The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (5 page)

A final limitation is that fossil and archeological evidence has proven much more informative about how our ancestors could afford the energy costs of large brains, than about what they actually used their brains for. Evidence in the last decade has revealed how our ancestors evolved the ability to exploit energy-rich foods such as game animals that could be hunted for meat,

and underground tubers that could be dug up and cooked. These energy-rich foods could also be digested using shorter intestines than other apes have. As anthropologist Leslie Aiello has argued, since guts use a lot of energy our smaller guts also increased our energy budget above what is available to other apes. The ability to exploit these new food sources, at a lower gut-cost, could have allowed our ancestors to afford larger bodies, larger brains, more milk production, or whatever other costly traits evolution might have favored. But a higher energy budget does not in itself explain why our brains expanded, or why any of our distinctive human abilities evolved. Sexual selection principles, not fossil evidence, may explain why we wasted so much of our energy on biological luxuries like talking, dancing, painting, laughing, playing sports, and inventing rituals.

An evolutionary account of the human mind cannot be constructed directly from fossils and stone artifacts. As archeologist Steven Mithen argued in his thoughtful book
The Prehistory of the Mind
, the physical evidence of prehistory must be interpreted in a much more sophisticated evolutionary psychology framework. Yet many scientists still have a special reverence for archeological evidence which is out of all proportion to what it can tell us about mental evolution. Fossils were certainly critical in convincing people that we had actually evolved in continuous stages from primate ancestors—almost 50 percent of Americans now accept the fossil evidence for human evolution. But evidence supporting the fact of human evolution is not always the best evidence for the mechanism of human evolution. A more fruitful place to start theorizing about the past is the present: the current capacities of the human mind (the adaptations to be explained) and the principles of current evolutionary biology (the selection pressures that can explain them). Bones and stones can be valuable sources of evidence, but they become most useful when combined with studies of other primates, and with studies of humans in tribal societies, modern societies, and psychology laboratories.

This may sound like a radical change in scientific method, but it isn't. In broadening the focus from stones and bones to the

comparative analysis of present adaptations, I am in fact proposing something rather conservative: that the evolutionary psychology of the human mind can play by the same scientific rules as the evolutionary biology that studies any other adaptation in any other species. It can present a bold theory about the function of the adaptation and the selection pressures that produced it, and see whether the adaptation has special features consistent with that function and those origins. Paleontology makes useful contributions to such studies, but it is not the most important source of data on the design and functions of biological adaptations. The details of an adaptation as it currently exists are often more informative than the fossilized remnants of its earlier forms. In this book I shall draw upon the fascinating discoveries of fossil-hunters and archeologists where appropriate, but I believe that the features of the modern human mind are often the best clues to its origin.

Show Me the Genes

From the 1980s, DNA evidence has become almost as important as fossil and archeological evidence in understanding human evolution. In the coming decades it is likely to become hugely more important, especially in tracing the human mind's origins. This is because evolved mental capacities depend on genes, even when they leave no fossil or archeological records. After the Human Genome Project identifies all 80,000 or so human genes in the next couple of years, we can look forward to three further developments that will allow much more powerful tests of my theory and other theories of mental evolution.

Neuroscientists will start to identify which genes underlie which mental capacities, by analyzing the proteins they produce, and the role those proteins play in brain development and brain functioning. (Of course there is no single gene for language or art—these are complex human abilities that probably depend on hundreds or thousands of genes.) Behavior geneticists will also identify different forms of particular genes that underlie individual differences in mental abilities such as artistic ability, sense of

humor, and creativity. Psychologist Robert Plomin and his collaborators have already identified the first specific gene associated with extremely high intelligence (a form of the gene labelled "IGF2R" on chromosome 6). Very little such work has been done so far, but the genes that underlie our unique human capabilities will be identified sooner or later, and evolutionary psychology will benefit.

Also, geneticists will find out more about which genes we share with other apes. Research centers in Atlanta and Leipzig are already pushing for the development of a Chimpanzee Genome Project. Since 1975, geneticists have been using a method called DNA hybridization to show that our DNA is roughly 98 percent similar to that of chimpanzees (compared to only 93 percent with most monkeys). However, this method is fairly crude, and we will not know exactly which of our genes are unique until the results of the Chimpanzee Genome Project can be compared to those of the Human Genome Project. Geneticists already know there are some significant differences: humans have 23 pairs of chromosomes whereas other apes have 24 pairs, and the genes on human chromosomes 4, 9 and 12 appear to have been reshuffled significantly compared with their arrangement on the chimpanzee chromosomes. There are plenty of genetic differences to account for our distinctive mental capacities, and the more we know about the unique human genes, the more we can infer about their evolutionary origins and functions.

Finally, it may be possible to recover more DNA from our extinct fossil relatives. DNA decays fairly quickly, and it is very hard to recover DNA from fossils older than about 50,000 years ago (
Jurassic Park
notwithstanding). However, Neanderthals survived until about 30,000 years ago, and a German team led by Svante Pääbo has already succeeded in recovering a DNA fragment from a Neanderthal's arm bone. This fragment, just 379 DNA base pairs long, showed 27 differences compared with modern humans, and 55 differences compared with chimpanzees. This substantial difference between humans and Neanderthals suggests that our lineages split apart at least 600,000 years ago—

much earlier than previously thought. It also shows that humans did not evolve from Neanderthals. Potentially, the same techniques could be applied to
Homo erectus
specimens from Asia, which also persisted until about 30,000 years ago, but which split off from our ancestors even earlier. It might even be possible, at some future date, to show which other hominids shared the genes underlying our apparently unique mental abilities. For example, if Neanderthals are found to share some of the same genes for language, art, music, and intelligence that modern humans have, then we could infer that those capacities evolved at least 600,000 years ago. Although behavior does not fossilize, some of the DNA underlying behavior does, and it can sometimes last long enough for us to analyze.

The DNA revolution will unveil many more aspects of human evolution and human psychology. I cannot yet show you the many genes that must underlie each of the human mental adaptations analyzed in this book. However, the genetic evidence that will emerge in the coming years will probably render my ideas—even the apparently most speculative ones—fully testable in ways I cannot anticipate. My sexual choice theory sometimes sounds as if it could explain anything, and hence explains nothing. This overlooks the fact that biologists are developing ever more sophisticated ways of testing which adaptations have evolved through sexual selection, and many of these methods—including a range of new genetic analyses—can be applied to human mental traits. Indeed, one goal of this book is to inspire other scientists to join me in testing these ideas.

What We Can Expect From a
Theory of Human Mental Evolution

Any theory of human mental evolution should, I think, strive to meet three criteria: evolutionary, psychological, and personal. The evolutionary criteria are paramount. Any theory of human mental evolution should play by the rules of evolutionary biology, using accepted principles of descent, variation, selection, genetics, and adaptation. It is best not to introduce speculative new

processes of the sort that have been touted recently, such as "gene-culture co-evolution," "cognitive fluidity as a side-effect of having a large brain," or "quantum consciousness." Complex adaptations such as human mental capacities need to be explained by cumulative selection for a function that promotes survival or reproduction.

This evolutionary criterion makes it much more important to identify the selection pressures that shaped each adaptation than to identify how the adaptation went through some series of structural changes, having started from some primitive state. Complex adaptations are explained by identifying functional features and specifying their fitness costs and benefits in a biological context. The emphasis is on what and why, rather than how, when, or where. For every theory of every adaptation, there is one demand that modern biologists make: show me the fitness! That is, show how this trait promoted survival or reproduction.

Psychologically, the human mind as explained by the theory should bear some resemblance to the minds of ordinary women and men as we know them. The mental adaptations described in the theory should fit our understanding of normal human abilities and personalities. If you're married, imagine your in-laws. If you commute by public transport, visualize your traveling companions. They're the kinds of minds the theory should account for: ordinary people, in all their variation. We should not worry too much about the minds of exceptional geniuses such as theoretical physicists and management consultants. We are not really trying to explain "the human mind" as a single uniform trait, but human minds as collections of adaptations with details that vary according to age, sex, personality, culture, occupation, and so forth. Still, differences within our species are minor compared with differences across species, so it can be useful to analyze "the human mind" as distinct from "the chimpanzee mind" or "the mind of the blue-footed booby."

Finally, any theory of human origins should be satisfying at a personal level. It should give us insight into our own consciousness- It should seem as compelling in our rare moments of

personal lucidity as it is when we are mired in that mixture of caffeine, television, habit, and self-delusion that in modern society we call "ordinary consciousness." It is so easy, when engaged in abstract theorizing about mental evolution, to forget that we are talking about the origins of our own genes, from our own parents, that built our own minds, over our own lifetime. Equally, we are talking about the origins of the genes that built the mind and body of the first person you ever fell in love with, and the last person, and everyone in between. A theory that can't give a satisfying account of your own mind, and the minds you've loved, will never be accepted as providing a scientific account of the other six billion human minds on this planet. Theories that don't fulfill this human hunger for self-explanation may win people's minds, but it will
not
win their hearts. The fact that 47 percent of Americans still think humans were created by God in the last ten thousand years suggests that evolutionary theories of human origins, however compelling at the rational level, have not proved satisfying to many people. We might as well admit that this is a third demand to impose on theories of human mental evolution, and see whether we can fulfill it. This criterion should not take precedence over evolutionary principles or psychological evidence, but I think it can be a useful guide in developing testable new ideas. If we cannot fulfill this criterion, perhaps we'll just have to live with the existential rootlessness that Jean-Paul Sartre viewed as an inevitable part of the human condition.

Working Together

In facing these three challenges, I have found my professional training as an experimental cognitive psychologist of limited value. What I learned about the psychology of judgment and decision-making was helpful in thinking about sexual choice. But most experimental psychology views the human mind exclusively as a computer that learns to solve problems, not as an entertainment system that evolved to attract sexual partners. Also, psychology experiments usually test people's efficiency and consistency when interacting with a computer, not their wit and

warmth when interacting with a potential spouse. These attitudes have carried over into fashionable new areas such as cognitive neuroscience.

Because cognitive psychology and neuroscience usually ignore human courtship behavior, this book discusses very little of the research areas I was trained to pursue. Such research reveals how human minds process information. But evolution does not care about information processing as such: it cares about fitness—the prospects for survival and reproduction. Experiments that investigate how minds process arbitrary visual and verbal information shed very little light on the fitness costs and benefits of the human abilities that demand evolutionary explanation, such as art and humor. Conversely, some less well-funded research on individual differences, personality, intelligence, and behavior genetics has proven surprisingly useful to me. Such research bears directly on the key questions in sexual selection: how do traits differ between individuals, how can those differences be perceived during mate choice, how are those differences inherited, and how are they related to overall fitness? Its conclusions are not always what we refer to nowadays as "politically correct." I would have been more comfortable combining evolutionary biology with a politically correct neuroscience that ignores human sexuality, individual differences, and genes. But in evolutionary psychology we have to deal with evolution, and that means paying attention to genetically heritable individual differences that give survival or reproductive advantages over other individuals.