carefully into ancient remains.
As DNA sequencing has become quicker and less costly, archaeologists, over recent years, assembled libraries of the DNA retrieved from ancient remains. From these libraries, researchers went looking for traces of HSV-1 in the archaeological record – and found it extremely scarce.
"We screened ancient DNA samples from around 3,000 archaeological finds and got just four herpes hits," said genomicist Meriam Guellil of the University of Tartu in Estonia.
Those four individuals spanned a timeframe of a thousand years. The most recent was a young male in the Netherlands, who was probably massacred during a French raid on his village in 1672. The wear on his teeth suggested he was a heavy smoker who used a clay pipe.
Two of the individuals hailed from Cambridge in the UK. One was a young adult male from the late 14th century, who was buried on the grounds of a medieval charitable hospital. His teeth showed significant signs of horrendous dental abscesses. The other, an adult woman, lived and died in Cambridgeshire in around the 6th to 7th centuries, and her dentition also showed signs of gum disease.
The oldest remains were of an adult male from Russia who lived and died around 1,500 years ago. Since HSV-1 tends to flare up when the patient has a mouth infection, finding traces of the virus in individuals with gum disease, abscesses, or who smoked tobacco is not hugely surprising.
With just these four cases, the team was able to sequence the herpes DNA, look at the differences between the four cases, and work out a mutation rate for today's strain of HSV-1.
This returned a timeline that suggested the form of HSV-1 plaguing the world today arose during the Bronze Age, after humans had migrated from the Eurasion steppe grasslands and into Europe, producing a population boom.
"Every primate species has a form of herpes, so we assume it has been with us since our own species left Africa," said archaeologist Christiana Scheib of the University of Cambridge and the University of Tartu.
"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."
The history of romantic kissing is a murky one, but previous research has found that it is not universal among humans. That same research found that the more socially complex a culture, the higher the frequency of romantic kissing. As humans migrated, spread, and settled in the Bronze Age, it's possible that kissing became more common too.
Without a clear method for tracing back the dawn of tonsil hockey, the speculation remains strictly hypothetical. Even conclusions on the origins of HSV-1 remain open to some tweaking, given the challenges involved in identifying the virus in ancient bones.
"Our work therefore highlights the need for more extensive coverage of modern HSV-1, particularly in regions such as Asia and Africa, together with additional observations provided by aDNA samples," the researchers wrote in their paper.
"Further ancient genomes, for example, from the Neolithic period, may further revise our understanding of the evolutionary history of this today ubiquitous pathogen and continue to inform on the nature of its association with human hosts."
The research has been published in Science Advances.