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Human nature simply cannot be understood in isolation from the rest of nature. This evolutionary approach is already difficult for many people to accept, but it is likely to generate even more resistance once its implications are fully grasped. After all, the idea that we descend from long-armed, hairy creatures is only half the message of evolutionary theory. The other half is continuity with all other life forms. We are animals not only in body but also in mind. This idea may prove harder to swallow.

We are so convinced that humans are the only intelligent life on earth that we search for other intelligent beings in distant galaxies. We also never seem to run out of claims about what sets us apart, even though scientific progress forces us to adjust these claims every couple of years. That is why we do not hear any more that only humans make tools, imitate each other, have culture, think ahead, are self-aware, or adopt another's point of view. It is the rare claim of human uniqueness that holds up for more than a decade.

If we look at our species without letting ourselves be blinded by the technological advances of the last few millennia, we see a creature of flesh and blood with a brain that, albeit three times larger than that of a chimpanzee, does not contain any new parts. Our intellect may be superior, but we have no basic wants or needs that cannot also be observed in our close relatives. I interact daily with chimpanzees and bonobos, which are known as anthropoids precisely because of their human-like characteristics. Like us, they strive for power, enjoy sex, want security and affection, kill over territory, and value trust and cooperation. Yes, we use cell phones and fly airplanes, but our psychological make-up remains that of a social primate.

To explain human behavior as a "mere" product of evolution, however, is often seen as insulting and a threat to morality, as if such a view would absolve us from the obligation to lead virtuous lives. The geneticist Francis Collins sees the "moral law" as proof that God exists. Conversely, I have heard people echo Dostoevsky's Ivan Karamazov, exclaiming that "If there is no God, I am free to rape my neighbor!"

Perhaps it is just me, but I am wary of anyone whose belief system is the only thing standing between them and repulsive behavior. Why not assume that our humanity, including the self-control needed to form a livable society, is built into us? Does anyone truly believe that our ancestors lacked rules of right and wrong before they had religion? Did they never assist others in need or complain about an unfair share? Human morality must be quite a bit older than religion and civilization. It may, in fact, be older than humanity itself. Other primates live in highly structured cooperative groups in which rules and inhibitions apply and mutual aid is a daily occurrence.

Even without claiming other primates as moral beings, it is not hard to recognize the pillars of morality in their behavior. These are summed up in our golden rule, which transcends the world's cultures and religions. "Do unto others as you would have them do unto you" brings together empathy (attention to the feelings of others) and reciprocity (if others follow the same rule, you will be treated well, too). Human morality could not exist without empathy and reciprocity, tendencies that have been found in our fellow primates.

After one chimpanzee has been attacked by another, for example, a bystander will go over to gently embrace the victim until he or she stops yelping. The tendency to console is so strong that Nadia Kohts, a Russian scientist who raised a juvenile chimpanzee a century ago, said that when her charge escaped to the roof of the house, there was only one way to get him down. Holding out food would not do the trick; the only way would be for her to sit down and sob, as if she were in pain. The young ape would rush down from the roof to put his arm around her. The empathy of our closest evolutionary relatives exceeds even their desire for bananas.

Reciprocity, on the other hand, is visible when chimpanzees share food specifically with those who have recently groomed them or supported them in power struggles. Sex is often part of the mix. Wild males have been observed to take great risks raiding papaya plantations, returning to share the delicious fruit with fertile females in exchange for copulation. Chimps know how to strike a deal.

Our primate relatives also exhibit pro-social tendencies and a sense of fairness. In experiments, chimpanzees voluntarily open a door to give a companion access to food, and capuchin monkeys seek rewards for others even if they themselves gain nothing from it. Perhaps helping others is self-rewarding in the same way that humans feel good doing good. In other studies, primates will happily perform a task for cucumber slices until they see others being rewarded with grapes, which taste so much better. They become agitated, throw down their measly cucumbers, and go on strike. A perfectly fine vegetable has become unpalatable! I think of their reaction whenever I hear criticism of the extravagant bonuses on Wall Street.

These primates show hints of a moral order, and yet most people still prefer to view nature as "red in tooth and claw." We never seem to doubt that there is continuity between humans and other animals with respect to negative behavior - when humans maim and kill each other, we are quick to call them "animals" - but we prefer to claim noble traits exclusively for ourselves. When it comes to the study of human nature, this is a losing strategy, however, because it excludes about half of our background. Short of appealing to divine intervention as an explanation, this more attractive half is also the product of evolution, a view now increasingly supported by animal research.

This insight hardly subtracts from human dignity. To the contrary, what could be more dignified than primates who use their natural gifts to build a humane society?

As I write this essay, my fingers hold a pen and my eyes scan the page - fingers that have evolved from fins, eyes that have developed from little more than pigmented spots. We may walk tall, but we cast a long evolutionary shadow. At the same time, my ears are distracted by bird-song from the yard outside. But why should I bother to waste my time listening to the birds? Why, indeed, should I be interested if three separate families of birds - songbirds, parrots, and hummingbirds - all evolved song independently, and why should I care that the manner in which some birds learn to sing is strikingly similar to the way that language emerges from babble in children?

The answer is that I am naturally curious and also that I appreciate beauty. The evolution of bird-song is not only a striking example of evolutionary convergence - that is, of unrelated organisms arriving at very much the same biological solution - but it has a much wider importance. It is an indication that at least some outcomes of the Darwinian process are more likely than others and, in some cases perhaps, are actually inevitable. The capacity for song points to even more striking similarities between birds and mammals in terms of overall cognitive capacity, not least with respect to play and the manufacture of tools. All of these developments have occurred independently and by the process of evolution.

So why quibble with the standard Darwinian formulation? Is it not obvious that the roots of human behavior and cultural sophistication lie in the rich loam of our evolutionary past? We are but a hair's breadth from our animal cousins. Such is evident in terms of their cognitive world (which many believe encompasses, at least in apes and some birds, a theory of mind), their capacity for self-recognition in mirrors, and the glimmerings among them not just of culture and its transmission but of crafted tools and even traits of personality. So what is the problem?

At one level, there is none. It would be strange if my fingers and eyes were to have an evolutionary origin but not my capacity to speak, to empathize, and even to deal with simple abstractions like numbers. And yet, though we may be just a hair’s breadth away from a chimp - not to mention a crow, a dolphin, an elephant, and even an octopus - we humans are still utterly and stupendously different. A seamless extrapolation from one species to another? That is what Darwin proposed, but pinning down how the glaring gaps - most obviously, language - were actually bridged remains almost entirely obscure.

Should we look, then, to human exceptionalism, to a freak mutation that suddenly propelled us into new worlds? It is possible, of course, but there is not a shred of evidence for it. Could it just be an illusion? Perhaps we think we are different, but the animals themselves know better. Is that credible? Not really. So profound is the gulf between us and the chimps that they might as well live in the Andromeda galaxy. Have you seen a chimp make a fire, let alone go to the library?

The late David Stove, an Australian philosopher, wrote a wonderful book entitled Darwinian Fairytales. How dare anybody use a word like "fairytale" in the same breath as the venerated Darwin? (See how the cage housing the ultra-Darwinists rocks and shudders, the occupants hurling themselves against the bars with cries of outrage.) But Stove was emphatically not a creationist or even a theist, let alone a Christian. And he had no quarrel with evolution. For him, the question was not where we came from but who we are now. In a piercing critique, he dismantled the Darwinian pieties purporting to show why we are so extraordinarily altruistic (not to mention our love of animals), demolished the absurdities of genetic determinism, exploded the naiveties of sociobiology, and laid waste the myth that we are "just another species."

But how did we come to be so different, in fact, so very odd? I would propose a radical alternative. We live in a world riddled with symbols and symbolic expression - a place where people kill for principle or engage in reckless altruism, where thousands cheer their teams while others choose monastic isolation. Our societies buzz with chatter, friendship, and laughter, but they are also haunted by terrible, reflective silences, echoing back through history for hundreds of years.

Somehow we have intuited the ineffable, matters that defy precise description but still resonate at the deepest levels. The world of myth is not just a set of superior fairy stories but rather an attempt to use language to describe our cosmic engagement. Is all this striving after ultimate meaning a massive delusion, a gigantic wish-fulfillment? Is this what happens when the brain gets too big: the puzzled and frightened ape stumbles across comprehension and just as suddenly realizes that his existence is entirely meaningless? Could our symbol-rich world be of interest only to a pitiless nihilist? I do not think so.

Suppose that the moral structure, the ethical voice, the heart-wrenching aesthetic, the haunting intuition that certain places are holy, the endless yearning for a world made good are not the fantasies of a deracinated ape but rather are signposts to deep realities in which our destiny may be involved. Suppose that evolution is like a search engine, always seeking the best solution. From this perspective, it is hardly surprising that scattered across the evolutionary landscape, among the grunts and howls, the dawning intelligence and the scarcely articulated emotions, we do indeed see the flickerings of ourselves.

The real question of how we came to be who we are does not revolve around a process of creeping Darwinian emergence, whereby the various components drifted together into a human whole with distinctive and (let's be honest) very odd powers all of its own. Rather it is a true story of discovery, of first detecting and then entering and finally enjoying entirely new worlds that were waiting for us all the time. We could not have arrived where we are except by evolution, and this is where we need to be. As rational creatures we now not only know evolution but we know how to transcend it.

Ever since Bishop Wilberforce asked, in a debate with Thomas Huxley, whether it was from his grandmother or grandfather that he claimed descent from a monkey, the sufficiency of evolutionary theory to explain humanity’s spiritual and moral qualities has been in question. Then, as now, the evolution of humans was a touchy subject, and after the publication of On the Origin of Species, Darwin devoted a separate work, The Descent of Man, to untangling how evolutionary understanding could be applied to humans and their special traits.

Since his account of "descent with modification" leaned heavily on natural selection of the individual, Darwin wondered how moral behaviors - which focus on others - evolved. When lying, cheating, manipulation, greed, and other less than admirable qualities seemed to benefit those individuals who practiced them, how could their opposites evolve? Pointing out that he "who was ready to sacrifice his life . . . would often leave no offspring to inherit his noble nature," Darwin pondered how members of a tribe became endowed with moral attributes.

His simple answers still apply. One who aids his fellows commonly receives aid in return. Darwin called this a "low motive" because it is self-regarding. So-called reciprocal altruism - I’ll carry your baby if you take my son on the hunt tomorrow at dawn - is operative in species whose members are capable of recognizing each others' faces. More important is the praise we love and the blame we dread, instincts that help bind tribe members who work together. Reciprocal acts of kindness and aid underlie families, tribes, and religious groups; they ensure survival and reproduction as "naturally selected" perpetuating, living entities.

Our human sort of mutual care, along with the strong feeling of life we have in the presence of sexual partners, family, friends, colleagues, classmates, and fellow citizens (in short, in the company of meaningful others), necessitates frequent communication: symbols, language, music, teaching, learning, etc. Do these activities fundamentally distinguish us from the non-human life forms with whom we share the planet and upon whom we depend for our survival? I doubt it.

This may sound inadequate to true believers in human uniqueness, especially on religious grounds. But religion serves an obvious evolutionary function: it identifies, unifies, and preserves adherents. Admonitions to desist from the seven deadly sins inhibit behaviors that threaten group solidarity and survival. Greed, for example, privileges the individual in seasons of limited resources. Lust - the biblical coveting of the neighbor’s wife (in its male-centered perspective) - interferes with ideals for the nurture of healthy children and effective warriors. Prohibiting sloth enhances productive work intrinsic to survival and reproduction of the social unit. Anger, perhaps useful in battle, destroys family and other social relationships. Envy and pride promote individual interests above those of the larger social unit. The survival value of prohibiting sin seems obvious.

By contrast, "love thy neighbor," interpreted from an evolutionary point of view, is an algorithm for social connectedness. The touted virtues of chastity, moderation, compassion, diligence, patience, moral commitment, and humility provide touchstones for effective group action. The intellectual historian Karen Armstrong, a former nun and the author of books on Christianity, Judaism, and Islam, argues that compassion is the crucial link among the major religions. The golden rule of Jesus, Confucius, and others is that we should not do to others what we would not want them to do to us. Is this not a clear precept for the evolutionary perpetuation of specific cohesive groups in familiar habitats?

We differ from other species in that fewer rules of social behavior are communicated only by shout, groan, touch, and facial expression and more by verbal explication. But all tend to maintain and perpetuate unity of the pack, gaggle, or herd. We people share a linguistic version of the universal tendency toward socio-ecological wisdom measurable in life forms at every level. After my collaborative scientific work for over a half century to detail the genetics, microscopy, and biochemistry of cells that adhere in their lives together, I consider the neo-Darwinist overemphasis on competition among selfish individuals - who supposedly perpetuate their genes as if they were robots - to be a Victorian caricature. Disease microbes that kill all their victims perish themselves as a result of their aggression.

I disagree with neo-Darwinist zoologists who assert that the accumulation of random genetic mutations is the major source of evolutionary novelty. More important is symbiogenesis, the evolution of new species from the coming together of members of different species. Symbiogenesis is the behavioral, physiological, and genetic fusion of different kinds of being; it leads to the evolution of chimeric new ones. One example is of originally pathogenic bacteria that invaded and killed many amoebae in the University of Tennessee laboratory of Kwang Jeon in the 1970s. He selected survivors, and eventually different amoebae with new species characteristics appeared among them. These had retained 40,000 bacteria in each amoeba!

A new type of fruit fly evolved after it acquired an insect-loving bacterium that prevented it from successfully mating with its old partners. Indeed, the only documented cases of the "origin of species" in real time involve not selfish genes but "selfless" mergers of different forms. Chemical and genetic evidence suggests that even mitochondria, bodies inside all of our cells that suffocate without oxygen, came from ancient mergers, truces between oxygen-respiring bacteria and the nearly poisoned cells of other kinds of microscopic beings. The mergers, naturally selected, survived to thrive and spread across the planet.

Gifted with large brains that permit us great neurological processing power, we humans plan further into the future. We recognize more of our own kind with whom, now via global communication, we establish relationships of identity and trust. But on a crowded planet, there has always been a premium on effective togetherness. Our moral nature reflects rather than conflicts with nature.

Free will may also be nature-deep. Large single-celled forams choose from brightly colored sand grains the correct ones with which to make shells. Aware of shape and color, they make choices and reproduce their kind. Awareness in some form has been naturally selected for at least 550 million years. For me, our spirituality and moral nature help perpetuate our living communities, just as similar attributes aided previous living communities whose evolution is chronicled in the fossil record.

Photo credit: Mariana Cook.

The evidence in support of Darwin's theory of evolution is overwhelming. In my own field of genomics, the digital record of the long history of life on this planet - a complex and awesome story of gradual change in DNA acted upon by natural selection - provides incontrovertible proof of descent from a common ancestor. As the noted geneticist and evolutionary theorist Theodosius Dobzhansky wrote several decades ago, "Nothing in biology makes sense except in the light of evolution." And that includes humankind.

But Dobzhansky believed in God. And so do I.

Regrettably, much of the current culture in the United States sees evolution as an affront to belief in God. But the 40 percent of working scientists who are believers have a different view. Most of us are theistic evolutionists. We see evolution as God's method for creation - and what an elegant method it is! Put another way, we see life (bios) as the consequence of God's Word (the Logos). Thus, I like to refer to theistic evolution as "biologos."

Scientists who share my view do not see evolution as incompatible with the Bible, and we are puzzled and distressed that so many modern-day Christians insist on an ultra-literal reading of Genesis, when thoughtful believers down through the centuries have concluded that this story of God's plan for creation was never intended to be read as a scientific textbook. We see science as the way to understand the awesome nature of God’s creation and as a powerful method for answering the "how" questions about our universe. But we also see that science is powerless to answer the fundamental "why" questions, such as "Why is there something instead of nothing?," "Why am I here?," and "Why should good and evil matter?"

Let's focus on this last question. One of the most notable characteristics of humanity, across centuries, cultures, and geographic locations, is a universal grasp of the concept of right and wrong and an inner voice that calls us to do the right thing. This is often referred to as the moral law. We may not always agree on what behaviors are right (which is heavily influenced by culture), but we generally agree that we should try to do good and avoid evil. When we break the moral law (which we do frequently, if we are honest with ourselves), we make excuses, only further demonstrating that we feel bound by the moral law in our dealings with others.

Evolutionary arguments, which ultimately depend on reproductive fitness as the overarching goal, may explain some parts of this human urge toward altruism, especially if self-sacrificing acts are done on behalf of relatives or those from whom you might expect some future reciprocal benefit. But evolutionary models universally predict the need for reflexive hostility to outside groups, and we humans do not seem to have gotten that memo. We especially admire cases in which individuals make sacrifices for strangers or members of outside groups: think of Mother Teresa, or Oskar Schindler, or the Good Samaritan.

We should be skeptical of those who dismiss these acts of radical altruism as some sort of evolutionary misfiring. And if these noble acts are frankly a scandal to reproductive fitness, might they instead point in a different direction - toward a holy, loving, and caring God, who instilled the moral law in each of us as a sign of our special nature and as a call to relationship with the Almighty?

Do not get me wrong. I am not arguing that the existence of the moral law somehow proves God’s existence. Such proofs cannot be provided by the study of nature. And there is an inherent danger in arguing that the moral law points to some sort of supernatural intervention in the early days of human history; this has the flavor of a "God of the gaps" argument. After all, much still remains to be understood about evolution's influence on human nature. But even if radically altruistic human acts can ultimately be explained on the basis of evolutionary mechanisms, this would do nothing to exclude God’s hand. For if God chose the process of evolution in the beginning to create humans in imago Dei, it would also be perfectly reasonable for God to have used this same process to instill knowledge of the moral law.

A deeper question raised by this debate is the fundamental nature of good and evil. Does morality actually have any foundation? To be consistent, a committed atheist, who argues that evolution can fully account for all aspects of human nature, must also argue that the human urge toward altruism, including its most radical and self-sacrificial forms, is a purely evolutionary artifact. This forces the conclusion that the concepts of good and evil have no real foundation, and that we have been hoodwinked by evolution into thinking that morality provides meaningful standards of judgment. Yet few atheists seem willing to own up to this disturbing and depressing consequence of their worldview. On the contrary, the most aggressive of them seem quite comfortable pointing to the evil they see religion as having inspired. Isn’t that rather inconsistent?

I was once an atheist myself, and so I understand the temptation to fall into a completely materialistic view of human nature. But seeing all of humanity's nobler attributes through the constricted lens of atheism and materialism ultimately leads to philosophical impoverishment and even to the necessity of giving up concepts of benevolence and justice. I found that a whole world of interesting questions opened up for me once I accepted the possibility of a spiritual aspect to humanity.

In the last two decades, evolutionary psychology has cast new light on ever more facets of human nature. And contrary to popular critiques of the field, it has done so in ways that are ever more intellectually thrilling, morally enlightening, spiritually satisfying, and socially progressive. What we mean by "evolution" and "human nature" continues to develop through mutual interaction, like the passions of a whispering couple in a close-embrace tango.

During the 1990s, biologists developed a whole new toolbox of ideas about the nature of evolution, including theories based on life history, multi-level selection, strong reciprocity, good-genes sexual selection, and costly signalling. These terms may be unfamiliar to non-specialists, but they represent a revolution in Darwinian theory and have proven their value again and again in understanding aspects of human nature that defy simplistic "survival of the fittest" reasoning.

Likewise, our understanding of human nature has been growing exponentially through work in evolutionary psychology, evolutionary anthropology, human evolutionary genetics, and primate behavior. Our model is no longer a tattered old treasure map of a few basic instincts (hunger, fear, lust) but a topographically detailed Google Earth panorama across a whole continent of familiar capacities (romantic love, moral commitment, self-deprecating humor, conspicuous charity, and many more). New theories have led researchers to acknowledge new aspects of human nature, and recognizing previously overlooked aspects of human nature has promoted new progress in evolutionary theory.

My own research has been inspired mostly by good-genes sexual selection theory (the idea that animals choose their partners based on cues about genetic quality) and costly-signalling theory (the idea that only animals in good condition can afford seemingly pointless displays like extravagant plumage). These theories have proved enormously useful in understanding a range of human behaviors that have seemed to have no clear survival payoffs, like music, dance, art, humor, verbal creativity, conspicuous consumption, and altruism.

Consider a few examples of new empirical discoveries from research I have done with various collaborators:

• Gil Greengross and I showed that women are more attracted to men who use self-deprecating rather than other-deprecating humor during courtship (but only if the men are fairly high in social status). This is consistent with the costly-signalling idea that self-mockery is a virtue that only the successful can afford.


• Martie Haselton and I showed that women at peak fertility, just before ovulation, show a stronger preference for creativity as opposed to wealth in potential mates. This supports the idea that creativity is an indicator of "good genes" rather than of potential as a "good provider."


• Vladas Griskevicius, several colleagues, and I showed that if men are put in a romantic mood rather than a neutral mood, they are more likely to spend money on conspicuous luxuries, whereas women spend more time on conspicuous charity, such that each sex is signalling a trait (social status or kindness) that is relatively more desired by the other sex.

Each new finding like this illustrates how new evolutionary theories can lead to discoveries that were never predicted by the standard "blank slate" view of human behavior.

Still, evolutionary psychologists must guard against complacency. We should not imagine that we have discovered every important facet of human nature, or that evolutionary theory as it exists circa 2009 has told us everything we need to know about the selection pressures that have shaped human nature.

Consider just one new development in biology: the whole new world of RNA, which may help explain the unique behavioral flexibility of the human brain. The "central dogma" of genetics since the 1950s was that DNA is transcribed into RNA, which is translated into proteins, which generate all the adaptive complexity of organic life. Thus, only the DNA sequences that code for proteins are important, and only evolutionary changes in protein-coding DNA are worth analyzing. When journalists report that humans have "only" some 25,000 genes - just a few more than the 20,000 of the C. elegans worm - they are referring to these protein-coding genes.

This "central dogma" has guided the Human Genome Project, the HapMap project, and even the genome-wide association studies that dominate the human genetics journals these days. But the idea has been decisively overturned in the last decade by new discoveries about the diversity of RNA that is transcribed from DNA but that is not, in turn, translated into proteins. Most of this "non-coding" RNA seems to constitute a genomic regulatory system of vast complexity - a system that determines the expression of different protein-coding genes in different cell types, tissues, and organs at different times during development and in response to different environmental changes. The human genome has a vastly more complex RNA system than C. elegans.

The molecular biologist John Mattick and others have argued that the evolution of this RNA system was crucial for three great innovations in life on earth: the emergence of the eukaryotic cell, the Cambrian explosion of multi-cellular life, and the complexity of the human brain. In this view, humans differ from other great apes not so much at the level of protein evolution but at the level of the RNA regulatory system that orchestrates the spatio-temporal patterning of gene expression and protein function. The inherited DNA that is translated into this RNA regulatory system does not just determine "innate instincts" or "hard-wired" behaviors; it also orchestrates dynamic changes in brain function and behavior under different circumstances.

Indeed, it seems likely that RNA is crucial in all sorts of behavioral flexibility that humans have, from feeling different moods (elation, love, depression, ambition) to laying down new memories, to super-charging our creativity, humor, and altruism when we are courting a new mate. All of this may be mediated by complex changes in gene expression throughout the brain, over time scales ranging from hours to decades. We are realizing that our genes do not just determine the blueprint for an infant’s brain; they are working actively throughout our lives, governed by this vast RNA regulatory system, giving us degrees of behavioral creativity and flexibility that it will take us decades to understand.

In short, evolution explains human nature very well indeed, but we are far from finished in the grand project of naturalizing human consciousness.

and almost surely never. Although human nature, like biological nature generally, results from a continuing process of evolution, the question before us is whether present-day evolutionary science explains human nature. Does it explain our religious beliefs and moral commitments as convincingly as it explains our more prosaic traits like, say, why we have four arms and legs instead of six? Obviously not. Evolutionary science has much more work to do before it can explain our more abstract traits. But how much more?

Religious beliefs, moral commitments, consciousness, and the free will to do right and wrong emerge in a social context. These traits are not properties of an individual like the ability to hear high notes or to taste bitter flavors. Social behavior develops as individuals acquire experience with one another. It is a system of traits that forms when individuals interact. A white-crowned sparrow learns its song by listening to others as it grows up. Unlike its vocal chords, a bird's song is a collective property belonging to its group.

What makes social behavior hard to understand is that interaction takes place during development rather than after it. By contrast, consider some socially important physical traits, like green or gray skin color in frogs. These traits are formed not during social interaction but prior to it. In wet years, with green moss on the trees, green frogs are more camouflaged and are able to fight longer for space than gray frogs before seeking cover from predators, whereas in dry years, gray frogs are able to defend their territory longer. The competitive balance point between green and gray frogs changes from year to year, depending on the year’s rainfall, favoring green frogs in wet years and gray frogs in dry years. At each year's balance point, the frogs occupy all of the living space according to a color ratio such that a newly arriving frog of either color has no advantage over another frog. Thus, the colors influence the outcome of territorial interactions, but the colors themselves are not generated by those interactions.

The competitive balance between socially important traits was studied by the late John Maynard Smith, a theoretical biologist who introduced mathematical game theory into evolutionary biology. Maynard Smith applied his analysis to the evolution of social behavior among competing individuals, assuming that their behavioral inclinations or "strategies" were already formed prior to their interaction. He famously discussed the evolutionary outcome of competition between "altruists" who interact with "selfish" individuals, as though the traits of altruism and selfishness were permanent characteristics of the actors, just as green or gray body coloration might be for a frog.

But in most social behavior, how an organism acts, whether it behaves altruistically or selfishly, depends in large part on its experience with others while maturing. Moreover, the Maynard Smith approach stipulates that behavioral interactions are inherently competitive because he considered their outcome to be a competitive balance point.

To go beyond the limitations of Maynard Smith’s model, my students and I have introduced the idea of "social selection." Our approach decomposes the evolutionary theory of social behavior into two levels or "tiers." The "lower" tier analyzes the development of behavioral actions using game-theory techniques but without Maynard Smith's assumption of inherently competitive behavior; we employ criteria for both cooperative and competitive endpoints. The "higher" tier analyzes the evolution of behavioral tendencies using population-genetic techniques.

With this approach, we have been able to show, for instance, that sexual conflict is not inevitable in the relationship between males and females in nature, as some evolutionary biologists claim, and we have demonstrated that some forms of sexual intimacy may be interpreted as mechanisms to enable friendship and teamwork among animals. All in all, our research suggests that the "selfish-gene" metaphor for evolution is misleading and inaccurate.

Still, the question remains whether evolutionary science, even after these and other improvements take root, will ever explain features of human behavior such as spirituality, morality, consciousness, free will, and so forth. But why stop there? Will evolutionary science ever explain most of the features of any species?

This question forces us to confront our own modest place in nature. The natural world is infinite, and even if the aggregate number of people who have ever lived were scientists working 24/7 on evolutionary research, their aggregate effort would be finite, leaving a still infinite set of evolutionary mysteries. Do we know why the chameleon evolved to catch bugs with its tongue instead of sneaking up and pouncing on them? No. Will we ever? Probably not. Do we know why and how humans have come to possess a sense of morality? Not yet. Will we ever? Almost surely not.

Scientific research requires the expenditure of scarce time and money, and for most people, the value of discovering the origins of our moral sense is dwarfed by the health benefits of curing cancer or the environmental benefits of conserving tropical forests. Questions about the evolution of morality seem destined to linger indefinitely on some back burner.

There is nothing inappropriate about asking how we evolved our sense of morality or any other aspect of human nature. Indeed, I believe that investigating how evolution occurs is a sacred calling and that our appreciation for every aspect of human life is enriched by an evolutionary perspective. But some parts of this enterprise are more practical than others - and also are far more likely to succeed.

I am deeply fascinated by evolution, and I wish to expand the boundaries of the evolutionary explanation as far as possible. Yet I do not think that all aspects of human nature can be explained by evolution. The question is subtle, and the answer depends on how we choose to define "human nature."

I like to think of human nature as a collection of thoughts, feelings, and actions that humans experience or perform. Language, for example, is a fundamental aspect of human nature. A child growing up in an environment of speakers develops a language faculty. The thoughts and ideas that are expressed in the languages of the world are all part of human nature. Similarly, we like to listen to music and perform it. A few of us compose music. Music is part of human nature. There is also something very intuitive about numbers and geometric objects, and the ability to do some basic math seems to be part of human nature.

Yet the great theorems of mathematics are statements of an eternal truth that comes from another world, a world that seems to be entirely independent of the particular trajectory that biological evolution has taken on earth. The great symphonies of Beethoven and Mahler capture glimpses of a beauty that is absolute and everlasting. Beyond the temporal, materialistic world there is an unchanging reality.

My position is very simple. Evolution has led to a human brain that can gain access to a Platonic world of forms and ideas. This world is eternal and not the product of evolution, but it does affect human nature deeply. Therefore evolution cannot possibly explain all aspects of human nature.

What is evolution? Evolution occurs whenever there is a population of reproducing individuals. Reproduction at different rates leads to natural selection. Mistakes during reproduction lead to mutation. Mutation and natural selection are two fundamental "forces" of evolution.

Reproduction can be genetic or cultural. The former gives rise to genetic evolution, which has molded life on earth over the last four billion years. The latter is the most decisive factor shaping human society. Humans with language invented a mechanism for nearly unlimited cultural evolution. New ideas and behaviors can spread rapidly by learning, teaching, and imitation. Cultural evolution allows rapid innovation and is responsible for the dramatic changes that have occurred on this planet in the last few millennia.

Sadly, humans do not use their evolved traits only for good ends. They wage wars of destruction. They fight each other, and they destroy the environment that is essential for their survival. Despite all of this, a flame of love is burning inside us that cannot be extinguished.

I am fascinated by questions concerning the evolution of cooperation and altruistic behavior. Natural selection is based on competition between individuals. It introduces conflict. Cooperation means that one individual pays a cost for another individual to receive a benefit. Cooperation is opposed by natural selection unless specific mechanisms are in place.

For humans, the fundamental mechanisms encouraging cooperation are direct and indirect reciprocity. Direct reciprocity is based on repeated interactions between the same two individuals: my behavior toward you depends on what you have done to me. Indirect reciprocity is based on repeated interactions in a group: my behavior toward you also depends on what you have done to others. Cooperation among humans is related to altruistic behavior. Loving others and trying to help them are important aspects of human nature.

Cooperation is, in my opinion, another fundamental "force" of evolution. Cooperation is needed for construction. Whenever evolution moves to higher levels of organization, cooperation is involved. The emergence of multi-cellular organisms, for example, requires cooperation among cells. And human language would not have evolved without sustained cooperation among potential speakers and hearers.

There is a fascinating additional problem concerning our present understanding of evolution. Evolution is a search process. Populations of reproducing individuals "search" for short-term solutions, such as adaptations to a new environment or modifications of a social system. But the search process has to operate within a given space of possibilities. This "search space" ultimately determines what can evolve. For example, evolution can find intelligent life, if it is part of the search space, but it cannot construct the possibility of intelligent life. For science to fully "explain" intelligent life (or other fundamental properties of living systems), we need not only a theory of evolutionary dynamics but also a theory describing how the fundamental laws of nature span the search space.

As a scientist, I could adopt the narrow position that I am exclusively interested in those aspects of human nature that can be analyzed by scientific methods. This is a valuable and useful perspective, and it will continue to generate much scientific progress. But in my Faustian search for truth, I realize that science does not give a complete analysis of human existence. We are all confronted by questions concerning the mystery and purpose of life, which cannot be answered by natural science alone.

I subscribe to the ideas of what Leibniz called "perennial philosophy": there is an unchanging reality beneath the world of change; this reality is also at the core of every human existence; and the purpose of life is to discover this reality. In the context of my own Christian faith, the fundamental aspect of human nature is our relationship with God and our participation in God's love and eternity. This particular aspect of human nature is also not a product of evolution.

Photo credit: Erik Jacobs.

Two centuries after the birth of Darwin, the Darwinian explanation of human nature is essentially complete. We now know why people everywhere - notwithstanding differences of culture and context - experience the same basic emotions, the same kinds of hopes and fears, even the same distortions of perception and cognition.

Ever since Darwin published On the Origin of Species in 1859, it has been clear that natural selection could explain the more obviously animal parts of human nature. Things like hunger and lust are no-brainers: genes that encourage you to ingest nutrients and have sex do better in the Darwinian marketplace than genes that counsel starvation and abstinence. Nor is it any great mystery how humans came to be socially competitive. High social status brings improved access to mates, so genes that fuel the pursuit of status fare well.

Much subtler legacies of evolution have come to light in recent decades as the modern science of evolutionary psychology has emerged. Not just animal appetites and drives, but fine-grained tendencies of emotion and cognition can now be ascribed with some confidence to natural selection. For example, genes inclining us to lower the social status of rivals by spreading unflattering gossip or harsh moral appraisals would be favored by natural selection. And, of course, the most effective propagandist is someone who believes the propaganda, so our everyday moral evaluations of people may be skewed by our genes.

Maybe the biggest accomplishment of post-Darwin Darwinians has come in explaining the mushy side of human nature: compassion, empathy, and so on. These emotions make obvious Darwinian sense only when they are directed toward those endearing little vehicles of genetic transmission known as offspring. But what about when they are directed toward collateral kin - siblings, cousins - or even non-kin? Over the past half-century, two theories - the theory of kin selection and the theory of reciprocal altruism, respectively - have answered these questions.

The theory of reciprocal altruism has also illuminated several other big parcels of the emotional landscape - gratitude, obligation, forgiveness, and righteous indignation. Even the sense of justice - the intuition that it is "right" for good deeds to be rewarded and for bad deeds to be punished - now makes sense as a product of natural selection.

The evolutionary roots of human nature have not been "proved" in the sense that theorems are proved, and they are not as firmly corroborated as, say, the first law of thermodynamics. But they grow increasingly plausible as more psychological experiments are done from a Darwinian angle, more evolutionary dynamics are modeled by computer, and the biochemical links between genes and behavior become clearer. One chemical alone - oxytocin - has been implicated in maternal bonding, romantic bonding, and the trust that undergirds friendship.

None of this is to say that no puzzles remain or that there are no disagreements among Darwinians. Spats between "group selectionists" and "individual selectionists," though often overstated and in some cases merely semantic, do sometimes have real consequences. Still, this infighting results from a surplus of serviceable Darwinian theories, not a shortage. There can no longer be reasonable doubt that the emotions and inclinations that people everywhere share are the legacy of natural selection. Darwin’s theory has illuminated and explained the fundamental unity of human experience.

Many people find it depressing that some of our noblest impulses are reducible to genetic self-interest - and, worse, that this self-interest can subtly corrupt our moral evaluations and our conduct. As it happens, the fact that they find this depressing is itself explicable in Darwinian terms. Natural selection has inclined us to present ourselves as public-spirited and even selfless, and in the service of that goal we are inclined to convince ourselves that we really are public-spirited and even selfless. In other words, we naturally consider ourselves noble, not just "noble."

But this points to the sense in which the Darwinian explanation of human nature is not depressing. If we are naturally inclined to overestimate our goodness, then a theory that exposes us to a truer view of ourselves has the potential to inspire self-improvement. What should depress us is how much time we spend deluding ourselves about our goodness, not the fact that we now have a chance to escape delusion and make amends.

Another dubious source of Darwinian depression is the idea that an evolutionary explanation of human nature leaves us with no great awe-inspiring mysteries about the human condition. Actually, Darwinism, while solving the mystery of human nature per se, has revealed deeper mysteries that it has no hope of solving.

For example: how on earth did the universe wind up generating an algorithm (natural selection) that turns an imperative of utter selfishness at the genetic level into altruism at the individual level? An algorithm this elegant is at least as awe-inspiring as more direct means of creating humanity and other species. Charles Kingsley, an Anglican clergyman and a naturalist, wrote in a letter to Darwin, "I have gradually learnt to see that it is just as noble a conception of Deity, to believe that He created primal forms capable of self-development into all forms needful pro tempore and pro loco, as to believe that He required a fresh act of intervention to supply the lacunas which he himself had made. I question whether the former be not the loftier thought."

Finally, there is the mystery of consciousness. I have said that natural selection readily explains emotions like compassion an

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