A child comes to the edge of deep water with a mind prepared for wonder.
—E. O. Wilson
She’s the czarina of cooperation. The sole sovereign of a society of millions. The most powerful potentate of pulling together. Measuring an inch or so long, the leaf-cutter queen lies at the heart of her sprawling subterranean empire. These dark red Atta ants abound in the New World tropics. Nests can be gigantic, stretching underground for up to eight meters in a network of tunnels, ducts, and chambers that extends over an area of some fifty square meters. All around lie tens of tons of soil that have been excavated by the queen’s much smaller, mostly female, subjects. During her reign of up to fifteen years, her 3 million short-lived inhabitants divide up dozens of jobs to work together as one. Hail Atta, queen of cooperation.
Each megalopolis of these social insects, coordinated by a complex chemical language, is greater than the sum of its parts, creating new levels of organization among its seven physical subcastes. The castes vary some 200-fold in terms of weight and eightfold in terms of head width. In all, the seven kinds of ant carry out a total of around thirty tasks.
One caste cuts foliage and leaves—their mandibular muscles make up one-quarter of their entire body mass—and some tropical ecologists estimate that the leaf-cutter colonies may harvest up to 17 percent of the total leaf production of a tropical rainforest where they thrive, in Mexico and Central and South America. In as little as a day, a nest’s denizens move back and forth along well-trodden foraging paths to defoliate a single tree. In a year, some species are able to harvest up to 470 kilograms of dry plant biomass.
Another caste hauls the leaf fragments back to the nest using horizontal foraging tunnels, superhighways that can extend up to six meters or more. A third dices the leaf up still further in this assembly line operation. However, the ants do not themselves eat the leaves they cut. By applying fecal droplets enriched with digestive enzymes, they turn the finely diced leaves into compost to grow fungus. Workers pluck nutrient-rich swellings known as gongylidia that form on the threadlike fungus and feed them to the colony’s larvae. They are peaceful mushroom farmers.
Remarkably, the colonies cannot survive without their Leucocoprinus fungus and the fungus is found nowhere but in these colonies. They do not do farm the precious fungus on the surface. Instead, they grow fungus in underground chambers that can reach the size of a football. In all, a single leaf-cutter nest may harbor a thousand such chambers. The smallest ants tend the fungus gardens and use antibiotic-producing bacteria to ensure their crop remains free from disease. They weed the gardens too, removing competing fungal strains, and keep it at an ideal, slightly acidic, pH. Thus the colony farms depend on three-way cooperation between ants, bacteria, and fungi.
Thanks to the digestive powers of the fungus, the ant larvae are able to consume the otherwise unpalatable harvest of tropical forests whose leaves are laden with chemicals designed to deter browsers, such as terpenoids and alkaloids. These ants are able to grow a monoculture year after year without disaster, and they use their antibiotic so prudently that there’s no sign of the antibiotic resistance that now plagues human medicine.
As a rule, riskier jobs are left to older workers who are destined soon to perish. Examples include waste disposal and defense. If the colony is disturbed, soldiers storm out of the nest and attempt to overpower the aggressor. While we send young men to war, ants send their old ladies. Soldiers are a caste of elderly females, each with a three millimeter wide head and well-developed mandibles. Their bite can penetrate human skin. So tenacious are their jaws that indigenous people in the Americas use them as sutures for holding the edges of wounds together.
Ant wonders do not end there. To establish new colonies, young males and females depart on a mating flight each year. A winged female ant mates with up to eight males, typically from other colonies, high in the air during a nuptial flight and stores all the accumulated sperm for the rest of her life. After the flight, all males die. The young queen digs a vertical shaft and creates a chamber at the bottom, which serves as the first of her own nests. There she deposits a fungus wad from her original colony (carried in a pouch on her body, called the infrabuccal pocket) to start a new fungus garden, the success of which is crucial for the future of the new colony. She removes her four wings, eats them, and lays her first handful of eggs. When the first workers emerge, they begin to eat, and to tend, the fungus culture. Groomed and fed by her workers, she can lay 20 eggs per minute, 28,800 per day, and 10.5 million each year. During her lifetime, a queen can produce more than 150 million daughters.
Ant colonies have much to teach us about the secrets of cooperation and advanced social behavior. They are one of the most successful forms of life, with at least 14,000 species. They have perfected ways to divide up labor to cooperate that appears more collegial than anything we do. They developed agriculture and architecture millions of years before our ancestors had even managed to walk upright. They are able to wage war. Unlike most ant species, army ants do not construct permanent nests but forage incessantly, pouring into an Atta nest and looting it if its citizens cannot mount an adequate defense against the marauders. Ants are also able to cooperate with other species, so that their lives are knotted together in a ruthless yet highly successful struggle for survival. For example, some species tranquilize aphids with drugs to keep them docile and “milk” them with their antennae for a treat of sugary honeydew.
In these eusocial societies, some group members surrender part or all of their personal genetic fitness to benefit fellow members other than their own direct descendants. It is the most advanced form of cooperation to be found in insects. And it works. Social insects are the most abundant of the land dwelling arthropods. Ants are perhaps the premier example, with the global mass of ants (up to 10 million billion of them, give or take) being roughly the same as the global mass of people. Even more impressive, these societies have thrived since the days of the dinosaurs. They offer an extraordinary glimpse of how cooperation can emerge from competition.
RISE OF THE SUPERORGANISM
A well-flavoured vegetable is cooked, and the individual is destroyed; but the horticulturist sows seeds of the same stock, and confidently expects to get nearly the same variety. . . . Thus I believe it has been with social insects: a slight modification of structure, or instinct, correlated with the sterile condition of certain members of the community, has been advantageous to the community: consequently the fertile males and females of the same community flourished, and transmitted to their fertile offspring a tendency to produce sterile members having the same modification.
—Charles Darwin, On the Origin of Species
Atta is not alone when it comes to being a master of cooperation. Think of a single worker bee. This solitary insect is as useless as a severed finger. But when attached to a colony, the bee becomes as useful as a digit on a hand. Now that bee can probe for the nectar of flowers and, once it has found a rich new source, point fellow hive members to these rich feeding grounds. Rather than gesture with wing or antenna, it uses a dance rich with symbolic information. In the same way that many factors and proteins coordinate the activity of cells in that organism known as the human body, so dozens of chemicals made by honeybee queens, workers, and brood play a role in social organization. Beehives are organized around an egg-laying queen tended by workers who, during their lifetime, make the transition from hivebound duties, such as nursing larvae, to more far-reaching jobs, such as foraging for food or defending the nest.
The different cell types in multicellular organisms are analogous to the different castes in a beehive, with workers constituting the soma— body tissue—and the queens representing the germ line, eggs and sperm. And, just as the body has mechanisms to weed out sickly cells, by apoptosis, a bee colony can regulate the lifespan of its members. The genome of our bodies is “optimized” by natural selection to build a good level of cooperation between germ line cells and soma cells with the help of apoptosis and various other processes. The same goes for breeding “good” workers and “good” queens. When I say good, I mean that they successfully reproduce and cooperate.
But, as we have seen in earlier chapters, there is a dark side to cooperation that comes in the form of parasites, cheats, defectors, and other lowlifes. In a healthy hive, workers identify and terminate cheats and abnormal colony members, ranging from embryos to adults. So long as this regulation continues, the colony thrives. However, if the types of worker that enforce order become too few, or if hive members change into malignant forms that can sidestep control mechanisms to replicate aberrantly, order is replaced by anarchy that ultimately leads to the decline and fall of the bee society.
There are some well-documented examples of the chaotic collapse of bee society. Take the relocation of the Cape honeybee by beekeepers from southern to northern South Africa in 1990. The result was the widespread death of managed African honeybees, Apis mellifera scutellata. The unhappy episode revealed the way in which insect society is susceptible to exploitation by rogue workers. In the African honeybees, these rogues began to harness the brood-rearing capacity of the colony to enhance their own personal reproductive success. It turns out that the billions of A. m. capensis workers now parasitizing South African honeybee colonies are all descendants of a single worker that was buzzing around during 1990. Their explosive growth has been likened to a social cancer.
Because of these many parallels between multicellular organisms and multicreature societies, from the division of labor to cancer, ant nests and beehives are known as “superorganisms.” The term (Latin super = above; Greek organon = tool) was coined in 1911 by the great American ant expert and biologist William Morton Wheeler (1865–1937) in an essay titled “The Ant-Colony as an Organism” and is defined as “a collection of single creatures that together possess the functional organization implicit in the formal definition of organism.” Wheeler was told, on receiving an honorary degree from Harvard, that his study of insects had shown how they, “like human beings, can create civilizations without the use of reason.”
But there is a puzzling feature of these societies that has been largely overlooked by investigators: the phylogenetic rarity of eusociality. By this I mean that of the 2,600 or so living taxonomic families of insects and other arthropods currently recognized, only fifteen are known to contain eusocial species. When it comes to vertebrate (backboned) creatures other than humans, only one—the naked mole rat—has achieved the same grade of social organization. Why is eusociality so rare? The mystery is deepened by the knowledge that once eusociality takes off, it is amazingly successful. The living mass of ants alone comprises more than half that of all insects and exceeds that of all terrestrial nonhuman vertebrates combined. The answer to this riddle lies in understanding how cooperation led to the emergence of superorganisms.
ANT WONDERS
The achievements of colonies of leaf-cutter ants have been hailed as “one of the major breakthroughs in animal evolution” by Edward Wilson at Harvard University. He should know. Wilson has studied ants for more than fifty years. His entire career seems to have been shaped by the sagacity of King Solomon, who remarked in Proverbs, “Go to the ant, thou sluggard; consider her ways, and be wise.” Wilson has invested much thought in the origins of eusocial species. Wilson, Corina Tarnita, and I have worked together on what he calls a “climactic project” that aims to explain the origin of eusociality using the mathematics of cooperation.
We considered two basic possibilities. Mutants that ensure that individuals “stay together’’ are a critical part of the evolution of eusociality (and, when it comes to the cells that make up creatures like you and me, of multicellularity too). If a gene makes offspring stay with their mother and help her—it could, for example, be a disruptive mutation in a gene that would normally make the offspring leave to set up their own nests—then we are dodging the Prisoner’s Dilemma. In this case, the workers are not independent agents. Their properties are determined by the genes that are present in the queen (both in her own genome and in that of the sperm she has stored). The workers can be viewed as “robots” built by the queen. They are part of the queen’s strategy for reproduction. This is not a cooperative dilemma, or even an evolutionary game.
There is a second possibility, wherein a gene makes individuals “come together.” In this encounter between players, we typically have a cooperative dilemma. For example, several fertilized queens cooperate to establish a new colony, which does indeed occur in the case of some ant species. These two mechanisms should be borne in mind when developing a theory for the evolution of eusociality.
SUPER NATURALIST
Wilson, the lord of the ants, grew up in rural Alabama, the only child of a government accountant. In 1936, when he was six years old, his parents divorced. He can vividly remember his encounters with wildlife at that early age. He studied a jellyfish one afternoon in the crystal waters of Perdido Bay as it floated absolutely still in the water. He had never dreamed of any such thing before, and so the suspended jellyfish, a sea nettle, came to symbolize “all the mystery and tensed malignity of the sea.” He was eager to know what else lurked in that deep, mysterious chamber of blue, speared with light.
When he was seven, he had an accident that, as he put it, “determined what kind of naturalist I would eventually become.” This mishap occurred at Paradise Beach, near Pensacola, Florida, fishing on a dock for pinfish. He jerked one out the water and one of the needlelike spines on its dorsal fin penetrated the pupil of his right eye. Eventually, the lens of the eye had to be removed in what was, at that time, a terrifying ordeal. Fortunately, he had close-up vision in his left eye, perfect for seeing the hairs on the body of an insect. He was now “committed to minute, crawling, and flying insects, not by touch of idiosyncratic genius, not by foresight, but by a fortuitous constriction of physiological ability.” The little pinfish had turned him into an entomologist.
Wilson also likes to joke that every kid has a bug period. “I just never grew out of mine.” As a ten-year-old, while exploring the National Zoo and nearby Rock Creek Park in Washington, he became enthralled by the “magic world” of insects. At the age of thirteen he made his first publishable discovery, a species of fire ant in Mobile, Alabama, that subsequently spread throughout the South. He would go on to study for his bachelor’s degree in biology at the University of Alabama and then move on to his master’s.
By then, his mastery of all things ant was apparent. His writings on the little-known dacetine ants prompted an entomologist to urge Wilson to transfer to Harvard, home to the world’s largest ant collection. There he studied the social behavior of ants, which he went on to show was influenced by chemical signals. Wilson can recall how, one day in 1959, he removed the Dufour’s gland from a fire ant, crushed it, and smeared it across a piece of glass. The fallen ant’s fellow workers rushed forth, following the smear to its end, where they milled around. The gland was clearly the source of a pheromone, one of a group of chemical substances secreted by ants to signal food, danger—even death.
In the next decade Wilson came across Bill Hamilton’s work on kin selection, which stirred his interest in what mathematics had to say about the world of the ant. Wilson was at that time highly receptive to the idea of a kind of Newtonian law of biology and, through his avid support, helped to establish kin selection as a dominant theory (which is why his current opposition to the idea is all the more striking). He had worked hard to put more mathematics into the discipline at the start of the 1960s with other thrusting young population biologists, such as Richard Lewontin, feeling that mainstream biology was lagging behind the amazing advances being made at that time in molecular biology.
Wilson recalls how, in the spring of 1965, he first read Hamilton’s paper on a train ride from Boston to Miami. As he set out from Boston, he was skeptical. But he gradually warmed to the idea during the following eighteen hours he spent in his roomette. By the time his train finally rolled into Miami, he was a convert to Hamilton’s dazzling “haplodiploid hypothesis” that initially gave kin selection its magnetic power. “It was brilliant,” says Wilson. “I still think it is.” His backing would catapult kin selection into mainstream discussions.
Here’s the essence of the hypothesis. Females develop from fertilized eggs, while males develop from unfertilized ones. As a result, females are diploid (they have two copies of the entire genetic code, or genome, just like people). Males are haploid; they have only one copy of the genome. This method of determining sex, called haplodiploidy, ensures that sisters are more closely related to each other than to their own offspring. And this means that the best chance they can give their own genes of surviving is to look after each other rather than lay eggs of their own.
The haplodiploid hypothesis works as follows. In a haplodiploid species sisters are 75 percent related, but mother and daughter are only 50 percent related. Hamilton’s rule predicted that hapolodiploid species would prefer to help raise their sisters rather than produce their own daughters. This is what provides the stability at the heart of the ant colony. Other insects also use haplodiploidy as the sex-determining mechanism, notably bees and wasps.
Wilson was “enchanted by [the idea’s] originality and seeming explanatory power.” From the occurrence of hapodiploidy alone he could draw a number of conclusions: societies of altruistic sisters might be expected to evolve more frequently among the ants, bees, and wasps than in others with conventional diploid sex determination, where both sexes have two sets of chromosomes. This seemed to be the case with Hymenoptera, an order of insects comprising the sawflies, wasps, bees, and ants, though it did not seem to fit the termites.
Unlike mathematicians, biologists do not dismiss an idea if there is a single exception. Indeed, they often resort to the baffling motto that declares that “one exception proves the rule.” Perhaps the insights that the rule can provide are so beautiful and intoxicating, it seems a shame to let an ugly and inconvenient fact get in the way. With kin selection one could, for example, predict that the worker castes of these species should be female and that males should be drones, whose sole function is to mate with the queen. The haplodiploid hypothesis felt like a magical key that could unlock new mysteries as effectively as the recently found structure of DNA.
In the fall of 1965, Wilson sailed on the Queen Mary to England to give an invited lecture on the social behavior of insects to the Royal Entomological Society in London. He remembers wandering around the great city with Hamilton, then a mere graduate student, telling him not to be discouraged by the initial indifference that had greeted his ideas about inclusive fitness. The two ended up promoting Hamilton’s work at the meeting of the society. Few of the panjandrums present were familiar with the Hamilton paper, and they were skeptical as a consequence. But Wilson was prepared. He had already thought about many of the objections. With Hamilton, “we carried the day.”
FROM ANTS TO SOCIOBIOLOGY
By the end of the 1960s, Wilson felt that the time had come to draw together the many threads of experimental and theoretical work on social insects. He wanted to create a synthesis of everything known about them that would spin “crystal-clear summaries of their classification, anatomy, life cycles, behavior and social organization.” Driven by what he called the “amphetamine of ambition,” he resolved to write a book on a discipline that he decided to dub sociobiology. Wilson’s research for his 1971 book The Insect Societies led him to the belief that behavior might result from genetic evolution, rather than from learning or cultural forces.
Wilson hoped that the fecund new gene-based ideas could be extended to provide the basis for understanding the evolution of social behavior, whether in social insects or social vertebrates, from murders of crows to flocks of sheep. At the back of his mind, he also thought that the concepts were powerful enough to apply to people (as he would put it, “Let us now consider man in the free spirit of natural history, as though we were zoologists from another planet”). By unpacking this explosive idea in his book Sociobiology: The New Synthesis in 1975, Wilson ensured that Hamilton’s kin selection theory penetrated deeply into the public consciousness.
In his book, Wilson pioneered and popularized attempts to build on this theory in order to explain the evolutionary mechanics behind behaviors such as aggression, altruism, promiscuity, even division of labor between the sexes. Although voted the most important book on animal behavior of all time by officers and fellows of the international Animal Behavior Society, Sociobiology became the object of bitter attacks by social scientists and other scholars, even from his former collaborator Richard Lewontin.
In a letter to The New York Review of Books, Lewontin and the Harvard evolutionary biologist Stephen Jay Gould were among a dozen professors, doctors, and students who condemned Sociobiology as providing “a genetic justification of the status quo and of existing privileges for certain groups according to class, race, or sex.” The left, sensitized by earlier false arguments about racial science, loathed the idea that human social behavior, indeed human nature itself, has a biological foundation. They feared that this kind of thinking was politically dangerous, the kind of idea that led to the establishment of gas chambers in Nazi Germany.
Wilson was denounced as a racist, a sexist, and a fascist (he is a Democrat, and a good-natured one at that). At one meeting of the American Association for the Advancement of Science, he was dowsed with a pitcher of ice water by demonstrators as they chanted, “You’re all wet!” At another meeting, this time of the American Anthropological Association, delegates considered a motion to censure sociobiology. Looking back on the furor, Wilson remarks that the book “was a hand grenade with the pin pulled out.”
Wilson resolved to answer his critics. In his next book, On Human Nature, he argued that most domains of human behavior—from child care to sexual bonding—are the result of deep biological predispositions consistent with genetic evolution. The danger of oppression lay not in sociobiological theory, but in uninformed views of man’s evolution— in particular, the kind of genetic pseudoscience that led to restrictive immigration laws in the United States and to the eugenic policies of Nazi Germany. The book won a Pulitzer Prize in 1979. But Wilson would return to his beloved insects, coauthoring a definitive 732-page book, The Ants. A second Pulitzer followed in 1991.
SPRING-LOADED SOCIETY
Hamilton’s dazzling haplodiploid hypothesis held true throughout the 1960s and 1970s, when it gave kin selection the same glossy sheen as a fundamental law of physics. But by the 1990s, the hypothesis began to dull and then to fail. At first, the termites were the lone, troublesome exceptions. Thanks to the efforts of one of Wilson’s students, Barbara Thorne at the University of Maryland, they were found to provide a good fit for group selection ideas instead. Then Wilson encountered the work of James Hunt at North Carolina State University and Raghavendra Gadagkar of the Indian Institute of Science, Bangalore. Both are wasp experts and both concluded that kin selection did not fit their observations.
At the same time, examples have emerged of eusocial creatures that are diploid rather than haplodiploid in sex determination. These include the platypoid ambrosia beetle, synalpheid sponge-dwelling shrimp, and bathyergid mole rats. Overall, half of all eusociality seems to have arisen in such diploid lineages. There are also societies that seem to have all the right ingredients to be eusocial but are not. For example, among the 70,000 or so known parasitoid and other apocritan Hymenoptera, one of the largest orders of insects, all of whom are haplodiploid, no eusocial species has been found. Nor has a single example come to light from among the 4,000 known hymenopteran sawflies and horntails, even though their larvae often form dense cooperative aggregations. Unsurprisingly, the haplodiploid hypothesis has now been abandoned by many who study social insects. Wilson often asks kin selectionists why they still cling to this idea: “They like to say, ‘Well, why bring that up?’”
Wilson’s current view about the origins of eusociality is profoundly different from the assessment in his seminal book Sociobiology: The New Synthesis. According to that widely accepted earlier account, selection acting on individuals that are related (kin selection), rather than on whole colonies, explains the origins of eusociality. Today, Wilson does not emphasize kin selection. Instead he focuses on ecological factors and on genes that predispose insects to colonial life.
By Wilson’s reckoning, the genesis of the ant colony begins with the nest. He points out that all of the branches or groups (technically called clades) known to have primitively eusocial species—aculeate wasps, halictine and xylocopine bees, sponge-nesting shrimp, termopsid termites, colonial aphids and thrips, ambrosia beetles, and naked mole rats—rely on colonies that build and occupy defensible nests. In a few cases, unrelated individuals join forces to create the little fortresses.
In most cases of animal eusociality in the Hymenoptera, the colony is begun by a single inseminated queen. Before the emergence of eusocial insects, a solitary insect species would reproduce by what Wilson calls progressive provisioning. Mated females build a nest, lay eggs, and feed the larvae. When the larvae hatch, the offspring leave the nest. Wilson and I believe that crossing the threshold to eusociality requires only that a female and her adult offspring do not depart to start new, individual nests but instead stay put. The high relatedness in the resulting colony is better explained as the consequence than the cause of eusociality. Once eusociality has evolved, colonies consist of related individuals, because daughters stay with their mother to raise further offspring.
Of course, for these extraordinary societies to evolve, there has to be an advantage to being social. The mathematical analysis shows that the fundamental question is how are the key demographic parameters of the eusocial queen (her fecundity and her risk of death) affected by the presence of workers? In the presence of workers, the eusocial queen has two fitness advantages over solitary mothers: she has increased fecundity—birth rate—and reduced mortality. While her workers forage and feed the larvae, she can stay at home, which reduces her risk of predation, increases her rate of egg laying, and enables her (together with some workers) to defend the nest.
We found that it is easier to maintain eusociality than to evolve it. For a wide range of circumstances it was unlikely that a eusocial mutation would take hold in a solitary society. Equally, however, once eusociality had evolved it could no longer be displaced by a solitary lifestyle. This explains in part why, even though eusociality is ecologically dominant, the condition has evolved rarely in the history of life.
What follows once a proto-eusocial nest has been established? The offspring of the queen would possess preexisting features—a “behavioral ground plan”—that could spring-load eusocial life. These traits include progressive provisioning of larvae, by which many can be reared at the same time, and flexibility in behavior that allows division of labor. For example, the females of solitary wasp species lose some of their young to predators when they forage for food, but if a second female were available to serve as a guard, these losses could be cut.
In the earliest stage of eusociality, the offspring that stayed with the nest would be expected to assume the worker role, following a preexisting genetically encoded behavioral rule. There is already good evidence of how such rules could spur cooperation—for instance to help care for offspring. Wilson likes to cite a beautiful Japanese experiment on solitary bees that were forced to build a nest on the same spot. Bees are programmed to make a nest in stages, so that when one had finished one construction job the second built on those efforts. At the end of their enforced partnership, one bee descends to the bottom of the nest—whether this is luck or dominance is not known—to become a queen and lay eggs. The second bee, seeing that the chore of producing offspring has been completed, takes on the role of foraging. Coerced partners have been forced to divide labor in guarding, tunneling, and foraging too.
As societies grow larger and more complex, however, competition between colonies grows fiercer, and as a consequence, group selection begins to act, spurring the emergence of a worker caste selected from genetic mutations within a group. This origin of an anatomically distinct worker caste appears to mark what Wilson calls the “point of no return” in evolution, at which eusocial life becomes irreversible. It is at this stage that insect societies made the transition to superorganisms. Eusociality, like multicellularity, is an important invention in evolution, one that shows the incredible power of cooperation.
GENESIS OF SUPERORGANISMS
Superorganisms arise from the initial formation of groups, along with a minimum and necessary combination of preadaptive traits, such as the creation and defense of a nest. Mutations guaranteed the persistence of the group, most likely by preventing ants from leaving the nest. As a result of what Wilson calls “spring-loaded preadaptations,” a primitive form of eusociality was established. This was honed and elaborated as emergent traits caused by the interaction of group members were shaped through natural selection by environmental forces. Finally group-level selection drives changes in the colony that result in sophisticated features such as fungi gardens and aphid herds.
Given the discussions about inclusive fitness in chapter 5, I should underline why this model is different. With our new mathematical model we see very clearly why it is not simply “relatedness” that drives the evolution of eusociality. In many solitary species there may have been mutations that make daughters stay with the nest. But whether or not one such species makes the step toward eusociality cannot be explained by relatedness alone because in all of those attempts the daughters are equally related to their mothers. Instead, it is down to parameters of life history, such as an increased rate of laying eggs and an increased longevity of the queen in the presence of workers, that really determine whether or not natural selection favors eusociality.
Finally, it is fascinating to contrast two-legged and six-legged society. Both owe their success to cooperation and division of labor. Both rely on multilevel selection, where there has been competition between groups. But, of course, ants are ruled by instinct alone while, thanks to language, we also have swiftly evolving cultures. Before we feel too smug, we should remember that after only 200,000 years, we humans are in danger of overwhelming our planet while the ants have lived in harmony with it for 100 million years.
Wilson likes to point out that both our civilization and that of the leaf-cutter owe their existence to agriculture. What is remarkable is that while our relationship with plants catapulted our species out of its hunter-gatherer lifestyle around 10,000 years ago, some social insects had achieved this transition 60 million years earlier. There may be parallels between the scenarios of animal and human eusocial evolution, and they are, we believe, well worth examining to shed light on how we made the step from wandering tribes of hunter-gatherers to hamlets, towns, and cities. Queen leaf-cutter, who rules over the greatest super-organism, still has much to teach us.