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وبخصوص الأصول الهندية لعلم الجبر هذا، فإن العرب، ”الذين عزي إليهم عن خطأ، ولحقبة طويلة، اختراع النظام العشري“، لم ينكروا هم أنفسهم مديونيتهم للهنود في هذا المجال، إذ أطلقوا على ذلك العلم اسم ”الهندسة“، أي ”الفن الهندي“، كما أنهم ”ما زالوا يكتبون أعدادهم من اليسار إلى اليمين، تماماً كما في النقوش الهندية، على حين أن الكتابة الأبجدية العربية تُقرأ هي نفسها من اليمين إلى اليسار“

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Having religious beliefs (including the promise of heavenly rewards for the disadvantaged) and rules, traditions, and institutions that allow a large number of non-kin to work together makes it easier for a group to defeat other groups with more divisive or inconsistent ideologies. Religions are a cultural feature that secondarily allow unrelated individuals to expand the circle of cooperation, exchange goods, and maintain a division of labor. And success in competing with other groups (and reducing conflict within one’s own group) may ultimately come to be reflected in our genes, insofar as certain genetic variants fare better in environments made more cooperative in the first place.

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The well-worked-out examples of the impact of culture on evolution are riveting. Once humans learned to control and then create fire, perhaps as early as 1.8 million years ago (the date is contested), they started to cook, and the caloric content of what they ate rose substantially because heating meat and vegetation makes food more nutritious. The kind of teeth, mouths, and stomachs hominids had at that time, suitable for chewing raw meat off bones and masticating twigs, could, and did, evolve in new directions once cooking arose. Teeth got daintier; masseter muscles (still the strongest muscle in the human body) got weaker (and jaw shape consequently changed); and the stomach got smaller (and the configuration of human ribs was consequently modified). Cooking freed up energy to power the demanding human brain, which got larger.

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Another very good example of gene-culture coevolution is lactose tolerance in adults. Lactose is the key sugar in milk. Using the enzyme lactase, babies can digest the lactose in breast milk, but they usually lose this ability as they get older because they do not need it once they are weaned. Worldwide, the ability to digest milk in adulthood is uncommon and is generally absent in East Asia and Africa. Overall, perhaps percent of the world’s population (including a significant fraction of North Americans and Europeans) lose their lactase enzymes by adulthood.56 The ability of adults to digest lactose confers evolutionary advantages only when a stable supply of milk is available. And this would not have been the case until after milk-producing animals (sheep, cattle, goats, camels) had been domesticated and a host of related cultural features, such as knowledge about how to care for such animals and collect their milk, had arisen. The advantages of being able to drink milk are numerous, ranging from a source of valuable calories to a source of necessary hydration during times of water shortage or spoilage. Amazingly, just over the past three thousand to nine thousand years, several adaptive mutations have occurred in the LCT gene (coding for lactase) independently in Africa, Europe, and Central Asia, all conferring this ability. And these mutations are principally seen in herders, not in nearby populations that have retained a forager lifestyle.

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A different evolved psychology is required for the prestige type of status. Our capacity for culture has refashioned the way our species’ brains work over the course of evolution. If we evolved to value prestige, we should see traces of this from a very young age. Cleverly designed experiments show just that.19 For instance, preschoolers are twice as likely to try to learn from a popular than an unpopular adult. This preference holds for copying the manipulation of toys and tastes for food. Insecure politicians exploit these types of cues, making ostentatious displays of fawning audiences in order to enhance their prestige in the eyes of others.

Prestige also works across domains. It’s commonplace that people perceived as competent in one domain will be presumed to be good models in other domains (“When you’re rich, they think you really know,” as Tevye observed in a song in Fiddler on the Roof). When I practiced medicine as a hospice doctor, I was commonly at the bedside of patients when they died. Possibly because I was good at providing end-of-life care, many families would ask me theological questions and put great stock in whether I thought there was an afterlife, which struck me as odd. In a study of three villages in Fiji, people were asked to whom they would go for advice regarding fishing, yam cultivation, and medicinal plants. Some people in the villages were seen as the optimal models for each practice, respectively, as you would expect if expertise was both concentrated and recognized. But prestige also transferred across domains. An individual seen as knowledgeable about yams was more than twice as likely to be regarded as someone from whom to learn to fish.

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This is cumulative culture. People endlessly contribute to the accumulated wealth of knowledge that belongs to humanity, and each generation is generally born into greater such wealth. (Of course, there are periodic reversals too, when information disappears, as happened after the collapse of the Roman Empire; for example, Europeans lost the knowledge of how to make concrete and for seven hundred years lived in Roman dwellings that they had no idea how to construct.)

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Interestingly, Belyayev’s ideas were not popular at the time he began his work, and he had to describe his experiment as being about physiology, not genetic inheritance. Soviet doctrine at the time was heavily influenced by Trofim Lysenko, a Russian scientist who rejected the genetic and evolutionary theories of Mendel and Darwin in favor of a theory of “acquired characteristics” similar to that advanced by Jean-Baptiste Lamarck. This theory posited, for example, that if a parent learned to run faster to catch its prey, then its children would inherit this newfound ability to run fast. Natural selection had nothing to do with it.

Lysenko wielded tremendous political power during the Stalinist era in the USSR, denouncing his scientific opponents, some of whom were executed. In 1948, the Soviet government outlawed opposition to his theories. Genetics was officially declared “a bourgeois pseudoscience” and virtually banned until the mid-1960s. Given the patently political ways in which his theories were advanced, the term Lysenkoism has come to be used to describe the manipulation of science to reach predetermined, ideological conclusions. I believe that the rejection of the role of genetics in contemporary human social life is a type of Lysenkoism.

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Snail flukes do more than affect snail shell thickness, however. They also exert control over the snail’s behavior. They somehow cause the snails to move toward light rather than, as they normally do, away from it. The flukes then migrate to the eyestalks of the snail and hang out there, pulsating and (it is thought) attracting the attention of hungry birds. All of this is bad for the snail because it means birds are more likely to see the snail and nibble the snail’s eyes. But it is good for the flukes, because it means they can enter the bird’s digestive system, which is essential to the next phase of the flukes’ own life cycle.

This behavioral control of the snails seems like science fiction, and, in fact, the evolution of parasites to modify their hosts’ behaviors (rather than just their bodies) is thought to be exceedingly rare, found in less than 0.5 percent of parasite species. But still, scientists are finding more and more examples. Mice infested with the parasite toxoplasma lose their innate aversion to cat urine, which sets them up to be eaten by cats, which become the next hosts of the parasite. Hairworms can cause their cricket hosts to jump into water and drown, which furthers the hairworms’ life cycle. Some parasites appear to achieve their objectives by evolving the capacity to release totally alien (to them) chemicals that imitate the hormones of their hosts, thus changing their behavior.26

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These observations can lead to a paradox: societies that stress uniqueness and individuality and that provide a fertile terrain for friendship based on the personal and specific can actually be those where our common humanity is more easily recognized. It is not a coincidence that the philosophers of the Enlightenment emphasized the special worth of each individual and, in parallel, highlighted the notion of universal human dignity (even though, at least initially, this was not applied to all classes of people). In fact, findings from cross-cultural studies suggest that in-group bias and an emphasis on the distinction between us and them is higher in collectivist societies (including Communist societies), which stress the importance of group membership and subsume the individual within the group, than it is in individualist societies (where social interdependence is less salient), which stress autonomy. Similarly, the more identities available for individuals to assume and the more cross-cutting they are (such that people who are discordant on religion might be members of the same political party, for instance), the more tolerant a society can be of outsiders and, hence, of everyone.

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But these experiments also found something that depresses me even more than the existence of xenophobia in the first place. People in other experiments who were given the opportunity to assign rewards to in-group and out-group members preferred to maximize the difference in amounts between the two groups rather than to maximize the amount their in-group got, reflecting a zero-sum mentality that one group’s gain is another group’s loss. What seems to be important to people is how much more members of one’s own group get compared to members of other groups, not how much one’s group has. Mere fealty to a collective identity is not able to explain this, because if all that mattered was how well off one’s group was, its standing relative to other groups would not matter. But both absolute and relative standings are important to people. Not only must one’s group have a lot, it must have more than other groups

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If banker’s-paradox dilemmas were features of our species’ evolutionary landscape, then we would expect to see the evolution of diverse adaptations to make more credit available. In tandem with friendship, humans might have a desire to be recognized and valued for their individuality and even to feel jealous (and perhaps act vindictively) when developments in their social lives threatened their special status. Referring to someone as “irreplaceable” is a common form of praise. And many psychological phenomena in our species reflect the threatening nature of social replaceability, including the fact that we seem to like to form groups that are small enough for individuality to be appreciated. Ironically, then, individuality is crucial to the formation of social groups and to how the whole emerges from the parts. From this perspective, the sense of alienation felt by many people who live in modern market economies makes sense. People are often dissatisfied with the feeling of anonymity engendered by formal institutions and bureaucracies. If your life is full of explicit and conditional exchanges with strangers that occur at a frequency and volume absent in our species’ evolutionary past, it might well make you miserable. Our species’ evolved psychology reads these exchanges as markers of how superficially, even meaninglessly, we are engaged with the people around us and how vulnerable we might be to a sudden reversal of fortune. Without friends, we feel naked.

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Working-class Americans report relying on their friends and neighbors for practical help such as child care, spiritual advice, car and home repairs, and cash gifts or loans more often than middle-class Americans do. This reliance makes them more rooted in their communities, with stronger ties there, whereas more affluent Americans tend to get support from formal institutions such as therapists, work colleagues and mentors, and legal and financial advisers. Being less reliant on the friends in their community, they can afford to be more geographically mobile. In this regard, the long sweep of modernity may be diminishing our natural inclination to make friends.

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Among Bozo fishermen in Mali, friends make lewd comments about the genitals of their friends’ parents. In Greece, male friends endlessly and publicly (and tiresomely, if you ask me) accuse one another of being masturbators. Some of these human behaviors, which can be seen as indicators of vulnerability or trust, may have parallels in behaviors seen in other primates. For instance, some groups of capuchin monkeys play a finger-in-the-mouth game in which one monkey places a finger into a friend’s mouth, and the friend clamps down on it so that it cannot be taken out but not so much that it hurts the finger-poker.

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This in turn suggests an even deeper observation about the role of friendship in our species. Our assembly into networks of friendship ties sets the stage for the emergence of moral sentiments. At their core, moral compunctions relate to how people interact with others, especially those who are not kin and for whom the bonds of kinship and the inexorable workings of inclusive fitness are not enough of a guide.

Most human virtues, I would argue, are social virtues. To the extent that we care about love, justice, or kindness, we care about how people enact these virtues with respect to other people. No one is interested in whether you love yourself, whether you are just to yourself, or whether you are kind to yourself. People care about whether you show these qualities to others. And so friendship lays the foundation for morality.

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Evolutionary biologist Jessica Flack and her colleagues manipulated the network structure of a group of eighty-four pigtailed macaques at the Yerkes National Primate Research Center near Lawrenceville, Georgia. They first measured connections based on whom the monkeys groomed or played with. Group leaders were identified by counting the number of times others silently bared their teeth at them during peaceful moments, an act of deference in this species, resembling a human smile. The scientists then strategically removed the highest-ranked individuals (they were knocked out of the group, as the scientists termed it) and then compared the resulting social networks to an unperturbed control condition.

When the high-ranking individuals were knocked out, chaos ensued. Conflict and aggression skyrocketed. This analysis sheds light on the impact of leaders on interactions within the group as a whole. First, after the leaders were removed, the group had fewer grooming and play interactions overall. That is, the remaining macaques became less connected to one another. This suggests that stable leadership promotes peaceful interactions not only between leaders and followers, but also between followers and other followers. The existence of popular leaders seems to facilitate social order throughout the group. And given that these interactions create opportunities for beneficial mutual support, it is easy to see why natural selection might favor the evolution of an individual interest in, and respect for, hierarchy.

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The story of the genetic roots of pair-bonding in men is even more speculative. But Larry Young has argued that the bonding and love that men feel for their partners might be an exaptation of the neural pathways that originally evolved in men for the guarding of territory. Male animals often identify, mark, and defend territory, and this behavior requires a host of related adaptations, including the ability to remember the territory and feel attached to it. According to Young, in male brains, females may have become an extension of the concept of territory. To be clear, Young is not even remotely suggesting that this is a conscious link in humans, or that women are possessions, or that this is the only factor at play in the evolution of love and pair-bonding in men. Still, the extent to which men’s sense of connection to women is tied to territorial feelings is illustrated by certain aspects of violence in our species; wars have often been fought over women in small-scale societies, and propaganda posters encouraging warfare even in the modern era have invoked the trope of defending women. The mass rapes committed by Soviet troops in Germany in World War II and by the Pakistani army during the 1971 war of Bangladeshi independence might also be seen as grotesque perversions of this evolutionary conflation of male territoriality and sexual behavior.

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stimulation of human breasts, which happens more easily during face-to-face sex (almost nonexistent in other primates), also releases oxytocin. The possibility that, evolutionarily speaking, heterosexual face-to-face intercourse activates maternal-child bonding pathways would help explain why human breasts are large compared to other primates’ as well as why they are large outside the period of lactation, which is also unusual.

Scientists speculate that stimulation of the vagina by the penis may prime similar bonding pathways. This in turn may help explain another mystery in evolutionary biology: why human penises are relatively large compared to those of our primate cousins. Gorilla penises, for instance, are only one and a half inches long. Young argues that human penises may have evolved to be larger than necessary because they stimulate the vagina in a way that simulates the birth process. Coupled with the unusual fact that humans often have sex face-to-face, women may register the context in which they are having this hormonal experience and thus associate their sexual encounter with a set of physiological experiences that originally evolved to facilitate mother-child bonding. This oxytocin reflex occurs while imprinting on a particular man, which supports pair-bonding.