Science Brain and Behavior contiuned 3

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HUMAN EVOLUTION
Anyone can see similarities among humans, apes, and monkeys, but some people believe
that humans are far too different from monkeys and apes to have an ancestor in
common with them. These skeptics have reasoned that the absence of a “missing link,”
or intermediate ancestor, further argues against the possibility of common descent. In
the past century, however, so many intermediate forms between humans and other
apes have been found in the fossil record that entire books are required to describe
them. Here we consider only the brains and behaviors of some of the more prominent
ancestors that link apes to us and to the human brain and behavior.
Humans: Members of the Primate Order
The human relationship to apes and monkeys places us in the primate order, a subcategory
of mammals that includes not only apes and monkeys, but lemurs, tarsiers, and
marmosets as well (Figure 1-10). In fact, we humans are only 1 of about 275 species in
the primate order. Primates have excellent color vision, with the eyes positioned at the
front of the face to enhance depth perception, and they use this highly developed sense
to deftly guide their hand movements.
Female primates usually have only one infant per pregnancy, and they spend a
great deal more time caring for their young than most other animals do. Associated
with their skillful movements and their highly social nature, primates’ brains are on average
larger than those of animals in other orders of mammals, such as rodents (mice,
rats, beavers, squirrels) and carnivores (wolves, bears, cats, weasels).
Humans are members of the suborder apes,which includes gibbons,orangutans, gorillas,
and chimpanzees as well (see Figure 1-10).Apes are arboreal animals, with limber
shoulder joints that allow them to brachiate (swing from one handhold to another) in
In Review .
Brain cells and nervous systems are relatively recent developments in the evolution of life on
Earth. Because they evolved only once, in the animal kingdom, a similar basic pattern exists
in the nervous systems of all animals. The nervous system becomes more complex with the
evolution of chordates, and this increase in complexity closely parallels increasingly complex
behavior. Particular animal lineages, such dolphins and primates, are characterized by
especially large brains and complex behaviors. These evolutionary developments in chordates
are closely tied to their bilateral symmetry, segmented spinal cord, brain encased in
cartilage or bone, crossed nervous system pathways, migration of the nervous system toward
the back of the body, and growth of the cerebral cortex and cerebellum.
WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 19
Visit the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
and go to the Chapter 1 Web links to view
a tutorial about human evolution.
Figure 1-9
Brain Evolution The brains
of representative chordates
have many structures in
common, illustrating a single
basic brain plan across
chordate species.
Fish Frog Human
Cerebrum Cerebellum Cerebrum Cerebellum Cerebrum Cerebellum Cerebrum Cerebellum
Bird
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20 ! CHAPTER 1
Common
ancestor
of primates
Apes
Lemurs Chimpanzees Humans
and lorises
Old World Gibbons Orangutans Gorillas
monkeys
New World
monkeys
Tarsiers
trees, a trait retained by humans, although they generally do not live in trees.Apes are distinguished
as well by their intelligence and large brains, traits that humans exemplify.
Among the apes, we are most closely related to the chimpanzee, having had a common
ancestor between 5 million and 10 million years ago. The family to which humans
belong is called Hominidae. In the past 5 million years,many hominids, primates that
walk upright, evolved in our lineage. Some extinct hominid species lived at the same
time as one another. At present, however, we are the only surviving hominid species.
Australopithecus: Our Distant Ancestor
One of our hominid ancestors is probably Australopithecus (from the Latin word austral,
meaning “southern,” and the Greek word pithekos, meaning “ape”) or a primate
very much like it. Figure 1-11 shows reconstructions of the animal’s face and body. The
name Australopithecus was coined by an Australian, Raymond Dart, for the skull of a
child that he found in a box 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.)
We now know that many species of Australopithecus existed,
some at the same time.
The skull of the “Taung child” did not belong to the earliest
species, which lived more than 4 million years ago. These early
hominids were among the first primates to show a distinctly
human characteristic: 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 fossilized footprints that
a family of australopiths left behind, walking through freshly
fallen volcanic ash some 3.6 million to 3.8 million years ago. The
footprints feature the impressions of a well-developed arch and
an unrotated big toe more like that of humans than of apes.
The evolutionary lineage from Australopithecus to humans
is not known precisely, in part because many Australopithecus
Hominid. General term referring to
primates that walk upright, including all
forms of humans, living and extinct.
Figure 1-10
Representatives of the Primate Order This cladogram illustrates hypothetical relationships
among members of the primate order. Humans are members of the family of apes. In general,
brain size increases across the groupings, with humans having the largest primate brains.
Figure 1-11
Australopithecus Australopithecus
(top) walked upright with free hands, as
do modern humans, but its brain was the
size of a modern-day ape’s, about onethird
the size of the human brain. Figure
comparison (bottom) based on the most
complete Australopithecus skeleton yet
found, a young female about 1 meter
tall popularly known as Lucy, who lived 3
million years ago.
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species evolved, some contemporaneously. One possible lineage is shown on the left in
Figure 1-12. A common ancestor gave rise to the Australopithecus lineage, and one
member of this group gave rise to the Homo lineage.
The last of the australopith species disappears from the fossil record about 1 million
years ago after coexisting with other hominids for some time. Also illustrated in
Figure 1-12, at the right, is the large increase in brain size that evolved in the hominid lineage.
The brain of Australopithicus was about the same size of that of nonhuman apes,
but succeeding members of the human lineage display a steady increase in brain size.
The First Humans
The oldest fossils designated as genus Homo, or human, are those found by Mary and
Louis 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,
but Mary Leakey argued that their dental pattern is more similar to that of modern
humans than to that of australopiths.More importantly, they made simple stone tools.
The Leakeys named the species Homo habilis (meaning “handy human”) to signify that
its members were toolmakers. Again, the precise relationships in the Homo lineage are
not known, because a number of early Homo species lived at the same time.
The first humans whose populations spread beyond Africa migrated into Europe
and into Asia. This species was Homo erectus (“upright human”), so named because of
the mistaken notion that its predecessor,H. habilis, had a stooped posture.Homo erectus
first shows up in the fossil record about 1.6 million years ago and lived until perhaps
as recently as 100,000 to 30,000 years ago. Its brain was bigger than that of any previous
hominid, overlapping in size the measurements of present-day human brains (see Figure
1-12, right).H. erectus also made more sophisticated tools than did H. habilis.
Modern humans, Homo sapiens, appeared in Asia and North Africa within about
the past 200,000 years and in Europe within the past 100,000 years. Most anthropologists
think that they migrated from Africa originally. Until about 30,000 years ago in
Europe and 18,000 ago in Asia, they coexisted with other hominid species. The Asiatic
species,Homo floresienses, found on the Indonesian island of Flores was, at about three
feet tall, an especially small subspecies of Homo erectus (Morwood et al., 2004).
In Europe, for example, H. sapiens coexisted with Neanderthals, named after Neander,
Germany, where the first Neanderthal skulls were found. Neanderthals had
WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 21
Brain size (in cubic centimeters)
4 3 2
Millions of years ago
A. africanus
1 0
1600
1400
1200
1000
800
600
400
200
0
Common
ancestor
Common ancestor
A. robustus
A. afarensis
A. africanus
H. habilis
H. habilis
H. erectus
H. erectus
H. neanderthalensis
H. neanderthalensis
H. sapiens
H. sapiens
Figure 1-12
The Origins of Humans (Left) The
human lineage and a lineage of extinct
Australopithecus probably arose from a
common ancestor about 4 million years
ago. Thus the ancestor of the human
lineage Homo was likely an animal
similar to Australopithecus africanus.
(Right) Brain size across this proposed
lineage has increased nearly threefold.
AFRICA
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22 ! CHAPTER 1
brains as large as or larger than those of modern humans, used tools similar to those
of early H. sapiens, and possibly had a similar hunting culture.We do not know how
H. sapiens completely replaced other human species, but perhaps they had advantages
in tool making, language use, or social organization.
EVOLUTION OF THE HUMAN BRAIN
Scientists who study brain evolution propose that a relative increase in the size and
complexity of brains in different species enabled the evolution of more complex behavior.
In this section, we consider the relation between brain size and behavior across
different species.We also consider leading hypotheses about how the human brain became
so large.
Brain Size and Behavior
In his book titled The Evolution of the Brain and Intelligence, published in 1973, Harry
Jerison uses the principle of proper mass to sum up the idea that species exhibiting more
complex behaviors will possess relatively larger brains than will species whose behaviors
are less complex. Jerison also developed an index of brain size to compare different
species’ brains, even though they differ in body size. He calculated that, as body size increases,
the size of the brain increases at about two-thirds the increase in body weight.
Using this index, plus an average brain-volume-to-body-weight ratio as a base, we
can quantify the expected brain size for a mammal of any given weight. The diagonal
line in Figure 1-13 plots expected brain size. Body size is on the x-axis, and brain size
is on the y-axis. The shaded polygon surrounding the diagonal line encompasses the
actual brain-to-body-size ratios of all mammals.
Animals that lie below the diagonal line have brains that are below the average
expected ratio for an animal of that size, whereas animals that lie above the diagonal
have brains that are larger than expected for an animal of that size. Notice that the
rat’s brain is a little smaller and the elephant’s brain is a little larger than the ratio predicts.
Notice also that a modern human is located farther to the upper left than any
0.05
0.1
0.5
1.0
5.0
10.0
50
100
500
1,000
5,000
10,000
Brain weight (in grams)
0.001 0.01 0.1 1 10 100 1,000 10,000 100,000
Body weight (in kilograms)
Mole
Rat
Cat
Lion
Chimpanzee
Gorilla
Wolf
Baboon
Vampire bat
Australopithecus
Homo sapiens Porpoise Elephant
Blue
whale
Opossum
The position of the modern
human brain, at the farthest
upper left, indicates that 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 1-13
Brain-to-Body-Size Ratios of Some
Familiar Mammals The axes use
logarithmic units to encompass the wide
range of body and brain sizes. The
shaded polygon includes the brain and
body sizes of all mammals. Adapted from
The Evolution of the Brain and Intelligence
(p. 175), by H. J. Jerison, 1973, New York:
Academic Press.
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other animal, indicating a brain that is relatively
larger for its body size than that of any other animal.
Using the ratio of actual brain size to expected
brain size, Jerison also developed an encephalization
quotient (EQ), a numerical value for the brain
size of each species. The top half of Figure 1-14 lists
the EQs for several familiar animals.Notice that a rat
has an EQ about one-half that of a cat, which is representative
of the average mammal on the diagonal
in Figure 1-13.
Crows have an EQ similar to that of monkeys,
and dolphins have an EQ comparable to that ofHomo
erectus. People who study crows would agree that they
are intelligent birds,whereas dolphins are both highly
intelligent and highly social mammals. The bottom
half of Figure 1-14 lists the EQs for a number of largebrained
species in the primate lineage.
Underlying Jerison’s principle of proper mass is
the idea that a larger brain is needed for increasingly
complex behavior.We can see some obvious relations
between larger brains and more complex behavior, or
movements, as we progress up the chordate ladder
from older to more recent classes of animals, with limb use as an example (see Figure
1-8).Among older chordates, cyclostomes, such as the lamprey,move by making snakelike,
side-to-side body movements, whereas the more recent fish species have fins that
enable more complex movements. In amphibians, fins evolved into limbs used in an
even more complex way than fins, to enable walking on land.
Birds and mammals use their limbs for still more complex movements, both for
locomotion and for handling objects. Being a primate is associated with many other
limb innovations, including extensive tool making. Each increase in behavioral complexity
is associated with increases in brain size.
Why the Hominid Brain Enlarged
The evolution of modern humans—from the time when humanlike creatures first appeared
until the time when humans like ourselves first existed—spans about 5 million
years. As illustrated by the relative size differences of skulls pictured in Figure 1-15,
much of this evolution entailed changes in brain size, which were accompanied by
changes in behavior.
The nearly threefold increase in brain size from apes (EQ 2.5) to modern humans
(EQ 7.0) has been a subject of extensive research and equally extensive speculation.One
line of evidence points to a series of rapid climate changes as a spur for behavioral
adaptation.Most likely, each new hominid species appeared after
climate changes produced new environments. Populations of existing hominids
were isolated, enabling a rapid selection for traits adaptive in each
new environment.
The first of these climate changes was triggered about 8 million years
ago. Before that time,most of Africa was rich forest inhabited by monkeys
and apes, among other abundant plant and animal species. Then a massive
tectonic event (a deformation of the earth’s crust) produced the Great
Rift Valley, which runs from south to north across the eastern part of the
African continent.
WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 23
Encephalization quotient (EQ).
Jerison’s measure of brain size obtained
from the ratio of actual brain size to the
expected brain size, according to the
principle of proper mass, for an animal
of a particular body size.
Common
animals
Human
lineage
Encephalization quotients
0 1 2 3 4 5 6 7
Rat
Cat
Elephant
Crow
Fruit bat
Dolphin
Monkey
Chimpanzee
Australopithecus
Homo habilis
Homo erectus
Homo sapiens
Sea lamprey
Figure 1-14
Comparative Encephalization
Quotients The EQs of some familiar
animals are compared in the top half of
the chart, and members of the primate
lineage are ranked in the bottom half.
Figure 1-15
The Course of Human Evolution The
relative size of the hominid brain has
increased nearly threefold, illustrated
here by comparing Australopithecus
afarensis (left), Homo erectus (center),
and modern Homo sapiens (right). A
missing part of the Australopithecus
skull, shown in blue, has been
reconstructed. From The Origin of Modern
Humans (p. 165), by R. Lewin, 1998, New York:
Scientific American Library.
K. O’Farrell/Concepts
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This reshaping of the African landmass left a wet jungle climate to the west and a
much drier savannah climate to the east. To the west, the apes continued unchanged in
their former habitat. But, in the drier eastern region, apes evolved rapidly into upright
hominids in response to the selective pressures of a mixture of tree-covered and grassy
regions that formed their new home.
Upright posture has a number of adaptive advantages, including being an efficient,
rapid means of locomotion across grass-covered areas. Such an upright posture may
have evolved in Australopithecus because these arboreal animals were forced to spend
more time on the ground moving between clumps of trees. Upright posture may also
have helped hominids to regulate their body temperature by reducing the amount of
body surface directly exposed to the sun and to improve their ability to scan the environment
for opportunities and threats; it may have been useful for tree climbing as
well.
Just before the appearance of Homo habilis 2 million years ago, the African climate
rapidly grew even drier, with spreading grasslands and even fewer trees. Anthropologists
speculate that the hominids that evolved into Homo habilis adapted to this new
habitat by becoming scavengers on the dead of the large herds of grazing animals that
then roamed the open grasslands.
The appearance of Homo erectus may have been associated with a further change
in climate, a rapid cooling that lowered sea levels (by trapping more water as ice) and
opened up land bridges into Europe and Asia. At the same time, the new hominid
species upgraded their hunting skills and the quality of their tools for killing, skinning,
and butchering animals. Archeologists hypothesize a number of migrations of hominids
from Africa into other parts of the world, with modern humans being the last
of these migrants.
A wide array of hypotheses seeks to explain why the modern human brain is so
large. One hypothesis contends that the primate life style favors an increasingly complex
nervous system. A second links brain growth to changes in hominid physiology.
And a third proposes that a slowed rate of maturation in a species favors larger brains.
We will now examine all these points of view.
THE PRIMATE LIFE STYLE
That the primate life style favors a larger brain can be illustrated by examining how primates
forage for food. Foraging is important for all animals, but some foraging activities
are simple, whereas others are complex. Eating grass or vegetation is not difficult;
if there is lots of vegetation, an animal need only munch and move on. Vegetation
eaters do not have especially large brains. Among the apes, gorillas, which are mainly
vegetation eaters, have relatively small brains (see Figure 1-13). In contrast, apes, such
as chimpanzees, that eat fruit have relatively large brains.
The relation between fruit foraging and larger brain size can be seen in a study by
Katharine Milton (1993), who examined the feeding behavior and brain size of two
South American (New World) monkeys that have the same body size—the spider
monkey and the howler monkey. As is illustrated in Figure 1-16, the spider monkey
obtains nearly three-quarters of its nutrients from eating fruit and has a brain twice
as large as that of the howler monkey,which obtains less than half of its nutrients from
fruit.
What is so special about eating fruit that favors a larger brain? The answer is not
that fruit contains a brain-growth factor, although fruit is a source of sugar that the
brain depends on for energy. The answer is that foraging for fruit is a complex activity.
Unlike plentiful vegetation within easy reach on the ground, fruit grows on trees,
and only on certain trees, in certain seasons. Among the many kinds of fruit, some are
better for eating than others, and many different animals and insects compete for a
24 ! CHAPTER 1
AFRICA
Wet
Great Rift Valley
Dry
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fruit crop. Moreover, after a fruit crop has been
eaten, it takes time for a new crop to grow. Each
of these factors poses a challenge for an animal
that eats mostly fruit.
Good sensory skills, such as color vision, are
needed to recognize ripe fruit in a tree, and good
motor skills are required to reach and manipulate
it. Good spatial skills are needed to navigate
to trees that contain fruit. Good memory skills
are required to remember where fruit trees are,
when the fruit will be ripe, and in which trees the
fruit 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 of
ripening fruit, having friends who can help
search also benefits a fruit eater. As a result,
successful fruit-eating animals tend to have complex
social relations and a means of communicating
with others of their species. In addition,
having a parent who can teach fruit-finding skills
is helpful to a fruit eater; so being both a good learner and a good teacher is useful.
We humans are fruit eaters and we are descended from fruit eaters, and so we are
descended from animals with large brains. In our evolution, we also exploited and
elaborated fruit-eating skills to obtain other temporary and perishable types of food as
we scavenged, hunted, and gathered. These new food-getting efforts required navigating
long distances, and they required recognition of a variety of food sources. At the
same time, they required making tools for digging up food, killing animals, cutting
skin, and breaking bones.
These tasks also require cooperation and a good deal of learned behavior.Humans
distinguish themselves from apes in displaying a high degree of male–male, female–female,
and male–female cooperation in matters not related to sexual activity (Schuiling,
2003). The elaboration of all these life-style skills necessitated more brain cells over
time. Added up, more brain cells produce an even larger brain.
CHANGES IN HOMINID PHYSIOLOGY
Researchers have attempted to relate physical differences between humans and other
apes to the evolution of a larger brain in humans. One adaptation that may have given
a special boost to greater brain size in our human ancestors was a new form of brain
cooling.Dean Falk (1990), a neuropsychologist who studies brain evolution, developed
her radiator hypothesis from her car mechanic’s remark that, to increase the size of a
car’s engine you have to also increase the size of the radiator that cools it.
Falk reasoned that, if the brain’s radiator, the circulating blood, adapted into a more
effective cooling system, the brain could increase in size. Brain cooling is so important
because the human brain works so hard.Although your brain makes up less than 2 percent
of your body weight, it uses 25 percent of your body’s oxygen and 70 percent of its
glucose.As a result of all this metabolic activity, your brain generates a great deal of heat
and is at risk of overheating under conditions of exercise or heat stress.
Falk argues that, unlike australopith skulls, Homo skulls contain holes through
which cranial blood vessels passed. These holes suggest that Homo species had a
much more widely dispersed blood flow from the brain than did earlier hominids, and
this more widely dispersed blood flow would have greatly enhanced brain cooling.
WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 25
Spider monkey diet
Howler monkey diet
Fruit
Leaves
Leaves
Flowers
Flowers
Fruit
A spider monkey,
with a brain size
of 107 g, obtains
72 percent of its
nutrients from fruit.
A howler monkey,
with a brain size of
50 g, obtains only
42 percent of its
nutrients from fruit.
Figure 1-16
Picky Eaters Katharine Milton
examined the feeding behavior and
brain size of two South American
(New World) monkeys that have the
same body size but different brain sizes
and diets.
P. A. Souders / Corbis K. Schafer / Corbis
Radiator hypothesis. Idea that
selection for improved brain cooling
through increased blood circulation in the
brains of early hominids enabled the
brain to grow larger.
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A second adaptation, identified by Hansell Stedman and his colleagues (2004),
stems from a genetic mutation associated with marked size reductions in individual facial
muscle fibers and entire masticatory muscles. The Stedman team speculates that
smaller masticatory muscles in turn led to smaller and more delicate bones in the head.
Smaller bones in turn allowed for changes in diet and an increase in brain size.
Stedman and his colleagues estimate that this mutation occurred 2.4 million years
ago, coinciding with the appearance of the first humans. The methodology used by
Stedman, in which human and ape genes are compared, will likely lead to future insights
into other differences between humans and apes, including those in brain size
and function.
NEOTENY
When a species’ rate of maturation slows down, a process called neoteny, juvenile
stages of predecessors become the adult features of descendants.Maturation
is delayed and the adult retains some infant characteristics. Because the
head of an infant is large relative to body size, neoteny would lead to adults
with larger skulls to house larger brains.
Many features of human anatomy besides a large brain-size-to-body-size
ratio link us with the juvenile stages of other primates. These features include
a small face, a vaulted cranium, an unrotated big toe, an upright posture, and
a primary distribution of hair on the head, armpits, and pubic areas.
Figure 1-17 illustrates that the head shape of a baby chimpanzee is more
similar to an adult human than it is to the head shape of an adult chimpanzee.
Humans also retain some behaviors of primate infants, including play, exploration,
and an intense interest in novelty and learning.Neoteny is common in the animal
world.Domesticated dogs are neotenic wolves, and sheep are neotenic goats.
Another aspect of neoteny related to human brain development is that a slowing
down of human maturation would have allowed more time for brain cells to be produced
(McKinney, 1998). Most brain cells in humans develop just before and after
birth; so an extended prenatal and neonatal period would prolong the stage of life in
which most brain cells are developing. This prolonged stage would, in turn, enable increased
numbers of brain cells to develop.
STUDYING BRAIN AND BEHAVIOR
IN MODERN HUMANS
The evolutionary approach that we’ve been using to explain how the large human brain
evolved is based on comparisons between species. Special care attends extending evolutionary
principles to physical comparisons within species, especially biological comparisons
within or among groups of modern humans.We will illustrate the difficulty
of within-species comparisons by considering misguided attempts to correlate human
In Review .
Constant changes in the environment eliminate some animal species and create new opportunities
for others to evolve. Among certain groups of animals, adaptations to these
changes include an increase in brain size. Thus, the large human brain evolved in response
to a number of pressures and opportunities, including changes in climate, the appearance
of new food resources to exploit, physiological adaptations, and neoteny.
26 ! CHAPTER 1
Neoteny. Process in which maturation is
delayed, and so an adult retains infant
characteristics; idea derived from the
observation that newly evolved species
resemble the young of their common
ancestors.
R. Stacks / Index Stock
C. A. Schmidecker / FPG
Figure 1-17
Neotony The shape of an adult
human’s head more closely resembles
the shape of the head of a juvenile
chimpanzee (left) than the shape of the
head of an adult chimp (right), leading
to the hypothesis that we humans may
be neotenic descendants of our more
apelike common ancestors.
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brain size with intelligence. Then we turn to another aspect of studying the brain and
behavior in modern humans—the fact that, unlike the behavior of other animal
species, so much of modern human behavior is culturally learned.
Fallacies of Human Brain-Size Comparisons
We have documented parallel changes in brain size and behavioral complexity through
the many species that form the human lineage. Some people have proposed that, because
brain-size differences between species are related to behavioral complexity, similar
comparisons might be made between individual members of a single species. For
example, some investigators have attempted to show that people with the largest
brains display the most intelligent behavior. Stephen Jay Gould, in his 1981 book titled
The Mismeasure of Man, reviewed many of these attempts to correlate human brain
size with intelligence and was critical of this research because of its faulty logic and
methods.
For one thing, determining how to measure the size of a person’s brain is difficult.
If a tape measure is simply placed around a person’s head, it is impossible to factor out
the thickness of the skull. There is also no agreement about whether volume or weight
is a better measure of brain size. And, no matter which indicator we use, we must consider
body size. For instance, the human brain varies in weight from about 1000 grams
to more than 2000 grams, but people also vary in body mass. To what extent should we
factor in body mass in deciding if a particular brain is large or small? And how should
we measure the mass of the body, given that a person’s total weight can fluctuate
widely?
Age and health affect the brain’s mass as well. People who suffer brain injury in
early life often have smaller brains and behavioral impairments. If we wait until after
death to measure a brain, the cause of death, the water content of the brain, and the
time elapsed since death will all affect the results.
Even if the problems of measurement could be solved, the question of what is causing
what remains. Exposure to a complex environment can promote growth in existing
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 the greater brain
mass enable the more complex behavior?
As if these factors were not perplexing enough, we must also consider what is
meant by intelligence.When we compare the behavior of different species, we are comparing
species-typical behavior—in other words, behavior displayed by all members
of a species. For example, lampreys do not have limbs and cannot walk, whereas salamanders
do have limbs and can walk; so the difference in brain size between the two
species can be correlated with this trait.
When we compare behavior within a species, however, we are usually comparing
how well one individual member performs a certain task in relation to other members—
for example, how well one salamander walks relative to how well another salamander
walks. In addition, for humans, individual performance on a task is influenced by many
factors unrelated to inherent ability, such as opportunity, interest level, training, motivation,
and health.
People vary enormously in their individual abilities, depending on the particular
task.One person may have superior verbal skills but mediocre spatial abilities, whereas
another person may be adept at solving spatial puzzles but struggles with written work,
and still another may excel at mathematical reasoning and be average in everything
else. Which of these people should we consider the most intelligent? Should certain
skills get greater weight as measures of intelligence? Clearly, it is difficult to say.
WHAT ARE THE ORIGINS OF BRAIN AND BEHAVIOR? ! 27
Species-typical behavior. Behavior
that is characteristic of all members of a
species.