About two-thirds of the human population is infected with at least one of the herpes simplex viruses. For many, it’s simply cold sores on the lips. Now, researchers show that one of our herpes viruses originated in chimpanzees, and it was passed into an extinct Homo precursor of modern humans 1.6 million years ago.
Herpes viruses have been infecting -- as well as co-diverging -- with their vertebrate hosts for hundreds of millions of years. The primate simplex viruses go as far back as the last common ancestor of monkeys and apes. We’re the only primate species to be infected with two distinct herpes simplex viruses: HSV-1 (mostly blisters) and HSV-2 (main cause of genital herpes). “We wanted to determine why,” says Joel Wertheim from the University of California, San Diego.
Our lineage split from apes about 6 million years ago. So Wertheim and his colleagues decided to see if the additional human simplex virus is the result of ancient viral lineage duplication or some cross-species transmission. So they compared the gene sequences of HSV-1 and HSV-2 to the family tree of simplex viruses from eight monkey and ape species. Using multiple models of molecular evolution, they estimated ancient viral divergence times -- accounting for natural selection over the course of the viral evolution.
They found that HSV-1 has been present in humans far longer than HSV-2. Furthermore, HSV-1 is the result of ancient co-divergence, infecting hominids before our spit from chimps. HSV-2, on the other hand, arose from a cross-species transmission event from the ancient ancestor of modern chimps to Homo erectus around 1.6 million years ago. Our species evolved in Africa about 200,000 years ago.
“Comparing virus gene sequences gives us insight into viral pathogens that have been infecting us since before we were humans,” Wertheim says in a news release.
The results also help us figure out how viruses evolve in various hosts and find their way into humans. Wertheim adds: “Animal disease reservoirs are extremely important for global public health. Understanding where our viruses come from will help guide us in preventing future viruses from making the jump into humans.”
The work was published in Molecular Biology and Evolution this week.