In the study, the researchers compared human herpes viruses with those of other primates. They used advanced models of molecular evolution to estimate when and how exactly the viruses had diverged from each other, and how they were introduced into humans.
According to a previous hypothesis, HSV-1 was thought to have been introduced to humans "potentially from another ape species, like orangutans," Wertheim said. And the split between HSV-2 and its chimpanzee counterpart was thought to have coincided with the split between humans and chimpanzees.
In contrast, the new study suggests that HSV-2 is the result of cross-species transmission to humans from ancestors of modern chimpanzees, and that HSV-1 is the result of a split between the human and chimpanzee viruses, he said.
Now, humans and chimpanzees have their own version of the HSV-1 virus, he said.
The results may help scientists better understand the mechanism of species-to-species transmission, which still happens, the study authors said. For instance, people today often get infected with a macaque simplex virus, which can result in severe illness, the researchers wrote in the study. But human-to-human transmission of this virus is extremely rare, they wrote.