Herpes simplex is a virus and there are two types. Oral is Type 1 and genital is Type 2 -- when it comes to oral we’ve always had it. The virus has been with us since we before we were considered “modern humans.” But Type 2, genital herpes, is new. Evidence shows that when we split from a common ancestor of the chimpanzee, about 7 million years ago, we did not have the herpes simplex 2 virus. Then somewhere between 3 and 1.4 million years ago, we got it.
The carrier, it seems, was an intermediate species unrelated to humans called Paranthropus boisei, which was “a heavyset bipedal hominin with a smallish brain and dish-like face.” This hominin likely contracted the virus by scavenging infected ancestral chimp meat. The virus would have passed from kill to killer through a bite or open sore. And the new virus then shared mouth-space with the simplex 1 virus. Over time found a happier home in a different "mucosal niche.”
It got to us because P. boisei and our direct ancestors Homo erectus lived in close enough proximity for the two genetic lines cross and share the virus. There’s evidence that the Homo erectus butchered and hunted its food, which means our ancestors likely got the herpes type-2 virus by eating infected meat. But, it’s also possible that we got it through interspecies intercourse. From there it was passed around from mother to child and also exposure.
It’s not common for a virus to transfer between species, though we are seeing a rise in this happening with things like swine flu, bird flu, ebola, and even HIV. It can, unfortunately, happen innocuously in some instances, say a chance encounter with a sick animal while traveling abroad.