The astonishing secrets of human evolutionary history revealed through comprehensive research by the greatest scholars of our time.
We are programmed to help, learn, and love each other!

“Do three-month-old babies know how to live in society?” “What kind of world would shipwreck survivors create?” “Why aren’t there any animals with wheels?” “Is kissing a universal behavior?” “Why is monogamy more prevalent than polygamy?” “Is a society without husbands and fathers possible?” “Why do we each have different faces?” “Why would we lay down our lives for others?” “Do animals share friendships?” “Is the enemy of my enemy my friend?” “How much of a share do we share when we have public money?” “How far do genes go?” “Are humans good or evil?” “Will the world get better or worse?”

Professor Nicholas Christakis, a peerless scholar of this era and a master of integration, embarks on an epic journey to uncover the secrets of human evolutionary history through integrated research that crosses the natural sciences, social sciences, and humanities.
After extensive and meticulous research, the author asserts:
Homo sapiens conquered the Earth not because of their brains or brawn, but because of their ability to create society.
If you look at the history of evolution, good things eventually win out, and the trajectory of evolution bends toward goodness.
The author scientifically and historically demonstrates that humans possess universal characteristics, such as the ability to help, learn, and love one another, and to create a good society.
This book vividly demonstrates that this shared humanity and bright side is genetically programmed as the driving force behind human evolution, unfolding a blueprint of grand optimism and hope for humanity and society.

As I write this, America seems to be split in two.
Left and right, city and country, religious and non-religious, insiders and outsiders, haves and have-nots.
Numerous analyses show that political polarization and economic inequality, which have persisted for a century, have reached a peak today.
Americans are engaged in a heated debate about their differences, about who can and should speak for whom, about the meaning and scope of personal identity, about the unstoppable power of tribalism to draw people's loyalties, and about whether it is possible, or even desirable, to ideologically embrace the melting pot that is America (and our shared American identity).
In this book, I want to demonstrate that there is more that unites us than divides us, and that society is fundamentally good.
--- p.44

Where does this cross-cultural similarity come from? Why can people be so different, even to the point of war, yet so similar? The fundamental reason is that within each of us lies an evolutionary "blueprint" for creating a good society.
Genes do amazing things in our bodies.
But what's even more surprising is what genes do outside our bodies.
Genes influence not only the structure and function of our bodies, but also the structure and function of our minds, and therefore the structure and function of our behavior, and even the structure and function of our society.
What we notice in people all over the world is the power of these genes.
This is the source of our common humanity.

Natural selection has shaped our lives as social animals, driving the evolution of traits I call the “social suite.”
Love, friendship, cooperation, the ability to learn, and even the ability to perceive the identities (personalities) of other individuals come from this “social cluster.”
Despite all the modern devices and artifacts (tools, agriculture, cities, nations), we have an innate tendency to express our natural social instincts.
This instinct is practical, and moreover, morally good.
Just as ants cannot suddenly create a beehive one day, humans cannot create a society that does not conform to these positive impulses.
I believe that we naturally develop these good tendencies just as we naturally develop more cruel tendencies.
We have no choice but to do so.
We feel a sense of accomplishment when we help others.
Our good deeds are not the product of the values ​​that blossomed during the 18th-century Enlightenment.
It originates from the deeper abyss, from prehistoric times.
--- pp.49~50

Chapter 1: Eight Social Traits Engraved Within Us
Although children's typical playmates, activities, toys, and playgrounds vary considerably across gender, age, and culture, their social behaviors and interaction patterns during play have always been remarkably similar.
Society itself can appear to be a mere extension of these children's play worlds.
In his 1938 classic on play, Homo Ludens, social historian Johan Huizinga even went so far as to say that “human civilization has contributed nothing to the essential features of the general concept of play.”
Children's behavior often reveals an innate tendency to create a kind of temporary microcosm of society.
Humans cannot learn without forming a society from a very young age.

--- p.57

In short, humans seem to be hardwired (in the sense of having a strong innate tendency) from a very young age to perceive other people's intentions and to care about fairness, interacting with others in a positive way.
So it's no surprise that all societies, though their specifics may vary from region to region, value kindness and cooperation, define and limit cruelty, and categorize people into good and bad.
Why are humans this way? Why do we exhibit such consistent social behaviors from birth? Where do the social principles that guide children's play and shape adult life come from? And how do people create similar types of social order, possessing the essential and familiar characteristics universally considered good across all societies?
--- p.60

Human society is so vibrant, complex, and all-encompassing that it is alive and moving in its own right.
It may seem like it was created by someone else, some powerful person, or some historical force beyond human comprehension.
When I was a child in the 1970s, some people were so impressed by the seemingly advanced ancient civilizations of Egypt and the Americas that they assumed they must have been created by aliens.
But human society does not come from somewhere else.
It comes from within us.

The ability to band together and form societies is a true biological hallmark of our species, just like the ability to stand upright and walk.
Thanks to this innate ability, which is so rare in the animal kingdom, humans have been able to:
We could do what Edward Osborne Wilson called “the social conquest of the earth.”
It is not our brains or brawn that enable us to rule the Earth, but this ability.
And like all other behaviors that have helped our species survive and reproduce, the ability to organize society has become instinctive.
Social formation is not something we can do, but something we must do.

I want to show that at the core of every society are eight “social clusters”:
(1) Personal identity possession and identification
(2) Love for one's partner and children
(3) Friendship
(4) Social network
(5) Cooperation
(6) In-group favoritism (preference for one's own group)
(7) Moderate hierarchy (relative egalitarianism)
(8) Social learning and social education
These characteristics are expressed within individuals, but they also characterize groups.
These eight things work together to create a society that is well-functioning, long-lasting, and even morally good.

--- pp.69~70

These traits that bind people together are very useful for survival in a world full of uncertainty.
Because it provides a more efficient way to acquire and transfer knowledge and enables people to address risks together.
In other words, these characteristics are Darwinian fitness.
Reproductive success.
It enhances the degree to which hereditary traits or genes are passed on to the next generation (translator's note) and contributes to the benefit of individuals and groups.
So it makes evolutionary sense.
In this way, our genes endow us with social sensitivity and behavior.
In doing so, it helps to structure society on both a small and large scale.

A social environment structured in this way creates a feedback loop throughout evolutionary history.
Throughout history, humans have lived surrounded by social groups, and the presence of fellow humans—people with whom we must interact, cooperate, or avoid—has shaped our genes as powerfully as predators.
Evolutionarily speaking, our social environment has shaped us as much as we have shaped it.

Moreover, although the physical, biological, and social environments have all played crucial roles in our evolution, they differ in one substantial respect.
Humans mastered fire a million years ago (a huge milestone).
However, it is only in the past few thousand years that humans have been able to significantly shape the physical and biological environment.
This is through activities such as building dams to block rivers, domesticating plants and animals, causing air pollution, and using antibiotics.
Before humans invented agriculture and cities, they did not construct their own physical environment.
I just chose the environment.
In contrast, however, humanity has always constructed its own social environment.

--- pp.71~72

Chapter 2: Accidental Communities: Surviving Disaster
Over the past several hundred years, there have been many groups that have voluntarily isolated themselves, either out of a desire for utopian, philosophical, or religious vision or out of a desperate need for real-world solutions, in an attempt to create a different type of community.
Many of the familiar utopian attempts originated in the United States.
The United States is rich in examples of self-governing communal living groups, including the Puritan and Shaker communities, and more recently, the various communities that became popular in the 1960s.
Another way to study social development is to examine cases in which people, like shipwrecked sailors faced with the challenge of building a functioning community through cooperation to survive, have attempted to create social order from unintentionally assembled groups.

We will examine several cases from these various angles in the future.
Let us just note here that the most striking feature of these cases, whether intentional or accidental, is that they produce completely predictable results.
Most attempts to create a society with fundamentally different rules have either failed completely or, as in the case of Taransey, ended up reverting to a form resembling the existing society.
As the world's vastly diverse cultures and the endless social changes that occur everywhere demonstrate, humanity, despite its extraordinary and singular capacity for innovation, is driven by certain fundamental and universal principles.
That is the “social group.”
Attempts to abolish this principle usually end in failure.
--- pp.80~81

What can we glean from all these isolated cases we've examined so far? First, there are two general observations that stand out.
First, there are groups that perform much better than other groups.
In particular, groups that stand out as “social groups” are more likely to succeed.
Second, we can confirm that there are commonalities in social behavior.
These are the characteristics expressed as “social group.”


But there is one thing that is equally striking, but that we cannot see.
Even if given the chance, small, isolated communities do not create entirely new types of efficient social order.
It is clear that this has something to do with the fact that these isolated people are products of the existing culture to which they belong.
Living in their own culture, they develop their own expectations of what society should be like.
As we saw in Chapter 1, psychologists studying social perception often attempt to study babies as young as three months old.
This is to minimize the effects of cultural background.
It's the same reasoning that prompts the idea of ​​conducting a forbidden experiment to raise a child in the wild.
--- pp.124~125

Chapter 3: The Intended Community: Dreaming of Utopia
Kibbutzim not only abandoned communal childcare, but eventually abandoned some of its other features as well.
In the 1970s, chores were transferred to the private sphere, and communal dining rooms and laundries were eliminated.
And since the 1990s, most kibbutzim have also been saying goodbye to the equitable sharing economic model.
By 2004, only 15 percent of the world's population was shared fairly.
Like 19th-century American communities, these utopian attempts reverted to adopting the norms of the societies they belonged to.

The kibbutz failed to reconstruct society as a whole.
Even gender roles could not be changed.
In part, this was because gender roles were so deeply ingrained, more deeply rooted than any other characteristic the kibbutz attempted to overturn.
But what I found most unrealistic from the beginning was the attempt to sever the attachment between adult and child.
As we will see in more detail later, close family love is one of the most important characteristics of the “social group.”
And even the members of this idyllic, cooperative community did not treat everyone equally.
One experiment found that kibbutz residents showed cooperative behavior when paired with other kibbutz residents, but not when paired with city residents.
This shows that psychologically, in-group favoritism is very strong.
Although the pioneers of the kibbutz movement successfully rejected the European urban culture in which they had grown up, conformity to the “social group” seemed inevitable.
--- p.150

New communities that accepted the necessity of a “social group” lasted longer than those that did not.
Brook Farm and the Shakers, for example, took seriously the notion that people were not interchangeable, uniform masses that could be molded into any type of society.
They viewed people as individuals with individuality and personality that deserved respect.
Balancing group identity and individuality is key to the success of any social system.
If we allow for greater diversity among individuals (and, by extension, their property rights), the challenge becomes how to integrate these individuals into society.
Society had to be structured so that individuals could suppress their own competitive selfishness.
To achieve this, it was crucial to encourage and utilize the cooperative instinct and to foster friendship and a sense of group belonging.
Likewise, excellent leadership was important for intended communities as well as unintended communities.

Various utopian communities took contradictory approaches to sexual relations.
Some communities emphasized sexual interaction between group members.
On the other hand, communities like the Shakers required strict abstinence.
But both strategies shared a common goal: to overturn the existing institution of marriage and weaken the deep personal connection between couples.
The goal of these strategies was to foster a sense of connectedness within the group as a whole.
It is for the same reason that many communities, like the kibbutzim, have attempted to break up the nuclear family and separate the living spaces of parents and children through communal childcare.
But as we have seen, these attempts almost always fail.
Because they tried to overturn the love instinct that was innate in our species.

Such attempts to deviate from the evolutionary blueprint seem doomed to failure, but strictly following the blueprint does not necessarily guarantee success.
External forces are also important.
Threats such as natural disasters, fires, economic and environmental constraints (even the availability of alcohol) can very effectively destroy even well-established communities.
In short, although individual circumstances vary, two types of enormous forces play a role in either leading to or undermining the communitarian dream of creating a new society.
These are the pressures of our innate biological nature and the pressures of the external environment.
It is not easy to abandon the “social group” because our internal blueprint pushes us and the forces around us pull us towards it.
No, it is not feasible.
--- pp.176~177

Chapter 4 Artificial Communities: Every World Imaginable
When people first assigned social connections, they usually began to be tolerant and cooperative.
But there were times when the newly assigned “friends” were unwilling to contribute.
In academic terms, they are called “defectors.”
People did not want to be used by traitors.
Participants were not allowed to change the initially assigned connection.
So, if someone betrayed, the only way to avoid being exploited by their neighbors was to betray them themselves (give up the act of generosity).
In this experiment, we saw that betrayal took over the society we created.
In rigid (and leaderless) social worlds where participants had no power to choose who they interacted with (and were therefore confined to the friend groups we assigned them), people stopped cooperating.

However, other experiments with different groups of participants gave them some control over who they interacted with.
With each new game, participants could choose not only whether to cooperate or betray, but also with whom they would form or break ties.
Naturally, people chose to form bonds with good people who cooperate and break ties with mean people who betray.
All this difference was made possible by allowing a degree of fluidity in social bonds and providing a degree of choice in friendships.
In such a society, cooperation persisted and people were kind to one another.
We also found that people who cooperated formed cliques to avoid neighbors who were mean and exploitative.
In other words, even the possibility of changing social connections can make a community better.

People often think that personality traits like kindness are fixed.
However, our group-based research suggests otherwise.
Altruism or exploitation can vary greatly depending on how the social world is structured.
So you can assign the same people to one social world or another and make them really generous to each other, or you can make them really mean or cold.
The important point is that this suggests that the tendency to cooperate is a characteristic of groups as well as individuals.
Cooperation depends on the rules that govern the formation of friendship bonds.
Regardless of what beliefs an individual or group holds or supports, good people can do bad things, and bad people can do good things, simply depending on which network structure they fit into.
It's not just a question of being associated with "bad" people.
The number and pattern of social connections are also important.7 Several characteristics of the “social group,” such as cooperation and social networks, work together.

Let me give you an analogy that might help.
When carbon atoms are connected in a specific way, they become graphite.
A soft, black material suitable for making pencil leads.
But when you link those same carbon atoms together in a different way, you get diamonds, which are hard, transparent, and beautiful, making them suitable for making jewelry.
There are two key concepts here.
First, the properties of softness, blackness, hardness, and transparency are not properties of carbon atoms.
These are properties of groups of carbon atoms.
Second, these properties are determined by how the carbon atoms are connected.
It's the same with social groups.
This phenomenon in which the whole possesses properties that each part does not have is called “emergence,” and such properties are called “emergent properties.”
When you connect people like this, they do well for each other.
If you connect like that, you're doing each other wrong.

--- p.186~187

The first two explanations for why so much of the morphospace is empty neatly distinguish between two streams of biological thought: natural selection and mutation.
The first explanation is that certain types of shells do not exist because there has never been enough fundamental genetic variation to allow such forms to arise.
This is called the “genetic-availability argument.”
This corresponds to the opaque hourglass concept.
A second explanation is that the shell had no need to explore such regions of morphological space at all.
That is to say, there was no environmental pressure whatsoever to favor that type of shell.
This is called the “selection or adaptationist argument.”
Considering all possible circumstances, these types of shells are simply unsuccessful.
This corresponds to the concept of an hourglass with a tight neck.

Now let's apply this concept outside the shell to see how tiny a portion of morphospace animals actually occupy.
Consider that no animal has ever evolved a wheeled means of locomotion or a means of flight by heating the air in the form of a hot air balloon.
The very notion that there are creatures with such means seems absurd to us.
But are such alternatives—animals with wheels instead of feet, birds with air sacs instead of wings—really impossible? Given the sheer diversity of phenomena that natural selection can generate—biological batteries, acid sprayers, light-bending lenses, swim bladders that allow them to float, and animals the size of massive buildings—one cannot help but wonder whether the absence of wheels truly reflects a fundamental limitation.

Then, wouldn't the second explanation be more plausible? Could it be that there were no circumstances in which wheeled travel was more advantageous than other available alternatives? When navigating rough, uneven terrain without roads, feet are far more convenient than wheels.
This is because, in general, the feet slip less and it is easier to overcome obstacles.
This is the adaptation argument, the claim that there has never been an environment in which wheels were functionally useful.
In fact, there are still many villages in the highlands of Greece and Turkey where it is more convenient to get around by donkey than by wheeled cart, and it is for the same reason that the US military is developing walking robots.

The genetic availability argument is supported by a minority of biologists.
Richard Dawkins wrote:
“They feel that the vast space of the museum permanently blocks natural selection.
Natural selection may knock hard on the door of a particular passage, but it can never enter.
“It’s simply that the necessary mutations can’t occur.” On the other hand, the adaptationist view, the adaptation argument, holds that organisms take certain forms because only certain forms are physically possible and, more importantly, because they have adaptive value.
Most biologists follow this view.

In our case, only certain types of social organization (patterns of relationships between individuals and groups) may help mammals, including humans, cope with their physical, biological, and social environments.
Only certain types of social organization make sense.
That is the “social group.”

--- pp.201~203

Chapter 5 Love Wins: The One and Only, Multiple Pairs
In the evolution of many species, parents initially developed a special bond with their children.
Our species includes the emotion of love.
This feeling toward the child may later be repurposed into a bonding experience.
It refers to the special feelings that humans have for their partners that go beyond the level of lust.
This dedicated process is often described as “exaptation.”
Preadaptation is the evolution of a trait that initially evolved for a specific purpose and is later used for a different purpose.
Bird feathers are a classic example.
Feathers may have initially evolved as a type of insulation and later were used for flight.

Researchers are working hard to identify the ultimate and proximate causes of pair bonding behavior in species other than humans.
These issues are not yet well understood scientifically.
Mating behaviors, including kissing, may be driven by several direct biological forces, such as the desire to detect the scent of a prospective mate.
But the ultimate cause of these phenomena, evolutionarily speaking, has to do with why humans prefer certain partners over others as mates, and why we evolved the ability to identify people or feel particularly attached to some.


Looking at evolutionary history, it seems that humans evolved to love their children first, then their mates, then their biological relatives, then their spouses, and finally their friends and groups.
I often wonder if we are in the midst of a long-term transition where we are becoming a species that feels more and more attached to people.
To understand human relationships with people other than sexual partners, we must first begin with the connection between sexuality and romance.
Because this bond appeared before all other types of bonds in the evolutionary process.
Unrequited love is a key element of this blueprint.

To summarize the story so far, I'll give you a very general timeline:
Our ancestors were polygamous until about 300,000 years ago, and from then until about 10,000 years ago, they were primarily monogamous.
Then, until about 2000 years ago, polygamy was again practiced, and after that, monogamy was adopted again.
Of course, there are many exceptions and the timeline is also approximate, but the overall picture was like this.

--- pp.225~226

Yet, formal institutions cannot completely eliminate this human desire—the desire to love and possess one's mate.
Because this desire originates from the most fundamental aspect of human nature.
People violate all types of norms in all societies.
Thus, the Na society, which could function perfectly well with only “secret visits,” is able to satisfy its desire for possession to some extent by institutionally permitting “public visits.”
Moreover, even in the Na society, there are lovers who are “swept up in the flames of love” and are not satisfied with just visiting each other, and are not interested in sleeping with multiple partners, but rather, they run away to possess each other completely.
This corresponds to the fact that many societies have established marriage systems that allow for partner swapping, such as allowing divorce or allowing men to have concubines.

Many have argued that the Na people's very peculiar sexual customs are a case in point against the universality of marriage.
There is absolutely no biological basis for monogamy.
But the existence of mutations doesn't mean that our species is completely devoid of any core traits.
As scientists, we do as much as we divide.
In other words, it not only looks for variations, but also looks for commonalities.
Our human blueprint is not the finished product of our reality, but a draft.
The fundamental motivation for the Na people to have this type of relationship structure is the basic human desire to have multiple partners, and the fundamental motivation for the institution of marriage is also the basic desire to possess a partner.
The exceptional case of the Na tribe demonstrates that the need for pair bonding, another aspect of our humanity as deep and fundamental as the need for attachment, is a nature that can never be completely suppressed or replaced, no matter what means are employed.
Even with the highly sophisticated cultural rules designed to break this connection, that is impossible.
--- pp.262~263

Chapter 6: Why We Are Attracted: The Evolution of Love
But there is one thing to note here.
If females come to prefer gifts to combative males, then lower-status males will be able to outcompete higher-status males over evolutionary time.
To the extent that a male's success is based on non-physical traits, female preferences change what is considered "dominant" to the male.
In such a scenario, the male's food supply and the female's trust could coevolve in a self-reinforcing manner.
Ultimately, the Gavrilets model suggests that our human and male ancestors (except for a few high-status males) evolved to secure mates by providing food, and that women evolved to maintain very high levels of fidelity to their mates in order to induce food provision.
As human brains grow larger and pregnancy and lactation become more demanding, it stands to reason that feeding becomes an attractive strategy.
This shift from strength to care also fits with the evidence, mentioned above, that the heterogeneity in body size and strength between men and women has been decreasing.

This type of analysis does not predict that women will be completely chaste.
Rather, the strength of female-male pair bonds likely depends on a balance between good genes (probably provided by top-ranking males) and better food and care (provided primarily by lower-ranking males).
Once this evolutionary process began, it would have led to a kind of “self-domestication” as more and more females bred with less aggressive males (a topic we will return to in Chapter 10).
As a result, humans would have become a group-living species, with mostly virtuous women forming pair bonds with mostly food-providing males.
Finally, we have entered the path of evolution of attachment and love.

Just as human anatomy is fundamental, human social systems (monogamy, group living, etc.) are subject to natural selection acting on our genes.
Therefore, we can see that our genes influence not only our body but also society.
For our species, the shift to pair bonding was a breakthrough biological adaptation (a groundbreaking turning point for our species).
And it still exists everywhere today as the foundation of marriage, one of our most core social institutions.

--- pp.278~279

As mentioned earlier, oxytocin is involved not only in the physical parts of reproduction but also in parts of the brain (such as bonding with offspring).
For example, rats cannot distinguish their own offspring from those of other rats.
But there is no need to distinguish between them.
Since baby rats cannot move, the mother rat only needs to know where her babies are.
On the other hand, lambs can walk immediately after birth, so mother lambs need to be able to identify their lambs by scent in a large flock of lambs.
Oxytocin is involved in this process.
Administering oxytocin to sheep can induce bonding with lambs other than their own.

This neurological function of oxytocin appears to have been diverted over evolution for purposes beyond identifying and caring for offspring.
The mechanisms involved in the mother-infant bond, shared by all mammalian species, have been modified in certain mammalian species (including our own) to give females the same feelings for their offspring as they do for their mates.
Females use several aspects of sex to establish or maintain this bond.
The same neural circuits are activated in a female's brain whether she is looking at her offspring or her mate.
--- p.292

Chapter 7: What We Date For: The Evolution of Friendship
The affection and affection we show to animals tells us a lot about our human capacity for love, friendship, and altruism.
I would argue that this ability to connect with other creatures is a hallmark of our humanity.
Before I elaborate on this argument, I want to focus on animals first.
Our pets (birds, dogs, horses, etc.) are often highly social beings, and they seem particularly capable of responding to our attempts to connect with them.
Yet wild animals like chimpanzees, elephants, and whales offer deeper insights into our social selves than the species we cohabitate with.
By forming friendships, humans form social networks that extend far beyond the family tree.
At this point, we act in a way that takes lessons from the examples of these other species, and thus in a way that natural selection has shaped.

In fact, elephants and whales have independently developed the ability to form friendships similar to ours through convergent evolution.
As we have seen, convergent evolution refers to the phenomenon in which unrelated species acquire identical characteristics through completely separate evolutionary paths.
This is the case with both birds and bats evolving to have the ability to fly, and with octopuses and humans evolving to have eyes with similar structures.
What do these similarities tell us? They tell us that these traits—flight, vision, friendship, etc.—are incredibly useful abilities, so well-suited to the opportunities offered by the environment that they seem almost inevitable.
Moreover, the existence of animal societies further reinforces the fact that various aspects of our sociality are of paramount importance.
So, if we look at what animals and we have in common, we can better understand what we humans have in common.
--- pp.312~313

When ethologists try to evaluate animal behavior in relation to friendship, a key question is, “When can we consider something to be a friend?”
One simple approach is to calculate an association index between two animals based on the amount of time they spend together.
If, during a week of sporadic observations, a couple is together for 4 hours, the husband is alone for 6 hours, and the wife is alone for 10 hours, the correlation coefficient is 4/(4+6+10)=0.20.
In other words, husbands and wives spend about 20 percent of their time together.
We can also compare these values ​​with those obtained from pairs of animals.
This strategy is similar to one of the most widely used methods to establish friendship in human groups.
As we've seen in studies of Antarctic base crews, this is a way to find out who they spend their free time with.

Many of our animal friends are also relatives.
This often includes siblings, aunts, cousins, and even grandmothers.
In fact, in non-human animals, especially those with long lifespans, matrilineal kinship is a good predictor of whether a friendship bond will last.
This is also true in chimpanzees and dolphins, where females tend to leave their original groups, making it more difficult for them to maintain such bonds.
Nonetheless, field observational studies have shown that animals such as chimpanzees and baboons form enduring friendships with at least one non-relative, especially when matrilineal kin are not around.
And friendships between male dolphins who are not related can last for decades.
--- pp.317~318

As we explore traits like friendship, cooperation, and social learning in other animals to understand the human blueprint, the conversation tends to veer toward semantics.
Do these traits really exist, or are we merely anthropomorphizing them? Do chimpanzees truly comfort and welcome each other? Do elephants truly rejoice at seeing their friends? Do whales truly care for their young together? The same questions arise when discussing animal personality, society, and culture.
Are these phenomena objective, or are they merely projections of our own minds, like seeing a human face in a rock?

Some critics argue (in my view, mistakenly) that the very term "animal friendship" reflects the observer's experience rather than the animal's experience or the actual function of the connection between the animals.
Others argue that the very concept of animal friendship is flawed, since even primates do not have a concept of the future.
Animals do not anticipate future social interactions with a particular individual, nor do they understand the concept of a future time when they will need or want reciprocal behavior from their friend.
Moreover, I don't think they have the sophisticated ability to even understand the concept of friendship, let alone the ability to declare to each other (in any explicit way) that they are friends.

But does this ability really enable friendship? This is part of a long list of reservations and caveats we impose on our animal brothers and sisters.
It's a list you wouldn't even dream of attaching to people who can't answer questions verbally, such as newborns, people with developmental disabilities, or people with memory impairments.
We believe that almost everyone can form friendships.
I believe that friendship can be formed even among those who have only a vague understanding of the concept of the future or who do not have a clear understanding of friendship.
And yet, to me, the attitude of being picky about animal friendship strikes me as arrogant.
--- pp.342~343

Chapter 8: How We Connect: The Evolution of Relationships
People sacrifice their lives for their mates, children, and relatives.
From an evolutionary perspective, this is not surprising given the various processes we have seen so far, including kin selection.
Although cases like Aurora are very touching.
But also, a man lays down his life for his friend.
This is much more difficult to explain.
Of course, in a war zone, there are times when people sacrifice their lives for someone who is not their relative.
But soldiers are trained to sacrifice themselves for one another against a common enemy.
The amazing thing is that people sometimes make these heroic sacrifices for friends, friends who are not relatives and who are not trained to protect them.
--- p.359

Tit-for-tat accounting is difficult when two people have a wide range of goods and services to exchange, the timing of help is uncertain, and reciprocation takes a long time.
But evolutionarily speaking, friendship is meant for just such cases, and that's why it's valuable.
A common way to tell who's a true friend around the world is to see if two people give to each other without expecting anything in return.
If someone you consider a friend explicitly says they expect something in return, it is effectively taken as a sign of a lack of friendship.
Of course, the extent to which this actually applies may vary somewhat depending on the individual, culture, and environment.
But friendship always relaxes even the expectations of exchange.
(…)

Humans have an emotional apparatus for making friends and supporting these relationships (similar to the feelings of attachment and love that accompany sexual relationships in our species).
This fact is evidence that the simple model of tit-for-tat give-and-take does not and cannot fully explain altruism and friendship in social life.
But why did this emotional mechanism evolve in humans? Evolutionary psychologists John Tooby and Leda Cosmides believe this capacity for friendship is a response to a kind of "banker's paradox" our species evolved to address.
This concept refers to the paradox that those who need resources most are those to whom bankers are most reluctant to lend.
Likewise, our hunter-gatherer ancestors may have had the hardest time receiving help from others when they most needed it, as they were perceived as unable to repay it.
Friendship may have evolved to cope with situations like this.
--- pp.368~369

The extreme and lethal intergroup conflicts we witness among humans, namely outright warfare, are extremely rare among animals.
It is therefore puzzling how humans can be so affectionate and kind on the one hand and so hateful and violent on the other.
The only species that exhibits a tendency that comes close to this duality is the chimpanzee.
So perhaps affection and hate are actually related.
Mathematical analyses of models of human evolution reveal the following:
In the past, the conditions were ripe for both altruism and ethnocentrism to emerge (this next point is important), but they emerged only when both existed.
In other words, the two needed each other.


Altruism is helping members of one's own group at the expense of oneself, while ethnocentrism or parochialism is hostility toward members of an out-group.
Periodic resource shortages (e.g., due to drought or flood) are a major predictor of conflict in modern hunter-gatherer groups.
And the scarcity of resources like oil remains a good predictor of war.
We know that during the Pleistocene epoch (the period from about 2.5 million to 10,000 years ago), climate change was severe, and that our ancestors living in this environment sometimes had to compete for scarce resources, which favored groups with brave and self-sacrificing members.
In situations where conflict arises with outgroups, ingroup altruism is useful.
Samuel Bowles and Choi Jeong-gyu's model shows that while altruism and ethnocentrism were unlikely to evolve independently, they could emerge together.
It seems that in order to be kind to others, we must first distinguish between “us” and “them.”

Political scientists Ross Hammond and Robert Axelrod also showed (again, using simple mathematical models) that ethnocentrism promotes cooperation between individuals, regardless of reciprocity in the form of “scratch my back and I’ll scratch yours.”
They found that even when they could only identify group members without knowing whether they had a history of cooperation, the largest group size was that of those who selectively cooperated with their own group members and not with other groups.
This analysis shows that simply presenting people with visible signs of being part of the same group can lead to the emergence of in-group favoritism and selective cooperation.
Thus, there is ample evidence of a relationship between in-group favoritism, altruism, and competition.

But I, along with my collaborators, the mathematical biologists Feng Fu and Martin Nowak, wrote a mathematical model to investigate whether in-group favoritism and cooperation could emerge without intergroup competition.
The results revealed that these characteristics can emerge as long as an individual has the ability to change group affiliation.
Fluid social dynamics can turn yesterday's enemies into today's friends.
--- pp.397~399

Chapter 9: One Way to Become a Social Person
Compared to the rest of the human body, the face is particularly varied and unique.
We have a hard time recognizing our friends by looking at pictures of their hands or knees, but we can recognize them very easily by looking at pictures of their faces (Figure [9-2]).
The ability to reveal and recognize individuality evolves when this ability is beneficial.
The features that animals use to recognize each other can be broadly divided into two types.
“Cue” and “signal”.
Identity cues are phenotypic traits that allow individuals to be distinguished but do not in themselves provide a survival advantage.
Although each person's fingerprints are unique and can be used to identify individuals, they have not evolved to send signals, so people do not usually use fingerprints to identify each other.
So, like the unique pattern of tiny blood vessels in the eye, fingerprints are merely possible clues.

Identity signals, on the other hand, are phenotypic traits that help animals survive and recognize individuals.
If you don't want others to mistake you for someone else, attack you, fail to reciprocate your kindness, forget that you had sex with them, or fail to recognize that you are their child, you need some way to let them know that you are you and not someone else.
To do this, the traits used there must have many distinct and memorable variations.


So, as you might expect, facial features are more diverse than other parts of our bodies.
And since every detail of our face can be useful in informing our identity, the more combinable traits we have, the better.
Because it makes it easier to make every face unique.
To make individuals identifiable and unique, every aspect of the face—from the distance between the eyes and the shape of the ears to the height of the forehead and the angle of the cheekbones—must be combined in as many different ways as possible.
This means that these various facial features should not be related to each other in different people.

--- pp.421~422

Cooperation is formally defined as “contributing to an outcome that benefits all members of a group (even a two-person group), regardless of whether others contribute to it.”
Those who contribute (collaborators) pay a cost by doing so, and those who do not contribute (traitors or free riders) pay no cost (pay no price), like the mean elephant mentioned above.
Traitors gain more than cooperators, so this advantage is evolutionarily advantageous if it increases survival and reproduction.
So it's surprising that not only humans but also other species exhibit so much cooperative behavior.

The decision to cooperate must have had a profound effect on the survival and reproduction of our ancestors.
“Should I join a dangerous hunting party to find food?” “Should I share the food I’ve foraged so I have less?” “Should I risk my life to defend my home if it comes under attack?” The answers to these questions are rooted in hundreds of thousands of years of evolution.
One might wonder how this logic applies to modern society, a society where human reproductive capacity is no longer closely tied to material gain.
But when we look at our history as a species, it is important to remember that for almost all of human history, until just two centuries ago, all of humanity lived in constant danger of death.
Evolutionarily speaking, the traces of this history remain engraved on us to this day.
So we must ask:
"Why couldn't a selfish traitor take control of the group and drive out the collaborators?" "Why aren't we all selfish today?"
--- pp.442~443

Identity, friendship, and cooperative interactions all serve another purpose.
It supports social education and social learning skills, and thus cultural skills.
And this cultural capacity reached its peak in our species.
One prominent reason why animals form social groups is that reinforcement learning can occur.
Social learning may be more effective than solitary learning when the cost of information acquisition is high and when peers are a reliable source of information.
If you find it difficult to learn how to make stone tools on your own, it is much better to imitate someone else.
If you see me putting my hand in the fire and it hurts, you can learn not to do that.
You are learning the knowledge I worked so hard to learn at almost no cost.
In this way, social learning is very effective.

Things can get better.
Education is a unique activity that can make learning more efficient.
If someone has the confidence to teach someone else, they can learn more easily.
Education can be formally defined as follows:
(1) An activity that occurs primarily or exclusively in the presence of beginners, (2) without cost or direct benefit to the teacher, and (3) enhances the learner's ability to acquire information or skills more efficiently than if the instruction had not been received.
In hunter-gatherer societies, formal schools are rare (or nonexistent).
However, education, and so-called natural pedagogy, is widely practiced from a very young age.

Education is actually a form of cooperative behavior, and is rare in the animal kingdom.
Because it costs money.
Yet it has evolved independently in animals such as ants (where experienced ants teach young ants where to find food by tandem running), meerkats (where they teach other meerkats how to handle dangerous food), pied babblers (where they teach their young to associate certain sounds with food), primates, and elephants.
This behavior, like other altruistic behaviors, could evolve through kin selection.
And animals that have already evolved the ability to socially learn the unconscious behavior of others seem poised to evolve further toward explicit instruction.
--- pp.456~457

Chapter 10: Remote-Controlled Genes
The effects of genes can be understood at many levels, often depending on which level scientists are interested in.
Biochemists investigating how genes affect cells can conclude their work by observing the very first step in the phenotype's development: the translation of a gene into its corresponding protein.
But why stop there? Medical geneticists can study how genes influence muscle function, brain structure, or disease symptoms, ignoring their influence on proteins.
Zoologists interested in all animals can breed animals in desired ways to study phenotypes of interest, such as the fur color of foxes or the monogamous behavior of field mice.
Furthermore, behavioral geneticists can ignore these intermediate levels and look at complex traits such as risk aversion or novelty seeking.

But if genes are expressed at various levels, from proteins to anatomical structures, physiological functions, and even behavior, why not go one step further (though admittedly a long way) and examine their effects outside the organism's body? In 2005, I, along with political scientist James Fowler, coined the term "exophenotype" to describe the effects of genes outside the body, particularly on the structure and function of social groups.
--- pp.477~478

One of the most bizarre examples of parasites controlling behavior is the so-called “zombie” ant.
Some ant species are unfortunately susceptible to the zombie ant fungus Ophiocordyceps unilateralis.
Ants infected with this fungus climb plants to a certain height and then bite the veins on the underside of the leaves.
The fungus then kills the ant and extends a long, mushroom-shaped stalk from the ant's head.
Soon, spores that can infect other ants fall from the bag like rain (see color plate [0-8]).
This phenomenon was first observed by Alfred Russel Wallace.
He is a natural historian who proposed the theory of natural selection at the same time as Darwin, but received much less recognition than his efforts.
Here we see an example of a species without a nervous system (a fungus) evolving to control the behavior of a species with a nervous system (an ant) ​​and turn the ants into a spore transmission platform.
Leaf fossils with ant bite marks on the veins suggest that this fungal phenotype has been around for tens of millions of years.

So, isn't it possible that even human traits and behaviors are actually genetic byproducts of genes from other organisms? Could it be that sneezing isn't intended to expel pathogens from the upper respiratory tract for our own benefit, as I learned in medical school, but rather for the pathogen's own benefit, allowing it to spread through the air? Could it be that these very pathogens are manipulating our behavior?
People believe that sneezing is a healthy behavior that expels annoying invaders, such as pathogens, from the body.
However, sneezing may be a behavior that pathogens use to manipulate us to spread their infection and increase their fitness.
Some intestinal parasites can reduce human fertility for their own benefit.

People infected with certain pathogens may act in a way that encourages loved ones to come to them and care for them (by manipulating people's tendencies to love, care, and cooperate).
This allows pathogens that make people sick to spread more widely.
It may not be a coincidence that sick people act like babies to elicit help from their caregivers.
Some scientists have even proposed the highly speculative hypothesis that microbes might enhance their evolutionary fitness by encouraging people to engage in certain religious behaviors, such as cutting themselves, rolling on the ground, or, in large numbers, kissing icons or sacred relics.
Microbes may even further shape our social lives by fostering our collective desires, while also facilitating their own spread.

--- pp.488~489

This process may have occurred in our species as well.
Through the picky selection of females and the collective opposition (and, in prehistoric times, killing) of overly aggressive individuals.
As a species, we are hardwired to tolerate only moderate hierarchies.
Anthropologist Richard Wrangham argues that this process of self-domestication has changed the behavior and biology of our species.
In fact, the unique genetic changes that occurred during the domestication of many animal species are seen in humans.
The fact that many similar genes have changed further supports the hypothesis that humans have also domesticated themselves.

Domestication may promote neurological changes that not only make people more docile, but also more attentive to others, thus making our species more receptive to training and suited to social learning.
Also, humans are more juvenile than their ancestors who belonged to the human genus.
And over the past few thousand years, this trend toward less aggressive behavior has accelerated, with interpersonal conflict plummeting to the lowest levels in human history.
In the Paleolithic Age, the number of people killed by deliberate violence could reach up to one-third of the population.
But now, even in the most violent societies, it's only about 1 in 1,000.


Whether it's a spider building a house, a bowerbird constructing a bower, a fungus controlling ants, or a human weaving a social network, animals are genetically programmed to interact with the world and shape it in ways that better suit them and enhance their survival.
The social environment that humans create is to some extent under the control of our genes.
And this social environment in turn influences us in some ways, making some social beings more fit than others and selecting for genetic variants that support this.
As humans, we have been changing ourselves.
Throughout evolutionary history, our genes (and those of our friends) seem to have been working hard to create a safer and more peaceful world.

--- pp.506~507

Chapter 11: Genes and Culture Coevolve
As we saw in Chapter 9, some animal species possess culture, albeit in a limited form.
However, cumulative cultures like ours, sophisticated forms of culture passed down from generation to generation, are very rare (though not unique).
But what's even more surprising is that culture not only influences the course of our lives, but also alters the evolutionary path of our entire species.
The cultural environment that humanity has created for itself and built over thousands of years has been a force driving natural selection.
The power to change our genetic heritage.

The concept we will examine from now on is different from the concept of external phenotype.
In the external phenotype, genes influence the evolution of organisms by encoding something specific, such as artifacts or social behavior.
But when it comes to culture, what our genes offer our species is different.
Genes give us the ability to be flexible in what we do.
While beavers are genetically programmed to build dams, humans are not genetically programmed to domesticate cattle.
But if we domesticate cattle, the very existence of domesticated cattle influences our evolution.

Our species' ability to survive in habitats ranging from the Arctic tundra to the African desert (hunting seals in the tundra and digging wells in the desert) relies only slightly on physiological adaptations.
This is an example of how polar people have more fat tissue and are shorter to conserve body heat.
But our species' ability to survive globally depends more on our ability to construct culture.
It is an innate ability that has led to amazing inventions such as the kayak (boat) and the parka (clothing).
No species relies as much on our ability to create and preserve cultural traditions as we do.

Ecologist Peter Richerson and anthropologist Robert Boyd define culture as “information that can influence an individual’s behavior that is acquired from members of his own species through education, imitation, and other forms of social transmission.”
One key element of this definition is the characteristic of “interpersonal relationships.”
In other words, culture is a characteristic of groups, not individuals.
Other scientists place greater emphasis on material artifacts, such as tools or works of art.
But of course, cultural knowledge precedes the creation of artifacts.

In exploring the blueprint of human society, we have so far looked beneath the thin veneer of culture.
I began Chapter 1 with the analogy that a culture might explain why two hills are 100 and 300 meters high, but not why they both sit on a plateau 3,000 meters high.
From this perspective, culture is superimposed on a set of more fundamental processes.
But the ability to create diverse cultures (our innate tendency to form cultures) is itself a crucial attribute of our species.
It is not the products of culture, but this capacity to form culture itself, that shows that we have evolved to have a propensity for social interaction, cooperation, and learning.

But it turns out that this cultural veneer goes deeper than simply shaping our lifelong behavior.
Culture can actually influence the genes we carry as a species.
It's as if a small hill can move a huge mountain beneath it.
This interaction between genes and culture is called “gene-culture coevolution theory” or “dual-inheritance theory.”
This refers to our ability to inherit both genetic and cultural information from our ancestors.
--- pp.516~518

It has been argued that culture began to play a role in the evolution of our species with the beginning of the agricultural revolution.
But cultural influences probably appeared much earlier, perhaps even as early as a million years ago, in the ancestral lineage leading to modern humans.
Even in the absence of historical or archaeological evidence, there are ways to understand the impact of culture on the human body by meticulously examining fossilized human bones.
We know that the climate fluctuated considerably from about 900,000 years ago to about 500,000 years ago.
Over time, this fluctuation would have created selection pressures that favored animals that were versatile enough to cope with a variety of environments rather than just one.
Social learning is particularly adaptive in situations where species face environments that change so frequently that genetic evolution (individual mutations leading to small differences in form and function) cannot cope quickly enough.

Case studies that have studied in detail the influence of culture on evolution are fascinating.
About 1.8 million years ago (the date is disputed), humans learned to control and subsequently light fire.
Since then, humans have begun cooking and the caloric content of food has increased significantly.
This is because heating meat and plants increases the nutrients available for absorption.
Until then, human teeth, mouths, and stomachs were adapted to chewing raw meat attached to bones and digesting small branches by gnawing on them, but once cooking emerged, they were able to evolve in a new direction.
Teeth have become more delicate.
The masseter muscle (still the strongest muscle in the body) became weaker (and thus the shape of the jaw changed).
The stomach has become smaller (which has changed the placement of the ribs).
Cooking gave us enough energy to sustain our energy-hungry brains, which in turn grew larger.

In this way, researchers can estimate when cooking began by examining fossil teeth and bones.
Similarly, examining the anatomy of the foot can help us estimate when running behaviors, which are crucial for long-distance tracking of animals, began.
Humans are unique among mammals in our ability to be marathon runners.
Although we can't even beat our pets in short distances, we have all sorts of adaptations that help us run for longer periods of time (like slow-twitch muscle fibers, which are useful for endurance, and the ability to regulate body temperature increases during prolonged exertion).
(…)

Although endurance running is important in this hunting style, it is not just the evolution of anatomical structure that is required.
Humans must be able to identify and track a specific animal as prey even when it is out of sight.
Every time you see a new kudu, you will be the first to tire yourself out and fall if you blindly chase after it, this one, then that one, then another one.
This is where culture comes into play.
The ability to track animals (based on knowledge of footprints, droppings, broken branches, and behavior) is painstakingly acquired over generations, carefully taught and passed on (which is why it takes many years to become a skilled hunter).
As this cultural invention took place, the physical changes that occurred to suit long-distance running became adaptive and useful.
Based on changes in foot anatomy preserved in the fossil record, scientists today can infer when the otherwise useless ability to run slowly over long distances emerged, and even when the cultural practice of tracking emerged.

--- pp.531~533

From a scientific perspective, gene-culture coevolution research offers the exciting potential of providing a unifying framework that brings together sociological and biological analyses of human nature.
Cultural evolution and genetic evolution should never be treated separately.
Because our cultural abilities are actually something we acquired through evolution.
The answer to the question, “Nature or nurture?” is simple.
“Both.”
--- pp.543~544

Chapter 12: How Natural Is a Good Society?
There is a long and sordid history of genetics being exploited to divide and alienate people.
Some have responded by simply ignoring the empirical evidence about the evolutionary origins of human behavior and social organization, hoping that the evidence will simply disappear.
But just because truth can be dangerous (misunderstood, misused, or combined with faulty moral premises) doesn't mean it should be suppressed.

I believe that a better way to find the origins of human similarities is to examine our common evolutionary heritage.
Genes are clearly something we all have.
And everyone's DNA is at least 99 percent identical.
Scientific understanding of humanity taps into the deep wellsprings of our common humanity, fostering a sense of fairness.
The foundation of society as we understand it, our blueprint, the “social complex,” has to do with our similarities, not our genetic differences.
--- p.569

If someone understood what it meant to be a watch (to tell time accurately), they would be in a position to say whether a watch's function was good or bad.
Likewise, if we understand what it means to be human, we might be in a position to say whether the human experience is good or bad.
For example, we might say that a person who lacks the capacity to love is not fully human, and that such an experience is bad.
From this perspective, these natural constraints and definitions can prevent the endless relativistic regression of morality.
We may say that a society is good when it promotes the happiness or survival of its members.
These are the very constraints that surround evolution and morality.
In fact, this concept is also old.
It goes back at least to Plato and Aristotle.

The philosopher Philippa Foot famously said something provocative:
“I believe that in moral philosophy it is useful to think about plants,” she said, arguing that there is no fundamental difference in the concept of “good” whether it is a tree with “good roots” or a person in a “good” state.
A root has a purpose, a logical constraint that it must satisfy, and this purpose sets the criteria for determining whether the root is good or bad.
For example, humans, animals, and plants are all living things.
In all three cases, we can tell whether they are healthy or unhealthy, or whether they are outstanding or flawed in their own kind.
This means we can identify characteristics that contribute to their health, excellence, and so on.
We can say the same thing about human virtues such as kindness and courage.


These values ​​are “natural excellences” and their opposites are “natural defects.”
Foote explained that “moral behavior is rational behavior,” and that morality can be determined through constraints imposed by the nature of our species.
In our case, rationality means that it is good for humans to live socially.
Because we are naturally pressured to do so.
In the matter of creating a good society, which is what makes us fully human, morality is guided by our past evolution.
--- pp.572~574

The trajectory of our evolutionary history is long.
But this trajectory is curved towards “goodness.”

--- p.582