By Annalee Newitz

THERE'S A STRANGE rumor about the famous philosopher of sex Michel Foucault. I can't remember where I first heard it, but I've listened to people tell it many times with slightly different embellishments. While he was teaching at UC Berkeley in the 1970s, Foucault gave a series of lectures on the history of sexuality. One of his main arguments was that people's beliefs about sex control their bodies in the same way chains restrict the movement of prisoners. It was a sort of top-down view of sex: erotic desires, Foucault postulated, originate from culture rather than from the body.

And Foucault would know, apparently. Legend has it that before his lectures, he would go to a certain gay bar in the South of Market area of San Francisco, get himself strapped to a barrel, and take it up the ass from any man willing to give it a go. In one version of the story, audience members at his subsequent colloquium noticed the distinct imprint of a buckle on his forehead when he arrived, although it gradually faded during his discourse on perversity. By 1980, Foucault's lectures were attracting so many students that Berkeley's huge Wheeler Auditorium would fill to bursting and eager young scholars were turned away at the door.

While Foucault's ideas about the social construction of sexual pleasure were earning him professional acclaim in California, across the country another scholar interested in the history of sexuality was getting a rather chillier reception. E.O. Wilson, a nerdy Harvard University entomologist, earned the lifelong hatred of many colleagues when he published a thick book in 1975 called Sociobiology. In it, he argued that social insects like ants prove that sexual and other behaviors are genetic. The implications were disturbing: his work seemed to suggest that men might be genetically programmed to dominate women, and blacks to be a subordinate race.

Responding to the conservative flavor of Wilson's ideas, a group of biologists (including his well-known Harvard colleague Stephen Jay Gould) berated him for engaging in "politics by scientific means." At a 1978 meeting of the American Association for the Advancement of Science, audience members heckled the outspoken sociobiologist, calling him a racist and dumping a jug of ice water on his head.

Only a few decades later, the dashing and popular Foucault is dead, an early casualty of AIDS. His ideas continue to seduce college students in the humanities, but to the general public he is either unknown or dismissed as just a wacky, queer postmodern type. Meanwhile, Wilson has lived to see his ideas accepted by many people as common sense. Articles about the genetic origin of human behaviors, from selfishness to promiscuity, are staples of science pages in newspapers and magazines everywhere. Wilson's greatest detractor, the eloquent and progressive Gould, died last year. And one of the most celebrated and widely read evolutionary theorists doing groundbreaking work today is an acolyte of Wilson's named Steven Pinker.

Something profound has happened to public perceptions of sex since the days when Foucault was happily soliciting sodomy and Wilson was getting doused. Our worldview has shifted: We no longer explain sex in reference to Foucault-era cultural forces like repression and liberation. Instead, we seek to understand it using biology, genomics, and Wilson's brand of evolutionary theory. We don't want to know about sexual revolution; we want to know about nature. In a few short years, sex has been lifted out of the realm of culture and placed back on the African savannas, where humans first developed languages, tools, and many other hallmarks of the modern Homo sapiens.

Books about sex in the 1960s and '70s reflected an interest in the cultural context of eroticism. Hits from this era included Eros and Civilization, Lovemaps, and The Sensuous Woman. Today's popular books on sex have titles with a distinctly scientific bent: The Red Queen: Sex and the Evolution of Human Nature; A Natural History of Rape; A Natural History of Love; and The Selfish Gene. Best-selling science writer Steve Jones recently published a book about masculinity called simply Y, in homage to the male chromosome. And one of the biggest sex-related best-sellers in recent years is Olivia Judson's "definitive guide to the evolutionary biology of sex," whimsically titled Dr. Tatiana's Sex Advice to All Creation.

Science historian and UC Irvine professor Jennifer Terry says our infatuation with sex and evolutionary biology is nothing new. "This goes all the way back to the 18th century and the dawn of the Enlightenment," she says. "At that time men of letters argued that law should be grounded in nature. Nature became a kind of projection screen for all kinds of notions about how we should live." As the science of biology emerged in the 19th century, naturalists like Charles Darwin continued the Enlightenment tradition by claiming that nature had shaped the way humans couple in a process called "sexual selection." Put simply, sexual selection holds that creatures drive their own evolution by choosing mates with certain attractive qualities. If, for instance, moose are really turned on by giant antlers, their booty-chasing habits will result in offspring with big antlers and, over time, all of moosekind will eventually evolve to have big horns on their heads.

But what makes us so sure that horny moose can tell us anything about why Sir Mix-a-Lot likes big butts or why so many people cheat on their partners? Why do we turn to evolution to explain rape, homosexuality, and pornography? Terry suggests that our renewed interest in the evolution of sex may be a response to the way biotechnology and other industries have blurred the distinction between nature and culture. "Biotech allows you to manipulate what used to be called 'nature.' We have a cultural practice that's transforming nature," she explains.

Suddenly, it's no longer so clear where nature ends and culture begins. If we can change the course of the potato's evolution in a lab, does that spell doom for old-fashioned sexual selection? What else will change? Will we no longer lust and fuck and breed the way nature intended?

Perhaps more to the point, many people are asking themselves: If we are no longer guided by nature, what does guide us? This burning question seems to be fueling our appetite for news about the degree to which evolution is still shaping our sex drives. As long as we can place our desires in the context of what our hunter-gatherer ancestors did 100,000 years ago, the bioengineered future in which sodomy is legal and men do dishes is no longer quite so scary. Evolutionary biology teaches us that sex is still governed by the immutable laws of nature. Right?

Well, it depends on what you mean by "nature."

The jungle and the supermarket

Most people know about Darwin's famous theory of natural selection. He argues that species evolve through "survival of the fittest." When several types of tiger compete for the same hunting grounds, for example, only the strongest and longest of tooth survive. The wimpier tiger species dies out. Lesser known is Darwin's theory of sexual selection. Detailed in The Descent of Man, it explains how a species changes the course of its own evolution through mate choice. While natural selection is about relationships between species, sexual selection is about why some members of a species win mates and pass on their genes while others don't. It's a scientific theory of desire.

Today sexual selection has become the focus of many evolutionary biologists' work. There are two basic models for how it operates – let's call them the jungle and the supermarket.

Experts who favor the jungle model emphasize human conflict and selfishness. Men and women struggle with each other the way wild animals do in a dangerous jungle. This version of the sexual story portrays people fighting tooth and nail for resources. When men and women do form social bonds, it's to trick or coerce a mate into rearing children with them so they can push their genes on the next generation. Pulitzer prize-winning author Jared Diamond makes a case for this in his book Why Is Sex Fun? He explains why families stay together in less-than-romantic terms: "Deserting a fertilized mate to pursue other females would bring no evolutionary gain to a male if his offspring thereby died of starvation. Thus, self-interest may force the male to remain with his fertilized spouse, and vice versa." In the jungle, only the most selfish and craftiest survive.

Richard Dawkins makes a similar case in The Selfish Gene, while Randy Thornhill and Craig Palmer give the jungle model an even darker cast in their work on how men evolved to rape women. Their idea is that men and women have developed competing mating strategies. Men learned to rape – and successfully perpetuated "rape genes" – to circumvent women's power to choose a mate. For Thornhill and Palmer, rape is part of male biology, just as being sexually picky is part of female biology. They recommend that women heed their research and keep it in mind when they decide what to wear to a nightclub.

Theorists who aren't convinced by this violent portrait of human sexuality have offered another way to understand our natural history. They espouse the supermarket model of sexual selection, which explains how eroticism grew out of cooperation and diversity. In this theory, our sexual habits resemble those of people shopping at a supermarket: some of us prefer melons, while others prefer pears. We are not all competing for the same fruits. Moreover, we gain access to all those luscious pieces of produce by cooperating to get them to the store: farmers work with suppliers, who work in turn with shop owners, and so on. At the supermarket, people seek many different kinds of sexual partners and cooperate with each other to find them.

Stanford University biology professor Joan Roughgarden is one of the most outspoken proponents of the supermarket theory of sexual selection. She argues that there is an evolutionary advantage to cooperation, because it solidifies social bonds and creates a safe environment for child rearing. According to Roughgarden, data gleaned from animals reveal that the genes of dominant (rapist) males are no fitter than those from subordinate (non-rapist) males.

The supermarket model also offers an evolutionary explanation for homosexuality, transgenderism, and other kinds of sexuality that cannot be accounted for in a strictly jungle mode of selection. More than 300 mammal species, including humans, engage in same-sex relationships. If these species evolved to cooperate, Roughgarden argues, it would explain the development of a form of sexuality whose purpose is purely social rather than reproductive.

"Homosexuality challenges the competition paradigm," she explains. "It demonstrates that animals are building friendships and exchanging pleasurable contact." And they aren't doing it in the service of their selfish genes.

Let's talk dirty to the animals

Biologist and UC Davis professor Marlene Zuk says people just won't stop asking her if infidelity is natural. Because she's written extensively about the sex lives of animals, people think she has special knowledge about the "natural" underpinnings of sexual behavior. Wilson's Sociobiology drew parallels between ants and humans, after all, and many fans of evolutionary biology expect that the key to understanding human desire lies in the world of animals.

But Zuk is wary of drawing comparisons between humans and other beings. She thinks that all too often people use examples from the animal world to make a political or moral argument about how humans should live. Right-wingers might use gorillas to justify rigid patriarchy; lefties like myself point to the promiscuous, bisexual bonobo as an example of how San Francisco culture is reproduced in nature. "The problem is that it's too easy to get only a limited view of what animals are really like," Zuk says. "You can find egalitarianism or you can find patriarchy and traditional families. Neither gives you a complete picture of what there is, and your preconceptions will prevent you from seeing the great diversity of sexual behaviors that animals express."

Understanding the true diversity of animal behaviors, however, can shed light on questions like why humans generally enjoy having more than one sexual partner over a lifetime. Perhaps it's because monogamy is a social behavior and not a sexual one. "Animals give us ideas about ways that things can exist that we'd never even thought about," Zuk enthuses.

In a recent book called The Myth of Monogamy, David Barash and Eve Lipton describe how genetic testing has revealed that many animals once believed to be monogamous are in fact having babies by individuals who are not their partners. Barash and Lipton argue that sexual promiscuity is good for the gene pool, while social monogamy is good for child rearing. For some species, fitness may depend on a mixture of fooling around and staying home with the kids.

Tom Cline, a UC Berkeley genetics professor, says that natural diversity forces us to ask a more basic question about sex. "Why have it at all?" he wonders. Enthusiastically, he answers himself: "Sex is a great way for organisms to exchange genetic information!" It's helpful to look at sex from this point of view because it shows how far humans have come. From nature's standpoint, sex is a good way to swap genes. Everything else is just cultural gravy.

One of Cline's major areas of research is the genetic system that causes fruit flies to develop into males or females. He points out that not all species have a male and female version. "Male and female come from the idea that there is a different investment in the [sexual cells called] gametes," he says. In species where each partner makes an equal investment in its gametes, there are only mates, not sexes. We describe humans in terms of male and female because the male invests very little energy in each sperm (there are thousands of sperm in every ejaculation), whereas the female invests a lot in the one or two eggs she produces per month. Although humans put great stock in the cultural differences between men and women, for Cline sex differences are simply a matter of chemistry and morphology. There is no such thing as culturally constructed gender among flies. "The bottom line here is that you're looking at how a species is designed to work in terms of reproducing."

Steve Jones, author of Y, would beg to differ. In his occasionally convoluted exegesis on the Y chromosome – the single piece of genetic material that distinguishes men from women – he teases cultural meanings out of the structure and composition of the male genome. Pointing out that the Y chromosome is small and full of junk DNA (not unlike every other chromosome), Jones postulates that men are much more vulnerable than was previously believed. Where Cline sees chemistry, Jones sees an explanation for men's sense of inadequacy.

Many conventional geneticists would probably take issue with Jones's correlation between the size of the Y chromosome and men's obsession with the sizes of certain other parts of their bodies. But his analysis sheds light on one of the major contributions evolutionary biology has made to public perceptions of sex. It reminds us that our culture grew out of our bodies, not the other way around. We started out as animals and developed culture out of our so-called natural urges. In fact, one of the few things that nearly all scientists agree on is that humans' large brains are a result of sexual selection – not natural selection. We might be humping more often, but if it hadn't been for the strange erotic desires of our distant forebears, we would have far fewer theories about it.

The latest mainstream author to take a stab at explaining how sex made us the smartest animals on the planet is Leonard Shlain, a surgeon at the California Medical Center of San Francisco. His new book is Sex, Time and Power: How Women's Sexuality Shaped Human Evolution. In a direct critique of the jungle model of sexual selection, Shlain argues that women's sexual choices guided humans to develop language and other complex symbolic systems that characterize contemporary culture.

An ability to use language was, according to Shlain, attractive to prehistoric women, because it allowed them to negotiate sex with potential mates. A pregnant woman needs help to survive, and any man who could easily communicate his willingness to assist her would have seemed a more enticing choice than the man who tried to bop her on the head and rape her. As women mated more often with men who could use language, they had children with those skills too. Eventually, when men became smart enough, according to Shlain, they invented patriarchy and took away women's right to choose their mates. (Shlain doesn't speculate about whether the invention of patriarchy precipitated a decline in male intelligence.)

Everything is natural

Of course, Shlain's account of human history is filled with the kind of wishful thinking that Terry and Zuk warn against. He claims that women discovered time by realizing they had periods once a month and that prehistoric men showed women they weren't selfish by sticking around after their ejaculations to give women orgasms. Obviously, there is no hard evidence for either of these assertions. They are simply imaginative reconstructions of what might have happened back in Africa more than 100,000 years ago. The book's preoccupation with men giving women orgasms sounds like something from the 1970s; it says more about Shlain's efforts to be a sensitive man than it does about what our ancestors might have done. Likewise, Thornhill and Palmer's account of the "naturalness" of rape might strike some as sexual conservatism posing as academic research.

Despite our best efforts, we may never figure out which parts of our sexual desires are cultural and which are inherited from our animal ancestors. And most evolutionary biologists are just fine with that. "If nonmonogamy occurs in animals and humans, so what? What's the point?" Zuk asks. Ultimately, animals are so different from us that their behavior proves nothing about humans.

Moreover, sometimes nature is so weird that we aren't sure what to make of it. Cline says the most illogical thing he's seen in his work is his favorite sex organ: the fruit fly's "sex comb." Scientists used to believe that this bristly structure on the fly's front legs allowed him to cling to the female while mating. But it turned out to be directly connected to the brain via the fly's nervous system. "Perhaps it gives the fly pleasure?" Cline speculates. The point is that the sex comb doesn't make the male fly live longer or stay healthier or pass on its genes more effectively. It's evolved principally to provide some kind of stimulation, sort of like a fruit fly version of the human clitoris. Neither the jungle nor the supermarket models of sexual selection can satisfactorily explain why some organs evolve purely to provide pleasure. Bacteria, after all, don't need orgasms to induce them to exchange genetic material. So why do flies? Why do we? "We just can't say for sure," Cline says.

Zuk hopes the wide variety of mating scenarios in nature will help people realize that, in a sense, nothing is unnatural. Especially today, when humans are affecting their evolution not just through sexual selection but also through direct manipulation of their genome in the lab, it seems foolish to think that there is some sort of superior natural way of life. "Lots of things are natural, like infanticide," Zuk says. "Do we want to emulate that?"

Knowing where we came from and understanding how our bodies gave birth to our cultures is important. But they can't answer the kinds of complicated political and moral questions people in San Francisco deal with everyday. How can nature teach us whether transgendered people should receive health benefits to cover sex-reassignment surgery, or whether same-sex marriage should be legal? And will nature, with its thousands of ways of creating families, show us what the "best" kind of kinship network is?

"I would always urge people not to look for simple lessons from nature," Roughgarden says. "When we are dealing with human affairs, we should stand up for ethical reasoning. We need to talk about how to have a just society."