The Evolution of Mind
Origin of man now proved.—Metaphysic must flourish.—He who understands baboon would do more towards metaphysics than Locke.
—Charles Darwin, 1838: Notebook M
What goes through a baboon's mind when she contemplates the 80 or so other individuals that make up her group? Does she understand their social relations? Does she search for rules that would allow her to classify them more easily? Does she impute motives and beliefs to them in order to better predict their behavior? Does she impute motives and beliefs to herself when planning a course of action? In what ways are her thoughts and behavior like ours, and in what ways—other than the obvious lack of language and tools—are they different? These are questions that also vexed Charles Darwin.
We have taken our title from one of Darwin's most memorable remarks. He wrote it on August 16, 1838, almost two years after returning from his voyage on the Beagle and 21 years before the publication of The Origin of Species. It was a time of vigorous intellectual activity, when Darwin read voraciously on many subjects, both within and beyond the sciences, and met and talked with many different people, from family friends to prominent literary and political figures. Despite this active intellectual life, however, it seems unlikely that he or anyone else had ever combined the words "baboon" and "metaphysics" in the same sentence. What was Darwin thinking?
Mind and behavior in Darwin's time
The Cambridge English Dictionary defines metaphysics as "the part of philosophy that is about understanding existence and knowledge." Writing in the Westminster Review in 1840, John Stuart Mill offered a summary of views on the origin of knowledge that were being discussed by Darwin and his contemporaries. "Every consistent scheme of philosophy requires, as its starting point, a theory representing the sources of human knowledge, and the objects which the human faculties are capable of [understanding]. The prevailing theory in the eighteenth century ... was that proclaimed by Locke, and attributed to Aristotle—that all our knowledge consists of generalizations from experience." According to this theory, Mill continued, we know "nothing, except the facts which present themselves to our senses, and such other facts as may, by analogy, be inferred from these. There is no knowledge a priori; no truths cognizable by the mind's inward light and grounded on intuitive evidence." Locke believed that the mind acts simply to associate events that have been joined together through proximity and repetition. From these associations it generates behavior. Anything we think or do can ultimately be traced to our experience.
Mill continued: "From this doctrine Coleridge with ... Kant ... strongly dissents. ... He distinguishes in the human intellect two faculties ... Understanding and Reason. The former faculty judges of phenomena, or the appearance of things, and forms generalizations from these: to the latter it belongs, by direct intuition, to perceive things, and recognize truths, not cognizable by our senses." In Kant's scheme, these perceptions exist a priori but are not completely innate because they require experience for their expression. For Kant, the mind was not a blank slate on which any sort of experience can write any kind of instructions. It is, instead, biased in the way it responds to features of the world—actively organizing experiences and generating behavior on the basis of preexisting schemes. To understand our thoughts, beliefs, and behavior, therefore, we must consider not only our own individual experiences but also the preexisting nature of the mind itself.
Empiricism and rationalism were hotly debated at the time. Mill reported that "between the partisans of these two opposite doctrines there reigns a bellum internecinum [in which] even sober men on both sides take no charitable view of each others' opinions." Darwin followed the debate, but with a more open mind and a much more zoological perspective than many of his contemporaries. While others debated the nature of the human mind, he also puzzled over the minds of bees, dogs, and baboons.
Darwin's interest in metaphysics was motivated by more than just idle curiosity—it was also fueled by excitement and personal ambition. By the late 1830s and 1840s, the theory of evolution by natural selection was beginning to take shape in his mind, and his notebooks are filled with many speculations about how his work might shed an entirely new light on the study of human knowledge.
Darwin had observed that every animal species engages in repeated, "habitual" behavior. Birds build nests, squirrels hoard seeds, and dogs raise the fur on their back when they feel threatened. He believed that these behaviors recurred because they were beneficial to the individuals involved and that, over generations, habitual behavior became "instinctive," or innate. Under the right conditions, instinctive behavior would appear automatically, even if the animal had never before had the appropriate experience. When they act by instinct, then, animals are not behaving according to Lockean reason, carefully weighing the information acquired from experience. Instead, they are governed by "hereditary tendencies" acquired over generations.
This is not to say that Darwin believed animals were slaves to their instincts, wholly devoid of learning or reason. Some of his contemporaries did hold such views, and used them to draw a sharp distinction between humans and other animals. The naturalist Edward Blyth (1837), for example, wrote that "whereas the human race is compelled to derive the whole of its information through the medium of its senses, the brute is, on the contrary, supplied with an innate knowledge of whatever properties belong to all the natural objects around." Darwin disagreed—both with the conclusion that animals' thoughts and behavior are entirely based on instinct and with the view that human thought and behavior are governed entirely by reason. "[It is] hard to say what is instinct in animals & what [is] reason, in precisely the same way [it is] not possible to say what [is] habitual in men and what reasonable. ... as man has hereditary tendencies, therefore man's mind is not so different from that of brutes." Like many of his contemporaries, Darwin was searching for an explanation of mind and behavior that would combine innate, inherited tendencies (a bit of rationalism from Kant) with reasoning based on experience (a bit of empiricism from Locke). In this as in so much else, Darwin was a man ahead of his time.
Darwin also realized that, whatever the exact balance between innate behavior and reason in any particular instance, his theory of evolution had important implications for the study of metaphysics. After all, thoughts and instincts came from the mind, and the mind could be studied like any other biological trait. It was different in different species, reflecting the particular adaptations of each, and it could change gradually over time, being transmitted from one generation to the next. In his notebook M (M for metaphysics), Darwin wrote: "We can thus trace causation of thought ... [it] obeys [the] same laws as other parts of structure."
With growing excitement, Darwin began to see that his theory might allow him to reconstruct the evolution of the human mind and thereby resolve the great debate between rationalism and empiricism. The modern human mind must acquire information, organize it, and generate behavior in ways that have been shaped by our evolutionary past. Our metaphysics must be the product of evolution. And just as the key to reconstructing the evolution of a whale's fin or a bird's beak comes from comparative research on similar traits in closely related species, the key to reconstructing the evolution of the human mind must come from comparative research on the minds of our closest animal relatives. "He who understands baboon would do more towards metaphysics than Locke."
Twentieth-century views: behaviorists and their critics
In the first half of the 20th century, research on the mind and behavior was dominated by modern-day empiricists like E. L. Thorndike, J. B. Watson, and B. F. Skinner, who together developed the doctrine of behaviorism. Like Locke, they believed that organisms come into the world with little a priori knowledge: behavior is the product entirely of experience. As an animal moves through its world, it encounters stimuli and responds to them. If its response is followed by something pleasant, like food, the response will be repeated whenever the animal encounters the same stimulus again. In this way, the animal quickly develops an array of behaviors that are well suited to its needs.
As the intellectual descendants of Locke, behaviorists believed that the mind is concerned primarily with the formation of associations: mechanical principles of attachment that develop as a result of experience. They saw the mind not as an active "thinking" organ, predisposed to organize incoming stimuli in certain ways, but instead as a rather passive arena in which stimuli from the environment are combined according to simple rules, thereby producing behavior. The behaviorists concluded that a few simple but powerful laws, like Pavlov's Law of Association and Thorndike's Law of Effect, could account for all behavior, in every species and every circumstance. They believed in the principle of equipotentiality. As Skinner famously remarked, "Pigeon, rat, monkey, which is which? It doesn't matter àonce you have allowed for differences in the ways they make contact with the environment, what remains of their behavior shows astonishingly similar properties."
The behaviorists saw little point in considering mental activities like thoughts, feelings, goals, or consciousness, for reasons that were both methodological and deeply philosophical. On the practical side, mental states like thoughts or emotions are private. They cannot be observed or measured, nor can one predict how they might be changed by experience. Under these circumstances, the mental activities of animals can hardly play a role in any scientific discipline. Even in humans, where introspection prompted some behaviorists to admit—grudgingly—that mental states might exist, the exact nature of these states are unknowable because they can never be verified by more than one person. Once again, this makes mental states unsuitable for scientific study. Some behaviorists went even further. In his 1974 book About Behaviorism, Skinner distinguished between "methodological behaviorists" who accepted the existence of mental states but avoided them because they could not be studied scientifically, and "radical behaviorists" like himself, who believed that "so-called mental activities" were an illusion—an "explanatory fiction." For Skinner, thoughts, feelings, goals, and intentions played no role in the study of behavior because they did not, in fact, exist.
Although behaviorism dominated 20th-century psychology, it was not without its critics. Perhaps the best way to understand them is to consider some classic observations and experiments that challenged the behaviorists' worldview.
Song sparrows (Melospiza melodia) and swamp sparrows (Melospiza georgiana) are two closely related North American birds with very different songs. Males in both species learn their songs as fledglings, by listening to the songs of other males. But this does not mean that the mind of a nestling sparrow is a blank slate, ready to learn virtually anything that is written upon it by experience. In fact, as classic research by Peter Marler and his colleagues has shown, quite the opposite is true. If a nestling male song sparrow and a nestling male swamp sparrow are raised side-by-side in a laboratory where they hear tape-recordings of both species' songs, each bird will grow up to sing only the song of its own species.
The constraints that channel singing in one direction rather than another cannot be explained by differences in experience, because each bird has heard both songs. Nor can the results be due to differences in singing ability, because both species are perfectly capable of producing each other's notes. Instead, differences in song learning must be the result of differences in the birds' brains: something in the brain of a nestling sparrow prompts it to learn its own species' song rather than another's. The brains of different species are therefore not alike. And the mind of a nestling sparrow does not come into the world a tabula rasa—it arrives, instead, with genetically determined, inborn biases that actively organize how it perceives the world, giving much greater weight to some stimuli than to others. One can persuade a song sparrow to sing swamp sparrow notes, but only by embedding these notes into a song sparrow's song. It is almost impossible to persuade a swamp sparrow to sing any notes other than its own. Philosophically speaking, sparrows are Kantian rationalists, actively organizing their behavior on the basis of innate, preexisting schemes.
In much the same way, human infants have their own sensory and cognitive biases. From the first days of life, they attend more readily to faces than to other visual stimuli and more readily to speech than to other auditory stimuli. This latter bias can apparently be traced to a preference for the intonation contours in spoken language: two-day-old babies show distinctive cerebral blood flow when they hear a normal sentence but not when the same sentence is played backward. Humans and sparrows are not alone in preferring their own species' sounds: when a rhesus macaque monkey (Macaca mulatta) hears a call given by a member of its own species, its brain exhibits activity that is markedly different from that shown in response to other sounds. Indeed, rhesus calls activate in the rhesus brain the same areas activated by human speech in the human brain.
Some of the most striking evidence for an innate predisposition to learn one's own species' communication comes from children who are born blind or deaf. Although they cannot see the objects in the world to which spoken words refer, blind children develop language at roughly the same age and in the same manner as children who can see. Data from children born deaf are even more striking. Lila Gleitman, Susan Goldin-Meadow, and their colleagues studied several deaf children born to hearing parents who did not themselves know ASL, the American Sign Language for the deaf. Although raised in loving, supportive environments, these children were deprived of any exposure to language. Nonetheless, they spontaneously invented a sign language of their own, beginning with single signs at roughly the same age that single words would ordinarily have appeared. And during the following months and years, as they developed more complex sentences, the children produced signs in a serial order according to their semantic role as subject, verb, and object.
The songs of sparrows, the calls of monkeys, and the language of human children could hardly be more different, yet they all lead to the same conclusion: Each species has a mind of its own that, like its limbs, heart, and other body parts, has evolved innate predispositions that cause it to organize incoming sensations in particular ways. The mind arrives in the world with constraints and biases, "prepared" by evolution to view the world, organize experiences, and generate behavior in its own particular way. And because each species is different, the behavior of different species is unlikely to be explained by a few general laws based entirely on experience. Although there may well be some general features of learning that are shared by many species, the behaviorists' principle of equipotentiality ("pigeon, rat, monkey ...") is understandable but incorrect.
But what of the behaviorists' second major premise, that the "mind" and "mental states"—if they exist at all—are private and unmeasurable, and cannot be studied scientifically? This view was also challenged, most prominently by the psychologist Edward C. Tolman (1932), who argued that learning is not just a mindless link between stimulus and response. Instead, animals acquire knowledge as a result of their experiences.
In 1928, Otto L. Tinklepaugh, a graduate student of Tolman's, began a study of learning in monkeys. His subjects were several macaques who were tested in a room in the psychology department at the University of California at Berkeley (sometimes the tests were held outdoors, on the building's roof, which the monkeys much preferred). In one of Tinklepaugh's most famous experiments, a monkey sat in a chair and watched as a piece of food—either lettuce or banana—was hidden under one of two cups that had been placed on the floor, six feet apart and several feet away. The other cup remained empty. Once the food had been placed under the cup, the monkey was removed from the room for several minutes. Upon his return, he was released from the chair and allowed to choose one of the cups. All of Tinklepaugh's subjects chose the cup hiding the food, though they performed the task with much more enthusiasm when the cup concealed banana.
To illustrate the difference between behaviorist and cognitive theories of learning, pause for a moment to consider the monkey as he waits outside the experimental room after seeing, for example, lettuce placed under the left-hand cup. What has he learned? Most of us would be inclined to say that he has learned that there is lettuce under the left-hand cup. But this was not the behaviorists' explanation. For behaviorists, the reward was not part of the content of learning. Instead, it served simply to reinforce or strengthen the link between a stimulus (the sight of the cup) and a response (looking under). The monkey, behaviorists would say, has learned nothing about the hidden food—whether it is lettuce or banana. His knowledge has no content. Instead, the monkey has learned only the stimulus-response associations, "When you're in the room, approach the cup you last looked at" and "When you see the cup, lift it up." Most biologists and laypeople, by contrast, would adopt a more cognitive interpretation: the monkey has learned that the right-hand cup is empty but there is lettuce under the left-hand cup.
To test between these explanations, Tinklepaugh first conducted trials in which the monkey saw lettuce hidden and found lettuce on his return. Here is his summary of the monkey's behavior:
Subject rushes to proper cup and picks it up. Seizes lettuce. Rushes away with lettuce in mouth, paying no attention to other cup or to setting. Time, 3–4 seconds.
Tinklepaugh next conducted trials in which the monkey saw banana hidden under the cup. Now, however, Tinklepaugh replaced the banana with lettuce while the monkey was out of the room. His observations:
Subject rushes to proper cup and picks it up. Extends hand toward lettuce. Stops. Looks around on floor. Looks in, under, around cup. Glances at other cup. Looks back at screen. Looks under and around self. Looks and shrieks at any observer present. Walks away, leaving lettuce untouched on floor. Time, 10–33 seconds.
It is impossible to escape the impression that the duped monkey had acquired knowledge, and that as he reached for the cup he had an expectation or belief about what he would find underneath. His shriek reflected his outrage at this egregious betrayal of expectation.
Many years later, Ruth Colwill and Robert Rescorla (1985) carried out a more controlled version of the same experiment. They began by training rats to make two responses, pressing a lever and pulling a chain. When the rats pressed the lever they received a small food pellet; when they pulled the chain they received liquid sucrose. By the behaviorist view, the rats had learned only to press the lever or pull the chain whenever they saw them. By the cognitive view, the rats had formed some kind of mental representation of the relation between a particular act and a specific type of food. To test between these hypotheses, Colwill and Rescorla made either the food pellet or the water unpalatable by adding lithium chloride, a substance that makes rats sick. If the rats had learned which food type was associated with which behavioral act, then those for whom the food pellet had been devalued would avoid the lever but continue to pull the chain, whereas those for whom the water had been devalued would do the opposite. This is exactly what happened.
The results of these experiments challenge the more extreme behaviorists' view that mental states like knowledge, beliefs, or expectations cannot be studied scientifically and may even be an illusion. Instead, they support Tolman's view that learning allows an animal to form a mental representation of its environment. Through learning, animals acquire information about objects, events, and the relation between them. Their knowledge has content, and this content can be studied scientifically.
This conclusion from the laboratory is important, because it encourages us to believe that Darwin was right: we can trace the causation of thought in different species, study its structure, and reconstruct its evolution. But while the scientific study of mind is an exciting prospect, a large dose of humility is in order. For all of their failings, the behaviorists did understand that, whereas behavior can be unambiguously observed and measured, knowledge and the content of mental states are abstract, hard to measure, and difficult even to define. Once you accept the existence of mental states and ascribe causal power to them, you have opened Pandora's box, releasing a host of fundamental questions that are difficult if not impossible to answer.
When we say that a song sparrow's brain "predisposes" it to attend to song sparrow song in a way that it attends to no other, what precisely do we mean? When we claim that a rat has formed an association between bar pressing and a particular type of food, what exactly is the nature of its knowledge? Does the rat think that the bar somehow stands for that food? Does it believe that pressing the bar causes the food to appear? Can rats distinguish between the relations A represents B and A causes B? When Pavlov's dog salivated at the sound of a metronome, was this an automatic, unthinking reflex, or did it occur because the metronome brought to mind an image of meat? None of these questions is easy to answer.
On first—and perhaps even further—inspection, baboons might seem less than ideal subjects for a study of the mind. Among other failings, they are not as closely related to humans as some other nonhuman primates. Baboons are members of the genus Papio, Old World monkeys that shared a common ancestor with humans roughly 30 million years ago. Baboons are more closely related to humans than monkeys of the New World, but they are much less closely related than the African apes—especially chimpanzees (Pan troglodytes)—which diverged from our own ancestors roughly five to seven million years ago. Moreover, the conservation status of baboons confers neither glamour nor prestige on those who study them. Far from being endangered, baboons are one of Africa's most successful species. They flourish throughout the continent, occupying every ecological niche except the Sahara and tropical rain forests. They are quick to exploit campsites and farms and are widely regarded as aggressive, destructive, crop-raiding hooligans. Finally, baboons are not particularly good-looking—many other monkeys are far more photogenic. Indeed, through the ages baboons have evoked as much (if not more) repulsion than admiration.
Baboons are interesting, however, from a social perspective. Their groups number up to 100 individuals and are therefore considerably larger than most chimpanzee communities. Each animal maintains a complex network of social relationships with relatives and nonrelatives—relationships that are simultaneously cooperative and competitive. Navigating through this network would seem to require sophisticated social knowledge and skills. Moreover, the challenges that baboons confront are not just social but also ecological. Food must be found and defended, predators evaded and sometimes attacked. Studies of baboons in the wild, therefore, allow us to examine how an individual's behavior affects her survival and reproduction. They also allow us to study social cognition in the absence of human training, in the social and ecological contexts in which it evolved.
In Darwin's theory of evolution by natural selection, necessity is the mother of invention. Traits arise or are maintained because they help the individuals who possess them to solve a problem, thereby giving those individuals an advantage over others in survival and reproduction. A blunt, heavy beak allows a finch to crush hard, dry seeds and survive a withering dry season; antlers enable a stag to defeat his rivals and mate with more females. The finch's beak and the stag's antlers did not arise at random; they evolved and spread because of their adaptive value. To understand the evolution of a trait, therefore, we need to understand how it works, and what it allows an individual to do that might otherwise be impossible.
And brains, Darwin realized, were biological traits like any other. To understand how they evolved, we must understand the problems they were designed to solve. In recent years, studies of the brain, intelligence, and evolution in animals have produced two general conclusions that will guide our study of baboon metaphysics.
First, natural selection often creates brains that are highly specialized. Arctic terns (Sterna paradisaea) migrate each year from one end of the earth to another, Cataglyphis ants navigate across the featureless Sahara, bees dance to signal the location of food, and Clark's nutcrackers (Nucifraga columbiana, a member of the crow, or corvid, family) store and recover tens of thousands of seeds during the fall and winter. Yet despite these specialized skills, there is no evidence that terns, ants, bees, or nutcrackers are generally more intelligent than other species. Instead, they are more like nature's idiots savants: brilliant when it comes to solving a specific, narrowly defined problem, but pretty much average in other domains.
Specialized intelligence may be widespread in animals because brain tissue is costly to develop and maintain. The human brain uses energy at a rate comparable to that used by the leg muscles of a marathon runner when running. If brain tissue is energetically expensive, the cheapest way to evolve a specialized skill may be through a small number of especially dedicated brain cells rather than a larger, general-purpose brain. For arctic terns, the ability to fly from pole to pole in the spring and fall is adaptive because it allows the birds to live in perpetual summer. As a result, selection has favored individuals with the neural tissue needed to navigate great distances using the sun, the stars, and the earth's magnetic field. But it has done so in the cheapest, most energy-efficient way possible—by selecting specifically for navigational skills.
The second general conclusion to emerge from recent research is that the domain of expertise for baboons—and indeed for all monkeys and apes—is social life. Most baboons live in multimale, multifemale groups that typically include eight or nine matrilineal families, a linear dominance hierarchy of males that changes often, and a linear hierarchy of females and their offspring that can be stable for generations. Daily life in a baboon group includes small-scale alliances that may involve only three individuals and occasional large-scale, familial battles that involve all of the members of three or four matrilines. Males and females can form short-term bonds that lead to reproduction, or longer-term friendships that lead to cooperative child rearing. The result of all this social intrigue is a kind of Jane Austen melodrama, in which each individual must predict the behavior of others and form those relationships that return the greatest benefit. These are the problems that the baboon mind must solve, and this is the environment in which it has evolved.
Social problems, of course, are not the only challenges. Baboons also need to solve ecological problems, like finding food and avoiding predators. But these problems are also overwhelmingly social. One of the most difficult aspects of finding food arises from the fact that as many as 100 other individuals in your group also want the food for themselves. And the best way to avoid being taken by lions, leopards, crocodiles, or pythons is to live in a group, with all of the opportunities and compromises that group life entails. Any way you look at it, most of the problems facing baboons can be expressed in two words: other baboons.
Excerpt from pages 1-12 of Baboon Metaphysics: The Evolution of a Social Mind by Dorothy L. Cheney and Robert M. Seyfarth, published by the University of Chicago Press. ©2007 by The University of Chicago.