Science Brain and Behavior contiuned 2

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WHAT ARE THE ORIGINS OF BRA IN
A ND
BEHAVIOR? .


9

Aristotle and Mentalism


The hypothesis thatthe mind
(or
soul or
psyche)
is responsible for
behavior
can
be traced
back more than
2000 years to
ancient
Greece.
In
classical mythology, Psyche was a mortal who
became the wife of
theyounggod
Cupid. Venus, Cupid’s mother, opposed
his marriage
to
a mortal, and
so
she harassed
Psyche with
countless, almost
impossible
tasks.

Aristotle
(384–322 BC)
Psyche performedthe tasks with
such
dedication,intelligence,
and
compassion
that
she was madeimmortal, thus removing Venus’s objection
to
her.
Theancient
Greek philosopher
Aristotle was alluding to
this storywhen
hesuggested
that
all human
intellectual functionsare produced
bya person’s psyche. The psyche,
Aristotle argued, is responsible for
life, and
its departurefrom
the body resultsin
death.
Aristotle’s account
of behavior
had
no
role for
the brain,whichhe thought
existed
to
cool the blood.To
him, the nonmaterial psyche was responsible for
human
thoughts,
perceptions, and
emotionsand
for
such
processes as imagination, opinion, desire,
pleasure, pain, memory, and
reason.The psyche was an
entityindependent
of the body.
Aristotle’s view that
a nonmaterial psychegoverns our
behavior
was adopted
byChristianityin
itsconcept
of thesouland
has been
widely disseminatedthroughoutthe
world.
Mind
is an
Anglo-Saxon
word
for
memoryand,when
“psyche” was translated
into
English,it
became mind.The philosophical position
that
a person’s mind, or
psyche,
is responsible for
behavior
is called
mentalism, meaning“of the mind.” Because
the mind
is nonmaterial, it
cannot
bestudied
with
scientific methods. Justthe same,
mentalism
has had
an
influence on
modern
behavioral science because many terms—

sensation, perception, attention, imagination, emotion, motivation, memory, and
volition


among them—arestill employed
as labels for
patterns of behavior
today, and
matters
relatedto
these behaviorsare thefocus ofcontemporary research
in
psychology.

Descartes and Dualism

In
the first
book on
brain
and
behavior,Treatise on
Man, René Des-
cartes (1596–1650),aFrench
physiologist, mathematician,andphilosopher,
proposed
a new explanation
of behavior
in
which
the brain
played
an
important
role. Descartes placedthe seat
of the mind
in
the brain
and
linkedthe mindto
the body. He saw mind
and
body as
separate but
interconnected. In
the first
sentence ofTreatise on
Man

René
Descartes
(1596–1650)
(1664), hestatedthat
mind
and
body“must
bejoined
and
unitedto
constitute people. . . .”
ToDescartes, most
of the activities of the bodyand
brain,including sensation, motion,
digestion, breathing, and
sleeping, could
be explained
by the mechanical and
physical principles current
in
seventeenth-centuryEurope. The mind, on
the other
hand, is nonmaterial, separatefrom
the body, and
responsible forrational behavior.
Descartes’s proposal that
an
entity calledthe minddirectsa machine calledthe body
was the first
seriousattempt
to
explain
the role of the brain
in
controllingintelligent
behavior.The problem
of howa nonmaterial mind
and
a physical brain
might
interact
has come to
be calledthe mind–body problem,andthe philosophical position
that
behavior
is controlled
by two
entities, a mind
and
a body, is called
dualism.
Figure1-4, an
illustration
from
Treatise on
Man,shows how, toDescartes, the
mind
receives information
from
the body. When
a handtouches a ball, for
example,

FrançoisGerard,
PsycheandCupid(1798)


E.
Lessing
Art
Resource,
New
York
Psyche. Synonym for mind,
an entity
once proposed to be the source of
human
behavior.


Mind. Proposed nonmaterial entity
responsible for intelligence,
attention,
awareness,
and consciousness.


Mentalism. O fthe mind; an explanation
of
behavior as
a function ofthe
nonmaterial mind.


Mind–body problem. Q uandary of
explaining a nonmaterial mind in
command of
a material body.


Dualism. Philosophical position that
holdsthat
both a nonmaterial mind and
the material body contribute to behavior.



Figure1 4
Dualism
Descartes
argued
that
the
pineal
body in
the
brainreceives
different
messages
from
a
hand
holding
a
fluteand
from
a
hand
touching
a
ball.
The
mind,
resident
in
the
pineal
body,
interprets
these
messages
and
so
learns
about
the
fluteand
ball.From
Treatise
on
Man,
by R.
Descartes,
1664.
Reprint
and
translation
(p.
60),
1972,Cambridge,
MA:
Harvard
University Press.




the mind learns through the brain that a ball exists, where the ball is located, and what
its size and texture are. The mind also directs the body to touch the ball, but again it
does so through the brain. The mind can command the brain to make the body carry
out a great variety of actions, such as running, changing breathing rate, or throwing
the ball across the room. The rational mind, then, depends on the brain both for information
and to control behavior.
Descartes was also aware of the many new machines being built, including clocks,
water wheels, and gears. He saw mechanical gadgets on public display in parks. In the
water gardens in Paris, one device caused a hidden statue 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 the sidewalk.
Influenced by these mechanical devices, Descartes used mechanical analogies, for
example, to describe how we automatically make decisions about distances and angles
on the basis of visual information and to explain why, when we focus directly on an
object, we are less aware of what surrounds it. He also considered in detail “mechanical”
physiological functions, such as digestion, respiration, and the roles of nerves and
muscles. To explain how the mind controls the body,Descartes suggested that the mind
resides in a small structure in the center of the brain called the pineal body, which is located
beside fluid-filled cavities called ventricles.
According to Descartes, the pineal body directs fluid from the ventricles through
nerves and into muscles. When the fluid expands those muscles, the body moves. In
Descartes’s theory, then, the mind regulates behavior by directing the flow of ventricular
fluid to the appropriate muscles. Note that, for Descartes, mind and body were
separate entities, and the pineal body was only a structure through which the mind
works.
Many problems in detail and logic corrupt Descartes’s theory.We now know that
people who have a damaged pineal body or even no pineal body at all still display normal
intelligent behavior. The pineal body plays a role in behavior related to biological
rhythms, but it does not govern human behavior. We now know that fluid is not
pumped from the brain into muscles when they contract. Placing an arm in a bucket
of water and contracting the arm’s muscles does not cause the water level in the bucket
to rise, as it should if the volume of the muscle increased because fluid had been
pumped into it.We now also know that there is no obvious way that a nonmaterial entity
can influence the body, because doing so requires the spontaneous creation of energy,
which violates the physical law of conservation of matter and energy.
Nevertheless, Descartes proposed scientific tests of his theory. To determine if an
organism possesses a mind, Descartes proposed two tests: the language test and the action
test. To pass the language test, an organism must use language to describe and reason
about things that are not physically present. The action test requires the organism
to display behavior that is based on reasoning and is not just an automatic response to
a particular situation. Descartes also assumed that animals are unable to pass the tests.
A good deal of experimental work that we will describe is directed toward determining
if he was right. For example, studies of sign language taught to apes are partly intended
to find out whether apes can describe and reason about things that are not
present and so pass the language test. “Origins of Spoken Language” on page 11 summarizes
the contemporary view on language in animals.
Descartes’s theory led to a number of unfortunate results. On the basis of it, some
people argued that young children and the mentally insane must lack minds, because
they often fail to reason appropriately.We still use the expression “he’s lost his mind”
to describe someone who is “mentally ill.” Some proponents of this view also reasoned
that, if someone lacked a mind, that person was simply an inhuman machine not due
normal respect or kindness. Cruel treatment of animals, children, and the mentally ill
10 ! CHAPTER 1
Ventricles
Pineal body
Visit the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
and go to the Chapter 1 Web links to read
a detailed history of the origins of the
mind–body question.
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has been justified by Descartes’s theory for centuries. It is unlikely that Descartes himself
intended these interpretations.He was reportedly very kind to his own dog, named
Monsieur Grat.
Darwin and Materialism
By the mid–nineteenth century, another theory of brain and behavior was emerging.
This theory was the modern perspective of materialism—the idea that rational behavior
can be fully explained by the working of the brain and the rest of the nervous
system,without any need to refer to an immaterial mind that controls our actions. This
perspective became especially prominent because it is supported scientifically by the
evolutionary theories of Alfred Russel Wallace and Charles Darwin.
WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 11
Origins of Spoken Language
Focus on New Research
Language is such a striking characteristic of our species that
it was once thought to be a trait unique to humans. Nevertheless,
evolutionary theory predicts that language is unlikely
to have appeared suddenly and full-blown in modern
humans. Language must have antecedents in other species,
especially the species most closely related to us.
The first attempt to teach human vocal language to
chimps was an abject failure. Not until 1971, when Beatrice
and Alan Gardner taught a version of American Sign Language
to a chimpanzee named Washoe, was it realized that
nonverbal forms of language might have preceded verbal
language. To test this hypothesis, Sue Savage-Rumbaugh and
coworkers began teaching a pygmy chimpanzee named
Malatta a symbolic language called Yerkish. (The pygmy
chimpanzee, or bonobo, is a species thought to be an even
closer relative of humans than the common chimp.)
Malatta and her son Kanzi were caught in the wild, and
Kanzi accompanied his mother to class. It turned out that,
even though he was not specifically trained, Kanzi learned
more Yerkish than his mother did. Remarkably, Kanzi also
displayed clear evidence of understanding quite complex
human speech.
Realizing that chimps in the wild have a rich vocal
repetoire and are especially vocal in producing food peeps
in association with food, Jared Taglialatela and coworkers
(2003) recorded the vocalizations made by Kanzi when he
was interacting with people and eating different kinds of
foods. From video records of many interactions with humans,
the scientists selected vocalizations associated with
“banana”, “grape”, “juice”, and “yes”. Spectral analysis of
the sounds associated with the semantic context or meaning
of each condition were analyzed to determine whether the
peeps uttered by Kanzi were similar in similar situations and
distinct in different situations.
The analyses revealed that the peeps were indeed similar
for vocalizations that occurred within a specific semantic
context and structurally different between the different
contexts. Although Kanzi is a language compentent chimp,
the finding that he uses “chimpanzeeish” in specific situations
in his interactions with humans provides further support
for the idea that human language may be derived from
more primitive forms of communication used by human
ancestors.
Great Ape Trust of Iowa
Materialism. Philosophical position that
holds that behavior can be explained as a
function of the nervous system without
explanatory recourse to the mind.
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Wallace and Darwin independently arrived at the same conclusion—the idea that
all living things are related. Each outlined this view in papers presented at the Linnaean
Society of London in July 1858. Darwin further elaborated on the topic in his book titled
On the Origin of Species by Means of Natural Selection, published in 1859. This
book presented a wealth of supporting detail, which is why Darwin is mentioned more
often as the founder of modern evolutionary theory.
Both Darwin and Wallace had looked carefully at the structure of plants and animals
and at animal behavior. Despite the diversity of living organisms, both men were
struck by the myriad characteristics common to so many species. For example, the
skeleton, muscles, and body parts of humans, monkeys, and other mammals are remarkably
similar.
Such observations led first to the idea that living organisms must be related, an idea
widely held even before Wallace and Darwin. But, more importantly, these same observations
led to Darwin’s explanation of how the great diversity in the biological world
could have evolved from common ancestry. Darwin’s principle of natural selection
proposes that animals have traits in common because these traits are passed from parents
to their offspring.
Natural selection is Darwin’s theory for explaining how new species evolve and
existing species change over time. A species is a group of organisms that can breed
among themselves but not with members of other species. Individual organisms within
any species vary extensively in their characteristics, or phenotypes, with no two members
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 individual organisms
whose characteristics best help them to survive in their environment are likely
to leave more offspring than are less-fit members. This unequal ability of individual
members to survive and reproduce leads to a gradual change in a species’ population,
with characteristics favorable for survival in that particular habitat becoming more
prevalent in succeeding generations.
Neither Darwin nor Wallace understood the basis of the great variation in plant
and animal species. The underlying principles of that variation were discovered by another
scientist,Gregor Mendel, beginning about 1857, through experiments that he did
with pea plants.Mendel deduced that heritable factors, which we now call genes, are related
to the various physical traits displayed by the species.
Members of a species that have a particular gene or combination of genes will express
that trait. If the genes for a trait are passed on to offspring, the offspring also will
have the same trait. New traits appear because new gene combinations are inherited
from parents, because existing genes change or mutate, because suppressed genes are
reexpressed, because expressed genes are suppressed, or because genes or parts of genes
are deleted or duplicated.
Thus, the unequal ability of individual organisms to survive and reproduce is related
to the different genes that they inherit from their parents and pass on to their offspring.
By the same token, similar characteristics within or between species are usually
due to similar genes. For instance, genes that produce the nervous system in different
kinds of animal species tend to be very similar.
Darwin’s theory of natural selection has three important implications for the study
of the brain and behavior:
l. Because all animal species are related, so too must be their neurons and their
brains. Today, brain researchers study animals as different as slugs, fruit flies, rats,
and monkeys, knowing that they can often extend their findings to human beings.
2. Because all species of animals are related, so too must be their behavior. Darwin
was particularly interested in this subject. In his book titled On the Expression of
12 ! CHAPTER 1
Charles Darwin
(1809–1892)
Alfred Wallace
(1823–1913)
Natural selection. Darwin’s theory
for explaining how new species evolve
and existing species change over time.
Differential success in the reproduction
of different characteristics (phenotypes)
results from the interaction of organisms
with their environment.
Species. Group of organisms that can
interbreed.
Visit the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
and go to the Chapter 1 Web links to view
more research about Charles Darwin.
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the Emotions in Man and Animals, he argued that emotional expressions are similar
in humans and other animals because we inherited these expressions from a
common ancestor. Evidence for such inheritance is illustrated in Figure 1-5, which
shows that smiling is common to people throughout the world. That people in different
parts of the world display the same behavior suggests that the trait is inherited
rather than learned.
3. Both the brain and behavior changed bit by bit in animals that evolved to greater
complexity, as humans obviously did. In the following section, we will trace the
steps in which the human nervous system evolved from a simple, netlike arrangement
of nerve fibers, 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 the
status of a theory: the brain theory. Donald O. Hebb in his influential book titled The
Organization of Behavior, published in 1949, described the brain theory as follows:
Modern psychology takes completely for granted that behavior and neural function
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 and then and
make neural cells do what they would not otherwise. (Hebb, 1949, p. xiii)
Some people reject the idea that the brain is responsible for behavior because they
think it denies religion. The biological explanation of brain and behavior, however, is
neutral with respect to religious beliefs. Fred Linge, the brain-injured man whose experience
begins this chapter, has strong religious beliefs, as do the other members of
his family. They used their religious strength to aid in his recovery. Yet, despite their religious
beliefs, they realize that Linge’s brain injury was the cause of his change in behavior
and that the process of recovery that his brain underwent is the cause of his
restored health. Similarly, many behavioral scientists hold strong religious beliefs and
see no contradiction between those beliefs and their use of the scientific method to examine
the relations between the brain and behavior.
EVOLUTION OF BRAIN AND BEHAVIOR
The study of living organisms shows that not all have nervous systems or brains and that
nervous systems and behavior built up and changed bit by bit as animals evolved.We will
trace the evolution of the human brain and behavior by describing (1) those animals that
first developed a nervous system and muscles with which to move and (2) how the nervous
system became more complex as the brain evolved to mediate complex behavior.
The popular interpretation of human evolution is that we are descended from
apes. Actually, apes are not our ancestors, although we are related to them through a
In Review .
We have considered three perspectives on how behavior arises. Mentalism is the view that
behavior is a product of an intangible entity called the mind (psyche); the brain has little
importance. Dualism is the notion that the mind acts through the brain to produce 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. Materialism, the view that
all behavior, language and reasoning included, can be fully accounted for by brain function,
guides contemporary research on the brain and behavior.
WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 13
Figure 1-5
Inherited Behavior Part of the
evidence supporting Darwin’s suggestion
that emotional expression is inherited
is the finding that people from all parts
of the world display the same emotional
expressions that they also recognize in
others, as is illustrated by these smiles.
J. Greenberg / Visuals Unlimited
A. Cassidy / Stone Images
O. Benn / Stone Images
J. Tisne / Stone Images
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14 ! CHAPTER 1
common ancestor, a forebear from which two or more lineages or family groups arise.
To demonstrate the difference, consider the following story.
Two people named Joan Campbell were introduced at a party, and their names afforded
a good opening for a conversation. Although both belong to the Campbell lineage
(family line), one Joan is not descended from the other. The two women live in
different parts of North America, one in Texas and the other in Ontario, and both their
families have been in those locations for many generations.
Nevertheless, after comparing family histories, the two Joans discovered that they
have ancestors in common. The Texas Campbells are descended from Jeeves Campbell,
brother of Matthew Campbell, from whom the Ontario Campbells are descended.
Jeeves and Matthew had both boarded the same fur-trading ship when it stopped for
water in the Orkney Islands north of Scotland before sailing to North America in colonial
times.
The Joan Campbells’ common ancestors, then, were the mother and father of
Jeeves and Matthew. Both the Texas and the Ontario Campbell family lines are descended
from this same man and woman. If the two Joan Campbells were to compare
their genes, they would find similarities that correspond to their common lineage.
In much the same way, humans and apes are descended from common ancestors.
But, unlike the Joan Campbells, we do not know who those distant relatives were. By
comparing the brain and behavioral characteristics of humans and related animals
and by comparing their genes, however, scientists are tracing our lineage back farther
and farther to piece together the story of our origins. In the following sections, we will
trace some of the main evolutionary events that led up to human brains and human
behavior.
Origin of Brain Cells and Brains
The earth formed about 4.5 billion years ago, and the first life forms arose about a billion
years later. About 700 million years ago, animals evolved the first brain cells, and,
by 250 million years ago, the first brain had evolved. A humanlike brain first developed
only about 3 million to 4 million years ago, and our modern human brain has been
around for only the past 100,000 to 200,000 years. As evolutionary history goes, that is
a short time span. Although life evolved very early in the history of our planet, brain
cells and the brain are recent adaptations, and large complex brains, such as ours, appeared
only an eye blink ago in evolutionary terms.
Classification of Life
Since the first living organism appeared, the divergence of life on Earth has been enormous.
Millions of species have evolved, and millions have gone extinct. As many as 30
million to 100 million species currently inhabit the planet. Scientists have described
only a small number of these species, about 1.5 million. The rest remain to be found,
named, and classified.
Taxonomy, the branch of biology concerned with naming and classifying species,
groups organisms according to their common characteristics and their relationships to
one another. As shown in Figure 1-6, the broadest unit of classification is a kingdom,
with more subordinate groups being phylum, class, order, family, genus, and species.We
humans belong to the animal kingdom, the chordate phylum, the mammalian class, the
primate order, the Hominidae family, the Homo genus, and the sapiens species. Animals
are usually identified by their genus and species name. So we humans are called Homo
sapiens, meaning “wise humans.”
Common ancestor. Forebear from
which two or more lineages or family
groups arise and so is ancestral to both
groups.
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WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 15
Species: Modern human
Genus: Human
Family: Hominidae
Order: Primates
Class: Mammals
Phylum: Chordates
Kingdom: Animals
Living organisms
Characteristics: Neurons and
muscles used for locomotion
Classified in five main kingdoms:
Monera (bacteria), Protista
(single cells), Plantae (plants),
Fungi (fungi), Animalia (animals)
Characteristics: Brain and
spinal cord
Characteristics: Large brains
and social behavior
Characteristics: Visual control
of hands
Characteristics: Tool use
Characteristics: Language
Characteristics: Complex
culture
Figure 1-6
Taxonomy of Modern Humans
Taxonomy classifies comprehensive
groups of living organisms into
increasingly specific subordinate groups.
Modern humans are the only surviving
species of the genus that included
numerous extinct species of humanlike
animals.
Galápagos woodpecker finch
This taxonomic hierarchy is useful in helping us trace the evolution of brain cells
and the brain. Brain cells and muscles first evolved in animals, allowing them to move.
The brain as an organ first evolved in chordates, allowing more complex movements; a
large brain with many different functions first evolved in mammals; a brain capable of
producing complex tools first evolved in apes; and a brain capable of written language
and complex culture first evolved in Homo sapiens. Although the most complex brain
and patterns of behavior have evolved in the human lineage, large brains and complex
behaviors have also evolved in some other lineages. Highly social dolphins have large
brains and some birds, such as the Galápagos woodpecker finch, use simple tools.
Evolution of Animals with Nervous Systems
A nervous system is not essential for life. In fact, most organisms in both the past and
the present have done without one. Of the five main taxonomic kingdoms of living organisms
illustrated at the top of Figure 1-6, only one, Animalia, contains species with
nervous systems. Taxonomists have so far identified about 1 million animal species and
CH01.qxd 1/28/05 9:14 AM Page 15

organized them into 15 phyla (the plural of phylum). Differences among the nervous
systems of these phyla illustrate the evolutionary markers of increasing nervous system
complexity.
Figure 1-7A shows the animal phyla in a chart called a cladogram (from the
Greek word clados, meaning “branch”). Cladograms display groups of related organisms
as branches on a tree; branch order represents how the groups are related evolutionarily,
as well as the traits that distinguish them. A cladogram is read from left to
right, with the most recently evolved organism or trait located at the right. The
branches for animals that possess neurons and muscle tissue diverge at the far right in
Figure 1-7A.
Neurons and muscles underlie the new forms of movement that distinguish the
animal kingdom. The simpler animal groups that branch off before this evolutionary
milestone have neither. Neurons and muscle tissue became more complex, as did the
behaviors that they control, as new groups of animals evolved. Animals without neurons
and muscles continue to exist and evolve, but we will follow the evolution of animals
with neurons and muscles.
As illustrated in Figure 1-7B, the nervous system representative of older phyla, such
as jellyfishes and sea anemones, is extremely simple. It consists of a diffuse nerve net,
with no structure that resembles a brain. (Compare the human nervous system illustrated
in Figure 1-2. Now imagine that the brain and spinal cord have been removed).
In moving to the right in Figure 1-7B, we find that species in somewhat more recently
evolved phyla, such as flatworms, are more complexly structured. These organisms
have heads and tails, and their bodies have bilateral symmetry (one-half of the
body is the mirror image of the other) and segmentation (the body is composed of
similarly organized parts). These animals also have segmented nervous systems that resemble
the human nervous system, with sensory and motor neurons projecting from
each segment. Recall that bilateral symmetry and segmentation are two important
structural features of the human nervous system.
Species in still more recently evolved phyla, such as clams, snails, and octopuses,
have clusters of neurons called ganglia in particular body parts. Ganglia resemble
primitive brains and function somewhat like them in that they are command centers.
In some phyla, encephalization, meaning “in the head,” becomes distinctive. For example,
insects, have ganglia in the head that are sufficiently large to merit the term
brain. One phylum, the chordates, of which humans are members, displays the greatest
degree of encephalization.
Chordates get their name from the notochord, a flexible rod that runs the length of
the back. In humans, the notochord is present only in the embryo; by birth, bony vertebrae
encase the spinal cord. Although the nervous systems of prechordates, such as
fruit flys, and chordates, such as humans, have many anatomical differences, the nervous
systems of both are formed under the instruction of similar sets of homeobox gene
clusters. These groups of genes specify the segmented organization of the nervous system
of insects and vertebrates and so are thought to have arisin in a common ancestor
to these lineages. Thus, the nervous systems of diverse animals are much more similar
than their superficial anatomical differences suggest.
The Chordate Nervous System
Figure 1-8 pictures representatives of seven of the nine classes to which the approximately
38,500 chordate species belong.Wide variation exists in the nervous systems of
chordates, but the basic pattern of a net that is bilaterally symmetrical, segmented,with
16 ! CHAPTER 1
Cladogram. Phylogenetic tree that
branches repeatedly, suggesting a
classification of organisms based on the
time sequence in which evolutionary
branches arise.
Nerve net. Simple nervous system that
has no brain or spinal cord but consists of
neurons that receive sensory information
and connect directly to other neurons that
move muscles.
Bilateral symmetry. Body plan in
which organs or parts present on both
sides of the body are mirror images in
appearance. For example, the hands are
bilaterally symmetrical, whereas the heart
is not.
Segmentation. Division into a number
of parts that are similar; refers to the idea
that many animals, including vertebrates,
are composed of similarly organized body
segments.
Ganglia. Collection of nerve cells that
function somewhat like a brain.
Chordates. Group of animals that have
both a brain and a spinal cord.
Homeobox gene cluster. A group
of genes that specify the segmental
organization of the nervous system of
insects and vertebrates and so are thought
to have arisen in a common ancestor to
these lineages.
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WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 17
Common
ancestor
of animals
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.
(A)
Common
ancestor
of animals
Sea
anemone
Flatworm Squid Frog
Nerve net: Simple nervous
system, organized as a net,
with no brain
Segmented nerve trunk:
Bilaterally symmetrical
organization
Brain: True
brain and
spinal cord
Ganglia: Structures that
resemble and function
somewhat like a brain
Ganglia
Complexity of movement
(B)
Figure 1-7
Evolution of the Nervous System (A) This cladogram relates the evolutionary sequence
connecting the five main kingdoms. (B) This cladogram shows the evolutionary relationships
among the nervous systems of the 15 animal phyla, from a nerve net, to a segmented nervous
system, to ganglia and nerve trunks, and finally to a brain and spinal cord.
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Common
ancestor
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 1-8
Representative Classes of Chordates
This cladogram illustrates the
evolutionary relationship among animals
having a brain and a spinal cord. Brain
size increased with the development of
limbs in amphibia. Birds and mammals
are the most recently evolved chordates,
and large brains relative to body size are
found in both classes.
a spinal cord and brain encased in cartilage or bone is common to all. This pattern is
found even in the earliest chordate species. Two additional features distinguish the
chordate nervous system:
1. The nervous system is “crossed,” that is, one hemisphere of the brain receives
most sensory signals from the opposite side of the body and sends motor commands
mainly to the opposite side of the body (left hemisphere controls right
body and vice versa). In prechordate animals, the nervous system does not cross
over. The reason that the cordate nervous system is crossed is not fully understood,
but crossed pathways allow for greater control of movements of the limbs
than for those of the trunk. Possibly, this crossed anatomical pathway permits
each hemisphere to hold one side of the body still while moving the limbs on the
other side.
2. The chordate spinal cord is dorsal (at the back) to the heart and gut, but the prechordate
nervous system is ventral (below the belly) to these structures. Some
anatomists speculate that this shift of the chordate nervous system toward the back
was a key adaptation in allowing the nervous system to grow larger.
The evolution of more complex behavior in chordates is closely related to the evolution
of the cerebral hemispheres, or cerebrum, and of the cerebellum (Latin, meaning
“little brain”). The relative differences of these two brain regions in different classes
of chordates are illustrated in Figure 1-9. The behaviors controlled by these regions include
new forms of locomotion on land, complex movements of the mouth and hands
for eating, improved learning ability, and highly organized social behavior.
The cerebrum and the cerebellum are proportionately small and smooth in the
earliest-evolved classes (e.g., fish, amphibians, and reptiles). In later-evolved chordates,
especially the birds and mammals, these structures become much more prominent.
Finally, in many large-brained mammals, both structures are extensively folded,
which greatly increases their surface area while allowing them to fit into a small skull
(just as folding a piece of paper enables it to occupy a small container such as an
envelop).
Increased size and folding become particularly pronounced in the dolphins and
primates, animals with the largest brains relative to their body size. Because relatively
large brains with a complex cortex and cerebellum have evolved in a number of animal
lineages, humans are neither unique nor special in these respects.We humans are
distinguished, however, in belonging to a lineage having large brains and are unique in
having the largest brain of all animals.
Cerebellum. Major structure of
the hindbrain specialized for motor
coordination. In large-brained animals, it
may also have a role in the coordination
of other mental processes.
To view a closeup of the cerebellum,
go to the brainstem and subcortical
structures section of the central nervous
system module on the Foundations CD.