Science / Medicine : Herpes Viruses Hold Clues to Early Human Sex Activity
New information about the earliest origins of human sexual activity is being obtained from viruses that cause cold sores and venereal disease.
By studying the composition of herpes viruses, researchers at the University of Mississippi Medical Center have concluded that distinctly human sexual behavior, including face-to-face mating and sexual attractiveness of females throughout their entire menstrual cycles, began more than 8 million years ago.
The new research also suggests that humans began to walk upright at about the same time.
The Mississippi researchers’ conclusions, published in the prestigious Proceedings of the National Academy of Sciences, are important because there is no other hard evidence to suggest when such behaviors began, said anthropologist C. Owen Lovejoy of Kent State University. “As anthropologists always say, ‘Behavior doesn’t leave fossils,’ ” he said.
The research is based on the fact that one of the two most common herpes viruses primarily infects the mouth while the other infects the genitals. Both are descended from a single virus that infected both areas. That virus was similar to a simian herpes virus found in nonhuman primates today.
Virologist Glenn A. Gentry and his colleagues used standard techniques to study differences in the amino acid composition of enzymes in the viruses and concluded that the two viruses must have diverged from their common ancestor 8 million years ago. That was about the same time that humans diverged from the great apes.
“We began to wonder what changed at that time to allow the separation of the two strains of herpes,” Gentry said in a telephone interview. “It became a problem of trying to figure out what the sexual and social behavior of our common ancestors was. The only thing we have to compare it with is the behavior of modern primates.”
Primates have a great deal of oral-genital contact, not only for sex but also for grooming and courting. This contact allows the simian virus--and presumably the ancestral virus--to populate both areas, Gentry said. For the two modern strains to diverge required that oral-genital contact be drastically reduced.
This indicates that human ancestors must have adopted face-to-face mating, which emphasizes kissing for oral-oral transmission. “Kissing is occasionally observed in orangutans and chimps, but is much more typical of humans,” he said.
The divergence also implies an increased incidence of sexual contact, Gentry added. Herpes is typically transmitted only during the period five to 10 days after the carrier becomes infected; then the virus hides in the nervous system for months or years until new sores are triggered by stress.
If the ancestral humans copulated only when the female was in heat, the genital form of the virus would have died out, he said. Thus, by about 8 million years ago, women must have become sexually receptive throughout the menstrual cycle. That implies that secondary sexual characteristics such as luxuriant hair and large breasts must have developed about the same time, Gentry said.
And finally, Gentry said, it is unlikely that the separation of the two viruses could have been maintained if the ancestral humans were walking on all fours, which brings the face into proximity with the genitals. Thus, he said, humans must have begun walking upright--which also facilitates face-to-face mating--at least 8 million years ago.