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Focus on Disorders: Closed Head Injury

Defining Brain and BehaviorWhat Is the Brain?How Is the Nervous System Structured?What Is Behavior?

Perspectives on Brain andBehaviorAristotle and MindDescartes and DualismDescartes’s LegacyFocus on Disorders: Linking Brain Function

to Brain DiseaseDarwin and MaterialismDarwin’s Legacy

The Evolution of Brain andBehaviorOrigin of Brain Cells and BrainsClassification SystemsEvolution of Animals with Nervous SystemsThe Chordate Nervous System

Human EvolutionHumans: Members of the Primate OrderAustralopithecus: Our Distant AncestorThe First HumansThe Evolution of the Human Brain

Studying Brain and Behavior in Modern HumansHuman Brain-Size ComparisonsCultureFocus on Disorders: Learning Disabilities

What Are the Origins of Brain and Behavior?

C H A P T E R

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Micrograph: Oliver Meckes/Ottawa/Photo Researchers

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Twelve years ago, I survived a serious head injury. In the sec-ond it took for my car to crash head-on, my life was perma-nently changed, and I became another statistic in what hasbeen called “the silent epidemic.”

During the next months, my family and I began to un-derstand something of the reality of the experience of headinjury. I had begun the painful task of recognizing and ac-cepting my physical, mental, and emotional deficits. Icouldn’t taste or smell. I couldn’t read even the simplestsentence without forgetting the beginning before I got to theend. I had a hair-trigger temper that could ignite instantlyinto rage over the most trivial incident.

During the first year, I could not take too much stimu-lation from other people. My brain would simply overload,and I would have to go off into my room to get away. Noisewas hard for me to take, and I wanted the place to be keptquiet, which was an impossibility in a small house withthree youngsters in it. I remember laying down some impos-sible rules for all of us. For example, I made rules thateverybody had to be in bed by 9:30 PM, that all lights had tobe out, and that no noise of any kind was permitted afterthat time. No TV, radios, or talking was allowed. Eventuallythe whole family was in an uproar.

Two years after my injury, I wrote a short article:“What Does It Feel Like to Be Brain Damaged?” At thattime, I was still intensely focusing on myself and my ownstruggle. (Every head-injured survivor I have met seems togo through this stage of narcissistic preoccupation, whichcreates a necessary shield to protect them from the painfulrealities of the situation until they have a chance to heal.) I had very little sense of anything beyond the material worldand could only write about things that could be describedin factual terms. I wrote, for example, about my various im-pairments and how I learned to compensate for them by avariety of methods.

At this point in my life, I began to involve myself withother brain-damaged people. This came about in part afterthe publication of my article. To my surprise, it was reprintedin many different publications, copied, and handed out tothousands of survivors and families. It brought me an enor-mous outpouring of letters, phone calls, and personal visitsthat continue to this day. Many were struggling as I had strug-gled, with no diagnosis, no planning, no rehabilitation, andmost of all, no hope.

The far-ranging effects of head injury on the survivor’slife and that of his or her family cannot be overemphasized.In my own case I realize that it was for me the single most

significant event of my lifetime. The catastrophic effect ofmy injury was such that I was shattered and then remoldedby the experience, and I emerged from it a profoundly different person with a different set of convictions, values,and priorities. Above all, I have learned that there is nolimit to the power of faith, hope, and love. With these, Imade the journey out of the shadows into a larger, brighterworld than the one I had left behind before my injury.(Linge, 1990)

This description of what it is like to be brain injured

was written by Fred Linge, a clinical psychologist

with a degree in brain research. (For an explanation

of how the brain can be injured in an accident, see

“Closed Head Injury” on page 2.) In the years after his in-

jury, Linge made an immense journey. He traveled from a

time before the car crash, when he gave less thought to

the relation between his brain and his behavior than he

did to the way in which he dressed. At the end of the jour-

ney, thoughts about his brain and his behavior dominated

his life. He became a consultant and advisor to many peo-

ple who also had suffered brain injury.

Most of you are like Fred Linge before he took that

journey. Your brain does its work so efficiently and unob-

trusively that you hardly give it any thought. You may be

unaware that the human brain has hundreds of parts, each

of which participates in certain tasks. You may have no

knowledge that the brain changes as you age, as you un-

dergo major life events, and even as you engage in seem-

ingly trivial behaviors, such as reading the words on this

page. In learning about the origins of the universe, the

world, and human beings, you may have encountered no

mention of the brain and its relation to behavior. Yet, if you

ever had first-hand experience with brain damage, you,

too, would be confronted with the workings of this most

wonderful and complex machine.

The purpose of this book is to take you on a journey

not unlike the one that Fred Linge took. Through it you,

too, will come to understand the link between brain and

behavior. Of course, we do not ask that you experience

brain damage to undertake this journey. The road that we

offer is simply one of information and discovery. Yet, along

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it, you will find that much of the evidence that we have

about the brain and behavior comes from the study of

changes in people who have suffered brain injury. At the

same time, we are also learning more and more about

how the brain works when we are healthy. This emerging

knowledge is changing how we think about ourselves,

how we structure education and our social interactions,

and how we aid those with brain injury.

In this chapter, we answer the question, What are the

origins of brain and behavior? We begin by defining both

the brain and behavior and outlining the nervous system’s

basic structure. We then look at how people through his-

tory have viewed the relation between brain and behavior,

starting with the mentalistic perspective of Aristotle and

progressing to the biological perspective of today. With

this background in mind, we explore the evolution of

2 ■ CHAPTER 1

Closed head injury results from a

blow to the head that subjects the

brain to a variety of forces. First, the

force exerted on the skull at the site

of the blow causes bruising (contu-

sion) known as a “coup.” Second,

the blow may force the brain

against the opposite side of the

skull, producing an additional

bruise called a “countercoup” (see

the accompanying illustration).

Third, the movement of the brain

may cause a twisting or shearing of

nerve fibers, causing microscopic

lesions. Such lesions may be found

throughout the brain, but they are most common in the

frontal and temporal lobes. Fourth, the bruises and strains

caused by the impact may produce bleeding (hemorrhage).

Because the blood is trapped within the skull, it acts as a

growing mass (hematoma), which exerts pressure on sur-

rounding brain regions. Finally, like blows to other parts of

the body, blows to the brain produce swelling (edema). This

swelling, which is a collection of fluid in and around dam-

aged tissue, is another source of pressure on the brain.

People who sustain closed head injury often lose con-

sciousness because the injury affects fibers in lower parts of

the brain that are associated with waking. The severity of

coma can indicate the severity of the injury. Closed head in-

juries resulting from motor vehicle accidents are particularly

severe because the head is moving when the blow is struck,

thereby increasing the velocity of the impact.

The diffuse effects of closed head injuries make diagno-

sis very difficult, which is why these kinds of injuries have

been collectively called a “silent epidemic.” Victims of se-

vere closed head injury can suffer serious repercussions in

their everyday lives. Like Fred Linge, many have difficulty re-

turning to their former levels of functioning, including carry-

ing out their previous jobs.

Closed Head Injury

Focus on Disorders

Shading (pink and blue) indicates regions of the brain mostfrequently damaged in closed head injury. A blow canproduce a contusion both at the site of impact and at theopposite side of the brain owing to compression of thebrain against the front (A) or back (B) of the skull.

(A) (B)

Direction of blow Direction of blow

A variety of mechanical forces cause closed head injuries as a result of a blow to the head.

The pressure resulting from a coup may produce a countercoup on the opposite side of the brain (shown in blue).

Movement of the brain may shear nerve fibers, causing microscopic lesions, especially in frontal and temporal lobes. Blood trapped in the skull (hematoma) and swelling (edema) cause pressure on the brain.

The damage at the site of impact is called a coup (shown in pink).

DEFINING BRAIN AND BEHAVIORBrain and behavior differ greatly but are linked. The brain is a physical object, a livingtissue, a body organ. Behavior is action, momentarily observable, but fleeting. Yet oneis responsible for the other, which is responsible for the other, which is responsible forthe other, and so on, and so on. We begin by defining first the brain, then behavior,and finally their interrelation.

What Is the Brain?For his postgraduate research, our friend Harvey chose to study the electrical activitythat the brain gives off. He said that he wanted to live on as a brain in a bottle after hisbody died. He expected that his research would allow his bottled brain to communi-cate with others who could “read” his brain’s electrical signals. Harvey failed in his ob-jective, in part because the goal was technically impossible but also because he lackeda full understanding of what “brain” means.

Brain is the Anglo-Saxon word for the tissue that is found within the skull, and itis this tissue that Harvey wanted to put into a bottle. Figure 1-1 shows a typical hu-man brain oriented as in the skull of an upright human. The brain has two relativelysymmetrical halves called hemispheres, one on the left and one on the right. So, justas your body is symmetrical, having two arms and two legs, so is the brain. If youmake your right hand into a fist and hold it up, the fist can represent the positions ofthe brain’s hemispheres within the skull, with the thumb pointing toward the front.

The entire outer layer of the brain consists of a folded tissue. The folds are calledgyri (singular, gyrus). This outer layer is known as the cerebral cortex (usually re-ferred to simply as the cortex). The word cortex, which means “bark” in Latin, is aptlychosen both because of the cortex’s folded appearance and because it covers most ofthe rest of the brain. The cortex of each hemisphere is divided into four lobes, namedafter the skull bones beneath which they lie. The temporal lobe is located approxi-mately at the same place as the thumb on your upraised fist. Because it points for-ward, it is a good landmark for identifying which part of the brain is the front. Thelobe lying immediately above the temporal lobe is called the frontal lobe because it islocated at the front of the brain, beneath the frontal bone of the skull. The parietallobe is located behind the frontal lobe, and the occipital lobe constitutes the area atthe back of each hemisphere.

It is clear that Harvey, who wanted to have his brain bottled after he died, wantedto preserve not just his brain but his self — his consciousness, his thoughts, all of hisintelligence. This meaning of the term brain refers to something other than the organfound inside the skull. It refers to the brain as that which exerts control over behavior.This meaning of brain is what we intend when we talk of someone being “the brainbehind the operation” or when we speak of the computer that guides a spacecraft asbeing the vessel’s “brain.” The term brain, then, signifies both the organ itself and thefact that this organ controls behavior. Could Harvey manage to preserve his control-exerting self inside a bottle? Read on to learn the answer to this question.

brain and behavior. Here we pay special attention to the

evolution of the human species, while still recognizing the

many traits that we have in common with other animals.

Finally, we look at several matters concerning the study of

the brain and behavior in modern humans, including the

matter of how our brains acquire the sophisticated skills of

human culture. In subsequent chapters, we will further de-

velop many of the ideas introduced here in addition to fill-

ing in a great many details about brain anatomy and func-

tion and how behavior is organized.

On the CD, visit the module onthe Central Nervous System. Look atthe 3-D view of the human cortex inthe section on the overview of thebrain for a hands-on view of what this organ looks like.

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 3

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Frontallobe

Parietallobe

Temporallobe

Occipitallobe

Sectional view

Folds in the brain's surface are called gyri.

Front Back

Bottom

Top

Your right hand, if made into a fist, represents the positions of the lobes of the left hemisphere of your brain.

Temporal lobe (thumb)

Frontal lobe (fingers)

Parietal lobe (knuckles)

Occipital lobe(wrist)

Lobes define broad divisions of the cerebral cortex.

(A) (B)

The brain is made up of two hemispheres, left and right.

Cerebral cortex is the brain’s outer “bark” layer.

Figure 1-1

(A) In this representation of the humanbrain, showing its orientation in thehead, the visible part of the brain is thecerebral cortex (cortex means “bark,”and the cortex resembles the bark of atree). The cortex is a thin sheet of tissuethat is folded many times so that it fitsinside the skull. The folds are called gyri.The brain consists of two symmetricalhalves called hemispheres. Eachhemisphere is divided into four lobes:frontal, parietal, temporal, and occipital.(B) The fist of your hand can serve as aguide to the orientation of the brain and its lobes. The thumb represents the temporal lobe and points forward,the flexed fingers represent the frontallobe, the knuckles represent the parietallobe, and the wrist represents theoccipital lobe.

Your right hand, if made into a fist, represents the positions of the lobes of the left hemisphere of your brain.

Temporal lobe (thumb)

Frontal lobe (fingers)

Parietal lobe (knuckles)

Occipital lobe(wrist)

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How Is the Nervous System Structured?Just like every other organ of the body, the brain is composed of cells. These braincells come in a variety of shapes and sizes. One type of brain cell is the neuron(sometimes called nerve cell), which has fibers projecting from it that make contactwith other cells. These interconnecting fibers make the brain a sensing, integrating or-gan that also instructs the body to move.

Most of the connections from the brain to the rest of the body are made throughthe spinal cord, which descends through a canal in the vertebrae (the bones that formthe backbone). The spinal cord is in the same orientation as that of the upright armsupporting your fist. The brain and spinal cord, which in mammals such as ourselvesare both protected by bones, together make up the central nervous system (CNS).

The central nervous system is connected to the rest of the body through nervefibers, as shown in Figure 1-2. To sense what goes on in the world around us and inour own bodies, these nerve fibers are extensively connected to sensory receptors onthe body’s surface, to internal body organs, and to muscles. All of these nerve fibersradiating out beyond the brain and spinal cord as well as all the neurons outside thebrain and spinal cord are referred to as the peripheral nervous system (PNS). Thecentral and peripheral nervous systems together make up the whole nervous system.

Networks of sensory pathways made up of bundles of nerve fibers and motorpathways also made up of bundles of nerve fibers of the peripheral nervous systemare connected to the central nervous system. Sensory nerves are those related to spe-cific senses, such as hearing, vision, and touch. They connect receptors for these

senses to sensory-processing areas of the brain. With information from sensory recep-tors, the brain constructs current images of the world, as well as memories of pastevents and expectations about the future. In addition, sensory information helps thebrain to construct a self-identity, as will be considered shortly. The motor nerves con-nect the brain and spinal cord to the body’s muscles. The movements produced bymotor pathways include the eye movements that you are using to read this book, thehand movements that you make while turning the pages, and the posture that youmaintain as you read.

Motor pathways are also used in the workings of your body’s organs, such as thebeating of your heart, the contractions of your stomach, and the movement of yourdiaphragm, which inflates and deflates your lungs. These pathways are part of anothersubdivision, called the autonomic nervous system, of the nervous system. The auto-nomic nervous system consists of all the neurons that receive messages from or sendcommands to the various organs of the body, including the heart, digestive system,sex organs, excretory organs, blood vessels, and glands. Your autonomic system allowsyou, for instance, to feel both hunger before eating and satisfaction after a meal. Italso enables you to digest and process food through your digestive tract. The auto-nomic nervous system, in short, regulates all your bodily functions, as well as theemotional responses associated with your voluntary actions.

Knowing that the nervous system is far more than just a brain, we can return tothe question of whether a brain kept alive in a bottle is a brain in the fullest sense ofthe word. What Harvey wanted to try in the “brain in a bottle experiment” and failed

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 5

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Central nervous system (CNS)The brain and spinal cord, thoseparts of the nervous system thatare encased by the skull andvertebrate bones

Peripheral nervous system (PNS)Parts of the nervous system that connect the brainand spinal cord to the rest of the body, including:

Autonomic connectionsto body organs

Sensory connectionsto sensory receptorsin the skin

Motor connectionsto body muscles

Figure 1-2

The human nervous system receivessensory information and producesbehavior. The nervous system consists of the central nervous system (the brainand spinal cord) and the peripheralnervous system (the many neurons thatconnect the brain and spinal cord to therest of the body).

Plug in the CD to look at how thesenerve networks work in our brains. Theoverview of the brain in the module onthe Central Nervous System includes arotatable, 3-D view of the brain that will help you visualize how all theseparts fit together

to consider was whether his brain could sustain intelligent behavior in the absence ofthe sensations and movements provided by the brain’s connections to the rest of thebody. We can probably never be really sure of the capacities of a bottled brain. Still,some research suggests that sensations and movements are essential for consciousness.

In one study in the 1920s, Edmond Jacobson wondered what would happen ifour muscles completely stopped moving. Jacobson believed that, even when we be-lieve that we are entirely motionless, we still make subliminal movements related toour thoughts. The muscles of the larynx subliminally move when we “think in words,”for instance, and we make subliminal movements of our eyes when we imagine a visual scene. So Jacobson had people practice “total” relaxation and later asked themwhat the experience was like. They reported a condition of “mental emptiness,” as ifthe brain had gone blank.

In another study in the 1950s, Donald O. Hebb and his coworkers investigated theeffects of sensory deprivation, as well as lack of movement, by having each subject lieon a bed in a soundproof room and remain completely still. Tubes covered the sub-jects’ arms so that they had no sense of touch, and translucent goggles cut off their vi-sion. The subjects reported that the experience was extremely unpleasant, not just be-cause of the social isolation, but also because they lost their normal focus in thissituation. Some subjects even had hallucinations, as if their brains were somehow try-ing to create the sensory experiences that they suddenly lacked. Most asked to be re-leased from the study before it ended.

These experiments suggest that the brain needs ongoing sensory and motor expe-rience if it is to maintain its intelligent activity. Thus, when we use the term brain tomean an intelligent, functioning organ, we should probably refer to a brain that isconnected to the rest of the nervous system. It seems very unlikely that a “brain in abottle” would continue to function in a normal way.

What Is Behavior?I. Eibl-Eibesfeldt began his textbook titled Ethology: The Biology of Behavior, pub-lished in 1970, with the following definition of behavior: “Behavior consists of pat-terns in time.” These patterns can be made up of movements, vocalizations, or changesin appearance, such as the color changes associated with blushing. The expression“patterns in time” can even include thinking. Although we cannot directly observesomeone’s thoughts, there are techniques for monitoring changes in the brain’s elec-trical and biochemical activity that may be associated with thought. So thinking, too,forms patterns in time.

A simpler definition of behavior is any kind of movement in a living organism.Such movements are limited to those that we can somehow see and measure. To dis-tinguish the movements of a living organism from the movements of other things,such as a falling leaf or waves rolling onto the seashore, we can add that the move-ments of a living organism have both a cause and a function.

For some animals, most movements are inherited ways of responding; but, forothers, movements entail both inherited and learned patterns of behavior. If all mem-bers of a species display the same behavior under the same circumstances, that specieshas probably inherited a nervous system designed to produce that behavior automati-cally. In contrast, if each member of a species displays a somewhat different responsein a similar situation, that species has inherited a nervous system that is much moreflexible and capable of allowing changes in behavior due to learning.

An example of the difference between a relatively fixed behavior pattern and amore flexible one is seen in the eating behavior of two different animal species —crossbills and roof rats — as illustrated in Figure 1-3. Crossbills are birds with beaks

6 ■ CHAPTER 1

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that seem to be awkwardly crossed at the tips; yet this beak is exquisitely designed toeat certain kinds of pine cones. When eating these pine cones, crossbills use largelyfixed behavior patterns that do not require much modification through learning. If acrossbill’s beak is changed even slightly by trimming, the bird is no longer able to eat.Roof rats, in contrast, are rodents with sharp incisor teeth that appear to be designedto cut into anything. Roof rats are also effective pine-cone eaters, but they can eatpine cones efficiently only if they are taught to do so by an experienced mother. Forroof rats, then, in contrast with crossbills with their relatively fixed action pattern,pine-cone eating is an acquired skill made possible by a flexible nervous system that isopen to learning. The behavior described here is limited to pine-cone eating, and wedo not intend to imply that all behavior displayed by crossbills is fixed or that all be-havior displayed by roof rats is learned. A central goal of research is to distinguish be-tween behaviors that are inherited and those that are learned and to understand howthe nervous system produces each type of behavior.

The complexity of behavior varies considerably in different species, largely de-pending on the degree to which a species is capable of learning and has flexibility inits responses. Generally, animals with smaller, simpler nervous systems have a nar-rower range of behaviors that they can use to react to a situation. Animals with com-plex nervous systems have more behavioral options in any given situation. We hu-mans believe that we are the animal species with the most complex nervous systemand the greatest capacity for learning new responses. Species that have evolved greatercomplexity have not thrown away their simpler nervous systems, however. Rather,complexity emerges in part because new nervous system structures are added to oldones. For this reason, although human behavior depends mostly on learning, we, likeother species, still possess many inherited ways of responding.

In ReviewThe brain consists of two hemispheres, one on the left and one on the right. Each has afolded outer layer called the cortex, which is divided into four lobes: the temporal, thefrontal, the parietal, and the occipital. The brain and spinal cord together make up thecentral nervous system, and all the nerve fibers radiating outward from the spinal cord toother parts of the body compose the peripheral nervous system. Networks of sensory andmotor nerves span these two systems. A simple definition of behavior is any kind ofmovement in a living organism. Although all behaviors have both a cause and a function,behaviors vary in their complexity and the degree to which they depend on learning.

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 7

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A crossbill's beak is specifically designed to open pine cones. This behavior is innate.

A baby roof rat must learn from its mother how to eat pine cones. This behavior is learned.

Figure 1-3

Some animal behaviors are largelyinnate, whereas others are largelylearned. The eating of pine cones bycrossbills is an example of an innatebehavior made possible by this bird’sspecially designed beak, the tips of whichare crossed, as illustrated by the parrotcrossbill on the left. In contrast, Rattusrattus, a roof rat that lives in pine treesin Israel, can become an effective pine-cone eater only if it learns the skill fromits mother. This learning is a form ofcultural transmission.Left: Adapted from The Beak of the Finch(p. 183), by J. Weiner, 1995, New York: Vintage. Right: Adapted from “CulturalTransmission in the Black Rat: Pinecone Feeding,” by J. Terkel, 1995, Advances in the Study of Behavior, 24, p. 122.

PERSPECTIVES ON BRAIN AND BEHAVIOR The central question in the study of brain and behavior is how the two are related.Fred Linge believed that both the behavioral problems that he suffered after his injuryand the gradual recovery that he made in the months that followed were in some wayrelated to changes that took place in his brain. This view has not always been generallyaccepted, however. More than 2000 years ago, Aristotle proposed that somethingcalled the mind, soul, or psyche produces behavior. In this section, we explore Aris-totle’s view and two other influential perspectives on how the brain and behavior arerelated. Certain older ideas that we will consider are still commonly held by contem-porary religions or by people who see them as “common sense.” Knowing the originof these ideas will allow you to see why some of them are useful to the modern scienceof the brain and behavior, whereas others are not.

Aristotle and MindThe hypothesis that the mind, soul, or psyche is responsible for behavior can betraced more than 2000 years to ancient Greece. (The terms mind, soul, and psyche areoften used interchangeably.) In classical mythology, Psyche was a maiden who becamethe wife of the young god Cupid. Venus, Cupid’s mother, opposed his marriage to amortal, and so she harassed Psyche with a number of almost impossible tasks. Psycheperformed the tasks with such dedication, intelligence, and compassion that she wasmade immortal, thus removing Venus’s objection to her. The ancient Greek philoso-pher Aristotle was alluding to this story when he suggested that all human intellectualfunctions are produced by a person’s psyche. The psyche, Aristotle argued, was re-sponsible for life, and its departure from the body resulted in death.

Aristotle’s account of behavior had no role for the brain, which he thought ex-isted to cool the blood. To Aristotle, the psyche was nonmaterial. To him, the psychewas responsible for human thoughts, perceptions, and emotions and for suchprocesses as imagination, opinion, desire, pleasure, pain, memory, and reason. Thepsyche was an entity, or “stuff,” as philosophers call it, that was independent of thebody. Aristotle’s view that a nonmaterial psyche governs our behavior was adopted byChristianity in its concept of the soul and has been widely disseminated throughoutthe Western world.

Mind is an Anglo-Saxon word for memory and, when “psyche” was translatedinto English, it became mind. The philosophical position that a person’s mind, or psy-che, is responsible for behavior is called mentalism, meaning “of the mind.” Mental-ism is not a scientific perspective. Because the mind is nonmaterial, it cannot be stud-ied with the use of scientific methods. Furthermore, most modern scientists believethat, when we understand how the nervous system produces behavior, it will not benecessary to explain behavior by the actions of a mind. But, despite this scientific re-jection of mentalism, terms that are mentalistic — such as sensation, perception, atten-tion, imagination, emotion, motivation, memory, and volition — are still used in psy-chology textbooks today. These terms, however, are usually employed as labels forpatterns of behavior, not in the Aristotelian sense of being products of some nonma-terial entity totally divorced from any part of the body.

Descartes and DualismAristotle’s mentalistic explanation of behavior survived almost unquestioned until the1500s. Then, in his book titled Treatise on Man, the first book on brain and behavior,René Descartes (1596 – 1650), a French physiologist, mathematician, and philosopher,

8 ■ CHAPTER 1

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Aristotle(384–322 BC)

Mind. A nonmaterial entity that is pro-posed to be responsible for intelligence,attention, awareness, and consciousness.

Mentalism. Of the mind; an explanationof behavior as a function of the mind.

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Link to a timeline on the history of brain research at www.worthpublishers.com/kolb/chapter1.

Click on the Web site to read a detailed history of the origins of themind–body question at www.worthpublishers.com/kolb/chapter1.

François Gerard, Psyche and Cupid

(1798).

proposed a new explanation of behavior in which the brain played an important role.Descartes placed the seat of the mind in the brain and linked the mind to the body.He saw mind and body as separate but interconnected. In the first sentence of Treatiseon Man, he stated that people must be composed of both a mind and a body.Descartes wrote:

I must first separately describe for you the body [which includes the brain];then, also separately, the mind; and finally I must show you how these twonatures would have to be joined and united to constitute people. . . .(Descartes, 1664, p. 1)

To Descartes, most of the activities of the body, including sensation, motion, di-gestion, breathing, and sleep, can be explained by the mechanical principles by whichthe physical body and brain work. The mind, on the other hand, is nonmaterial, sepa-rate from the body, and responsible for rational behavior. Figure 1-4, an illustrationfrom Descartes’s book, shows how the mind receives information from the body.When a hand touches a ball, for example, the mind learns through the brain that aball exists, where the ball is located, and what its size and texture are. The mind alsodirects the body to touch the ball, but again it does so through the brain. The mindcan command the brain to make the body carry out a great variety of actions, such asrunning, changing breathing rate, or throwing the ball across the room. The rationalmind, then, depends on the brain both for information and for control of behavior.

The age in which Descartes lived was exciting because scientists and engineerswere making advances in understanding the universe, in constructing mechanical de-vices, and in comprehending the functions of the body. Descartes, who dissected ani-mals that he obtained from butcher shops, was recognized to be among the bestanatomists of his day. He was also aware of the many new machines being built,including clocks, water wheels, and gears. He saw mechanical gadgets on public dis-play in parks, such as those in the water gardens in Paris. One device caused a hiddenstatue to approach and spray water when an unsuspecting stroller walked past it.The statue’s actions were triggered when the person stepped on a pedal hidden in thesidewalk.

Influenced by these mechanical devices, Descartes proposed that the functions ofthe body were produced by similar mechanical principles. For example, he used me-chanical analogies both in describing how we automatically make decisions about dis-tances and angles on the basis of visual information and in trying to explain why,when we look at an object, we are less aware of what surrounds it. He also consideredin detail “mechanical” physiological functions, such as digestion, respiration, and theroles of nerves and muscles.

To explain how the mind controls the body, Descartes suggested that the mind re-sides in a small part of the brain called the pineal body, which is located in the centerof the brain beside fluid-filled cavities called ventricles. According to Descartes, thepineal body directs fluid from the ventricles through nerves and into muscles. Whenthe fluid expands those muscles, the body moves. In Descartes’s theory, then, themind regulates behavior by directing the flow of ventricular fluid to the appropriatemuscles. Note that, for Descartes, mind and body were separate entities and the pinealbody was only a structure through which the mind works.

Descartes’s proposal that an entity called the mind directs a machine called thebody was perhaps the most influential idea ever proposed in philosophy or neuro-science. It was the first serious attempt to explain the role of the brain in controlling in-telligent behavior. The problem of how a nonmaterial mind and a physical brain mightinteract has come to be called the mind – body problem, and the philosophical posi-tion that behavior is controlled by two entities, a mind and a body, is called dualism.

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 9

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René Descartes(1596–1650)

Figure 1-4

Descartes proposed that humans haveboth a mind and a brain. He argued thatthe pineal body in the brain receivesdifferent messages from a hand holdinga flute and from a hand touching a ball.The mind, located in the pineal body,interprets these messages and so learnsabout the flute and ball. From Treatise on Man, by R. Descartes, 1664.Reprint and translation (p. 60), 1972, Cambridge, MA: Harvard University Press.

Ventricles

Pineal body

Mind–body problem. The problem ofhow to explain how a nonmaterial mindcan command a material body.

Dualism. A philosophical position thatholds that both a nonmaterial mind andthe material body contribute to behavior.

Descartes’s theory has many problems in its details and logic. With respect to its de-tails, we now know that people who have a damaged pineal body or even no pineal bodyat all still display normal intelligent behavior. The pineal body has a role in biologicalrhythms, not in governing all of human behavior. Furthermore, we now know that fluidis not pumped from the brain into muscles when they contract. Placing an arm in abucket of water and contracting the arm’s muscles does not cause the water level in thebucket to rise, as it should if the volume of the muscle increased because fluid had beenpumped into it. With respect to its logic, Descartes’s theory was also flawed. There is noobvious way that a nonmaterial entity could influence the body, because doing so wouldrequire the creation of energy, which would violate the laws of physics.

Descartes’s LegacyTo determine if an organism possesses a mind, Descartes proposed two tests: the lan-guage test and the action test. To pass the language test, an organism must use lan-guage to describe and reason about things that are not physically present. The actiontest requires the organism to display behavior that is based on reasoning and is notjust an automatic response to a particular situation. Descartes believed that, even if anengineer made a robot that appeared very human, it could be distinguished from areal human because it would fail these two tests. Descartes also assumed that animalsare unable to pass the tests. A good deal of experimental work today is directed to-ward determining if he was right in this assumption. For example, studies of sign lan-guage taught to apes are partly intended to find out whether apes can describe andreason about things that are not present and so pass the language test.

Descartes’s theory had a number of unfortunate results. On the basis of it, somepeople argued that young children and the mentally insane must lack human mindsbecause they often fail to reason appropriately. We still use the expression “he’s lost hismind” to describe someone who is “mentally ill.” Some proponents of this view alsoreasoned that, if someone lacked a mind, that person was simply a machine and notdue normal respect or kindness. Cruel treatment of animals, children, and the mentallyill was justified by Descartes’s theory. It is unlikely that Descartes himself intended theseinterpretations. He was reportedly very kind to his own dog, named Monsieur Grat.

Without being aware of the source of their ideas, some people still hold a verydualistic notion about the mind and the body, including the brain. For instance, peo-ple still refer to the “mind’s ideals” on the one hand, and the body’s “animal instincts”on the other. John M. Harlow used such language to describe his now-famous brain-injured patient Phineas Gage, who lived a century ago. Gage was a 25-year-old dyna-mite worker who survived an explosion that blasted an iron tamping bar (about a me-ter long and 3 centimeters wide) through the front of his head (Figure 1-5). Gage hadbeen of average intelligence and very industrious and dependable. He was describedas “energetic and persistent in executing all of his plans of operation.” But, after theaccident, his behavior changed completely. As Harlow wrote:

The equilibrium or balance, so to speak, between his intellectual faculties andanimal propensities seems to have been destroyed. He is fitful, irreverent, in-dulging at times in the grossest profanity, manifesting but little deference tohis fellows, impatient of restraint or advice when it conflicts with his desires,at times perniciously obstinate, yet capricious and vacillating, devising manyplans of operation, which are no sooner arranged than they are abandoned inturn for others appearing more feasible. A child in his intellectual capacityand manifestations, he has the animal passions of a strong man. (Blumer andBenson, 1975, p. 153)

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Figure 1-5

When Phineas Gage died in 1861, noautopsy was performed, but his skull waslater recovered. Measurements fromGage’s skull and modern imagingtechniques were used to reconstruct theaccident and determine the probablelocation of the lesion. The frontal cortexof both hemispheres was damaged.From “The Return of Phineas Gage: Clues About the Brain from the Skull of a FamousPatient,” by H. Damasio, T. Grabowski, R. Frank, A. M. Galaburda, and A. R. Damasio, 1994, Science, 20, p. 1102.

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This description embraces the idea that we humans are composed of both a mindthat controls our rational behavior and a body that expresses our animal passions.The description would be more accurate, though perhaps less colorful, without refer-ring to this dualism of mind and body. After all, people are animals, so all of their be-haviors are animal behaviors. Some behaviors are “high minded,” whereas others arelower, or “animalistic.” Furthermore, this appeal to dualism detracts from the mostimportant aspect of Harlow’s findings. Because Gage’s brain damage was in thefrontal lobes, Harlow was providing evidence that the frontal lobes were locations offoresight and planning. “Linking Brain Function to Brain Disease,” above, describesthe beginnings of the important discovery that particular parts of the brain regulatespecific kinds of behaviors.

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 11

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With the growth of cities in Europe in the 1500s, there were

too many mental patients to be accommodated in home

care. The mentally ill were placed in hospitals, monasteries,

and other buildings that had been converted into asylums.

There, patients were little more than prisoners. They lived

under appalling conditions and were often chained and

placed on public view. They were poorly treated in part be-

cause many people at the time accepted Descartes’s belief

that an entity called the mind controls rational behavior.

Having “lost their minds,” these patients were considered lit-

tle more than animals.

At the end of the 1700s, the French Revolution brought

in a new social order and new attitudes toward freedom, in-

cluding the idea that all people were equal before the law,

even those in asylums. The French physician Philippe Pinel

reformed two large asylums in Paris—La Bicêtre and La

Salpêtrière. He released many of the patients and greatly im-

proved conditions there for those too ill to leave. Neverthe-

less, as cities continued to grow, so too did the size of asy-

lums. For example, La Salpêtrière (which was originally a

saltpeter factory) was the size of a small city, housing as

many as 5000 female patients.

When Jean Charcot came to La Salpêtrière Hospital in

the 1860s, he and his staff began to document the symptoms

of the patients. Then, when the patients later died, the scien-

tists examined their nervous systems and correlated the

abnormalities that they found with the patients’ previous

behavior. Charcot described the results in his lectures to

others in the hospital and published them in his textbooks

and journals. Many scientists, attracted by Charcot’s approach,

visited and studied with him, including Sigmund Freud, the

founder of psychoanalysis. They, in turn, began to apply his

method in other countries and in other ways. Many of Char-

cot’s coworkers are well known today for the diseases that

they identified, many of which were named after them.

The method of relating nervous system abnormalities to

behavioral abnormalities contributed greatly to scientific

knowledge. It led to the understanding that an intact brain is

essential for normal behavior. It also showed that, when the

link between symptoms and nervous system pathology is

known, patients can be diagnosed and given more intelli-

gent treatment. At the same time, systematic study linking

brain pathology to behavioral symptoms contributed to the

downfall of Descartes’s view that the pineal body was cen-

tral to the control of behavior.

Linking Brain Function to Brain Disease

Focus on Disorders

Philippe Pinel supervising the unchaining of the insane in LaBicêtre asylum in Paris in 1793.

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Darwin and MaterialismBy the middle of the nineteenth century, the beginnings of another theory of thebrain and behavior were emerging. This theory was the modern perspective of mate-rialism — the idea that rational behavior can be fully explained by the working of thebrain and the rest of the nervous system, without any need to refer to a mind thatcontrols our actions. This perspective had its roots in the evolutionary theories of Al-fred Russel Wallace and Charles Darwin.

Wallace and Darwin independently arrived at the same conclusion — the ideathat all living things are related. Each outlined this view in a paper presented at theLinnaean Society of London in July 1858. Darwin further elaborated on the topic inhis book titled On the Origin of Species by Means of Natural Selection, published in1859. This book presented a wealth of supporting detail, which is why Darwin is men-tioned more often as the founder of modern evolutionary theory.

Both Darwin and Wallace had looked carefully at the structure of plants and ani-mals and at animal behavior. Despite the diversity of living organisms, they werestruck by the number of characteristics common to so many species. For example, theskeleton, muscles, and body parts of humans, monkeys, and other mammals are re-markably similar. These observations led first to the idea that living organisms mustbe related, an idea widely held even before Wallace and Darwin. But, more impor-tantly, these same observations led to Darwin’s explanation of how the great diversityin the biological world could have come from common ancestry. Darwin’s principleof natural selection proposes that animals have traits in common because traits arepassed from parents to their offspring.

Darwin believed that all organisms, both living and extinct, are descended fromsome unknown ancestor that lived in the remote past. In Darwin’s terms, all livingthings are said to have common descent. As the descendants of that original organismspilled into various habitats over millions of years, they developed different structuraland behavioral adaptations that made them suited for specific ways of life. But, at thesame time, they retained many similar traits that reveal their relatedness to each other.Brain cells are one such characteristic common to animal species. Brain cells are anadaptation that emerged only once in animal evolution. Consequently, all brain cellsthat living animals possess are descendants of that first brain cell.

Natural selection is Darwin’s way of explaining how new species evolve and ex-isting species change over time. A species is a group of organisms that can breedamong themselves, but not with members of other species. Individual organismswithin any given species vary extensively in their characteristics, with no two mem-bers of the species being exactly alike. Some are big, some are small, some are fat,some are fast, some are lightly colored, and some have large teeth. Those individualorganisms whose characteristics best help them to survive in their environment arelikely to leave more offspring than are less-fit members. This unequal ability of indi-vidual members to survive and reproduce leads to a gradual change in a species’ pop-ulation, with characteristics favorable for survival in that particular habitat becomingmore prevalent over generations. If some part of a species population then becomesreproductively isolated — say, because it is separated by a physical barrier such as anocean or a mountain range — that subgroup over time could evolve into a newspecies, different from the species from which it originated. For example, imagine achubby primate living in an environment of dense vegetation that is faced with grad-ual drying of the climate so that its environment eventually becomes a savanna withonly scattered trees. Those members of the species that are thinnest and so can runquickly, are lightly colored and so blend with the grass, and have fine teeth, which arebetter for eating insects, will prosper and so leave the most descendants.

12 ■ CHAPTER 1

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Materialism. The philosophical positionthat holds that behavior can be explainedas a function of the nervous system with-out explanatory recourse to the mind.

Common descent. Refers to individualorganisms or families that descend fromthe same ancestor.

Natural selection. Differential successin the reproduction of different pheno-types resulting from the interaction of or-ganisms with their environment. Evolutiontakes place when natural selection causeschanges in relative frequencies of allelesin the gene pool.

Charles Darwin(1809–1892)

Alfred Wallace(1823–1913)

Link to more research about Charles Darwin on the Web site atwww.worthpublishers.com/kolb/chapter1.

Neither Darwin nor Wallace understood the basis of the great variation in plant andanimal species. The underlying principles of that variation were discovered by anotherscientist, Gregor Mendel, beginning about 1857, through experiments that he did withpea plants. Mendel deduced that there are heritable factors, which we now call genes, re-lated to the various physical traits displayed by the species. Members of a species thathave a particular gene or combination of genes will express that trait. If the genes for atrait are passed on to offspring, the offspring also will have the same characteristic. Newtraits appear because genes combine in new ways, because existing genes change or mu-tate, or because new genes are formed. Thus, the unequal ability of individual organismsto survive and reproduce is related to the different genes that they inherit and pass on totheir offspring. By the same token, similar characteristics within or between species areusually due to similar genes. For instance, genes that produce the nervous system in dif-ferent kinds of animal species tend to be very similar to one another.

Darwin’s LegacyDarwin’s theory of natural selection has three important implications for the study ofthe brain and behavior. First, because all animal species are related, so too must betheir brains. Darwin himself did not say much about this topic, but contemporaryscientists do. Today, brain researchers study a wide range of animals, including slugs,fruit flies, rats, and monkeys, knowing that they can often extend their findings to hu-man beings. Second, because all species of animals are related, so too must be theirbehavior. Darwin was particularly interested in this subject. In his book titled On theExpression of the Emotions in Man and Animals, he argued that emotional expressionsare similar in humans and other animals because we inherited these expressions froma common ancestor. Evidence for such inheritance is illustrated in Figure 1-6, whichshows that smiling is common to people throughout the world. That people in differ-ent parts of the world display the same behavior suggests that the trait is inheritedrather than learned. The third implication of Darwin’s theory is that both the brainand behavior were built up bit by bit in animals that evolved to greater complexity, ashumans obviously did. Later in this chapter, we will trace how the human nervoussystem evolved from a simple net of nerves, to a spinal cord connected to that net,and finally to a nervous system with a brain that controls behavior.

Evidence that the brain controls behavior is today so strong that the idea has thestatus of a theory: the brain theory. Donald O. Hebb in his influential book titled TheOrganization of Behavior, published in 1949, described the brain theory as follows:

Modern psychology takes completely for granted that behavior and neuralfunction are perfectly correlated, that one is completely caused by the other.There is no separate soul or life force to stick a finger into the brain now andthen and make neural cells do what they would not otherwise. (Hebb, 1949,p. xiii)

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 13

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Figure 1-6

Darwin proposed that emotionalexpression is inherited. Part of theevidence supporting this suggestion isthe finding that people from all parts ofthe world use the same emotionalexpressions that they also recognize inothers, as is illustrated by these smiles.

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Some people reject the idea that the brain is responsible for behavior, because theythink it denies religion. Their thinking, however, is mistaken. The biological explanationof brain and behavior is neutral with respect to religious beliefs. Fred Linge, the brain-injured man described in the beginning of this chapter, has strong religious beliefs, asdo the other members of his family. They used their religious strength to aid in his re-covery. Yet, despite their religious beliefs, they realized that Linge’s brain injury was thecause of his change in behavior and that the process of recovery that his brain under-went was the cause of his restored health. Similarly, there are many behavioral scientistswith strong religious beliefs who see no contradiction between those beliefs and theiruse of the scientific method to examine the relations between the brain and behavior.

In ReviewWe have considered three perspectives on how behavior arises. Mentalism is the viewthat behavior is a product of an intangible entity called the mind, implying that the brainhas little importance. Dualism is the notion that the mind acts through the brain to pro-duce language and rational behavior, whereas the brain alone is responsible for the“lower” kinds of actions that we have in common with other animal species. Finally,materialism is the view that all behavior, language and reasoning included, can be fullyaccounted for by brain function. Materialism is the perspective that guides contemporaryresearch on the brain and behavior.

THE EVOLUTION OF BRAIN AND BEHAVIORThe popular interpretation of human evolution is that we are descended from apes. Ac-tually, apes are not our ancestors, although we are related to them. To demonstrate thedifference, consider the following story. Two people named Joan Campbell were intro-duced at a party, and their names afforded a good opening for a conversation. Althoughboth belonged to the Campbell lineage (family line), one Joan was not descended fromthe other. The two women lived in different parts of North America. One was from Texasand the other was from Ontario, and both their families had been in those locations formany generations. But, after comparing family histories, the two Joans discovered thatthey had ancestors in common. The Texas Campbells were descended from Jeeves Camp-bell, brother of Matthew Campbell, from whom the Ontario Campbells were descended.Jeeves and Matthew had both boarded the same fur-trading ship when it stopped for wa-ter in the Orkney Islands north of Scotland before sailing to North America. The JoanCampbells’ common ancestors, then, were the mother and father of Jeeves and Matthew

14 ■ CHAPTER 1

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Common ancestor. An ancestor fromwhich two or more lineages or familygroups arise and so is ancestral to bothgroups.

Origin of

earth

4500

Firstsingle-celled

animals

Originof life

3500 2500

Millions of years ago

PRECAMBRIAN

Campbell. Both the Texas Campbell family line and the Ontario one were descendedfrom this same man and woman. In much the same way, humans and apes are descendedfrom common ancestors. But, unlike the Joan Campbells, we do not know who those dis-tant relatives were. By comparing the characteristics of humans and related animals,however, scientists are tracing our lineage back farther and farther. In this way, they canpiece together the story of the origin of the human brain and behavior.

Origin of Brain Cells and BrainsThe brain and brain cells go back a very long time. The earth originated about 4500million years ago, and the first life forms arose about 3500 million years in the past.About 700 million years ago, animals evolved the first brain cells, and, by 250 millionyears ago, the first brain had evolved. A humanlike brain, however, first developedonly about 3 million to 4 million years ago, and our modern human brain has beenaround for only the past 100,000 to 200,000 years. As evolutionary history goes, that isa rather short amount of time. Although life evolved very early in the history of ourplanet, brain cells and the brain are more recent adaptations, and large complexbrains, such as ours, evolved only very, very recently. Figure 1-7 shows this evolution-ary time line.

Classification SystemsSince the first appearance of a living organism, the diversity of life on earth has beenenormous. Millions of species have evolved, some of which have become extinct. Asmany as 30 million to 100 million species currently inhabit the planet. Scientists havedescribed only a small number of these species, about 1.5 million. The rest remain tobe found, named, and classified.

Taxonomy, the branch of biology concerned with naming and classifying species,groups organisms according to their common characteristics and their relationshipsto one another. As shown in Figure 1-8, the broadest unit of classification is a king-dom, with more subordinate groups being a phylum, class, order, family, genus, andspecies. We humans belong to the animal kingdom, the chordate phylum, the mam-malian class, the primate order, the Hominidae family, the Homo genus, and the sapi-ens species. Animals are usually identified with a first and second name, their genusand species name. So we humans are called Homo sapiens, meaning “wise humans.”

This hierarchy of categories helps us trace the evolutionary history of our humanbrain and behavior. Brain cells and muscles first evolved in animals; the brain as anorgan first evolved in chordates; a large brain with many different functions firstevolved in mammals; a brain capable of producing complex tools first evolved in

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 15

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Firstsimple nervous

systems

Firstbrains

Firsthumanbrains

1500 500 0

PALEOZOIC MESOZOIC

CENOZOIC

Figure 1-7

Relative to the origin of the earth 4500million years ago, the first single-celledanimals 3500 million years ago, theorigin of neurons 700 million years ago,and the first brain about 250 millionyears ago, the appearance of the firsthuman brain within about the past 4million years is a very recent event.

Taxonomy. The branch of biology con-cerned with naming and classifying thediverse forms of life.

apes; and a brain capable of language and culture first evolved in Homo sapiens. Al-though the most complex brain and patterns of behavior have evolved in the humanlineage, large brains and complex behaviors have also evolved in some other lineages.Some birds, such as the Galápagos woodpecker finch, use simple tools, and dolphinshave surprisingly large brains.

Evolution of Animals with Nervous SystemsA nervous system is not essential for life. In fact, most organisms in both the past andthe present have done without one. Of the five major taxonomic kingdoms, only onecontains species with nervous systems. Figure 1-9 shows these five kingdoms in achart called a cladogram (from the Greek word clados, meaning “branch”). A clado-gram displays groups of organisms as branches on a tree in such a way that branch

16 ■ CHAPTER 1

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Species: Modern human

Genus: Human

Family: Hominidae

Order: Primates

Class: Mammals

Phylum: Chordates

Kingdom: Animals

Living organisms

Characteristics: Neurons and muscles used forlocomotion

Characteristics: Brainand spinal cord

Characteristics: Large brains and social behavior

Characteristics: Visual control of hands

Characteristics: Tool use

Characteristics: Language

Characteristics: Complexculture

Figure 1-8

Taxonomy classifies animals into groupssubordinate to more comprehensivegroups. (From bottom to top) Modernhumans are the only surviving species ofthe genus that included numerousextinct species of humanlike animals.Humans belong to the ape (Hominidae)family, which includes a number of livingmembers, among them chimpanzees.Hominidae are but one of many familiesof the primate order. The primates aremembers of the class of mammals, whichin turn belongs to the chordate phylum,which in turn belongs to the animalkingdom. A characteristic feature ofanimals is a nervous system, and inchordates the nervous system includes abrain and spinal cord. A distinctivefeature of mammals is a large brain, and,among the mammals, the primates are aparticularly large-brained order, with thelargest brains being found in humans.

Galápagos woodpecker finch

Cladogram. A phylogenetic tree thatbranches repeatedly, suggesting a classification of organisms based on thetime sequence in which evolutionarybranches arise.

order represents how the groups are related. The five kingdoms shown are: Monera(simple cells, such as bacteria), Protista (more complex cells, such as protozoa), Plan-tae (plants), Fungi, and Animalia (animals). Looking at this chart tells you at a glancehow uncommon the nervous system is in the living world: it is found only in animals.You can also see that neurons are associated with muscles, suggesting that the func-tions of both were to enable movement. This cladogram also reinforces our earlierpoint that the nervous system is a late evolutionary development. Life first appearedwith Monera, and it thrived for millions of years with nothing but single-celled or-ganisms. The first multicellular organisms arose in Plantae and Fungi. Brain cells, ner-vous systems, and muscles did not appear in Animalia until much later. They arestructures that life did without for eons.

Brain cells, nervous systems, and muscles are what give animal species their char-acteristic feature: the ability to move. This ability has enabled animals to occupy manydifferent biological niches, where they use their movements for food gathering, repro-duction, and self-preservation. The animal kingdom contains a great many species.Taxonomists have so far identified about 1 million animal species and organized theminto 15 phyla (phyla is the plural of phylum).

Figure 1-10 shows the evolution of the nervous system in animal phyla. The nervoussystem in species in older phyla, such as jellyfishes and sea anemones, is extremely sim-ple. It consists of a nerve net, with no structure that resembles a brain. (The net looks alittle like a human nervous system from which the brain and spinal cord have been re-moved). Species in somewhat more recent phyla, such as flatworms, are more complexlystructured. These organisms have heads and tails, are bilaterally symmetrical (one-halfof the body is the mirror image of the other), and are segmented (the body is composedof similarly appearing segments). They also have segmented nervous systems that re-semble the human nervous system, with sensory and motor neurons projecting fromeach segment. Bilateral symmetry and segmentation are two important structural features of the human nervous system. For example, just as our body is bilaterally

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 17

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Commonancestor

Animalia(animals)

Monera(bacteria)

Protista(single cells)

Plantae(plants)

Fungi(fungi)

True cells (nuclei and organelles)

Muscles and neurons

Multicells

Brain cells, nervous systems, and muscles first evolved in animals.

Figure 1-9

In this cladogram of the five kingdoms ofliving organisms, the line connecting thekingdoms represents the sequence inwhich the kingdoms evolved. Animalsare members of the most recentlyevolved kingdom, and theircharacteristics include brain cells, nervoussystems, and muscles.

Bilateral symmetry. Refers to organs orparts that are present on both sides of thebody and very similar in appearance oneach side (For example, the hands are bi-laterally symmetrical, whereas the heart isnot.)

Segmentation. Refers to animals thatcan be divided into a number of parts thatare similar; also refers to the idea thatmany animals, including vertebrates, arecomposed of similarly organized bodysegments.

symmetrical, our brain has two bilaterally symmetrical hemispheres. The brain itself evolved by growth in the most anterior (front) segments of the nervous system.

Species in still more recently evolved phyla, such as clams, snails, and octopuses, havean additional feature in their nervous systems: collections of neurons called ganglia (sin-gular, ganglion), some of which are in the region of the animal’s head. Ganglia resemble abrain and function somewhat like one, but, unlike a brain, they are not a central struc-ture coordinating all of the animal’s behavior. Only species in one phylum, the chor-dates, have a true spinal cord and brain. Chordates get their name from the notochord, aflexible rod that runs the length of the back. In humans, the notochord is present only inan embryo; by birth, it has been replaced by vertebrae that encase the spinal cord.

There are several important differences between the chordate nervous system andthe nervous system in older phyla. Whereas the chordate nervous system is locatedalong the back, in other animals the nervous system is located below the gut. In addi-tion, the brain is a central processing structure in the chordate nervous system. It is lo-cated in the animal’s head (the body part that arrives first as the animal moves). It is alsoin close proximity to the animal’s many sensory-receptor systems, such as those for vi-sion, hearing, taste, and smell. This positioning of the brain makes it quick to receiveand respond to sensory information as the animal travels from one place to another.

The Chordate Nervous SystemAlthough much variation exists in the nervous systems of chordates, the basic pattern of abrain attached to a spinal cord is common to all of them. This pattern is found even in theearliest chordate species. Figure 1-11 shows seven of the nine classes of chordates to whichthe approximately 38,500 chordate species belong. In each class, the nervous system consists

18 ■ CHAPTER 1

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Commonancestor

Sea anemone

Flatworm Squid Frog

Nerve net: Simple nervous system, organized as a net,with no brain

Segmented nerve trunk: Bilaterally symmetricalorganization

Brain: True brain andspinal cord

Ganglia: Structures that resemble and function somewhat like a brain

Ganglia

Figure 1-10

This cladogram illustrates theevolutionary relationship of the 15animal phyla, showing the evolution ofthe nervous system from a nerve net, toa segmented nervous system, to anervous system consisting of ganglia andnerve trunks, and finally to a nervoussystem featuring a brain.

of a brain and a spinal cord as well as sensory and motor connections, but, in each class,there is an enormous range of brain sizes. Distinguishing features of chordates are limbs forlocomotion along with some very large-brained species, especially the birds and mammals.

The relative difference in brain size in different classes of chordates is illustratedin Figure 1-12, which shows representative brains of a fish, an amphibian, a bird, anda mammal (in this case, a human). The cerebrum, of which the cerebral cortex is theexternal structure (described earlier as being folded and covering most of the rest ofthe brain in humans), is proportionately small and smooth in the earliest-evolvedchordate shown in Figure 1-12. In later-evolved chordates with larger brains, the cor-tex is disproportionately larger and begins to cover other structures. Finally, in large-brained mammals, the cortex is folded. By folding, the cortex is able to greatly in-crease its size while still fitting into a small skull (just as a folded piece of paper canoccupy a small container). Toward the back of the brain, a structure called the cere-bellum (which means “little brain” in Latin) also has increased in size, taking on theappearance of a little cortex. Figure 1-12 shows the human brain cut longitudinallythrough its center to illustrate how the cortex has grown to cover the rest of the brain,including the cerebellum. The evolution of more complex behavior in chordates isclosely related to the evolution of both the cerebral cortex and the cerebellum. Thelarge differences in brain size that exist in the 4000 species of mammals are duemainly to differences in the size of these two brain regions.

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 19

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Commonancestor

Agnatha(lampreys and hagfish)

Mammalia(mammals)

Aves(birds)

Reptilia(reptiles)

Amphibia(frogs and salamanders)

Limbs

Large brains

Osteichthyes(bony fishes)

Chondrichthyes(sharks and rays)

Figure 1-11

This cladogram illustrates theevolutionary relationship betweenclasses of chordates, animals having abrain and a spinal cord. Brain sizeincreased with the development of limbsin amphibia. Birds and mammals are themost recently evolved chordates, andlarge brains are found in both classes.

Fish Frog Human

Cerebrum Cerebellum Cerebrum Cerebellum Cerebrum Cerebellum Cerebrum Cerebellum

Bird

Figure 1-12

The brains of representative chordateshave many structures in common. Theseside views of the brains of some repre-sentative chordates show that the cere-brum and cerebellum account for mostof the increase in brain size. The humanbrain has been cut through the center toillustrate how the cerebral cortex enfoldsthe rest of the brain. The similar parts ofthe brain found in these diverse animalspecies also illustrate that there is a basicbrain plan.

Turn on the CD to view a close-up of the cerebellum in the module on theCentral Nervous System in the section onthe brainstem and subcortical structures.

In ReviewBrain cells and the nervous system are relatively recent developments in the evolution oflife on this planet. Because they evolved only once in the animal kingdom, a similarbasic nervous system pattern exists in all animals. The nervous system did become morecomplex in certain groups of animals, and this increase in complexity closely parallelsincreasingly complex behavior. Particular lineages of animals, such as the lineage towhich humans belong, are characterized by especially large brains and complex behav-ior. These evolutionary developments are closely tied to the growth of the cerebral cortexand cerebellum.

HUMAN EVOLUTIONAlthough everyone can see similarities among humans, apes, and monkeys, manypeople once believed that humans are far too different from monkeys and apes tohave had a common ancestor with them. These skeptics also reasoned that the ab-sence of a “missing link,” or intermediate form of ancestor, further argued against thepossibility of common descent. In the past century, however, so many intermediateforms between humans and other apes have been found in the fossil record that entirebooks are required to describe them. Here we consider only some of the more promi-nent ancestors that link apes to ourselves.

Humans: Members of the Primate OrderThe human relationship to apes and monkeys places us in the primate order, a subcat-egory of mammals that includes not only apes and monkeys, but lemurs, tarsiers, andmarmosets as well. In fact, we humans are only 1 of about 275 species in the primateorder, some of which are illustrated in Figure 1-13. Primates have excellent vision —including color vision and eyes in the front of the face to enhance depth perception —

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Commonancestor

Apes

HumansChimpanzeesLemursand lorises

GorillasOrangutansGibbonsOld Worldmonkeys

New Worldmonkeys

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Figure 1-13

This cladogram illustrates a hypotheticalrelationship between the members ofthe primate order. Humans are membersof the family of apes. In general, brainsize increases across the groupings, withhumans having the largest brains.

Visit the Web site for links to a tutorial about human evolution atwww.worthpublishers.com/kolb/chapter1.

and they use this excellent vision to deftly guide their hand movements. Female pri-mates usually have only one infant per pregnancy, and they spend a great deal moretime caring for their young than most other animals do. Associated with their skillfulmovements and their highly social nature, primates on average have larger brainsthan animals in other orders of mammals, such as rodents (mice, rats, beavers, squir-rels) and carnivores (wolves, bears, cats, weasels).

Humans are members of the suborder apes, which in addition to us includes gib-bons, orangutans, gorillas, and chimpanzees. Apes are arboreal animals, with limbershoulder joints that allow them to brachiate (swing from one handhold to another) intrees. Among the apes, we are most closely related to the chimpanzee, having had acommon ancestor between 5 million and 10 million years ago. The family to whichhumans belong is called Hominidae. In the past 5 million years, there have been manyhominids, or humanlike animals that are members of the Hominidae family. Someextinct hominid species lived at the same time as one another. At present, however, weare the only surviving hominid species.

Australopithecus: Our Distant AncestorOne of our hominid ancestors is probably Australopithecus (Australo meaning“southern,” pithecus meaning “ape”) or a primate very much like it. A reconstructionof what the animal looked like is shown in Figure 1-14. The name Australopithecuswas coined by an Australian, Raymond Dart, for the skull of a child that he found in abox of fossilized remains from a limestone quarry near Taung, South Africa, in 1924.(The choice of a name to represent his native land is probably not accidental.) Wenow know that there were many species of Australopithecus, some of which existed atthe same time. The skull of the “Taung child” did not belong to the earliest species,which lived about 4 million years ago.

These early australopiths are the first primates to show a distinctly human charac-teristic: they walked upright. Scientists have deduced their upright posture from the shape of their back, pelvic, knee, and foot bones and from a set of 3.6-million- to3.8-million-year-old fossilized footprints that a family of them left behind when walk-ing through freshly fallen ash from a volcano. The footprints feature a well-developedarch and a big toe that was more like that of humans than of apes. The most completeAustralopithecus skeleton yet found is that of a young female, popularly known as“Lucy.” This skeleton was 40 percent complete even after having been buried for 3 mil-lion years. Lucy was only about 1 meter tall and had a brain about the size of a modernchimpanzee’s brain, which is about one-third the size of a modern human brain.

The evolutionary sequence from Australopithecus to humans is not known pre-cisely, in part because there were a number of species of Australopithecus alive at thesame time. One possible lineage is shown in Figure 1-15. A common ancestor gaverise to Australopithecus, and one member of this group gave rise to the Homo lineage.The last of the australopith species disappeared from the fossil record about 1 millionyears ago after coexisting with hominids for some time. Also illustrated in Figure 1-15is the large increase in brain size that evolved in the hominid lineage.

The First HumansThe oldest fossils to be designated as Homo, or human, are those found by Mary andLouis Leakey in the Olduvai Gorge in Tanzania in 1964, dated at about 2 million years.The primates that left these skeletal remains had a strong resemblance to Australopithecus,from which they were thought to descend. But Mary Leakey argued that their dental pat-tern is more similar to that of modern humans than to that of australopiths and, more

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Hominid. General term referring to pri-mates that walk upright, including allforms of humans, living and extinct.

Figure 1-14

Reconstruction of Australopithecus, oneof the oldest known species in thehominid family, to which modern humansbelong. Australopithecus walked uprightwith free hands, as do modern humans,but its brain was the size of an ape’s,about one-third the size of our brain.

important, that they apparently made simple stone tools, which were found near theirbones. The Leakeys named the species Homo habilis (meaning “handy human”) to sig-nify that its members were tool users. Again, the precise relationships in the Homo lineageare not known, because there were a number of species of Homo alive at the same time.

The first humans whose populations spread beyond Africa migrated into Europeand into Asia. This species was Homo erectus (“upright human”), so named becauseof the mistaken notion that its predecessor, Homo habilis, had a stooped posture.Homo erectus first shows up in the fossil record about 1.6 million years ago and lastsuntil perhaps as recently as 100,000 to 30,000 years ago. Homo erectus has a pivotalposition in our evolutionary history. Its brain was bigger than that of any previoushominid, overlapping in size the measurements of present-day human brains. Homoerectus also made more sophisticated tools than did Homo habilis.

Modern humans, Homo sapiens, appeared in Asia and North Africa about 200,000 to100,000 years ago and in Europe about 40,000 years ago. Most anthropologists think thatthey migrated from Africa. Until about 60,000 years ago, perhaps even as recently as30,000 years ago, they coexisted with other hominid species in Africa, Europe, and Asia.For example, in Europe they coexisted with Neanderthals, named after Neander, Ger-many, where the first Neanderthal skulls were found. Neanderthals had brains as large asor larger than those of modern humans, used tools similar to those of early Homo sapi-ens, and possibly had a similar hunting culture. We do not know how Homo sapiens com-pletely replaced other human species, such as Neanderthals. Homo sapiens may have beenmore aggressive and killed off competing species. Or they may have been more skilled attoolmaking and so were better at getting food. Or perhaps the various human species in-terbred so completely that characteristics of all but ourselves simply disappeared.

The Evolution of the Human BrainScientists who study the evolution of the brain propose that a relative increase in thesize and complexity of the brain in different species is what enabled the evolution ofmore complex behavior. In this section, we consider the relation between brain sizeand behavior across different species. We also consider a number of hypotheses abouthow the human brain became so large.

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Figure 1-15

The origins of humans. The humanlineage and a lineage of extinctAustralopithecus likely arose from acommon ancestor about 4 million yearsago. Thus the ancestor of the humanlineage Homo was likely an animalsimilar to Australopithecus africanus. Theprobable sequence of human evolutionwas from Homo habilis to Homo erectusto Homo sapiens. Connecting lines arenot shown, because a number of speciesof each group were alive at the sametime. Homo neanderthalensis isconsidered a subspecies of Homo sapiens,but how closely they were related isunclear. There was a very large increasein brain size in this proposed lineage.

“Lucy”Homo sapiens

AFRICA

BRAIN SIZE AND BEHAVIORIn his book titled The Evolution of the Brain and Intelligence, published in 1973, H. J.Jerison uses the principle of proper mass to sum up the relation between increasedsize of the nervous system and increased complexity of behavior. This principlestates that the amount of neural tissue responsible for a particular function is equiv-alent to the amount of processing that the function requires. So, as behaviors be-come more complex and require more neural processing, a greater amount of neuraltissue must be allocated to them. It follows that species exhibiting more complex behaviors will possess relatively larger brains than those of species whose behaviorsare simpler.

Jerison found support for the principle of proper mass by comparing brain sizein a wide variety of animal species. Jerison calculated that, as body size increases, thesize of the brain increases at about two-thirds the increase in body weight. With theuse of this formula, plus an average brain-volume-to-body-weight ratio as a base, it ispossible to determine the expected brain size for a mammal of any given weight. Thisexpected brain size is plotted in the diagonal line shown in Figure 1-16, a graph inwhich body size is on the x-axis and brain size on the y-axis. The diagonal line repre-sents the expected increase in brain size as body size increases. The polygon sur-rounding the diagonal line encompasses the brain and body sizes of all mammals. Theactual brain and body sizes of a number of representative animals also are plotted onthis graph. Animals that lie below the diagonal line have brains that are below averagefor an animal of that size, whereas animals that lie above the diagonal line have brainsthat are larger than expected for an animal of that size. Notice that the rat has a brainthat is a little smaller and the elephant a brain that is a little larger than expected. No-tice also that a modern human is located farther to the upper left than any other ani-mal, indicating a brain that is relatively larger for its body size than that of any otheranimal.

To further illustrate the relative sizes of brains, Jerison developed a numerical system that eliminates body weight as a factor and adjusts brain sizes according to a

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 23

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Principle of proper mass. The ideathat complex behavioral functions areproduced by a larger brain or brain regionthan that in which simple behavioral functions are produced; usually used torefer to the idea that the brain size of an animal species is proportional to its behavioral complexity.

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The position of the modern human brain, at the farthest upper left, indicates it has the largest relative brain size.

Deviation from the diagonal line indicates either larger (above) or smaller (below) brain size than average, relative to body weight.

The average brain size relative to body weight is located along the diagonal line.

Figure 1-16

Brain and body sizes of some commonmammals. The axes are in logarithmicunits to represent the wide range ofbody and brain sizes. The polygonincludes the brain and body sizes of allmammals. The line through the polygonillustrates the expected increase in brainsize as body weight increases. Thelabeled dots show the brain and bodysizes of a number of representativespecies. Animals that lie above thediagonal line have brain sizes that arelarger than would be expected for ananimal of that size. Modern humanshave the largest brain relative to bodysize of all mammals. Adapted from The Evolution of the Brain andIntelligence (p. 175), by H. J. Jerison, 1973, NewYork: Academic Press.

scaling factor. He calls this system an encephalization quotient (EQ). Figure 1-17(top) lists the EQs for several common animals. Notice that a rat has an EQ aboutone-half that of a cat, which is representative of the average mammal. Interestingly, acrow has an EQ similar to that of a monkey, and a dolphin has a very large EQ indeed.People who study crows would agree that they are intelligent, whereas people whostudy dolphins, although admitting that they are intelligent social creatures, are stilluncertain about the reason for their need of such a large brain. Figure 1-17 (bottom)lists the EQs for a number of species in the human lineage. Clearly, we modern humans are descended from a lineage of large-brained animals, but we have thelargest brain.

Underlying the principle of proper mass is the idea that a larger brain is neededfor increasingly complex behavior. Although it is not always easy to compare the com-plexity of behavior in different animal species, there are some fairly obvious examplesof more complex behaviors that correlate with greater brain size. These examples areseen as we progress up the chordate ladder from older to more recent groups of ani-mals. For instance, among the older chordates, cyclostomes, such as the lamprey,move by making snakelike, side-to-side body movements, whereas the more recentfish have fins with which to move, in addition to using these lateral movements of thebody. Fish, significantly, have relatively larger brains than those of cyclostomes. Finsevolved into limbs in amphibians, and these limbs are used in a more complex waythan fins are to enable walking on land. Amphibians, significantly, have relativelylarger brains than those of fish. Birds and mammals use their limbs for still morecomplex movements, both for locomotion and for handling objects. Birds and mam-mals, significantly, have relatively larger brains than those of amphibians. Being a pri-mate is associated with many other behavioral innovations. Primates engage in com-plex social and sexual behavior, as well as complex feeding habits, care of their young,defense of their territories, and, in regard to humans, the use of language. Primates,significantly, have relatively larger brains than other mammals do.

Why is there this trend toward greater brain size and behavioral complexity inchordates? Why, in the course of evolutionary history, did animals not retain verysimple nervous systems? A number of factors have created relatively constant oppor-tunities and pressures for animals to modify their behavior and thus their nervoussystems. For example, because the first animals were relatively simple, they left a widevariety of more complex behaviors available as potential means of gaining a survivaladvantage. Filling these behavioral niches was often favored through natural selection.In addition, various cataclysmic events have wiped out large numbers of species,changing the environment and offering opportunities for new evolutionary adapta-tions. For example, a comet that struck the earth about 60 million years ago probablyled to the dinosaurs’ extinction and opened up new opportunities for the rapid evolu-tion of mammals. There have also been many changes in the earth’s landmass and cli-mate that have challenged animals to evolve in order to adapt and survive. As a resultof all these factors, behavioral and structural complexity has grown.

WHY THE HOMINID BRAIN ENLARGEDThe evolution of modern humans—from the time when humanlike creatures first ap-peared until the time when humans like ourselves existed—took about 5 million years.As illustrated by the relative size differences of skulls presented in Figure 1-18, much ofthis evolution entailed a change in brain size, which was accompanied by changes in be-havior. There was a nearly threefold increase in brain size from apes (EQ 2.5) to mod-ern humans (EQ 7.0). What caused this substantial growth of the brain? Most likely, the

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Figure 1-17

The EQ (encephalization quotient) of(top) some common animals and (bottom)members of the human lineage.

Encephalization quotient (EQ). Ameasure of brain size obtained from theratio of actual brain size to the expectedbrain size for an animal of a particularbody size.

Sea lamprey

appearance of each new hominid species was associatedwith climate changes that produced new environmentsthat isolated populations of existing hominids and pro-duced a rapid selection of traits that were adaptive for thenew environment.

The first of these climate changes was triggeredabout 8 million years ago. Before that time, most ofAfrica was a rich forest inhabited by monkeys and apes,as well as other animal species. Then a massive tectonicevent (a deformation of the earth’s crust) produced theGreat Rift Valley, which runs from south to north acrossthe African continent. This reshaping of the Africanlandmass left a wet jungle climate to the west and a muchdrier climate to the east. To the west, the apes continuedunchanged in their former habitat, but, in the drier re-gion to the east, apes had to evolve rapidly to adapt to the mixture of tree-covered andgrassy regions that formed their new home. An upright posture is an efficient means ofrapid locomotion across grass-covered areas. Such an upright posture may haveevolved in Australopithecus because these animals were forced to spend more time onthe ground moving between clumps of trees. In addition, an upright posture may havehelped to regulate body temperature by reducing the amount of the body’s surface di-rectly exposed to the sun.

Just before the appearance of Homo habilis, the African climate changed again,becoming drier, with increasing amounts of grassland and even fewer trees. Anthro-pologists speculate that a group of hominids that evolved into Homo habilis adaptedto this new habitat by becoming scavengers on the dead of the large herds of grazinganimals that then roamed the open grasslands. The appearance of Homo erectus mayhave been associated with further change in climate that opened up land bridges intoEurope and Asia. At the same time, the new hominids added hunting to their behav-ioral skills and were constantly upgrading the quality of their tools for killing, skin-ning, and butchering animals. Archeologists think there were a number of migrationsof hominids from Africa into other parts of the world, with modern humans beingthe last of these migrants. Each of the migrations may have been forced by changes inclimate that altered the habitat to which the animals residing there were adapted. Inany case, modern humans eventually replaced all other species of hominids every-where on earth.

At least three factors are thought to be related to the development of a largerbrain as our species evolved. The first is the primate life style, which favored a morecomplex nervous system. The second is the development of a new way of cooling alarger brain mass. And the third is neoteny, a process by which maturation is delayedand an adult retains infant characteristics.

The Primate Life Style That the primate life style favors a larger brain can be illus-trated by examining how primates forage for food. Foraging is a very important activ-ity for all animals, but some foraging activities are quite simple, whereas others aremore complex. Eating grass or vegetation is not an especially difficult task; if there islots of vegetation, an animal need only munch. Vegetation eaters do not have espe-cially large brains. Among the apes, gorillas, which are mainly vegetation eaters, haverelatively small brains. In contrast, apes that eat fruit, such as chimpanzees, have rela-tively large brains. The relation between fruit foraging and larger brain size can beseen in a study by Katharine Milton (1993), who examined the feeding behavior and

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Figure 1-18

In the course of human evolution, therelative size of the brain increasedthreefold, as illustrated here by acomparison between Australopithecusafarensis (left), Homo erectus (center),and a modern human. The part of theAustralopithecus skull shown in blue is missing.From The Origin of Modern Humans (p. 165), by R. Lewin, 1998, New York: Scientific AmericanLibrary.

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brain size of two South American (New World) monkeys that have the same bodysize — the spider monkey and the howler monkey. As is illustrated in Figure 1-19, thespider monkey obtains 72 percent of its nutrients from eating fruit and has a brainthat is twice as large as that of the howler monkey, which obtains only 42 percent ofits nutrients from fruit.

What is so special about eating fruit that favors a larger brain? The answer is notthat fruit contains a brain-growth factor, although fruit is a source of sugar, on whichthe brain depends for energy. The answer is that foraging for fruit is a difficult, com-plex activity. Unlike plentiful vegetation within easy reach on the ground, fruit growson trees, and only on certain trees, in certain seasons. There are many kinds of fruit,some better for eating than others, and many different animals and insects competefor a fruit crop. Moreover, after a fruit crop has been eaten, it takes time for a newcrop to grow. Each of these factors poses a challenge for an animal that eats mostlyfruit. Good sensory skills, such as color vision, are needed to recognize ripe fruit in atree, and good motor skills are required to reach and manipulate it. Good spatial skillsare needed to navigate to trees that contain fruit. Good memory skills are required toremember where fruit trees are, when the fruit will be ripe, and in which trees thefruit has already been eaten. Fruit eaters have to be prepared to deal with competitors,including members of their own species, who also want the fruit. To keep track ofripening fruit, it also benefits a fruit eater to have friends who can help search. As a re-sult, successful fruit-eating animals tend to have complex social relations and a meansof communicating with others of their species. In addition, it is very helpful for a fruiteater to have a parent who can teach fruit-finding skills, so it is useful to be both agood learner and a good teacher. Each of these abilities requires the evolution of newbrain areas or more brain cells in existing brain regions. Added up, more brain cellsproduce a larger brain.

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Spider monkey diet

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A howler monkey, with a brain size of 50 g, obtains only42 percent of its nutrients from fruit.

A spider monkey, with a brain sizeof 107 g, obtains72 percent of its nutrients from fruit.

Figure 1-19

Katharine Milton examined the feedingbehavior and brain size of two SouthAmerican (New World) monkeys thathave the same body size. She found thatthe spider monkey obtains 72 percent ofits nutrients from eating fruit and that ithas a brain that is twice the size of thatof the howler monkey, which obtainsonly 42 percent of its nutrients from fruit.

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We humans are fruit eaters and we are descended from fruit eaters, so we are de-scended from animals with large brains. In our evolution, we also exploited and elabo-rated fruit-eating skills to obtain other temporary and perishable food items as wescavenged, hunted, and gathered. These new food-getting efforts required navigatingfor long distances, and they required recognition of a variety of food sources. At thesame time, they required making tools for digging up food, killing animals, cuttingskin, and breaking bones. These tasks also required a good deal of cooperative behav-ior. The elaboration of all of these skills necessitated new brain areas or more braincells in existing brain regions. Added up, more brain cells produce an even larger brain.

The Radiator Hypothesis An event that may have given a special boost to greaterbrain size in our human ancestors was a new form of brain cooling. Dean Falk (1990), aneuropsychologist who studies brain evolution, came up with this idea from somethingthat her car mechanic told her. He said that, to increase the size of a car’s engine, youhave to also increase the size of the radiator that cools it. Consequently, Falk proposedthe radiator hypothesis, the theory that a change in blood flow around the brain in-creases the rate at which the brain is cooled, thus allowing the brain to become larger.

Why is brain cooling so important? The answer is the tremendous amount ofwork done by a human brain. Although your brain makes up less than 2 percent ofyour body, it uses 25 percent of your body’s oxygen and 70 percent of its glucose. As aresult of all this metabolic activity, your brain generates a great deal of heat and is atrisk of overheating under conditions of exercise or heat stress. This risk of overheating,Falk argues, places a limit on how big the brain can be. In animals with less efficientbrain-cooling systems than we have, the size limit on the brain is even lower. This morestringent constraint has kept the brain of the chimpanzee at its current size.

Falk speculates that a more efficient brain-cooling mechanism arose in the hom-inid line between Australopithecus and more humanlike species, such as Homo habilis.When examining hominid skeletons, Falk noticed that, unlike australopith skulls,Homo skulls contain holes through which blood vessels pass. These holes suggest thatHomo species had a much more widely dispersed blood flow from the brain than didearlier hominids, and this more widely dispersed blood flow would have greatly en-hanced brain cooling. The increase in brain cooling, in turn, would have allowed thebrain to become larger, which it apparently did in response to evolutionary pressuresposed by the new environments exploited by these animals.

Neoteny One mechanism through which an increase in brain size could have takenplace is through neoteny, in which a species’ rate of maturation slows down, so juve-nile stages of predecessors become the adult features of descendants. Because thehead of an infant is large relative to body size, this process would have led to adultswith larger skulls and larger brains. Many other features of human anatomy, besidesa large brain-size-to-body-sizeratio, also link us with the juve-nile stages of other primates.These features include a smallface, a vaulted cranium, an un-rotated big toe, an upright pos-ture, and a primary distributionof hair on the head, armpits,and pubic areas. Figure 1-20illustrates that the head shapeof a baby chimpanzee is more

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Neoteny. A process in which maturationis delayed, so an adult retains infant char-acteristics; the idea derived from the ob-servation that newly evolved species re-semble the young of their ancestors.

Figure 1-20

An adult human more closely resemblesa juvenile chimpanzee than an adultchimp. The rounder head of the babychimpanzee compared with the moreelongated head of the adult chimpanzeeleads to the hypothesis that we humansmay be neotenic descendants of ourmore apelike distant ancestors.

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similar to an adult human head shape than it is to the head shape of an adult chim-panzee. We also retain behavioral features of primate infants, including play, explo-ration, and an intense interest in learning new things. Neoteny is a very common occurrence in the animal world. Domesticated dogs are thought to be neotenicwolves, and sheep are thought to be neotenic goats.

Another aspect of neoteny in relation to human brain development is that a slow-ing down of human maturation would have allowed more time for brain cells to beproduced (McKinney, 1998). Most brain cells in humans develop just before and afterbirth, so an extended prenatal and neonatal period would have prolonged the stage oflife in which brain cells are developing. This, in turn, would have enabled the creationof increased numbers of brain cells.

In ReviewConstant changes in the climate and physical features of the earth have eliminated cer-tain animal species and created new opportunities for other species to emerge. The largehuman brain evolved in response to a number of pressures and opportunities. Theyincluded changes in climate, the appearance of new food resources to exploit, a morewidely dispersed blood flow from the brain that enabled better brain cooling, and theretention of certain juvenile physical and behavioral traits.

STUDYING BRAIN AND BEHAVIOR IN MODERN HUMANSSo far, we have taken an evolutionary approach in exploring the human brain. But be-cause this approach is designed mainly for comparisons between species, special caremust be taken in extending its principles to comparisons within species, especiallycomparisons within groups of modern humans. We will illustrate the difficulty ofwithin-species comparisons by considering attempts to correlate human brain sizewith intelligence. Then we will turn to another aspect of studying the brain and be-havior in modern humans — the fact that, unlike the behavior of other animalspecies, so much of modern human behavior is culturally learned.

Human Brain-Size ComparisonsIn comparisons between animal species, larger brain size correlates with more com-plex behavior. Does the same relation hold true in comparisons between individualmembers of a single species? For instance, do people with the largest brains displaythe most complex and intelligent behavior? Stephen Jay Gould (1981), in his book ti-tled The Mismeasure of Man, reviewed numerous attempts to correlate human brainsize and intelligence. He is critical of this research because its logic and methods leaveconclusions completely muddled.

For one thing, it is difficult to determine how to measure the size of a per-son’s brain. If a tape measure is simply placed around a person’s head, it is impos-sible to factor out the thickness of the skull. There is also no agreement aboutwhether volume or weight is a better indicator of brain size. And, no matterwhich indicator we use, we must consider as well the relation between body sizeand brain size. For instance, the human brain varies in weight from about 1000

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grams to more than 2000 grams, but people also vary in body mass. To what ex-tent should we factor in body mass in deciding if a particular brain is large orsmall? And how should we measure the mass of the body, given that a person’s to-tal weight can fluctuate quite widely? There is also the matter of when in lifebrain size should be measured, because age and health affect the brain’s mass. Ifwe wait until after death to measure a brain, the cause of death, the water contentof the brain, and the time after death will all affect the results. And, even if theproblems of measurement could be solved, there remains the question of what iscausing what. Exposure to a complex environment can promote growth in exist-ing brain cells. So, if larger brains are found to correlate with higher intelligence,does the complex problem solving cause the greater brain mass or does thegreater brain mass enable the more complex behavior?

As if these matters were not perplexing enough, there is also the question ofwhat is meant by intelligence. When we compare the behavior of different species,we are comparing species-typical behavior — in other words, behavior displayed byall members of a species. When we compare behavior within a species, however, weare usually comparing how well one individual member performs a certain task inrelation to other members. This comparison is a completely different kind of mea-sure. In addition, individual performance on a task is influenced by many factorsunrelated to inherent ability, such as interest level, training, motivation, and health.People vary enormously in their individual abilities, depending on the particulartask. One person may have superior verbal skills but mediocre spatial abilities,whereas another person may be adept at solving spatial puzzles but struggle withwritten work, and still another excels at mathematical reasoning but is average ineverything else. Which of these people should we consider the most intelligent?Should certain skills get greater weight as measures of intelligence? Clearly, it is diffi-cult to say.

Given these questions, it is not surprising that brain size and intelligence withinthe human species do not seem particularly related to each other. People who virtu-ally everyone agrees are very intelligent have been found to have brains that vary insize from the low end to the high end of the range for our species. For instance, thebrilliant physicist Albert Einstein had a brain of average size. Similarly, women havebrains that weigh about 10 percent less than men’s brains (roughly equivalent to theaverage difference in female and male body size), but there is little evidence that thetwo sexes differ in average intelligence.

The lack of correlation between brain size and intelligence within a single speciesis found not only in humans. For instance, Figure 1-21 plots the average brain size in

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Species-typical behavior. A behaviorthat is characteristic of all members of aspecies.

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100

PekineseBulldog

Old Englishsheepdog St. Bernard

Miniatureschnauzer

Labradorretriever

Standardpoodle

80

60

40

20

0

Inte

llige

nce

(Hig

her-

rank

ed)

(Low

er-

rank

ed)

Dachshund

Figure 1-21

In a comparison of intelligence rankingsand brain size in breeds of dogs—animals that are all members of the samespecies— the correlation is r = 0.009,which indicates no relationship (an r = 1would show a perfect relationship). Ifyour dog does not show well, do not bedistressed, because there are a lot ofproblems in determining dog intelligence. Intelligence rankings are from The Intelligence of Dogs, by S. Coren, 1994, Toronto: The Free Press. Size measures are from “Brain Weight–Body Weight Scaling in Dogs and Cats,” by R. T. Bronson, 1979, Brain, Behavior and Evolution, 16, 227–236.

different breeds of dogs against each breed’s level of intelligence as ranked by dog ex-perts. The brain sizes, which range from less than 50 grams to nearly 130 grams, werenot adjusted for the breeds’ body sizes, because such adjustments are not made instudies of humans. As you can see, there is no relation at all between the overall size ofa breed’s brain and that breed’s intelligence ranking.

There must be differences of some kind in the brains of individual personsbecause people differ in behavior and talents. At present, though, we are not yetsure of what structural or functional measures are related to behavioral traits.However, it is very unlikely that gross brain size will provide an accurate or usefulmeasure. Researchers who study this question believe that measures of the relativesize and function of particular brain regions will be more helpful. Important, too,will probably be differences in how the various parts of the brain are organizedand interrelated.

CultureThe most remarkable thing about the brains of modern humans is that they have al-lowed us to develop an extraordinarily rich culture. Human culture consists of thecomplex learned behaviors characteristic of a group of people who pass those behav-iors on to one another and from generation to generation. Biologist G. P. Murdockcompiled the following list, in alphabetical order, of major categories of behavior thatare part of human culture:

Age-grading, athletic sports, bodily adornment, calendar [use], cleanli-ness training, community organization, cooking, cooperative labor, cos-mology, courtship, dancing, decorative art, divination, division of labor,dream interpretation, education, eschatology, ethics, ethnobotany, eti-quette, faith healing, family feasting, fire making, folklore, food taboos,funeral rites, games, gestures, gift giving, government, greetings, hairstyles, hospitality, housing, hygiene, incest taboos, inheritance rules, jok-ing, kin groups, kinship nomenclature, language, law, luck, superstitions,magic, marriage, mealtimes, medicine, obstetrics, penal sanctions, per-sonal names, population policy, postnatal care, pregnancy usages, prop-erty rights, propitiation of supernatural beings, puberty customs, reli-gious ritual, residence rules, sexual restrictions, soul concepts, statusdifferentiation, surgery, toolmaking, trade, visiting, weaving, and weathercontrol. (Murdock, 1945)

Not all the items in this list are unique to humans. Many other animal species displayelements of some of these behaviors. For example, many other animals display agegrading (any age-related behavior or status), courtship behavior, and rudimentary el-ements of language. To the extent that the behaviors of other animals are similar tothose of humans, it is likely that humans inherited both the behaviors and the relatedbrain circuitry.

Despite such behavioral similarities across species, humans clearly have pro-gressed much further in the development of culture than other animals have. Forhumans, every category of activity on Murdock’s list requires extensive learningfrom other members of the species, and exactly how each behavior is performedcan differ widely from one group of people to another. A human brain must func-tion adequately to acquire these complex cultural skills. When its functioning is in-adequate, a person may be unable to learn even basic elements of culture. “Learning

30 ■ CHAPTER 1

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Culture. Behaviors that are learned andpassed on from one generation to the nextthrough teaching and learning.

Disabilities,” above, describes how incapacitating it can be to have a brain that hasdifficulty in learning to read.

Because of vast differences in cultural achievements, the behavior of modernhumans is completely unlike that of Homo sapiens 100,000 years ago. Granted,simple toolmaking and tool use predate modern humans, but art, such as carvingsand paintings, dates back only some 30,000 years, well after the appearance ofHomo sapiens. Agriculture appears still more recently in human history, about10,000 to 15,000 years ago. And reading and writing, the foundations of our mod-ern literate and technical societies, were invented only about 7000 years ago. St.Ambrose, who lived in the fourth century, is reported to be the first person whocould read silently. So silent reading is an even more recent behavior than writing.Most forms of mathematics, another basis of modern technology, were inventedstill more recently than reading and writing were. Although most researchers thinkthat the origin of verbal language coincided with the beginning of modern hu-mans, it may have developed much later. If we consider that most of our culturewas invented long after our brains evolved into their present form, most of ourverbal language skills also could have been learned long after Homo sapiens firstevolved. It is likely that, when you have completed your college or university cur-riculum, your vocabulary will be larger and your language will be richer than thoseof your parents and grandparents.

A remarkable feature of the modern human brain is that it can do so many thingsfor which it was not seemingly designed. The brains of early Homo sapiens certainly didnot develop to help program computers or travel to distant planets. And yet the brainsof modern humans are capable of both these complex tasks and more. Apparently, the

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 31

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During the first third of their lives, children expend much

energy in learning a society’s culture, and acquiring the lan-

guage skills of their culture is a challenge that we expect

them to meet. Yet some people have difficulties in mastering

language-related tasks. These difficulties and others that af-

fect performance in school are classified under the “um-

brella” of learning disabilities.

One of the most common learning disabilities is dyslexia

(from the Latin dys, meaning “poor,” and lexia, meaning “read-

ing”), an impairment in learning to read. Not surprisingly, chil-

dren with dyslexia have difficulty learning to write as well as to

read. In 1895, James Hinshelwood, an eye surgeon, examined

some schoolchildren who were having reading problems, but

he could find nothing wrong with their vision. Hinshelwood

was the first person to suggest that these children had an im-

pairment in brain areas associated with the use of language.

In more recent times, Norman Geshwind and Albert

Galaburda proposed a way in which such an impairment

might come about. These researchers were struck by the

finding that dyslexia is far more common in boys than in

girls. Perhaps, they reasoned, excessive amounts of the

hormone testosterone, which produces male physical char-

acteristics early in development, might also produce ab-

normal development in language areas of the brain. Pursu-

ing this hypothesis, they examined the brains of a small

sample of dyslexic people who had died. They found ab-

normal collections of neurons, or “warts,” in the language

areas of the left hemisphere. This relation between struc-

tural abnormalities in the brain and learning difficulties is

further evidence that an intact brain is necessary for nor-

mal human functioning.

Learning Disabilities

Focus on Disorders

things that the human brain did evolve to do contained all the elements necessary forthe invention and use of far more sophisticated skills. The human brain is apparently ahighly flexible organ that allows the great variety of knowledge and achievements thatare part of modern culture.

The acquisition of a complex culture was a gradual, step-by-step process, withone achievement leading to another. The development of language, for instance,must have started when our early ancestors began to use concepts to represent thethings important to them in their world. In her book titled The Chimpanzees ofGombe, primatologist Jane Goodall described the process by which such conceptsmight have developed in chimpanzees. She used the development of the concept of“fig” as an example, explaining how a chimp might progress from knowing a figonly as a tangible, here-and-now entity to having a special vocal call that repre-sents this concept symbolically. Goodall writes:

We can trace a pathway along which representations of . . . a fig becomeprogressively more distant from the fig itself. The value of a fig to a chim-panzee lies in eating it. It is important that he quickly learn to recognize as fig the fruit above his head in a tree (which he has already learned toknow through taste). He also needs to learn that a certain characteristicodor is representative of fig, even though the fig is out of sight. Food calls made by other chimpanzees in the place where he remembers the figtree to be located may also conjure up a concept of fig. Given the chim-panzees’ proven learning ability, there does not seem to be any great cog-nitive leap from these achievements to understanding that some quitenew and different stimulus (a symbol) can also be representative of fig.Although chimpanzee calls are, for the most part, dictated by emotions,cognitive abilities are sometimes required to interpret them. And the in-terpretations themselves may be precursors of symbolic thought.(Goodall, 1986, pp. 588 – 589)

Presumably, in our own distant ancestors, the repeated acquisition of con-cepts, as well as the education of children in those concepts, gradually led to theacquisition of language and other aspects of a complex culture. The study of thehuman brain, then, is not just the study of the structure of an organ that evolvedthousands of years ago. It is also the study of how that organ acquires sophisticatedcultural skills — that is, of how the human brain functions in today’s world.

In ReviewCare must be taken in extending principles learned in studying the evolution of the brainand behavior. What is true for comparisons across different species may not be true forcomparisons within a single species. For instance, although a larger brain correlates withmore complex behavior when comparing different species, brain size and intelligenceare not particularly related when looking at individual persons within the species ofmodern humans. We humans are also distinguished in the animal kingdom by theamount of our behavior that is culturally learned. We have progressed much further inthe development of culture than other species have.

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SUMMARY1. What is the brain and what is behavior? Behavior can be defined as any kind of

movement in a living organism. As such, a behavior has both a cause and a function. The flexibility and complexity of behavior vary greatly in differentspecies, with human behavior being highly flexible and complex. Located insidethe skull, the brain is the organ that exerts control over behavior. The brainseems to need ongoing sensory and motor activity to maintain its intelligent activity.

2. How is the nervous system structured? The nervous system is composed of the cen-tral nervous system, which includes the brain and the spinal cord, and the periph-eral nervous system, through which the brain and spinal cord communicate withsensory receptors and with muscles. The brain and spinal cord communicate notonly with the skeletal muscles that enable movement of the body, but also withthe body’s many internal organs. The nerve pathways taking part in regulating internal bodily functions, including emotional responses, are collectively calledthe autonomic nervous system.

3. How have people through history viewed the relation between the brain and behav-ior? Aristotle believed that the brain has no role in behavior, but rather that be-havior is the product of an intangible entity called the psyche or mind. Descartesmodified this theory, proposing that only rational behavior is produced by themind, whereas other behaviors are produced mechanically by the brain. Finally,Darwin’s proposal that all living things are descended from a common ancestorled to the conclusion that the source of all behavior is the brain.

4. How did brain cells and the nervous system evolve? Brain cells and the nervous system evolved in animals over millions of years. The evolutionary stages throughwhich the brain evolved can be traced though groups of living animals. The nervous system evolved in the animal kingdom and the brain and spinal cordevolved in the chordate phylum. Mammals are a class of chordates characterizedby especially large brains.

5. What species were the early ancestors of modern humans? One of our early hom-inid ancestors was probably Australopithecus or a primate very much like it,who lived in Africa several million years ago. From an australopith species, morehumanlike species likely evolved. Among these species are Homo habilis andHomo erectus. Modern humans, Homo sapiens, did not appear in Asia and NorthAfrica until about 200,000 to 100,000 years ago.

6. How did the human brain evolve? The human brain evolved through the sequenceof hominid species that are the ancestors of modern humans. Since Australopithe-cus, the brain has increased in size almost threefold. This evolution was stimulatedby the natural selection of more complex behavior patterns. It was also madepossible by changes in blood circulation that enabled a larger brain to be adequately cooled.

7. What are some important considerations in studying the brain and behavior ofmodern humans? It is important to realize that principles learned in studying theevolution of the brain and behavior may not apply to the brain and behaviorwithin a single species, such as Homo sapiens. As animals evolved, a larger brainwas associated with more complex behavior, yet, within our species, the most ableand intelligent people do not necessarily have the largest brains. In the study ofmodern humans it is also important to recognize the great extent to which ourbehavior is not inherent in our nervous systems, but rather is culturally learned.

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 33

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34 ■ CHAPTER 1

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There are many resources available for

expanding your learning online:

■ www.worthpublishers.com/kolb/chapter1

Try some self-tests to reinforce your

mastery of the material in Chapter 1.

Look at some of the news updates

reflecting current research on the brain.

You’ll also be able to link to other sites

which will reinforce what you’ve

learned.

■ http://neurolab.jsc.nasa.gov/timeline.htm

Review a timeline of important people

in the study of the brain from René

Descartes to Roger Sperry put together

by the Spotlight on Neuroscience,

National Aeronautics and Space Admin-

istration (NASA).

■ http://serendip.brynmawr.edu/Mind/Table.html

Visit this site from R. H. Wozniak of

Bryn Mawr College to learn more about

the philosophical underpinnings of

dualism and materialism and to read a

detailed history of the origins of the

mind–body question and the rise of

experimental psychology.

On your CD-ROM you’ll be able to quiz

yourself on your comprehension of the

chapter. You’ll be able to begin learning

about the anatomy of the brain in the

module on the Central Nervous System.

This module is composed of a rotatable,

three-dimensional brain as well as a

number of sections of the brain that you

can move through with the click of a

mouse. In addition, the Research Methods

module contains various CT and MRI

images of the brain, including a video

clip of a coronal MRI scan.

neuroscience Interact ive

REVIEW QUESTIONS1. Summarize the ideas of Aristotle, Descartes, and Darwin regarding the relation

between the brain and behavior.2. Trace your own lineage by using the taxonomic system described in this chapter.3. Can you recall the number of species of living organisms in each taxonomic

subgrouping? What do you think accounts for the apparent relation betweennumbers of species and brain size?

4. We suggested that brain size is one way of accounting for behavioral complexityfor interspecies comparisons but not for intraspecies comparisons. Why did wemake this distinction?

5. How does culture increase the difficulty of understanding human brain function?

FOR FURTHER THOUGHTDarwin’s principle of natural selection is based on there being large individual differenceswithin species. There are large individual differences in the brain size of modern humans.Under what conditions could a new human species with a still larger brain evolve?

RECOMMENDED READINGCampbell, N. A. (1999). Biology, 2nd ed. Menlo Park, CA: Benjamin Cummings. This

introductory biology textbook provides a comprehensive overview of the structure andfunction of living organisms.

Coren, S. (1994). The intelligence of dogs. Toronto: The Free Press. This very popular book includes a number of tests that are supposed to tell you how smart your dog is. Thebook also provides comparisons of intelligence for different dog breeds as rated by dogtrainers. Check your dog out against other breeds by using easy-to-perform tests.Remember, if your dog is not well trained, it might not do well on the tests.

Darwin, C. (1965). The expression of the emotions in man and animals. Chicago: University ofChicago Press. (Original work published 1872) If a dog growls at you, is the dog angry?Darwin thought so. Darwin’s only book on psychology is one in which he argues thatthe expression of emotions is similar in animals, including humans, which suggests theinheritance of emotions from a common ancestor. This view is becoming popular today,but Darwin proposed it more than 100 years ago.

Darwin, C. (1963). On the origin of species by means of natural selection, or the preservation offavored races in the struggle for life. New York: New American Library. (Original workpublished 1859) This book is the most important one ever written in biology. Darwinextensively documents the evidence for his theory of natural selection. The book is anenjoyable account of natural life, and one chapter, titled “Instincts,” describes behaviorin both wild and domesticated animals.

KEY TERMS

bilateral symmetry, p. 17cladogram, p. 16common ancestor, p. 14common descent, p. 12culture, p. 30dualism, p. 9encephalization quotient

(EQ), p. 24

hominid, p. 21materialism, p. 12mentalism, p. 8mind, p. 8mind – body problem,

p. 9natural selection, p. 12neoteny, p. 27

principle of proper mass,p. 23

segmentation, p. 17species-typical behavior,

p. 29taxonomy, p. 15

Goodall, J. (1986). The chimpanzees of Gombe. Cambridge, MA: Harvard University Press.Goodall’s three-decade-long study of wild chimpanzees, begun in 1960, rates as one ofthe most scientifically important studies of animal behavior ever undertaken. Learnabout chimpanzee family structure and chimpanzee behavior, and look at the beautifulphotographs of chimpanzees engaged in various behaviors.

Gould, S. J. (1981). The mismeasure of man. New York: Norton. Gould criticizes and repudiatesextensive literature of the nineteenth and twentieth centuries that claims that differencesin human intelligence and differences in the intelligence of the sexes are due to differ-ences in brain size. The appealing feature of this book is that Gould is highly critical ofthe methodology of the proponents of the brain-size hypothesis while also giving rea-sons derived from modern genetics to criticize their position.

Lorenz, K. Z. (1981). The foundations of ethology. New York: Springer Verlag. Learn how tostudy animals and learn how they behave from one of the founders of ethology, thestudy of animal behavior.

Martin, R. D. (1990). Primate origins and evolution: A phylogenetic reconstruction. Princeton,NJ: Princeton University Press. Martin provides a detailed description of the origins andthe evolution of primates. This book is an excellent primate reference.

Weiner, J. (1995). The beak of the finch. New York: Vintage. This book is a marvelous study of evolution in action. Weiner documents how the populations of Galápagos finches areaffected by changes in the availability of certain kinds of food. Careful measurements ofthe finches’ beaks demonstrate that certain beak sizes and shapes are useful when certainkinds of food are available; however, when the appropriate food becomes unavailable,populations of birds with differently shaped beaks become favored.

WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ■ 35

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