Central Park

Central Park divides two of Manhattan's greatest treasure collections. On the West Side stands the American Museum of Natural History, with its dinosaur fossils, stuffed African elephants, dioramas of apes, and displays of ancient human remains. On the East Side stands the Metropolitan Museum of Art, with its Rembrandt self-portraits, peacock-shaped sitar, gold rapiers, Roman temple, Etruscan mirrors, and Jacques Louis David's Death of Socrates.

These works symbolize our unique human capacities for art, music, sports, religion, self-consciousness, and moral virtue, and they have troubled me ever since my student days studying biology at Columbia University. It was easy enough for me to take a taxi along the West Seventy-ninth Street transverse (the natural history museum) to East Eighty-first Street (the Met). It was not so easy for our ancestors to cross over from the pre-human world of natural history to the world of human culture. How did they transform themselves from apes to New Yorkers? Their evolutionary path seems obscure.

Yet we know there must have been a path. The human mind evolved somehow. The question scientists have asked for over a century is: How? Most people equate evolution with "survival of the fittest," and indeed most theories about the mind's evolution have tried to find survival advantages for everything that makes humans unique. To extend the metaphor, one kind of theory suggests our problem was not following the transverse to a collection of decorative arts, but traveling a different route to some useful inventions. Perhaps the human mind evolved for military prowess, symbolized by the Sea-Air-Space Museum on the aircraft carrier USS Intrepid, docked at Pier 86. Or perhaps our minds evolved for reciprocal economic advantage, symbolized by the World Trade Center and Wall Street, or through a thirst for pure knowledge, as housed in the New York Public Library. The survival advantages of better technology, trade, and knowledge seem obvious, so many believe the mind's evolution must have been technophilic and survivalist.

Ever since the Darwinian revolution, this survivalist view has seemed the only scientifically respectable possibility. Yet it remains unsatisfying. It leaves too many riddles unexplained. Human language evolved to be much more elaborate than necessary for basic survival functions. From a pragmatic biological viewpoint, art and music seem like pointless wastes of energy. Human morality and humor seem irrelevant to the business of finding food and avoiding predators. Moreover, if human intelligence and creativity were so useful, it is puzzling that other apes did not evolve them.

Even if the survivalist theory could take us from the world of natural history to our capacities for invention, commerce, and knowledge, it cannot account for the more ornamental and enjoyable aspects of human culture: art, music, sports, drama, comedy, and political ideals. At this point the survivalist theories usually point out that along the transverse lies the Central Park Learning Center. Perhaps the ornamental frosting on culture's cake arose through a general human ability to learn new things. Perhaps our big brains, evolved for technophilic survivalism, can be co-opted for the arts. However, this side-effect view is equally unsatisfying. Temperamentally, it reflects nothing more than a Wall Street trader's contempt for leisure. Biologically, it predicts that other big-brained species like elephants and dolphins should have invented their own versions of the human arts. Psychologically, it fails to explain why it is so much harder for us to learn mathematics than music, surgery than sports, and rational science than religious myth.

I think we can do better. We do not have to pretend that everything interesting and enjoyable about human behavior is a side-effect of some utilitarian survival ability or general learning capacity. I take my inspiration not from the Central Park Learning Center on the north side of the transverse but from the Ramble on the south side. The Ramble is a 37-acre woodland hosting 250 species of birds. Every spring, they sing to attract sexual partners. Their intricate songs evolved for courtship. Could some of our puzzling human abilities have evolved for the same function?

A Mind for Courtship

This book proposes that our minds evolved not just as survival machines, but as courtship machines. Every one of our ancestors managed not just to live for a while, but to convince at least one sexual partner to have enough sex to produce offspring. Those proto-humans that did not attract sexual interest did not become our ancestors, no matter how good they were at surviving. Darwin realized this, and argued that evolution is driven not just by natural selection for survival, but by an equally important process that he called sexual selection through mate choice. Following his insight, I shall argue that the most distinctive aspects of our minds evolved largely through the sexual choices our ancestors made.

The human mind and the peacock's tail may serve similar biological functions. The peacock's tail is the classic example of sexual selection through mate choice. It evolved because peahens preferred larger, more colorful tails. Peacocks would survive better with shorter, lighter, drabber tails. But the sexual choices of peahens have made peacocks evolve big, bright plumage that takes energy to grow and time to preen, and makes it harder to escape from predators such as tigers. The peacock's tail evolved through mate choice. Its biological function is to attract peahens. The radial arrangement of its yard-long feathers, with their iridescent blue and bronze eye-spots and their rattling movement, can be explained scientifically only if one understands that function. The tail makes no sense as an adaptation for survival, but it makes perfect sense as an adaptation for courtship.

The human mind's most impressive abilities are like the peacock's tail: they are courtship tools, evolved to attract and entertain sexual partners. By shifting our attention from a survival-centered view of evolution to a courtship-centered view, I shall try to show how, for the first time, we can understand more of the richness of human art, morality, language, and creativity.

A 1993 Gallup Poll showed that almost half of all Americans accept that humans evolved gradually over millions of years. Yet only about 10 percent believe that natural selection, alone and unguided, can account for the human mind's astounding abilities. Most think that the mind's evolution must have been guided by some intelligent force, some active designer. Even in more secular nations such as Britain, many accept that humans evolved from apes, but doubt that natural selection suffices to explain our minds.

Despite being a committed Darwinian, I share these doubts. I do not think that natural selection for survival can explain the human mind. Our minds are entertaining, intelligent, creative, and articulate far beyond the demands of surviving on the plains of Pleistocene Africa. To me, this points to the work of some intelligent force and some active designer. However, I think the active designers were our ancestors, using their powers of sexual choice to influence—unconsciously—what kind of offspring they produced. By intelligently choosing their sexual partners for their mental abilities, our ancestors became the intelligent force behind the human mind's evolution.
Evolutionary Psychology Turns Dionysian

The time is ripe for more ambitious theories of human nature. Our species has never been richer, better educated, more numerous, or more aware of our common historical origin and common planetary fate. As our self-confidence has grown, our need for comforting myths has waned. Since the Darwinian revolution, we recognize that the cosmos was not made for our convenience.

But the Darwinian revolution has not yet captured nature's last citadel—human nature. In the 1 990s the new science of evolutionary psychology made valiant attempts. It views human nature as a set of biological adaptations, and tries to discover which problems of living and reproducing those adaptations evolved to solve. It grounds human behavior in evolutionary biology.

Some critics believe that evolutionary psychology goes too far and attempts to explain too much. I think it does not go far enough. It has not taken some of our most impressive and distinctive abilities as seriously as it should. For example, in his book How the Mind Works, Steven Pinker argued that human art, music, humor, fiction, religion, and philosophy are not real adaptations, but biological side-effects of other evolved abilities. As a cognitive scientist, Pinker was inclined to describe the human mind as a pragmatic problem-solver, not a magnificent sexual ornament: "The mind is a neural computer, fitted by natural selection with combinational algorithms for causal and probabilistic reasoning about plants, animals, objects and people."

Although he knows that reproductive success is evolution's bottom line, he overlooked the possible role of sexual selection in shaping conspicuous display behaviors such as art and music. He asked, for example, "If music confers no survival advantage, where does it come from and why does it work?" Lacking any manifest survival function, he concluded that art and music must be like cheesecake and pornography—cultural inventions that stimulate our tastes in evolutionarily novel ways, without improving our evolutionary success. His views that the arts are "biologically frivolous" has upset many performing artists sympathetic to evolutionary psychology. In a televised BBC debate following the publication of How the Mind Works, the theatrical director and intellectual polymath Jonathan Miller took Pinker to task for dismissing the arts as non-adaptations without considering all their possible functions. One of my goals in writing this book has been to see whether evolutionary psychology could prove as satisfying to a performing artist as to a cognitive scientist. It may be economically important to consider how the mind works, but it is also important to consider how the mind mates.

The view of the mind as a pragmatic, problem-solving survivalist has also inhibited research on the evolution of human creativity, morality, and language. Some primate researchers have suggested that human creative intelligence evolved as nothing more than a way to invent Machiavellian tricks to deceive and manipulate others. Human morality has been reduced to a tit-for-tat accountant that keeps track of who owes what to whom. Theories of language evolution have neglected human storytelling, poetry, wit, and song. You have probably read accounts of evolutionary psychology in the popular press, and felt the same unease that it is missing something important. Theories based on the survival of the fittest can nibble away at the edges of human nature, but they do not take us to the heart of the mind.

Moreover, the ritual celibacy of these survivalist doctrines seems artificial. Why omit sexual desire and sexual choice from the pantheon of evolutionary forces that could have shaped the human mind, when biologists routinely use sexual choice to explain behavioral abilities in other animals? Certainly, evolutionary psychology is concerned with sex. Researchers such as David Buss and Randy Thornhill have gathered impressive evidence that we have evolved sexual preferences that favor pretty faces, fertile bodies, and high social status. But evolutionary psychology in general still views sexual preferences more often as outcomes of evolution than as causes of evolution. Even where the sexual preferences of our ancestors have been credited with the power to shape mental evolution, their effects have been largely viewed as restricted to sexual and social emotions—to explain, for example, higher male motivations to take risks, attain social status, and demonstrate athletic prowess. Sexual choice has not been seen as reaching very deep into human cognition and communication, and sexuality is typically viewed as irrelevant to the serious business of evolving human intelligence and language.

In reaction to these limitations, I came to believe that the Darwinian revolution could capture the citadel of human nature only by becoming more of a sexual revolution—by giving more credit to sexual choice as a driving force in the mind's evolution. Evolutionary psychology must become less Puritan and more Dionysian. Where others thought about the survival problems our ancestors faced during the day, I wanted to think about the courtship problems they faced at night. In poetic terms, I wondered whether the mind evolved by moonlight. In scientific terms, sexual selection through mate choice seemed a neglected factor in human mental evolution. Through ten years of researching sexual selection and human evolution, since the beginning of my Ph.D., it became clear to me that sexual selection theory offered valuable intelligence about aspects of human nature that are important to us, and that cry out for evolutionary explanation, but that have been ignored, dismissed, or belittled in the past.

Trying a Different Tool

The human brain and its diverse capacities are so complex, and so costly to grow and maintain, that they must have arisen through direct selection for some important biological function. To date, it has proven very difficult to propose a biological function for human creative intelligence that fits the scientific evidence. We know that the human mind is a collection of astoundingly complex adaptations, but we do not know what biological functions many of them evolved to serve.

Evolutionary biology works by one cardinal rule: to understand an adaptation, one has to understand its evolved function. The analysis of adaptations is more than a collection of just-so stories, because according to evolutionary theory there are only two fundamental kinds of functions that explain adaptations. Adaptations can arise through natural selection for survival advantage, or sexual selection for reproductive advantage. Basically, that's it.

If you have two tools and one doesn't work, why not try the other? Science has spent over a century trying to explain the mind's evolution through natural selection for survival benefits. It has explained many human abilities, such as food preferences and fear of snakes, but it consistently fails to explain other abilities for decorative art, moral virtue, and witty conversation. It seems reasonable to ask whether sexual selection for reproductive benefits might account for these leftovers. This suggestion makes sexual selection sound like an explanation of last resort. It should not be viewed that way, because sexual selection has some special features as an evolutionary process. As we shall see, sexual selection is unusually fast, powerful, intelligent, and unpredictable. This makes it a good candidate for explaining any adaptation that is highly developed in one species but not in other closely related species that share a similar environment.
What Makes Sexual Selection So Special?

In the 1930s, biologists redefined natural selection to include sexual selection, because they did not think sexual selection was very important. Following their precedent, modern biology textbooks define natural selection to include every process that leads some genes to out-compete other genes by virtue of their survival or reproductive benefits. When one biologist says "evolution through natural selection," other biologists hear "evolution for survival or reproductive advantage." But non-biologists, including many other scientists, still hear "survival of the fittest." Many evolutionary psychologists, who should know better, even ask what possible "survival value" could explain some trait under discussion. This causes enormous confusion, and ensures that sexual selection continues to be neglected in discussions of human evolution.

In this book I shall use the terms "natural selection" and "sexual selection" as Darwin did: natural selection arising through competition for survival, and sexual selection arising through competition for reproduction. I am perfectly aware that this is not the way professional biologists currently use these terms. But I think it is more important, especially for non-biologist readers, to appreciate that selection for survival and selection for attracting sexual partners are distinct processes that tend to produce quite different kinds of biological traits. Terms should be the servants of theories, not the masters. By reviving Darwin's distinction between natural selection for survival and sexual selection for reproduction, we can talk more easily about their differences.

One difference is that sexual selection through mate choice can be much more intelligent than natural selection. I mean this quite literally. Natural selection takes place as a result of challenges set by an animal's physical habitat and biological niche. The habitat includes the factors that matter to farmers: sunlight, wind, heat, rain, and land quality. The niche includes predators and prey, parasites and germs, and competitors from one's own species. Natural selection is just something that happens as a side-effect of these factors influencing an organism's survival chances. The habitat is inanimate and doesn't care about those it affects. Biological competitors just care about making their own livings. None of these selectors cares whether it imposes evolutionary selection pressures that are consistent, directional, efficient, or creative. The natural selection resulting from such selectors just happens, willy-nilly.

Sexual selection is quite different, because animals often have very strong interests in acting as efficient agents of sexual selection. The genetic quality of an animal's sexual partner determines, on average, half the genetic quality of their offspring. (Most animals inherit half their genes from mother and half from father.) As we shall see, one of the main reasons why mate choice evolves is to help animals choose sexual partners who carry good genes. Sexual selection is the professional at sifting between genes. By comparison, natural selection is a rank amateur. The evolutionary pressures that result from mate choice can therefore be much more consistent, accurate, efficient, and creative than natural selection pressures.

As a result of these incentives for sexual choice, many animals are sexually discriminating. They accept some suitors and reject others. They apply their faculties of perception, cognition, memory' and judgment to pick the best sexual partners they can. In particular, they go for any features of potential mates that signal their fitness and fertility.

In fact, sexual selection in our species is as bright as we are. Every time we choose one suitor over another, we act as an agent of sexual selection. Almost anything that we can notice about a person is something our ancestors might have noticed too, and might have favored in their sexual choices. For example, some of us fall in love with people for their quick wits and generous spirits, and we wonder how these traits could have evolved. Sexual choice theory suggests that the answer is right in front of us. These traits are sexually attractive, and perhaps simpler forms of them have been attractive for hundreds of thousands of years. Over many generations, those with quicker wits and more generous spirits may have attracted more sexual partners, or higher-quality partners. The result was that wits became quicker and spirits more generous.

Of course, sexual selection through mate choice cannot favor what its agents cannot perceive. If animals cannot see the shapes of one another's heart ventricles, then heart ventricles cannot be directly shaped by sexual selection—vivisection is not a practical method for choosing a sexual partner. A major theme of this book is that before language evolved, our ancestors could not easily perceive one another's thoughts, but once language had arrived, thought itself became subject to sexual selection. Through language, and other new forms of expression such as art and music, our ancestors could act more like psychologists—in addition to acting like beauty contest judges—when choosing mates. During human evolution, sexual selection seems to have shifted its primary target from body to mind.

This book argues that we were neither created by an omniscient deity, nor did we evolve by blind, dumb natural selection. Rather, our evolution was shaped by beings intermediate in intelligence: our own ancestors, choosing their sexual partners as sensibly as they could. We have inherited both their sexual tastes for warm, witty, creative, intelligent, generous companions, and some of these traits that they preferred. We are the outcome of their million-year-long genetic engineering experiment in which their sexual choices did the genetic screening.

Giving so much credit to sexual choice can make sexual selection sound almost too powerful. If sexual selection can act on any trait that we can notice in other individuals, it can potentially explain any aspect of human nature that scientists can notice too. Sexual selection's reach seems to extend as far as psychology's subject matter. So be it. Scientists don't have to play fair against nature. Physics is full of indecently powerful theories, such as Newton's laws of motion and Einstein's theory of general relativity. Darwin gave biology two equally potent theories: natural selection and sexual selection. In principle, his two theories explain the origins of all organic complexity, functionality, diversity, and beauty in the universe. Psychologists generally believe that so far they have no theories of comparable power. But sexual selection can also be viewed as a psychological theory, because sexual choice and courtship are psychological activities. Psychologists are free to use sexual selection theory just where it is most needed: to explain mental abilities that look too excessive and expensive to have evolved for survival.

This sexual choice view also sounds rather circular as an explanation of human mental evolution. It puts the mind in an unusual position, as both selector and selectee in its own evolution. If the human mind catalyzed its own evolution through mate choice, it sounds as though our brains pulled themselves up by their own bootstraps. However, most positive-feedback processes look rather circular, and a positive-feedback process such as sexual selection may be just what we need to explain unique, highly elaborated adaptations like the human mind. Many theorists have accepted that some sort of positive-feedback process is probably required to explain why the human brain evolved to be so large so quickly. Sexual selection, especially a process called runaway sexual selection, is the best-established example of a positive-feedback process in evolution.

Positive-feedback systems are very sensitive to initial conditions. Often, they are so sensitive that their outcome is unpredictable. For example, take two apparently identical populations, let them undergo sexual selection for many generations, and they will probably end up looking very different. Take two initially indistinguishable populations of toucans, let them choose their sexual partners over a thousand generations, and they will evolve beaks with very different colors, patterns, and shapes. Take two populations of primates, and they will evolve different hairstyles. Take two populations of hominids (bipedal apes), and one may evolve into us, and the other into Neanderthals. Sexual selection's positive-feedback dynamics make it hard to predict what will happen next in evolution, but they do make it easy to explain why one population happened to evolve a bizarre ornament that another similar population did not.