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The next time you savor a mouthful of semen (which, if it's not your own, isn't necessarily a great idea), pause to consider that you are tasting what could be the most highly evolved human tissue.

Cum-hounds won't need convincing, but the point has been given tentative validation in a recent issue of Nature, the renowned British science journal. University of Chicago researchers have discovered that some of the genes that code for proteins regulating sperm production have evolved with unusual speed, changing much faster than other human genes. The speed of evolutionary change suggests that semen is the fruit of a particularly fierce biological arms race. This in turn leads to the conclusion that pre-historic humans did a lot of sleeping around.

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Humans have been competing since time immemorial to survive and reproduce. But the fast pace of sperm evolution points to a scenario in which that, additionally, once males scored with a female, their semen was in competition with that deposited by other males with whom the female had also recently had sex. With rampant promiscuity, rapid evolution of sperm was required, the researchers theorize, to knock out the other guy's sperm and win the egg.

Genes may fail to pass on to future generations when the creature to whom they belong gets, say, eaten by a predator. But such weeding out by "natural selection" is only part of the evolutionary story. Genes also face what is known as sexual selection. If peahens prefer peacocks with brightly colored feathers, genes for colored feathers will pass more frequently to offspring, even if they serve no other function. Dependent sometimes only on creaturely whim, sexual selection can wreak genetic changes in a population with the speed that Parisian fashion houses embrace tweed body wraps or reject chartreuse.

The University of Chicago researchers concluded that the genes for sperm production were evolving so fast thanks to sexual selection after comparing their rate of change in humans with that of two of our close primate cousins: chimpanzees and gorillas. Each of these primates has strikingly different mating patterns. Gorillas organize their social life in harems, in which one male sexually dominates a relatively stable group of females. Among chimpanzees, by contrast, social structure is more fluid, more evenly gender-mixed, and promiscuous. For the genes in question, chimps showed as rapid an evolution as humans, while the gorillas' changed at a rate much slower.

This correlation doesn't clinch the case that the rapid evolution of these human genes follows from sperm competition, but it's good evidence. Together with their patriarchal harems that only a Mormon or Islamic fundamentalist could love, gorilla males also sport relatively smaller testicles and less copious ejaculate than humans or chimps. With female gorillas so thoroughly dominated, the males just don't need the extra sexual equipment. Humans could have a history of promiscuity and sexual selection to thank not just for a mouthful of sperm's je ne sais quois, but also for the comparatively large and comely human testicles and penises that deliver it.