Humans: the Party Animal

Michael S. Gazzaniga

Humans: the party animal

By de½nition, the species Homo sapiens there are other animal and insect spe- is unique. Over a time course of approxi- cies that are social, our species takes so- mately seven million years, humans have ciability to a previously unknown level. evolved into quite a different animal from We are party animals, and on our way what was the last common ancestor we to becoming such we have evolved a share with our closest surviving relative, whole host of unique features–features the chimpanzee. Trying to ½gure out how so unique that we humans are playing we came to be what we are, and identify- in another ballpark. ing what aspects, both physical and behav- Although many may suggest that hu- ioral, we share with other animals, most mans act “like a bunch of animals,” and especially the chimpanzee, and those that the daily news intimates that we are end- are uniquely human has been of ongoing lessly ½ghting with one another, it is by interest.1 cooperating with and helping unrelated others that we are unparalleled among Take a minute the next time you go animal species. Something is markedly to a dinner party, barbecue, wedding different in our brains: we are “wired” reception, or baby shower–run-of-the- differently. The results of this altered mill events for us Homo sapiens–to pon- wiring allow humans to read books, or der the fact that such events are com- to go to the symphony, school, or jail. pletely unheard of in any other species. That is not to say humans are 100 per- What other animal would plan an event, cent different. In fact, most of our auto- provide food to unrelated others, and matic processing is much the same as sit together and share it without a food in other animals. ½ght, all while laughing about stories Although all species are unique unto of the past and hopes and dreams of the themselves, all have a common origin future? There is none. No matter how and are made up of the same materials. smart your family dog may be, he would It isn’t surprising that when Charles not divvy up a prime rib roast and pass Darwin ½rst proposed that humans it out to the other dogs of the neighbor- were descended from the great apes, he hood with a happy little bark; neither thought that the difference between us would our closest relatives, the chimps. and our closest relatives, the chimpan- Humans are social beings, and although zees, was a quantitative difference, not a qualitative one. We were just fancier © 2009 by Michael S. Gazzaniga apes with bigger brains, Darwin rea-

Dædalus Summer 2009 21

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 Michael S. soned. In the mid-1960s, however, parison to a chimp’s brain, which Gazzaniga Ralph Holloway added to Darwin’s the- weighs about 400 grams, an average on being human ory, concluding that brain reorganiza- human brain weighs about 1,300 grams. tion, rather than brain size alone, result- Homo neanderthalensis, however, had a ed in the evolutionary changes in cogni- bigger brain than modern-day humans, tive capacity. Evidence for Holloway’s and although it is clear through fossil insight is accumulating. evidence that their culture was more What exactly is brain reorganization, advanced than that of the chimp, they and how has it affected brain computa- were not in the same league as H. sapiens. tions and the human mind? Cognitive Thus brain size is not the only variable scientists Derek Penn, Keith Holyoak, in human uniqueness. and Dan Povinelli, “happy to be the hos- In non-primate mammals, the brain’s tage[s] of empirical fortune,”2 claim: prefrontal cortex has two major regions that work together to contribute to the The profound biological continuity be- “emotional” aspects of decision-making. tween human and nonhuman animals We do, of course, make many of our de- masks an equally profound functional cisions quickly and based on our emo- discontinuity between the human and tions, and so still utilize these two evolu- nonhuman mind. . . . [That discontinuity] tionarily older regions. However, some pervades nearly every domain of cogni- decisions are based on rational thinking. tion–from reasoning about spatial rela- Only primates possess a third, evolu- tions to deceiving conspeci½cs–and runs tionarily newer region, the lateral pre- much deeper than even the spectacular frontal cortex, where the intriguing scaffolding provided by language or cul- Brodmann Area 10 is located. One hun- ture alone can explain. dred years ago, German neurologist Kor- This “discontinuity of human cogni- binian Brodmann identi½ed ½fty-two tion,” they propose, was a watershed distinct regions of the human cerebral change that occurred after the hominid cortex, based on the underlying cytoar- line diverged from our last common an- chitectonics. Area 10 in humans is dis- cestor with the chimp, and it resulted proportionately larger compared to the in our exceptional relational ability. We rest of the great ape brains, and is dense- far exceed other species in our ability to ly interconnected with other still larg- grasp analogies and to combine relations er regions in human brains. Area 10 is into higher-order structures. Mindful concerned mainly with the “rational” that there are no “unbridgeable gaps” in aspects of decision-making and is in- evolution, ½guring out how this came to volved with all sorts of abilities in which be is the question. Regardless of whether humans excel: memory and planning, or not our ability to form higher-order cognitive flexibility, abstract thinking, relations is the basis for our cognitive initiating appropriate behavior and in- differences, something very different is hibiting inappropriate behavior, learning going on in the human brain. rules, and picking out relevant information that is perceived through the senses. There is no question that the human The posterior parietal cortex is anoth- brain is big. After the hominid line di- er disproportionately enlarged area. One verged from the last common ancestor of our hominid ancestors, Australopithe- we share with the chimps, the brain un- cus africanus, displayed the ½rst signs of derwent a huge growth spurt. In com- enlargement of this cortex, the part of

22 Dædalus Summer 2009

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 the brain active when objects are manip- Over the last several years, we have Humans: ulated and motor activities are planned. also learned of many specialized brain the party animal Also disproportionately large are the functions that are lateralized in the hu- connective pathways of the temporal man brain. This means that one hemi- lobe, indicating increased local con- sphere of the brain may perform a spe- nectivity between neighboring cortical ci½c function that the other hemisphere ½elds, which support the formation and cannot perform. The ½rst hint of human processing of declarative memory; self- lateralization of function came in 1836, recognition; visual, auditory, and lan- when a French neurologist observed that guage processing; and the detection of three of his patients that had lesions biological motion.3 of their left hemispheres had speech dis- Neurobiologist Georg Striedter sug- turbances. Twenty-½ve years later, Paul gests that the human brain has not en- Broca, after studying the postmortem larged randomly, but that an entire cir- brains of aphasic patients, concluded cuit that has made humans more flexi- that the speech center was located in ble and capable of ½nding novel solu- the left hemisphere. There are some tions to problems has enlarged. Perhaps anatomical asymmetries to be found in one of the most important abilities in- the non-human primates and, more no- cluded in this circuit is that of inhibit- tably, in the great apes; however, there ing automatic response, which is noto- is scant evidence for lateralization of riously dif½cult or impossible for other function in other mammalian species. animals, including our chimp relatives. The corpus callosum, the great track Only with this ability can one respond of neurons that transmits information in novel ways, and utilize the cognitive from one hemisphere to the other, may flexibility that we uniquely possess. have provided the evolutionary innova- (These systems are not fully developed tion that allowed cortical capacity to in adolescent humans, offering a possi- expand. Without increasing brain size, ble explanation for their impulsive it allowed mutations leading to innova- ways.) tions in one half of the brain, while pre- When brain size increases, what actu- serving cortical function in the other ally increases is the number of neurons, half. There are also microscopic asym- their width, and their connections. In metries in the cellular organization of general, the larger the area, the better the neocortex that have not been found connected it is. Each neuron, however, in other species and are thought to be can connect to only a limited number uniquely human.4 Clearly, our brains of other neurons, a number that does are physically different. It should come not change as the overall number of as no surprise, therefore, that they also neurons increases. As absolute brain function differently. size increases, proportional connectivity tends to decrease, and the internal Our brains and abilities evolved be- structure changes as the connectivity cause our bodies evolved along with pattern changes. In turn, less dense them: changes in one happened in con- connections force the brain to special- cert with changes in the other. Between ize, create local circuits, and automate. ½ve and seven million years ago (some The human brain has billions of neu- recent studies suggest that it may have rons that are organized into local, spe- been more than ten million)5 we shared cialized circuits, known as modules. our last common ancestor with the

Dædalus Summer 2009 23

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 Michael S. chimpanzee. For some unknown reason, possible.) Birth became more dif½cult Gazzaniga most likely climactic changes resulting as brains and heads enlarged. In com- on being human in a change in the food supply, the hom- parison to other apes, human babies are inid line split. The branch of the family born one year prematurely, and, unlike that produced the chimps stayed in the chimps, their heads and brains continue tropical forest and remained much the to grow for several years. same. The other branch stepped out into From our current viewpoint, bipedal- the woodlands, where life was quite dif- ism seems only advantageous; yet the ferent, evolving to become bipedal and, late psychologist Leon Festinger saw the over time, undergoing a host of other proverbial fly in the ointment. He point- changes as well that have led to our cur- ed out that “bipedalism, in and of itself, rent abilities. must have been a nearly disastrous dis- Particularly important among the advantage,”6 making us slower and less anatomical results of bipedalism are able to climb, and its evolution needed a our elongated necks and the fact that special explanation. our tongue and pharynx dropped lower Before we leave Festinger’s bipedal down into the throat. Unlike the chimps quandary, we should pause to consider that have two separate passageways for a study done by the evolutionary biolo- food and air, we developed a unique sys- gists Willem de Winter and Charles tem, in which air and food share a com- Oxnard. Rather than looking at overall mon pathway in the back of the throat. brain size, de Winter and Oxnard sug- We have a structure, the epiglottis, that gested that a brain part’s size is, to a cer- closes the pathway to the lungs when tain extent, related to its functional rela- we swallow and opens when we breathe. tionships with other brain parts. Using The unique anatomy of the pharynx, brain-part ratios from 363 species, they speci½cally the larynx, enables us to ut- ran multivariate analyses, with fascinat- ter the wide variations in sound that ing results. Groups emerged based on we can and makes speech possible. No similar lifestyles (locomotion, foraging, doubt, the survival advantage we gained and diets), rather than on phylogenetic was an increased ability to communi- relationships. For instance, New World cate, even though we face an increased insectivorous bats had brain-part ratios risk of death by choking. Bipedalism more closely linked with Old World car- also set our hands free, and our thumbs nivorous bats, rather than with their became unique. Both humans and phylogenetically closer relatives, the chimps have opposable thumbs; chimps, New World fruit eating bats. though, don’t have ulnar opposition: The primates fell into three groups, they can’t arc their thumbs across to also based on lifestyles that cut across their baby ½ngers. Hence, we can pick phylogenetic lines: those with hind- up objects with the tips of our ½ngers, limb-dominant locomotion; the four- not with just the sides, as chimps do. We limb dominant; and the upper-limb also have more sensitive ½ngertips, with dominant–those that hang from thousands of nerves per square inch that branches while eating, reach above send information to the brain. their heads for fruit, and escape by One major physical problem presented upper-limb acrobatic activities. (This by bipedalism was a smaller pelvis, and group included chimps and gorillas, thus birth canal. (A wider pelvis would along with the phylogenetically dis- have made bipedalism mechanically im- tant spider and wooly monkeys.)

24 Dædalus Summer 2009

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 Oxnard’s analysis revealed that the spe- and gathering more ef½cient, thus pro- Humans: cies within a lifestyle group had similar viding more food for the growing brain. the party animal brain organizations: that the conver- Over the years there have been many gence and parallels in brain relation- suggestions as to what forces were driv- ships are most likely associated with ing the relentlessly enlarging brain. It convergences and parallels in lifestyles is coming to be accepted that, through that cut across phylogenetic groups. the process of natural and sexual selec- Humans, however, the only species tion, two factors were pushing the in- of the 363 studied that have a bipedal crease in overall brain size: a diet that lifestyle, fell into a group unto them- provided the added calories needed to selves, with a highly signi½cant 22 stan- feed the metabolically expensive big- dard deviation difference between them ger brain; and the challenges originat- and chimpanzees. Oxnard concluded, ing from living in those large groups– “The nature of human brain organiza- “the social world”–necessary to guard tion is very different from that of chim- against predators. panzees, which are themselves scarcely Many proposals have been tendered different from the other great apes and as to what diet provided the necessary not too different even from Old World amount of calories to feed that metabol- monkeys.”7 Something about the hu- ic furnace of a growing brain. Richard man bipedal lifestyle is related to our Wrangham, a primatologist who has very different brain organization. studied chimpanzees in Uganda for over thirty years, suggests that Homo sapiens Festinger suggested that the primary are uniquely biologically adapted to factor allowing this “seriously handi- eat cooked food, and that cooked food, capped species” to survive was an in- which has more calories than raw food ventive brain and neural system that and is faster to eat, drove the expansion could ½gure out just what to do with of the brain by increasing calories and those appendages that would be fruit- decreasing the amount of time and ener- ful and adaptive. But perhaps it was gy it takes to ingest and digest them–in the disadvantage of being slower that turn freeing up more time for hunting later resulted in so many cognitive and socializing. And, as the saying goes, changes. The open woodlands were a free time is the devil’s playground. radically different environment. Food Banding together in social groups sources were highly scattered, and al- for protection against predators pre- though there were more animals for sents its own set of problems, such as our ancestors to hunt, there were also competition with conspeci½cs for re- more animals hunting them. And just sources (both food and prospective when our ancestors were being exposed mates). Thus the cognitive challenge to bigger and more dangerous preda- of surviving in increasingly larger so- tors, they could no longer run as fast cial groups was likely the other driver or climb as well. There are two ways to of increasing brain size, as psycholo- discourage predators: either be bigger gists Richard Byrne and Andrew Whiten and faster than they, or live in a larger suggest. Their proposal, now dubbed group. Unable to choose the former, the Social Brain Hypothesis, states: the early hominids banded together in Most monkeys and apes live in long-last- large groups not only to provide better ing groups, so that familiar conspeci½cs protection, but to make both hunting

Dædalus Summer 2009 25

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 Michael S. are major competitors for access to re- ships. The other social skills that he has Gazzaniga on being sources. This situation favours individ- correlated with brain size are the num- human uals that can offset the costs of competi- ber of individuals with whom an animal tion by using manipulative tactics, and can simultaneously maintain a cohesive skillful manipulation depends on exten- intimate relationship; how much social sive social knowledge. Because compe- skill is required in male mating strategy; titive advantage operates relative to the the ability to manipulate others in the ability of others in the population, an social group without the use of force; “arms race” of increasing social skill re- and the frequency of social play. sults, which is eventually brought into While a chimp maxs out juggling a so- equilibrium by the high metabolic cost cial group size of about 55 individuals, of brain tissue. Dunbar has calculated from the brain size of humans that we have a social Successfully living in social groups group size of about 150. Initially this involves more than competition; it seems rather surprising when you think also involves cooperation. Very recent- of the huge cities that many humans in- ly, developmental and comparative psy- habit; but when you look more closely, chologists Henrike Moll and Michael it begins to make more sense. One hun- Tomasello proposed the Vygotskian In- dred ½fty individuals is the typical size telligence Hypothesis. They assert that of hunter-gatherer clans, also the typical although cognition in general was driv- number of individuals on a modern-day en mainly by social competition, the Christmas card list or in military units unique aspects of human cognition– and businesses that can be run informal- the cognitive skills of shared goals, joint ly. It appears to be the maximum num- attention, joint intentions, and cooper- ber of people an individual can keep ative communication needed to create track of and for whom he would be such things as complex technologies, willing to do a favor. And the extent cultural institutions, and systems of to which humans do favors is unique. symbols–were driven, even constitut- ed, not by social competition, but so- Doing favors is altruistic, and Dar- cial cooperation.8 win himself could never quite ½gure Anthropologist and evolutionary bi- out how it occurred through natural ologist Robin Dunbar has been search- selection. Why would an individual do ing for social and ecological indices that anything that would increase the sur- correlate with primate brain size. He vival of another to his disadvantage? has found ½ve aspects of social behavior The late William Hamilton, an evolu- that correlate with brain size, the ½rst of tionary biologist, realized that altruis- which is social group size: the bigger the tic behavior could evolve if the bene½t- neocortex, the larger is the social group. ing individuals were genetically related The great apes require a bigger neocor- to the provider, because helping one’s tex per given group size than other pri- close relatives survive and reproduce mates, indicating that the social milieu also passes your genes on to the next of the great apes is more cognitively generation. taxing. Dunbar’s research has shown Humans, however, help unrelated that the factor limiting social group size others all the time. We are the superla- is the ability to manipulate and coordi- tive Helpful Henrys of the animal world, nate information and social relation- and this behavior has its foundations in

26 Dædalus Summer 2009

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 reciprocal altruism, ½rst described by less but receive an equal bene½t, will Humans: sociobiologist Robert Trivers. If an indi- out-compete the non-cheaters and take the party animal vidual does a favor for an unrelated indi- over. If cheaters take over, no one foots vidual and is sure it will be returned at a the bill and reciprocity crumbles. Hu- later date, then that could provide a sur- mans have evolved two abilities that are vival advantage. Reciprocal altruism is necessary components for prolonged re- very rare in the animal world, and sev- ciprocal social exchange and are on the eral problems have to be overcome in short list of uniquely human capacities: order for it to work. First, the commit- the ability to inhibit actions over time ment problem: how could any individ- (a.k.a. delayed grati½cation) and punish- ual be sure (trust) that the favor would ment of cheaters in reciprocal exchange. be returned? Second, an individual has The importance of reciprocal ex- to be able to recognize another individ- change should not be understated. Cos- ual speci½cally; third, he has to be able mides observes, “As humans, we take to remember a favor was done and what for granted the fact that we can help it was; fourth, he has to live in close each other by trading goods and ser- enough contact that predictable occa- vices. But most animals cannot engage sions will arise for reciprocation; and in this kind of behavior–they lack the ½fth, he has to be able to evaluate the programs that make it possible. It seems cost of his favor and make sure he gets to me that this human cognitive ability one back of equal value. Marc Hauser, is one of the greatest engines of coop- a professor of psychology and evolu- eration in the animal kingdom.”9 Moll tionary biology at Harvard University, and Tomasello think that the unique as- thinks that our impressive mathemat- pects of human cognition were driven ical abilities evolved with the emergence by social cooperation. of social exchange systems. However, Indeed, cooperation is rare in our because there is a time lag between the chimp relatives, and recent studies completion of a favor and its reciproca- show that it only happens in competition, cheating can occur. Not surprising- tive situations, and only under certain ly then, a species that practices recipro- circumstances with certain individu- cal altruism has mechanisms to identi- als.10 Brian Hare and Michael Toma- fy cheaters. Evolutionary psychologist sello suggest that the temperament of Leda Cosmides has developed a test the chimp constrains his behavior, and that indicates that the human mind has that the human temperament might a speci½c module that detects individu- be necessary for the evolution of more als who cheat in social exchange situa- complex forms of social cognition. Be- tions. She has found that cheater detec- fore hominids were able to work coop- tion develops at an early age, operates eratively, they had to become less ag- regardless of experience and familiar- gressive and competitive and more tol- ity, and detects cheating, but not unin- erant and friendly with one another. tentional violations. Hare and Tomasello suggest that this Identifying cheaters is only half may have been achieved by a kind of the job. Game theory researchers have self-domestication process that select- shown that for prolonged social reci- ed for systems that controlled emotion- procity to exist, not only must cheaters al reactivity, such as aggression. Perhaps be detected, but they also must be pun- individuals in a group would either os- ished; otherwise, cheaters, who invest tracize or kill overly aggressive or des-

Dædalus Summer 2009 27

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 Michael S. potic others. Dubbed the “emotional that may motivate an individual to ac- Gazzaniga reactivity” hypothesis, it is based on tion. We share many of our so-called on being human continuing studies done by geneticist moral intuitions with other animals. Dmitry Balyaev, who began domesti- Humans differ by way of the reasoning cating foxes in Siberia in 1959 by select- about the judgment or action that ing only for a single criterion: whether comes afterward, as the brain seeks a they exhibited fearless and non-aggres- rational explanation for an automatic sive behavior toward humans. In other unconscious reaction. This is when words, he selected for fear and aggres- the uniquely human left brain’s inter- sion inhibition. The experimentally do- preter device (see below) provides an mesticated foxes are as skilled at using explanation for the moral emotion, in- human communicative gestures–point- tuition, and the action. Also unique to ing and gazing, for example–as domes- humans, some moral emotions can evi- tic dogs.11 These results suggest that so- dence themselves in blushing or tears, ciocognitive evolution has occurred in and are dif½cult to counterfeit. As a re- the experimental foxes as a correlated sult, these behaviors are a good adver- by-product of selection on systems me- tisement that an individual has a con- diating fear and aggression. Dog domes- science or is compassionate. Such vi- tication is thought to have occurred by sual proof of a moral emotion can indi- a similar process. Wild dogs that were cate that the individual would be trust- less fearful of humans were the ones worthy and a good partner for recipro- that approached them, stuck around, cal exchange. and reproduced. It appears that the moral emotions of shame, embarrassment, guilt, dis- Reciprocal exchange is not only an gust, contempt, sympathy, and compas- engine of cooperation, it is one of the sion are also uniquely human. Moral driving forces behind our innate moral- emotions solve the commitment prob- ity. Many researchers studying morals lem presented by social exchange and and ethics propose that we have “ethi- allow the ½rst move. Jonathan Haidt, cal” modules. These modules have been a social psychologist, points out that derived from the common emotions moral emotions aren’t just for nice and the behaviors they engender, which guys, and this is a very important point. we share with other social species and Some moral emotions can also lead to which include being territorial; having ostracism, shaming, and murderous dominance strategies to protect territo- vengeance. Oddly enough, it may be ry; forming coalitions to garner food, that those moral emotions are what ac- space, sex; and reciprocity. These mod- tually made us nicer: moral emotions ules have evolved to deal with speci½c motivate the punishment of cheaters, circumstances, common to our hunter- which is necessary to sustain reciproc- gatherer ancestors, that involved what ity and cooperation. Perhaps they were we now consider moral or ethical issues. part of the self-domestication process An environmental trigger activates proposed by Hare and Tomasello. We these modules, which induce an auto- know that our relatives the chimps un- matic judgment of approval or disap- derstand intention and are vengeful. In proval. If the trigger is strong enough, experiments with humans, chimpanzees a moral emotion is elicited. The emo- will become more upset when a human tional state produces a moral intuition intentionally interrupts a feeding ses-

28 Dædalus Summer 2009

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 sion, than when he accidentally does12 Theory of Mind, and language, and Humans: the party and will retaliate against personally there are those who think the neurons animal harmful actions.13 were fundamental for human conscious- But all this does not explain why peo- ness. Scientists studying macaque mon- ple will leave a tip in a restaurant that keys discovered these premotor neu- they will never return to, or why people rons, which ½red both when a monkey will cooperate with unknown others or observed or heard another manipulat- sacri½ce for non-kin. Haidt suggests that ing an object with his hand or mouth sociologist Emile Durkheim’s insight, and when he himself manipulated an that morality binds and builds groups, object. Mirror neurons are the ½rst con- can answer this question: “Morality crete evidence of a neural link between constrains individuals and ties them observation and imitation of an action. to each other to create groups that are Subsequently, more extensive mirror emergent entities with new properties.” neuron systems have been described in He suspects that reciprocal altruism is the human, where they are not restrict- supplemented with a type of “indirect ed to just hand and mouth movements, reciprocity.” Here, it pays to be virtuous as in the monkey. They correspond to by following the morals of the commu- movements all over the body; in fact, nity, because such behavior enhances the same neurons are active even when your reputation or status, and rewards we only imagine an action. you with increased future cooperation.14 Mirror neurons are implicated not Certain abilities are necessary to feel only in the imitating of actions, but also the uniquely human moral emotions: in understanding the intention of actions. Humans also appear to have mir- • In order to feel sympathy and compas- ror systems in the insula involved with sion, one must understand that anoth- understanding and experiencing the er has feelings, and be able to identify emotions of others, mediated through them and take another’s perspective, the viceromotor response. Such systems, which requires inhibiting the default by unconsciously internally replicating mode of self-perspective. actions and emotions, may be the mech- • In order to feel the “self-conscious” anism behind what gives us an implicit emotions of guilt, shame, and embar- grasp of how and what other people feel rassment, one also has to be self-aware or do, and contribute input used in our and conscious of these emotions. theorizing about the reason (the why) for the actions and emotions of others. • In order to cooperate with another, one Giacomo Rizzoletti, who ½rst discov- must share intentions, attention, goals, ered mirror neurons, and Michael Arbib, and possess Theory of Mind, the un- director of the University of Southern derstanding that the other has beliefs, California Brain Project, suggest that goals, and intentions. Once again, in the mirror system was fundamental for order to enact this suite of behaviors, the development of speech and, before one has to inhibit self-perspective. speech, for other forms of intentional communication, such as facial expres- How might this all work in the brain? sion and hand gestures. Mirror neurons Many neuroscientists think that the so- also serve to synchronize our feelings called mirror neurons are fundamental and movements with those of others for the development of self-awareness, around us: for example, everyone in the

Dædalus Summer 2009 29

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 Michael S. audience claps in unison to bring the states of other people. Having a Theory Gazzaniga performer back out for one more rendi- of Mind (ToM), also known as “intui- on being human tion of their favorite song. Mirror neu- tive psychology,” is our intuitive un- rons are one of the psychological mech- derstanding that others have invisible anisms that create group cohesion. states–beliefs, desires, intentions, and But if the same neurons are active goals–that can cause behaviors and when I observe an action as when I per- events. Some cognitive psychologists form the action, how can I tell who has think ToM is the foundation of what done it? Beyond the shared neural net- is unique about the human mind. Chil- works that are active in both cases, when dren slowly develop the full suite of we take a personal perspective, parts of ToM abilities over the ½rst ½ve years the somatosensory cortex, the part of of age, but some of the abilities are up the brain with speci½c areas that map and running as early as nine months. to speci½c parts of the body, are also ac- While we share some aspects of ToM tive. When we take another’s perspec- with our chimp relatives, other aspects tive, however, we activate one area of are uniquely human. Chimps and chil- the temporal parietal junction that has dren less than four can understand input and output connections with many what others perceive, and the perceiv- parts of the brain and plays a part in dif- able goals of their actions, but they can’t ferentiating self from other, as well as a understand that another may have a false part of Area 10 in the prefrontal cortex. belief. This ability in children is evident The activation of Area 10 is what inhib- between four and ½ve years old, when its self-perspective; damage to the area they begin to understand that what oth- can lead to excessively egocentric behav- ers believe may not actually be true. A ior. It has been suggested that errors in full-blown ToM is needed for manipu- assessing another’s perspective are a fail- lating others’ thinking, which is the ure of suppressing one’s own. Inhibiting basis of classroom learning. Actively our own perspective gives us flexibility teaching is a uniquely human ability. to take another’s, and although chimps appear to be able to do it to a limited ex- Humans possess not only an intuitive tent (even then only while in competi- psychology, but also an intuitive biolo- tion), humans can do it voluntarily with- gy and physics, some aspects of which out constraints. are shared by other animals, and some Neither reality nor visibility con- which are uniquely human. We humans strains humans. We can feel an emotion automatically categorize whatever we through abstract input, such as reading, run across as either an animate or inani- or by merely imagining it. The emotion mate object. In every society, people in- of disgust activates the same brain cir- tuitively think about animate objects– cuitry (the operculum) whether one is plants and animals–in the same special experiencing it oneself or observing or hierarchical way. This intuition is the imagining the disgust of others. The out- hardwired knowledge that animate ob- put of this region, however, is connect- jects have an underlying causal nature, ed to the rest of the brain in a modality or essence, which is responsible for their speci½c way, so these modalities feel dif- appearance and behavior. Harvard ferent.15 researchers Alfonso Caramazza and Complex social interactions depend Jennifer Shelton claim that there are on our ability to understand the mental domain-speci½c knowledge systems

30 Dædalus Summer 2009

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 (modules) for animate and inani- the causal forces that underlie their Humans: mate categories that have distinct neu- observations. They appear to know by the party animal ral mechanisms. These domain-speci½c observation that fruit will fall to the knowledge systems aren’t actually the ground, but they don’t reason that if knowledge itself, but systems that make they are reaching for something and you pay attention to particular aspects drag it across a hole in a table, that it, of situations in order to increase your too, will fall into the void. Povinelli speci½c knowledge. For instance, we and Vonk suggest that humans are intuitively understand that a large ani- unique in their ability to reason about mal that has forward facing eyes and causal forces, and this extends to the sharp teeth and that stalks is a predator; psychological realm and is used to pre- so, too, do other animals. We aren’t dict and explain events or psychologi- born knowing what speci½c predator it cal states. is. If an object meets the innate criteria Intuitive psychology is a separate do- for the animate category and has bio- main from intuitive biology and phys- logical motion, then we place it in the ics. A “desire” or a “belief” isn’t labeled animal category and we automatically with physical properties such as “has infer it has speci½c properties that all gravity” or “is solid,” or biological prop- such things have: it’s alive, it requires erties such as “walks,” “breathes,” or, food and water, it can die, it has goals, most importantly, “dies.” This separate intentions, and, inaccurate though it processing of object understanding from may be, ToM! This automatic bestow- psychological understanding is what al of ToM on all animals is why it is so Yale psychologist Paul Bloom says gives easy to anthropomorphize our pets, rise to our “duality of experience.” Hu- and why it is so dif½cult to believe that mans are dualists; they act as if (and humans have a different psychology usually believe that) a person has both a from other animals. physical body and another part–a soul, This, however, is different from spirit, or “essence” that de½nes that per- how we think about inanimate ob- son. The body, an animate object, gets jects. If something is placed in the in- tagged by our intuitive biology as some- animate category, a different set of thing that eats, sleeps, walks, has sex, properties are inferred, such as “is sol- and dies. However, because the psycho- id,” or “won’t disappear.” The full ex- logical part is not visible and does not tent of the intuitive physics in other an- have an obvious physical substance, it is imals is not known, but as Marc Haus- subject to different inferences; “it dies” er suggests, along with ½ve-month-old is not one of them. Humans have an in- babies, other animals must understand tuitive belief that one’s body and one’s object permanence, otherwise if an an- essence are separate. imal didn’t understand that the lion Because the mental separation hap- that went behind the bush is still there, pens automatically, it is easy to think there would be no prey animals left. that either the body or the essence can Daniel Povinelli and Jennifer Vonk have exist separately, hence the concepts of reviewed what is known about the phys- a zombie, the body without the mind, ical knowledge of non-human primates or the soul, spirit without the body. Hu- and have concluded that even though mans, unsurprisingly, have been even they can reason about the causes of ob- more creative, inventing other essences served events, they do not understand such as ghosts, angels, demons, the dev-

Dædalus Summer 2009 31

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 Michael S. il, and gods or God. If Povinelli and sphere will smoothly make up a reason Gazzaniga Vonk are correct that other animals why an action, which was initiated by on being human cannot form concepts about imper- the right brain and of which it has no ceptible entities or processes, and they knowledge, was done. do not possess a full ToM, then being a This device, which we dubbed “the in- dualist and conceiving of such entities terpreter,” takes all incoming informa- as spirits or God are uniquely human tion, assembles it into a “makes sense” qualities. explanation, and spews it out. It can only work with the information that it re- Humans endlessly generate explana- ceives, and if there are gaps in this infor- tions and reasons for everything. My mation, it is of no consequence: it will colleagues and I noticed this tenden- generate a story to ½t the information cy while studying split-brain patients. it has. For example, in one experiment The surgical procedure to cut the cor- with a split-brain patient, we showed pus callosum is a last-ditch treatment a command to the right hemisphere to for patients with severe intractable epi- laugh, and she did. When we asked her lepsy, for whom no other treatments why she was laughing, instead of the have worked. Very few patients have left brain answering that it didn’t know, had this surgery, and it is done even she said, “You guys are so funny!” The more rarely now because of improved speech center in the left hemisphere had medications and other modes of treat- not seen the command to laugh, but cer- ment. The treatment has been very suc- tainly was receiving the input that its cessful, and most patients seemed com- person was laughing. Since that was all pletely unaware of any changes in their the information it had, it had to come mental processes. Cutting the corpus up with a “makes sense” answer. It will callosum isolates the right hemisphere also explain emotional states. In anoth- from the speech center, which usually er experiment, we used a visual stimulus is in the left hemisphere, so not only to trigger a negative mood in the right can the right hemisphere not commu- hemisphere. This time, although the pa- nicate to the left hemisphere, it can’t tient denied seeing anything, she sud- talk to anyone else either. With special denly said that she was upset and it was equipment, you can give a command to the experimenter who was upsetting the right hemisphere only. For example, her. She felt the emotional response to you could ask the right hemisphere to the stimulus, all of the autonomic re- pick up an apple from a bowl of fruit. sults, but her left hemisphere had no The right hemisphere controls the left idea what caused them. hand, so the patient would pick up the The interpreter is the device that puts apple with his left hand. When you ask all the incoming information together; the patient why he picked up the apple, it creates order out of chaos, and creates his speech center, in the left hemisphere, a narrative of and explanation for our answers. The left hemisphere, however, actions, emotions, thoughts, memories, doesn’t know why the left hand picked and dreams. It is the glue that keeps us up an apple, because it didn’t see the feeling uni½ed and creates the sense that command. This is no problem for the we are rational agents. It tells our story. I speech center; it will answer anyway. propose that the left-brain interpreter is It may say, “ I’m hungry,” or “I prefer uniquely human. Receiving input from a apples.” In these patients, the left hemi- wide variety of sources–the same sourc-

32 Dædalus Summer 2009

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 es that are available to other animals– dation block of our unique cognition. Humans: it integrates that informational input But neuroscientists are not alone in try- the party animal in a unique way to create our self-con- ing to divine ancient secrets. scious self, giving humans a distinct There is a section in the Grand Can- type of self-awareness that goes beyond yon called the “Great Unconformity,” the physical self-awareness exhibited which is the surface between the rock by mirror self-recognition. strata called the Tapeats Sandstone, which averages 545 million years old, The evolutionary changes that the and the 1.8 billion-year-old metamor- hominid line has undergone have phic rock called Vishnu schist that it brought us to our current state of be- sits upon. This unconformity repre- ing as a species, Homo sapiens. We share sents a time gap of 1.2 billion years of many features with our distant relatives, unknown geologic history–or about the chimpanzees, but we also have many 25 percent of the earth’s history. I guess unique qualities, ranging from differ- if geologists can keep plugging away at ences in our brain anatomy (on both a the mysteries of the 1.2 billion years macro- and microscopic level) to differ- missing in the geologic record, then ences in behavior and cognition. How- we neuroscientists can keep plugging ever, the mystery remains of what ex- away at the 7 million years making up actly that change was that occurred be- the Great Discontinuity, that unknown tween our last common ancestor with record of the evolution of human cog- the chimps, and that perhaps is the foun- nition.

ENDNOTES 1 Michael S. Gazzaniga, Human: The Science Behind What Makes Us Unique (New York: Ecco, 2008). 2 Derek C. Penn, Keith J. Holyoak, and Dan J. Povinelli, “Darwin’s Mistake: Explaining the Discontinuity between Human and Nonhuman Animals,” Behavioral and Brain Sciences 31 (2008): 109–130. 3 Natalie M. Schenker, Anne-Marie Desgouttes, and Katerina Semendeferi, “Neural Connec- tivity and Cortical Substrates of Cognition in Hominoids,” Journal of Human Evolution 49 (2005): 547–569. 4 There is more neuropil, the space between cell bodies that is ½lled with dendrites, axons, and synapses, in the left hemisphere’s speech center, in the area of the primary motor cortex designated to the hand, the primary visual cortex, and extrastriate areas. 5 See discussion in Charles Oxnard, “Brain Evolution: Mammals, Primates, Chimpanzees, and Humans,” International Journal of Primatology 25 (2004): 1127–1158. 6 Leon Festinger, The Human Legacy (New York: Columbia University Press, 1983), 4. 7 Oxnard, “Brain Evolution.” 8 Henrike Moll and Michael Tomasello, “Co-operation and Human Cognition: The Vygot- skian Intelligence Hypothesis,” Philosophical Transactions of the Royal Society 362 (2007): 639–648. 9 Leda Cosmides, El Mercurio, October 28, 2001. 10 For a review, see Moll and Tomasello, “Co-operation and Human Cognition.”

Dædalus Summer 2009 33

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021 Michael S. 11 Brian Hare et al., “Social Cognitive Evolution in Captive Foxes is a Correlated By-Product Gazzaniga of Experimental Domestication,” Current Biology 15 (2005): 226–230. on being human 12 Michael Tomasello, Josep Call, and Brian Hare, “Chimpanzees Understand Psychological States: The Question is Which Ones and to What Extent,” Trends in Cognitive Sciences 7 (2003): 153–156. 13 Keith Jensen, Josep Call, and Michael Tomasello, “Chimpanzees are Vengeful but not Spiteful,” Proceedings of the National Academy of Sciences 104 (2007): 13046–13050. 14 Jonathan Haidt, “The New Synthesis in Moral Psychology,” Science 316 (2007): 998–1002. 15 Mbemba Jabbi, Jojanneke Bastiaansen, and Christian Keysers, “A Common Anterior Insula Representation of Disgust Observation, Experience and Imagination Shows Divergent Functional Connectivity Pathways,” PLoSONE3 (8) (2008): e2939.

34 Dædalus Summer 2009

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/daed.2009.138.3.21 by guest on 29 September 2021