Blame prehistoric chimps.
Or perhaps our ancestors who hunted them.
Researchers say it appears the modern version of the herpes virus was transmitted from chimpanzees to humans tens of thousands of years ago.
The scientists add that the current version of genital herpes may have evolved from a mixture of two strains of the virus.
They published their findings today in the journal Molecular Biology and Evolution.
Herpes’ evolutionary path
There are two main types of herpes virus around today.
The herpes simplex virus 1 (HSV-1) is transmitted mostly by mouth and is found most often in cold sores. This ailment affects about two-thirds of the world’s population.
The herpes simplex virus 2 (HSV-2) is the main source for genital herpes. It affects about 11 percent of people around the globe.
The HSV-2 strain has been used to help verify the “Out of Africa” theory that humans began migrating from that continent more than 50,000 years ago.
In general, HSV-1 and HSV-2 are considered cousins and have been thought to have evolved separately.
However, the researchers of the study took a closer look at the evolution of the herpes virus, building on the work done in a 2014 study.
The team, led by Sebastien Calvignac-Spencer, PhD, of the Robert Koch Institut in Germany, examined the whole genome sequencing data of 18 HSV-2 isolates.
The researchers say they determined the two main lineages of HSV-2 began diversifying about 30,000 years ago. One strain was restricted to Sub-Saharan Africa, while the other spread globally.
Calvignac-Spencer told Healthline in an email that the HSV-2 strain eventually mixed its genome with the HSV-1 strain.
He said this mixture did not occur in all HSV-2, but the HSV-2 lineage that spread around the world contains the presence of HSV-1 recombinant fragments.
“We don't know whether there is a causal relationships and these HSV-1 fragments provided a selective advantage to this lineage, but that is clearly one intriguing possibility,” Calvignac-Spencer said.
How was the virus spread?
Calvignac-Spencer said his team did not investigate how herpes spread from chimps to humans.
He said it’s unclear how the virus moved from one species to another.
Part of the reason is that the transmission happened so long ago.
However, he said, there are a number of possible routes.
One is aggressive interaction between prehistoric humans and apes could have resulted in direct skin contact with blood.
Humans also hunted apes, so the virus could have been transmitted while chimp meat was being butchered.
Humans could have also consumed food such as fruit wedges contaminated with chimpanzee saliva.
The significance of the research
Calvignac-Spencer said there are several important observations to come out of his team’s research.
One is that scientists now know that coinfection with HSV-1 and HSV-2 could result in the mixing of genomes, something not considered possible before this study.
“Since this may have important functional consequences and alter the properties of the virus[es], I think it is clearly a question that warrants some follow-up,” he said.
Calvignac-Spencer added that his team was able to identify an “ancestral” pre-recombination version of HSV-2.
“It turns out it is quite divergent from the ordinary HSV-2. So it might well be that some clinical tests designed to detect the ordinary HSV-2 are not so good at detecting the African HSV-2,” he said.
Calvignac-Spencer said his team plans to explore these and other issues in the near future.