The genes involved in sperm production have been evolving at a much faster rate than most other human genes, according to a study published yesterday in the journal Nature by Dr. Chung-I Wu and colleagues at the University of Chicago.
The finding implies that, at least in the human evolutionary past, women generally had many sexual partners, enough that the males' sperm needed to compete with one another.
The phenomenon of sperm competition is well known among humans' closest living relatives, chimpanzees and gorillas, as well as in humbler creatures like fruit flies. Fruit fly sperm are positively toxic, a chemical weaponry acquired because several males will mate with a female, and the male fly whose sperm knocks out the rest has the best chance of fatherhood.
In the primate world, gorillas practice a harem system in which one male controls access to many females. Presumably because males can be sure of their paternity, gorillas do not need to produce much sperm and so have small testicles.
Chimpanzees, on the other hand, have huge testicles in relation to their body size.
Under the lax rules of the chimp mating system, a female is likely to be inseminated several times, and males need to deliver competitive volumes of ejaculate to have a serious chance at paternity.
Dr. Wu is an evolutionary biologist who studies male reproduction as a driving force in creating new species. "We have built up a notion that what drives species apart is sexual selection in the form of competition among males," he said.
After testing this theory in fruit flies, he realized he could explore it further in people because of the vast amount of DNA information now building up in computer databanks as a result of the human genome project, the federally sponsored effort to decode the three billion-unit sequence of the human genetic playbook.
Choosing a group of genes known to produce proteins involved in male reproduction, Dr. Wu showed that they had evolved considerably faster than other human genes.
He and his colleagues, Dr. Gerald J. Wyckoff and Dr. Wen Wang, then looked at three genes involved in sperm production. Two of the genes make special DNA-packing materials called protamines that govern the shape and size of the sperm. They found that human protamine genes had evolved very rapidly.
Looking at the counterpart genes in other primates, the Chicago biologists calculated that the protamine genes of chimpanzees had evolved just as fast, but that those of gorillas much more slowly.
In Dr. Wu's view, the finding strongly suggests, though does not yet prove, that human sperm have needed to be competitive for the same reason as chimpanzee sperm: they have to fight off rival sperm from other males in the female reproductive tract. He believes his findings apply not necessarily to today's world but certainly to the period in which humans evolved.
Dr. Bobbi S. Low, a biologist at the University of Michigan and author of a new book on the evolution of human sexual behavior, "Why Sex Matters" (Princeton University Press), said the Chicago biologists' work "is fabulous for its connection of genetic evidence on the intensity of sexual selection to things we already knew from physical and behavioral data."
"I'm really excited by it," she added.
Behavioral and biological evidence suggests that sexual selection is an intense force on male primates, and that the intensity might vary with the social system. This idea is supported, Dr. Low said, by Dr. Wu's finding that gorillas, with their harem system, have been under less evolutionary pressure than humans and chimpanzees, which live in multimale groups.
In chimpanzee societies, male chimps try to control access to a small number of females, but not with complete success. Females will often solicit sneak copulations with younger males outside their group. Primatologists believe they may do this to buy insurance for their children, because adult males are more protective toward the children of females they have mated with.
"No male can completely control access to females all the time," Dr. Low said, "and that is the situation in which sperm competition arises."
The evolution of human sexual behavior and social organization is far from resolved, but Dr. Wu's work shows that DNA can speak to matters on which fossils are mute. It suggests that men vied for reproductive success at the level of sperm competition as well as in other ways and that in this respect human social and sexual structure has been closer to the chimpanzee model than the gorilla one.