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Evolution of the Brain:Neuroanatomy, Development, and Paleontology

Daniel D. White, Ph.D.University at Albany, State University of New York



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CONTENTS

INTRODUCTION 1

A look at brain size 1

BASIC NEUROANATOMY 7

The basic building blocks of the central nervous system 7

Major subdivisions of the brain 11

Major cerebral subdivisions and their functions 13

HUMAN BRAIN DEVELOPMENT 17

LATERALIZATION, HANDEDNESS, AND LANGUAGE 18

SEXUAL DIMORPHISM IN THE HUMAN BRAIN21

COMPARING NONHUMAN PRIMATE AND HUMAN BRAINS 22

The indirect or comparative neuroscientific method 22

The direct or paleoneurological method 24

PRIMATE PALEONEUROLOGY 26

Adaptive shifts in primate evolution that impact the brain 27

HOMINID PALEONEUROLOGY 30

Robust and gracile australopithecine endocast morphology 31

Early Homo 34

Late Homo 35

Homo floresiensis 35

SUMMARY 36

GLOSSARY AND KEY WORDS 37

QUESTIONS FOR REVIEW 40

SUGGESTED FURTHER READING 41

REFERENCES CITED 41

FIGURE TITLES AND CREDITS 46



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PREFACE

Today, the brain is considered an organ of paramount importance for those

of us who study human behavior, evolution, and health. Most people would agree

that it is the organ that makes us human. It is the organ that houses ourpersonalities, memories, and thoughts. It is essentially, us.

Historically, this was not always the case though. The Egyptians were

famous for scooping the brains out of the skulls of deceased pharaohs and

disposing of them during the mummification process. The early Greeks didn’t

think so highly of the brain either. Aristotle thought that it was simply the organ

that cooled the heat generated by the thinking…heart.

The study of the brain has come a long way since those early days and things

are moving quite fast in the brain sciences. The field is moving so fast, in fact, that

it is often difficult even for professionals to keep up with all of the new

discoveries.I have written this module for the introductory student of human evolution

and physical anthropology. In it you will be introduced to how the human brain is

put together, how it functions, and how it got to be the way it is today. By

studying these basic building blocks, I hope that you will be able to better

appreciate and understand the other subjects covered in your course.

As you read through the text, think about what parts of your brain are

working to see the words, think the thoughts, and create the questions. I hope you

get as much out of reading it as I did out of writing it.

Dan White

ACKNOWLEDGEMENTS

I would like to thank Drs. Jurmain, Kilgore, and Ciochon for giving me the

opportunity to share my knowledge of and passion for the study of human brain

evolution with such a large audience. This module has been improved greatly by

the careful editorial comments of Bob and Lynn. Any mistakes that may stillremain are entirely my responsibility. Thanks also go to Lin Marshall at Thomson

Wadsworth who patiently and cheerily guided me through the CRC process for the

first time. Finally, thank you to my mentor and friend, Dr. Dean Falk, whose

research continues to inspire me to learn more about how the human brain evolved.



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INTRODUCTION

The human species is the only species that has ever asked the question,

where do I come from? It is the only species that creates symphonies, poetry, and

literature. It is the only species that has developed mathematical equations andexplored the moon. No other species on earth is likely to ever match these

achievements. The biological structure that has enabled humans to perform these

feats of intelligence is, of course, the human brain.

The adult human brain is a three-pound, intricately folded structure that

makes up about 2% of the body’s total mass. Although relatively small in

proportion to other structures in the body, the resting brain consumes as much

energy as all of the resting skeletal muscles in the body put together (Aiello and

Wheeler, 1995). This energetically expensive and complex organ is built on a

typically primate structural plan, but also possesses some unique qualities.

Studying human brain evolution through comparison with living nonhumanprimates and fossilized cranial material allows paleoanthropologists to explore the

questions that intrigue us most of all. When did we first become truly human?

When did we first utter the sounds that everyone would consider true language?

What is it about our brain that gives us the extraordinary intellectual abilities we

possess today? What did the brains of our earliest ancestors look like, and what

were they capable of?

A look at brain size

Absolute Brain size

Humans have big brains. The average adult human brain measures

approximately 1350 cubic centimeters (cm3

or cc) and contains as many as 20

billion neocortical neurons (Pakkenberg and Gundersen, 1997). Human brains are

by far the largest of any extant primate, weighing about three times more than the

brains of our closest living primate cousin, the chimpanzee (Stephan et al., 1981).

Figure 1 shows schematically how much larger the human brain is in comparison

to the brains of prosimians (strepsirhines), New World monkeys, Old World

monkeys, lesser apes, and great apes. This is what we call an absolute difference

in brain size.For such a simple statistic, the absolute size of the brain can tell researchers

a great deal about the attributes of a particular species. For example, absolute

brain size (meaning the total weight or volume of all brain structures) has been

shown to predict mental flexibility in nonhuman primates (Gibson et al., 2001).

For our purposes, we can consider mental flexibility as a type of intelligence.

Tests of mental flexibility demonstrate the cognitive power of a species by



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challenging individuals to make decisions in a changing test environment in the

laboratory. The more flexible the individual is, the better the test score, and the

higher the intelligence. Just as brain size increases from prosimians to monkeys to

apes, so does the cognitive flexibility of these animals.

This type of experimental research is fascinating to paleoanthropologists

because it takes a statistic that we can glean from fragmentary fossil evidence and

allows us to make at least general predictions about the cognitive abilities of our

early ancestors. We know that cranial capacity increases during human evolution

from the small australopithecine brains to larger Homo habilis, Homo erectus, and

eventually to the large brains of modern Homo sapiens. Experimental evidence

from nonhuman primates tells us to predict greater cognitive flexibility as well.

Figure 1. Brain volume measurements from Stephan et al. (1981) data set. (After Rilling, 2006).



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Relative brain size

There are certain minimum requirements for brain size in any mammal. At

the lowest level, there must be enough brainpower to keep the body in

homeostasis. (Homeostasis is the body’s ability to maintain internal equilibrium

even as the environment changes.) There must also be enough brain and nervoustissue to sense the outside world and to respond appropriately to the stimuli from

both outside and inside. These basic functions allow an animal to feed, flee

danger, and reproduce. Animals that have more brain tissue than just the basic

level necessary for survival may have an advantage over animals that survive with

the minimum requirements for their body size. In order to determine which

animals have more brainpower than is strictly necessary, we need to know the

relationship between the size of the brain and the size of the body. This is called

relative brain size. There are many ways to define relative brain size but the

simplest is brain size divided by body size.

Mammalian bodies and brains don’t scale to each other in a one to one ratio.Small-bodied animals like mice have relatively large brains for their body size,

while large-bodied animals like elephants or whales have relatively small brains

for their body size. We see a similar phenomenon during growth. A small-bodied

baby has a much larger head in relation to its body than a large-bodied adult does.

This same relationship holds true for primates. As primate body size increases, the

ratio of brain size to body size decreases. This is an example of allometric scaling

(See Box 1).

If we were to graph the relationship between body weight and brain weight

in primates, it would look like a curve that flattens out as body weight increases.In order to better visualize our data, we can straighten out the allometric curve

mathematically by transforming the brain and body size measurements to

logarithms.

Figure 2 shows the relationship between body weight and brain weight in a

sample of 45 primate species drawn from the most extensively used postmortem

primate data set in existence (Stephan et al., 1988; Stephan et al., 1981). The line

(of regression) that runs through the data points indicates the expected size of the

brain for any particular body size. For example, if we wanted to grow a 1kg (2.2

lb) galago (bush baby, see Chapter 6) to a hypothetical 60kg (130 lb) super galago,

we could predict from the line its expected brain size (about 500 cm3 or 17ounces).



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Box 1 Scaling brains and bodies: Isometry and allometry

The best way to understand the scaling principles of isometry and

allometry is by visualization. The diagrams below demonstrate isometry in

A and allometry in B.

Isometric scaling means that the shape and proportions (size ratios) of theorganism in time T1 are exactly the same as the shape and proportions in

times T2 and T3. The ratio of head to body size is exactly the same although

both head and body size increase in absolute size.

Allometric scaling means that the shape and proportions of the organism

change through time. In this case, time can mean developmental time

(ontogeny) or evolutionary time (phylogeny). In B, we see that the

proportion of head to body size changes as the organism gets bigger. Body

size increases faster than brain size and the shape of the organism in time T1

is different from the shape of the organism in times T2 and T3. This type of

allometric scaling is what occurs in human development and primate brainevolution.



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The relationship between the regression line and a data point can tell us

something interesting about the brainpower of that individual or group. If we look

at the diamonds that represent lemurs in Figure 2, we see that, in general, these

primates fall below the line whereas the human point is well above the line. This

means that lemurs have smaller brains than would be predicted for a primate of

equal body size, and humans have much larger brains than would be predicted for a

primate of equal body size. That is, humans are more encephalized than the

lemurs. In fact, humans are more encephalized than all other primates.

Figure 2. Double logarithmic scale comparing body weight in grams to brain weight inmilligrams of extant primates. The line represents a linear regression that shows theexpected brain size for any particular body size desired. For example, the X marks theexpected location on the line of human brain size. The fact that actual human brain sizeis higher than the line indicates that there was selective pressure for more brain tissueduring human evolution. Notice also that, in general, prosimians fall below the line and

anthropoids rise above the line. Data from (Stephan et al., 1988).

Harry Jerison, one of the world’s foremost experts in brain evolution and

intelligence, first developed what is called an encephalization quotient or EQ to

compare how much a species diverges from a similar line. Jerison’s line included

data on a large sample of living mammals. The encephalization quotient is thus the

ratio of actual brain size to predicted brain size for his mammalian data set.



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Encephalization quotients can give us an idea of how much more or less

brainpower a species has in comparison with other mammals. As you might

expect, primates have high EQ scores with humans at the top of the range. EQ

scores tend to correlate highly with ecological factors like feeding behaviors and

the types of food an animal eats.

For example, monkeys that eat leaves have lower EQ scores than monkeys

that eat fruits or apes that are omnivorous. Fruit-eating monkeys and omnivorous

apes need to work harder cognitively to get their food than monkeys who eat

abundant, widely available leaves. Early members of the genus Homo appear to

have had a varied diet that included all sorts of food from meat to fruits and tubers.

This type of diet calls for more brainpower than a simple vegetarian diet.

Brain size and reorganization in the fossil record

If we look at hominid phylogeny for evidence of brain size, we see that not

all hominids had such large brains. Figure 3 shows that absolute brain size hasbeen increasing since at least 1.8-2.0 million years ago when we have the first

evidence of the genus Homo. Before this time, early fossil hominids like the

australopithecines had brains only slightly larger than that of an average

chimpanzee. Based on absolute brain size, determined by cranial capacity in

fossils, it appears that members of our own genus were more mentally agile than

the australopithecines and earlier members of the hominid lineage. As Kathleen

Gibson and her colleagues put it, “bigger is better” when it comes to brain size and

cognition (Gibson et al., 2001). Is an increase in absolute and relative brain size

the only explanation for the presumed boost in intelligence throughout hominid

brain evolution?

Ralph Holloway from Columbia University would say that a rearrangement

or “reorganization” of brain tissue and brain circuitry is also essential in the

evolution of the human brain (Holloway et al., 2004). It appears that as brains get

bigger there’s also a change in the way they are wired up. Bigger brains require

more wiring, but more wiring can become inefficient (Hofman, 2001). In order to

maintain efficiency, the brain becomes reorganized. Holloway and his colleagues

would say that the human brain is not just a super-sized monkey brain. To better

grasp the topic of brain reorganization in hominid brain evolution, it’s important to

first have knowledge of basic neuroanatomy and brain development. The nextsection provides some of these basics.



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Figure 3. Cranial capacity (a measure of brain size) of fossil hominid specimens derivedfrom Holloway et al. (2004) and Falk et al. (2005). Notice that brain size increases morethan 3 fold during hominid evolution.

BASIC NEUROANATOMYThe human nervous system can be broken down into two main components,

the central nervous system and the peripheral nervous system. The central

nervous system is made up of the brain and the spinal cord, which together serve asa central processing system. The peripheral nervous system is made up of nerve

fibers that extend out from the brain and spinal cord to the rest of the body. These

nerves interact with the external environment through sensory organs (like the skin

and eyes) to let our brains know what is happening around us (exteroception).

They relay information about our movement and our location in the environment

(proprioception) and they communicate with organs (over which we don’t have

voluntary control) such as the stomach, intestines, heart, and kidneys.

The basic building blocks of the central nervous system

The brain and spinal cord are made up of two basic types of cells, neurons

and neuroglia. Neurons are the most basic functional unit of the nervous system

and are classified into three different categories: motor, sensory, and interneurons.

These cells carry electrochemical impulses, called action potentials that transmit

information (for example the sight and smell of your favorite food) within the brain

and to other parts of the nervous system. Neurons share characteristics with other



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cells in the body but also possess specializations that allow them to do the work of

thinking, moving the body, and sensing the outside world. Neuroglia (or just glia

meaning, “glue”) are supporting cells that allow neurons to do their job properly

(Barr and Kiernan, 1993). Among other functions, these cells serve as structural

scaffolding for neurons and produce the insulation (myelin) that helps neurons

conduct action potentials.

Neurons, like other cells in the body, possess a nucleus, cytoplasm, and a

selectively porous cell membrane. However, neurons also have some special

qualities that allow them to carry action potentials (or more commonly – nerve

impulses) throughout the brain and body. These specializations are the small

branch-like processes called dendrites at each end of the cell, an enlarged cell body

containing a cell nucleus, and a long tail-like extension called an axon (Figure 4).

Figure 4. The basic components of a motor neuron.

In most cases, dendrites that branch off of the cell body are the receptor sites

for nerve impulses coming from adjacent neurons. If a neuron adjacent to our

neuron in Figure 4 is stimulated to send a nerve impulse, special signaling

chemicals called neurotransmitters are released into the space that separates the

two neurons. This space, called the synaptic cleft, is filled with neurotransmitters

which act like keys to locked chemical gates in our neuron. Under the right

conditions, these gates are opened and the process of transmitting the nerve

impulse from the cell body of one neuron through the axon and on to anotheradjacent neuron is set in motion. This is called a chemical synapse. (There are

also electrical synapses that cause action potentials but these are much less

frequent than chemical synapses.)

An insulation layer that surrounds the axons of most neurons called a myelin

sheath allows the nerve impulses to pass rapidly through the cell. The myelin

sheath acts much like the insulation around an electrical wire and just like poorly



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insulated electrical wires, poorly myelinated neurons are inefficient. The

symptoms of the chronic disease - multiple sclerosis (MS) are caused by the

destruction of the myelin sheath surrounding axons throughout the nervous system

(Fox, 1990). People with MS may experience blurred or double vision, poor

balance, and muscle weakening to name only a few of the diffuse and numerous

malfunctions caused by this disease.

There are places in the central nervous system where neuron cell bodies and

their accompanying dendrites are found in high density. These areas are

commonly referred to as gray matter (Figure 5). Like yin and yang, where there is

gray matter there must also be white matter. White matter is made up of the long

myelinated axonal extensions from neurons that tend to run together in bundles or

tracts. In fact, white matter is white because the myelin sheath surrounding the

axons is white. One of the best examples of white matter is the corpus callosum,

which serves to relay information from one side of the brain to the other.

Figure 5. The human brain in occipital view. Left side shows the cortex (gray matter)

and white matter in the cerebral hemisphere. (Modified from the ComparativeMammalian Brain Collections website http://brainmuseum.org from the University of Wisconsin-Madison Brain Collection.).

The neocortex of the cerebrum is made of six layers of cells distinguished

from each other by cell type and cell density. The term neocortex (meaning “new

cortex”) refers to this structure’s relatively recent phylogenetic expansion. The



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neocortex is relatively large in mammals (including primates) and makes up as

much as 80% of total brain volume. Evolutionary expansion of the neocortex

comes in the form of increased surface area and not increased cortical thickness

(Rakic and Kornack, 2001). It’s the increase in surface area of the neocortex that

gives the human brain its complicated pattern of gyrification.

The cell composition or cytoarchitecture of the neocortex changes from

one region of the brain to another and can be distinguished in histological sections.

The best known and oldest map of these cytoarchitectural zones was developed by

Korbinian Brodmann in his 1909 treatise (Figure 6). These zones correspond

basically to functional areas of the brain such as vision, hearing, and motor control.

Figure 6. Brodmann’s 1909 cytoarchitectonic map of the human brain. The numbers inthe illustration designate areas that contain different combinations of cells and celltypes. These areas have also been linked to various functions, such as vision (Modifiedfrom Garey, 2006).



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Major subdivisions of the brain

The brain can be divided into four major regions, the brain stem (including

the medulla oblongata, the pons, and the midbrain), the diencephalon, the

cerebellum, and the cerebrum (Figure 7). These regions are derived from the basic

hindbrain, midbrain, forebrain pattern of all vertebrate animals. The retention of this basic pattern is considered an ancestral trait in mammals. The human brain

and the brains of other mammals have also evolved some derived traits that allow

them to survive in their environment. Comparative neuroanatomy indicates that

within the primate lineage the cerebrum (specifically the neocortex) and its

components have increased in size and changed in organization more than any

other structure in the brain (Rakic and Kornack, 2001). Other regions, like the

olfactory bulb, have remained relatively unchanged or diminished in relative size

through time (Radinsky, 1975; Stephan et al., 1988).

Figure 7. The major subdivisions of the human brain. The curved white band is thecorpus callosum that links the left and right cerebral hemispheres.

The brain stem contains structures that make connections with the spinal

cord and peripheral nervous system, as well as the cerebrum and cerebellum

(Figure 7). The medulla is made up of nerve fibers from the spinal cord and

bundles of neurons called nuclei transmit information to higher parts of the brain



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like the cerebrum. The pons (meaning “bridge”) carries fibers from the medulla

and spinal cord to the rest of the brain, but also links sensory information

processed in the cerebrum with the cerebellum. The midbrain relays some visual

and auditory information from the brain to the sense organs as well as information

coming from the cerebellum (Barr and Kiernan, 1993). Your midbrain is activated

when you try to watch a tennis match and track the ball going back and forth over

the net. The diencephalon is located at the center of the brain and contains the

thalamus and hypothalamus among other important components. The thalamus is

a central relay station for information passing from the peripheral nervous system,

midbrain, cerebellum, and cerebrum. It’s composed of densely packed nuclei that

connect to areas in the cerebral cortex that process sensory (such as visual)

information. The hypothalamus produces proteins called releasing factors that

stimulate the pituitary gland to release hormones such as follicle-stimulating

hormone, luteinizing hormone, growth hormones, and thyroid-stimulatinghormones into the blood. These hormones activate target cells throughout the body

that control reproduction, growth, and metabolism (Barr and Kiernan, 1993).

The cerebellum is the second largest component of the brain and is best

known for its involvement in keeping the body balanced and allowing it to make

smooth, coordinated movements. Think about the combination of breathing,

muscle-control, and balance it takes for a major league pitcher to hurl a baseball

across the plate or the agility required for a gibbon to swing itself, Tarzan-like,

through the forest canopy. Without a properly functioning cerebellum, this

symphony of movement would be more like a hurky-jerky cacophony. Over the

past several years, neuroscientists have also linked the verbal acrobatics of humans

to parts of the cerebellum. It seems that searching for the right verb with the

correct conjugation also requires the processing expertise of the cerebellum.

The cerebrum is the largest component of the brain and contains as many as

100 distinct cortical areas that process information and initiate interaction with the

environment. The next section discusses the major subdivisions of the cerebrum in

more detail.



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Box 2 Functional mapping of the brain

How do we know what parts of the brain serve which functions?

The most celebrated early evidence for localizing a function to a

specific area of the brain was described by Paul Broca in the early 1860s.

Broca, a French surgeon and anthropologist, autopsied the brain of a manwho was both paralyzed on his right side and essentially speechless. The

man, who Broca called “Tan,” could understand when he was spoken to but

could only utter one word – “tan”. Broca observed that an injury had

destroyed the left inferior part of the frontal lobe (the inferior frontal gyrus)

of Tan’s brain. Armed with behavioral, clinical, and anatomical evidence,

Broca documented the existence of the motor language area located in the

inferior frontal gyrus of the left side of the brain that now bears his name

(Cooper, 2006).

Today, with the aid of noninvasive advanced medical imaging

technology, neuroscientists and physicians are able to expand and deepenour understanding of where functions are processed in the living, thinking

brain. Cognitive neuroscientists are actively designing experiments that are

used to map brain activation during motor and cognitive tests to specific

regions of the brain. Of greatest importance to anthropologists are those

behaviors that are either adaptive or unique to humans and nonhuman

primates such as advanced problem solving skills, planning behavior,

complex reward seeking, and even recognizing the faces of familiar and

unfamiliar species members.

Occipital lobe

The occipital lobe is most closely associated with the sense of vision. It’s

located at the rear, or more correctly caudal (meaning “toward the tail”), portion of

the brain. Here we find the primary visual cortex (PVC) that receives visual

stimuli from the eyes. This region of the brain is also sometimes referred to as

Brodmann’s area 17 (Figures 6). The PVC is delineated in primates by the lunate

sulcus and is surrounded by the secondary association cortex (Brodmann’s areas

18 and 19) that serves to add detail and context to the raw image received by thePVC. Much of the human visual cortex is buried deep in the interhemispheric

space between the two cerebral hemispheres. This makes the human visual cortex

look small on the external surface of the brain compared to other primates, but it is

in fact larger in absolute volume (Stephan et al., 1988). During primate

evolutionary history, it’s possible that the visual cortex was “pushed around” by

increasing association areas in other parts of the cerebrum.



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Temporal lobe

The temporal lobe is located below (inferior to) the Sylvian fissure and

contains the primary auditory cortex, as well as auditory association cortex. The

left temporal lobe contains a number of areas devoted to language such as the

planum temporale and Wernicke’s area (Figure 9). These regions are alsorepresented on the right hemisphere but don’t perform the same function. Most of

the primary auditory cortex and all of the planum temporale are buried within the

Sylvian fissure and aren’t visible on the exterior surface of the brain.

Lesions (or pathological alterations) in the association cortex around the

primary auditory cortex may lead to a bizarre condition called auditory agnosia or

word deafness. A patient with auditory agnosia experiences spoken words as just

random sounds or noises. Damage to Wernike’s area causes both word deafness

and word blindness (alexia). Patients with damage to this area can speak fluently

but incoherently, producing what is described as “word salad”. The temporal lobe

is clearly essential for integrating sound information with meaning (Falk, 2000).

Figure 9. Primary areas and regions of interest in primate and hominid brain evolution.

The border of the Visual area is delineated by the lunate sulcus.Another condition that’s caused by damage to either the left or right

temporal lobe is called prosopagnosia. Prosopagnosia is the inability to recognize

and identify faces. A patient with prosopagnosia is able to visually perceive faces

but finds it impossible to use the details of a person’s face to determine his or her

identity (Purves et al., 1999). From an evolutionary perspective, we can see how

the function of the temporal lobe is integral to the social lives of primates. A



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highly developed ability to recognize group members and take cues from

conspecifics must have provided a strong selective pressure in the highly social

primate order.

Parietal lobe

The parietal lobe lies caudal to the central sulcus, rostral to (in front of) the

lunate sulcus, and superior to (above) the Sylvian fissure (Figure 8). It contains

the primary sensory area as well as a large association area. The primary sensory

area receives and interprets input from the sense organs throughout the body.

The human parietal association cortex has been shown to be involved in a

diverse array of functions such as attention focusing, visuomotor control, and the

discrimination of size and orientation of objects (Culham and Kanwisher, 2001).

This multimodal association region has increased in size throughout human

evolution. The parietal lobe is a good region to look for derived characteristics

during primate evolution because of the diverse functions it controls. The volumeof the parietal lobe may not be much larger relative to total brain size than that of

living great apes but reorganization of the internal wiring could have played a

major role in the behavioral and intellectual advancements seen in the genus Homo

(Semendeferi, 2001; Bruner, 2004; Holloway et al., 2004; Falk, 2006)

Frontal lobe

The frontal lobe is the region of the brain most often associated with an

individual’s personality and character. This region receives input from the outside

world and from primary and association areas all over the brain. The frontal lobe

is our executive director and modifies sensory and association information before

it’s acted upon. Contained in this lobe are the primary motor cortex located rostral

to the central sulcus, its adjacent premotor association areas, Broca’s motor speech

area, and another region of special importance to hominid brain evolution, the

prefrontal cortex.

The functional importance of the frontal lobe is revealed when it’s damaged

by a tumor, surgical intervention, or more temporarily with recreational drugs. We

all know that we have thoughts and emotions that we control based on the social

setting in which we find ourselves. Someone with a damaged frontal lobe

(particularly if the damage is bilateral) is less well able to govern the expression of thoughts and emotions that rise up in daily life. For example, people with

bilaterally impaired frontal lobes, who in the past were modest and reserved, may

become boastful and outrageous. These people may not be able to adjust their

behavior based on whether they’re surrounded by friends or in the company of

their employer. They may also lose the ability to make future plans and to adapt to



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changing environments. All of these impairments take place while relative

intelligence, memory, and ability to learn remain intact (Purves et al., 1999).

When we look at these functions from an evolutionary perspective, we can

see how important the frontal lobes have been in the development of our species.

The frontal lobes are an integral part of language, the manipulation of and the

adaptation to social environments, and the ability to plan ahead. All of these

behaviors and skills are highly adaptive in an ever-changing environment.

HUMAN BRAIN DEVELOPMENT

Now that we’ve explored the basic anatomy and function of the human

brain, we turn to the topic of how the brain grows. The study of growth and

development from infancy to adulthood is called ontogeny. With the aid of three-

dimensional MRI technology, neuroscientists have been able to provide detailed

evidence of the maturation rates of different cortical regions of the brain and have

even linked the development of the cortex to measures of intelligence. The morewe understand about the ontogeny of human brain development, the easier it is to

interpret the developmental processes of extinct hominid species.

At birth, the human brain has achieved only about 24-31% of its adult

volume (DeSilva and Lesnik, 2006), but it’s blessed with an overabundance of

gray matter. The neurons in this cortical gray matter come together to form an

equally abundant network of synapses. After birth the brain continues to grow at a

relatively fast pace but it’s accompanied by a period of selective pruning as well

(Feldman, 2004). The overabundance of neurons, like the sucker branches of a

fruit tree, are pruned away to create an efficient (and fruitful) result.In an impressive and rare longitudinal study1, Gogtay and colleagues

scanned the brains of 13 healthy children every two years for 8 to 10 years. The

resulting brain development maps show the trajectory of brain maturation from

childhood (around 5 years old in this case) to adulthood (age 20). The researchers

hypothesized that the refinement of gray matter throughout the brain would follow

the functional developmental pattern seen in children. In fact, this is exactly what

they found (Gogtay et al., 2004).

The first areas to begin to mature are the primary sensory cortices in the

parietal lobe and the frontal and occipital poles. The primary sensory cortex is

considered an ancestral cortical region and is therefore shared with the earliestcommon ancestor of primates. Maturation progresses in a back to front direction

over the primary motor cortex and eventually, at the end of adolescence, to the

prefrontal cortex. In general, association cortex matures later than primary cortex

1 A longitudinal study is a type of research method that enables scientists to observe and measure the same study

participants over long periods of time. This type of research is valuable but also very challenging because

researchers and participants both need to be committed to the project for many years.



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Out of 100 people, only about 10 are more comfortable using their left hand

to write or do other basic tasks like turning a key in a lock. The remaining 90

people are much more comfortable with their right hand. This is what scientists

call functional lateralization. A great deal of research has shown that other

primates have hand preferences when performing certain tasks (like reaching for

food or carrying an infant), but no other species is as lateralized for handedness as

humans. Nonetheless, the segregation of tasks within the brain of monkeys and

apes suggests that lateralization is a very old primate characteristic. I am reminded

of how lateralized I am everyday when I unlock the door of my car. In a strict

sense, it’s possible for me to use my left hand to turn the key, but the action seems

effortless with my right hand.

How do we explain such clear functional lateralization in handedness? From

an evolutionary perspective, there doesn’t appear to be any compelling selective

pressure to use one hand over another. In fact, we might even argue the opposite.

Think about a switch-hitting batter in baseball. A switch hitter can choose to batlefty or righty as a consequence of the pitcher’s handedness. Being comfortable in

all tasks with both hands would seem to be more beneficial for survival. There

must be another reason why humans have become so lateralized for hand

preference.

Another functional lateralization that you have already read about is the

location of the language centers in the brain. Broca’s and Wernicke’s areas are

located in the left hemisphere in the vast majority of people. Language is clearly

an adaptive trait in humans and must have had positive selective pressure through

human evolution. Interestingly, the motor language area and the primary motor

area for the right hand are quite close to each other in the left hemisphere of the

brain. Perhaps these areas developed synergistically and influenced the strong

lateralization effects we see today (Falk, 1987). This hypothesis may work for

humans but other primates don’t possess the same language skills that humans do.

So why are side preferences and brain asymmetries found in other primates?

It turns out that as brain size increases (and remember primates in general

have big brains), lateralization also increases, in part, because of the mechanical

demands of brain wiring (Falk, 2006; Hofman, 2001). In order to maintain

efficient communication between the parts of the brain, the whole brain becomes

modularized. The left and the right sides take on different responsibilities in orderto reduce the need for excess and inefficient wiring. This also has the fortunate

consequence of opening up “functional space” for more and varied association

areas. Language areas that control comprehension and the motor component of

speech are on the left, while areas that control the tone of language and other more

subtle functions reside in the right hemisphere (Falk, 1992). Table 1 provides a list

of functional differences between the left and right sides of the human brain.



20

Table 1 Functions of the Left and Right cerebral hemispheres (Falk, 1992)

Functions associated with the left

hemisphere

Functions associated with the right

hemisphere

Analytical processing

LanguageRight hand

Time sequencing

Global, holistic processing

Visuospatial skillsLeft hand

Recognizing faces

Tone of voice

Musical ability

Emotions

Humor/ Metaphor

These functional asymmetries are also accompanied by morphological

asymmetries that can be detected in whole brains or endocranial casts (discussed

below). If we look at the shape of the human brain from above, we often see thatthe frontal pole (or tip of the lobe) on one side extends a bit beyond the frontal

pole of the other side and is a little wider laterally (Le May, 1976). The same is

true for the occipital pole. These asymmetries are called petalias. The right

frontal/ left occipital petalia combination is correlated with right-handedness in

humans. Left-handed people tend to be more symmetrical in this aspect of brain

morphology, but are more likely to show the opposite pattern to that depicted in

Figure 10. Interestingly, men tend to show a greater degree of frontal/occipital

petalias and are more lateralized, in general, than women. Perhaps this is why I

have such a hard time opening my car door with my left hand? If an oldNeandertal male had had a car, it looks like he would have had a hard time opening

the door with his left hand too (Le May, 1976).



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Figure 10. Human endocast in superior view. Arrows indicate right frontal and leftoccipital petalias.

SEXUAL DIMORPHISM IN THE HUMAN BRAIN

Just as we cannot say that the brains of small children are simply small

versions of the adult brain, we cannot say that the brains of women are just smaller

versions of the brains of men. To put a twist on a favorite old cliche, it might be

more accurate to say that women’s brains are from Venus and men’s brains arefrom Mars. This is perhaps an exaggeration, but it makes the point that when

studying brain anatomy, function, and evolution, biological sex matters (Cahill,

2006).

Humans are a sexually dimorphic species. On average, men are larger than

women and have larger brains. Statistics vary from study to study, but in one large

sample the brain weight of men was approximately 1392 grams and that of women

was approximately 1252 grams (Ho et al., 1980). This difference has often been

attributed simply to the effects of brain to body scaling. Interestingly, Dean Falk

and her colleagues have shown that in a large sample of 414 human males and390 females , the overwhelming majority of the difference between brain weight in

men and women is due to sex difference rather than simply to body size difference

(Falk, 2001). At the simplest level of size, the brains of men and women differ.

Hundreds of neurological experiments have also shown that males tend to

excel on tests of visuospatial skills, like mental rotation of objects, map reading

and maze tracing, while females tend to excel in language-oriented skills like



22

reading comprehension, writing, and word puzzles. There is, however, extensive

overlap between the two. So it’s not unusual to see a female airline pilot or a male

talk show host, but the (average) differences are real (Falk, 1987; Falk, 2004a).

Underlying these functional contrasts in the brains of men and women is

another anatomical difference. Men have less gray matter as a total percentage of

volume than women and women have less white matter as a percentage of total

volume than men. In other words, women, even though they have smaller brains,

have a higher percentage of that volume dedicated to the job of processing

information. Males, on the other hand, have a larger portion of brain volume

dedicated to passing information back and forth between the different cortical

regions (Falk, 2001; Gur et al., 1999). Let’s put it this way, if you and your family

are lost in the middle of the woods and you need to pick a partner to go search for

help, you should probably choose a male relative. When you get back from your

harrowing adventure, choose a female relative to help you write the book.

When did this variation in skills and brain anatomy appear in humanevolution? We can only speculate. Some researchers have promoted the

manufacture and use of tools as an important selective pressure for visuospatial

skills in males. Other researchers prefer the idea that males were most likely to

hunt and search out mates in other groups. Still others focus on the complexity of

the social group and the need to band together to be successful as a selective

pressure for this pattern of functional abilities. No matter what the “true” answer,

the more we know about the differences between male and female primates, the

better informed our hypotheses will be. I should also note that although the

differences between male and female brains is real, these differences in no way

imply superiority or inferiority of one over the other.

COMPARING NONHUMAN PRIMATE AND HUMAN BRAINSScientists interested in the evolution of the human and nonhuman primate

brain follow two main methods of inquiry. The first method is the indirect or

comparative method of study, which uses the brains of living species of primates

and compares them to modern human brains. The second method is called the

direct method and uses endocranial casts (endocasts) derived from fossil crania to

make comparisons with endocasts of living and extinct species. The terms

endocranial and endocast refer to structures on the inside of the skull.

The indirect or comparative neuroscientific method

The indirect or comparative method of studying brain evolution allows

scientists to examine both the external, internal (hidden), and microscopic structure

of the brain in a full range of primate species. The comparative method is called

“indirect” when applied to evolutionary studies because it uses living species to



23

determine the structure and function of the common ancestor of the groups being

studied. For example, to understand something about the brain of the common

ancestor of chimpanzees and humans, we would study the brains of humans,

bonobos, and chimps.

In this approach, basic anatomical measurements can be paired with

ecological, behavioral, and experimental data about these species to build a fuller

picture of how the brain is influenced by the environment. For example, the

strongly delineated cortical layer IV of the visual cortex in tarsiers can be linked

directly to the large eyes and nocturnal behavior of this primate (Zilles, 2005) (see

Chapter 6 to see a tarsier.)

Examples of functional reorganization in the brain

Until quite recently, neuroscientists generally agreed that the visual cortex of

monkeys, apes, and humans were essentially identical (Preuss, 2001). This makes

sense if we remember that stereoscopic vision and color vision are primaryfunctions that characterize the primate pattern and are vital to the survival of these

species. By studying the cortical microanatomy (meaning the pattern and types

of cells) of the visual cortex, Todd Preuss and his colleagues have shown that

information passing into the visual cortex from the brain’s main visual relay station

is distributed differently in humans than in monkeys and apes. This different

distribution pattern contradicts the idea of a uniform visual cortex in all primates

and raises the possibility that some selective pressure unique to human evolution

influenced this alteration. The researchers hypothesize that the human ability to

analyze fast moving visual stimuli may aid in the process of deciphering the rapid

movements of the mouth (and body) with speech (Falk, 2006; Preuss, 2001).

Katerina Semendeferi and her colleagues, in a series of groundbreaking

studies, utilized 3-dimensional MRI technology to compare sulcul patterns and

brain volumes in humans and great apes. Table 2 shows results from a

comparative analysis of whole brain and brain component measurements

(Semendeferi and Damasio, 2000). As you can see from the table, human brain

size is much larger than even the gorilla, whose body size is considerably larger

than the average human’s. The same can be said for all of the major subdivisions

of the brain. However, there is a twist.

As you read earlier, the frontal lobe is the region that controls highercognitive functions, such as decision-making, planning, and the regulation of

emotions. Scientists have long suspected that the frontal lobe is, if not entirely

unique to humans, larger and more complex than in any other primate species. If

we look solely at absolute brain volume, this is certainly the case. However,

Semendeferi and Damasio also looked at what percentage of total brain size is

comprised of frontal lobe and found that great apes and humans hardly differ. The



25

1950). Figures 11 and 12 demonstrate the morphology that is visible on a typical

human endocast.

Figure 11. A rubber copy of the human brain and an endocast of the internal cranium of the same individual. Notice that there are very few distinguishable sulci and gyri in thelarger human endocasts. Also note that the base of the human cast reproduces moredetail than the sides or top of the cast.

Endocasts can also be formed naturally through the fossilization process as

seen in the famous Taung child from South Africa. In the case of Taung, after the

flesh and brain tissue decayed away, the cranium of the three year-oldaustralopithecine tipped over on its right side and sediment filled the cavity to form

a hemi-cast of the right endocranial surface. Fortunately, the fossil’s discoverer,

Raymond Dart, was an expert in neuroanatomy and immediately recognized that

the endocast and its associated cranium were the remains of a creature that had

never before been described (Dart, 1925) (see Chapter 11).

The most recent and most technologically advanced method for making

endocasts comes from the use of 3-dimensional computed tomography (CT).

Three-dimensional CT technology allows researchers to create “virtual” endocasts

of precious fossil crania without the fear of damaging or destroying delicate

internal cranial structures in the casting process (Falk, 2004b). Virtual endocasts

can be manipulated with computer software in a variety of ways to obtain

morphometric data (volume, linear measurement), to “virtually” remove tough

sedimentary matrix, and to facilitate reconstruction of partial or warped casts

(Conroy et al., 2000).



26

Figure 12. Lateral view of a human endocast.

Paleoneurology, the study of fossil endocasts, can sometimes be

controversial because not all scientists agree about what the bumps and grooves on

endocasts represent (Holloway et al., 2002; White and Falk, 1999). Similarly,

fragmentary fossil crania need to be reconstructed in order to evaluate endocranialmorphology, and reconstructions leave room for disagreement as well (Falk et al.,

2000).

PRIMATE PALEONEUROLOGYPaleoanthropologists are interested in the evolutionary history of the Primate

Order because it allows them to track the sequence and the timing of changes over

primate evolutionary history. In the case of hominid brain evolution, we are

particularly interested in when and why the primate brain changed from a small,

rather smooth-surfaced, and tapered appearance to the large, convoluted, androunded shape of modern ape and human brains. The history of these changes is

wrapped up in the adaptive shifts that took place during primate evolution,

bringing nocturnal animals into the day and arboreal animals onto the ground.



27

Adaptive shifts in primate evolution that impact the brain

There are three main shifts in brain morphology that took place during

primate evolution: a reduction in the relative importance of olfaction, an increase

in the relative importance of vision, and an increase in the relative importance of

the neocortex (Radinsky, 1975) (Figure 13). What are the ecological factors thatinfluenced these changes in brain morphology?

Figure 13. The brains of A. Bushbaby B. Macaque C. Chimpanzee drawn toapproximately the same size to eliminate the effects of body size. (In reality, thebushbaby brain is 9 times smaller than the macaque brain and 40 times smaller thanthe chimpanzee brain.) F=frontal lobe, OB=olfactory bulb, cb=cerebellum. Notice thesize of the olfactory bulb and its position in front of the frontal cortex, the size of thefrontal cortex, and the position of the cerebellum under the cerebrum in the monkey andape. (Modified from the Comparative Mammalian Brain Collections websitehttp://brainmuseum.org from the University of Wisconsin-Madison Brain Collection.).

Modern pottos, lorises, galagos, tarsiers, and some lemurs have retained the

primitive adaptation of being active at night. These prosimians are generally small

and susceptible to predators like cats and large birds. Nocturnal activity paired

with cryptic coloration or super-leaping ability helps these primates avoid

predators. In order to communicate with each other, especially when it comes to

finding mates, nocturnal prosimians use olfactory cues like urine-scenting and

other odiferous secretions. This reliance on olfaction is seen in the relatively large

olfactory bulbs of prosimians (Figures 13 and 14).

With the exception of the South American owl monkey, all anthropoids(monkeys, apes, and humans) are diurnal. Diurnal species rely on visual cues to

navigate their world. Because of their visual acuity and color vision, primates can

easily detect things like the bright coloration of a ripe fruit, or the red swelling of

potential mate’s rump; thus they use and rely upon visual input to be successful in

their environment. The ability to see in three dimensions is also a highly adaptive

trait because primates spent much of their evolutionary history up in trees. Falling



28

is a major selective pressure in arboreal animals. This pressure is mitigated by

visual acuity and stereoscopic vision (as well as, of course, grasping hands, feet,

and sometimes, tails). The relative increase in the size and complexity of the

visual cortex in primates and anthropoids, in particular, is an adaptive shift of

major importance.

The third adaptive shift relates to the increase in both relative and absolute

size of the neocortex. The neocortex, as we have already seen, is involved in the

processing of information from all of the different senses. During anthropoid brain

evolution, primates with more association areas in which to process information

were more likely to pass on their genes to the next generation. They were cleverer,

they had greater behavioral flexibility, and they were better able to adapt to

changing environments.

Figure 14. Percentage of brain volume dedicated to olfactory bulbs, visual cortex, andneocortex from the Stephan et al. (1988) data set.

Eocene (56 – 33 m.y.a.)

The first major adaptive radiation of primates began in the Eocene epoch 55

million years ago. This epoch saw the spread of small tarsier-like (omomyid) and

larger lemur-like (adapid) primates around the world. The endocranial evidence

for these groups of early primates is fragmentary and scarce, yet it reveals that

brain size relative to body size was small, that olfaction was still quite important,

and that vision was becoming better developed.



29

Similar to modern prosimians, the olfactory bulb impressions of both

omomyid and adapid endocasts are relatively large and protrude beyond the frontal

lobe (see Chapter 7 in your text for a modern example). The surfaces of the

endocasts show only a few sulci such as the Sylvian fissure and a typically lemur-

like coronolateral sulcus in the adapids (Figure 15). The coronolateral sulcus

courses antero-posteriorly or front to back along the cortex. This is different from

the more complex mediolateral direction seen in monkeys. This simpler sulcul

pattern is, in part, due to a simpler brain anatomy but is also a product of small

brain size. (Remember, smaller brains tend to have fewer sulci and gyri than larger

brains, even within the same phylogenetic grade.) The estimated cranial capacity

of the largest Eocene specimen is less than 11 cm3

or about a third of an ounce

(Falk, 2006; Gurche, 1982; Radinsky, 1975).

The frontal lobes of Eocene primates aren’t well developed, but the parietal

and occipital regions are enlarged in comparison to other early mammals. This

small but detectable increase in the visual cortex of the brain foreshadows furtheradaptive changes in visual cortex during the evolution of monkeys and apes.

Figure 15. A. Prosimian brain showing the direction of the coronolateral sulcus; B. NewWorld monkey brain showing the central sulcus separating the primary sensory andprimary motor areas; C. Old World monkey brain. Notice the increase in the number of sulci and the shifted direction of the central sulcus in comparison to the coronolateralsulcus of the prosimian brain.



30

Oligocene (33-23 m.y.a)

The best paleoneurological evidence from the Oligocene is for the species

Aegytopithecus zeuxis from the Fayum in Egypt. The cranial capacity of this small

monkey-sized arboreal primate is only about 30 to 34 cm3or about an ounce and it

shares some primitive characteristics with early prosimians (Radinsky, 1973).Although slightly reduced in comparison with Eocene primates, the olfactory bulbs

are still relatively large and extend beyond the frontal lobe in a characteristically

prosimian manner. However, the endocast also reveals some morphology that

foreshadows the brain development of both monkeys and apes.

Miocene (23 – 5 m.y.a.)

The Miocene has been called the “golden age of hominoids” because of the

diversity of species, habitats, and fossil skeletons that come from this epoch. This

is also the epoch where we begin to see the divergence of the ancestors of great

apes and Old World monkeys from the main trunk of the hominid family tree. Thefossil specimen that best exemplifies the paleoneurological evidence of the

Miocene is the endocast of Proconsul .

The endocast of Proconsul africanus has been reconsidered many times

since its original publication in the 1950s. Compared to Aegytopithecus, Proconsul

had a much larger body size and a cranial capacity that was close to 150 cm3. The

endocast is warped and fragmentary, so a definitive capacity is difficult to

determine. Nonetheless, the sulcul pattern of this 18 MY old African Miocene

fossil indicates that the frontal lobe had begun to expand and become more

complex. The central sulcus of this endocast is transverse, the visual cortex islarge, and there are additional frontal sulci that reveal a more complex neocortex

(Falk, 1982).

The Proconsul endocast indicates that the brain of this Miocene primate was

not as derived as modern ape brains, but does follow the trend of increasing size,

increasing visual cortex, and increasing neocortical complexity seen in later

hominoids.

HOMINID PALEONEUROLOGY

Early fossil hominid remains from all over central, eastern, and southern

Africa show that hominids were bipedal long before there was any significantincrease in absolute brain size. Fossil hominids (Sahelanthropus, Orrorin,

Ardipithecus) from the earliest phase of hominid evolution, between 7 and 4

million years ago, so far have not provided a great deal of paleoneurological

evidence. The evidence of brain evolution in the hominid phylogeny becomes

more complete about a million years later with the remains of australopithecines.



31

Aside from the important 3-fold increase in size between the

australopithecines and modern Homo sapiens, there are several other important

trends that can be elucidated by studying fossil hominid endocasts. Endocasts are

judged on their level of apelike and humanlike qualities throughout hominid

phylogeny. Apelike or humanlike appearance in overall shape or sulcul pattern

suggests a certain level of cognitive achievement or behavioral flexibility. When

we use “apelike” or “humanlike” to describe an endocast, we simply mean that the

hominid being studied shares some primitive traits with the common ancestor or

derived traits with later specimens. These terms don’t suggest that fossil hominids

were apes or more closely related to living apes. After all, living chimpanzees

have been evolving for the past 6 or 7 million years too.

The shape of the human endocast is rounded and globular with the

cerebellum tucked up underneath the cerebral hemispheres. The endocasts of apes

are more elongated and lower, with the cerebellum extending more toward the

back of the cerebrum. The shape of early hominid endocasts tends to fall inbetween these two morphologies. We can also examine the pattern of sulci that is

reproduced on the endocast surface. Although controversial, early hominids tend

to show a more apelike pattern, reminiscent of the common ancestor with

chimpanzees, while later fossils classified in the genus Homo tend to show more

derived sulcul patterns (Falk, 1980). To illustrate these trends, we will look at a

few examples.

Robust and gracile australopithecine endocast morphology

Chapter 11 of your textbook outlines the basic morphological and

behaviorial differences between robust and gracile australopithecines. Research

over the past 20 years has also revealed paleoneurogical differences between these

two groups of hominids.

Typical of paleoanthropological research, the similarities and differences

between the gracile and robust australopithecines have generated a great deal of

scientific debate. Still, it’s well established in paleoanthropology that the line

leading to the specialized robust australopithecines eventually became extinct. On

the other hand, some researchers argue that the gracile australopithecines may be

the progenitor of the human lineage (Falk, 2004a). If the gracile australopithecines

are related to members of the genus Homo, we might expect to find some derivedcharacteristics that are not shared with the robust forms. However, the

paleoneurological record is up for interpretation and cannot be simply read like a

book as the following examples will show.



32

Taung

The Taung child provides an excellent example of the kind of interpretation

difficulties that can arise in paleoneurology. When Raymond Dart originally

described this fossil, he mistook the impression of the lambdoid suture (the

location where the parietal and occipital bones knit together) for the lunate sulcus(Falk, 2004a). The lunate sulcus, you will remember, is the depression that

delineates the primary visual cortex from secondary association areas in the

occipital lobe. By misinterpreting the location of this sulcus, the endocast of

Taung looked to have a humanlike occipital lobe. This was, of course,

tremendously important from a psychological point of view because it gave the

small “man-ape” from South Africa an air of legitimacy in the human family tree.

In a reanalysis of South African fossil hominid natural endocasts, Falk (1980)

demonstrated that a small dimple in an apelike position was more likely the

position of the lunate sulcus. Holloway disputed this location and preferred a

rearward position that would make the gracile australopithecine occiput look morehumanlike. No one will ever know the true location of the lunate sulcus in the

Taung specimen, but further evidence from the front part of the endocast suggests

that if Taung had a humanlike occiput, it had an apelike frontal lobe.

Thus, the Taung lunate sulcus is only part of the story. The relatively

complete frontal lobe of this specimen reveals both a sulcul pattern and sulcul

density that is reminiscent of ape morphology (Falk, 1980). One example of this

apelike morphology is the presence of a fronto-orbital sulcus that is nonexistent in

human brains and shared with great ape brains (Figure 16). The fronto-orbital

sulcus of a chimpanzee brain is depicted in Figure 16B. This suggests that thesulcul pattern of australopithecines had a primitive characteristic shared with the

common ancestor of hominids and chimpanzees. Humans have a derived pattern

of gyri and sulci caused by reorganization frontal cortices (Figure 16A).



33

Figure 16. A. Human brain B. Chimpanzee brain. fi = inferior frontal sulcus, R’=horizontal branch of the Sylvian sulcus, R= ascending branch of the Sylvian sulcus, S=Sylvian fissure, ls = lunate sulcus, fo = fronto-orbibital sulcus. Notice the relativelyforward position of the lunate in the chimpanzee brain.

Reconstructing robust endocasts

Reconstructing fragmentary fossil hominid endocasts is a tricky business.

The process takes both artistic talent and an intimate knowledge of human, ape,

and fossil hominid endocranial anatomy. It is therefore crucial that early

reconstruction attempts are analyzed critically when new fossil hominid material is

discovered. In 2000, a group of researchers did just this with robust

australopithecines.

Early reconstructions of robust australopithecine endocasts were apparently

based largely on more complete endocranial remains from gracileaustralopithecines like the famous Mrs. Ples fossil, Sts 5 from Sterkfontein, and

thus resembled gracile australopithecines in some ways (Falk et al., 2000). With

the discovery of several new robust australopithecine remains, Falk and her

colleagues showed that the frontal lobes of robust australopithecines were much

different in shape than gracile australopithecines. It appears that the robust



35

described as Broca’s motor speech area. It would therefore be very easy to jump to

the conclusion that ER 1470 was capable of humanlike speech but this conclusion

would be hasty. We don’t know if Brodmann’s areas 44 and 45 in humans are

directly comparable to the same areas in ER 1470, so we can only cautiously state

that the sulcul pattern of ER 1470 more closely resembles the pattern of modern

humans than modern apes and australopithecines.

Late Homo

The endocasts of later hominids like Homo erectus and Homo

heidelbergensis (as well as Neandertals) are large and reproduce humanlike

asymmetries (petalias and “Broca’s caps”), but tend to differ in sulcul details seen

in earlier smaller-brained hominids. The reason for this variability is unknown, but

it may have something to do with the thickness and durability of the brain’s

protective coverings called the meninges. The shape of Homo erectus endocasts is

reminiscent of the cranium’s wide base and long, low profile. The leap towardhuman morphology is evident in H. erectus, but the frontal lobes don’t appear as

round and prominent as in the modern endocast. Neandertal endocasts are large

and clearly encephalized but possess uniquely protuberant occipital lobes that may

reveal an expansion of visual cortex in these extinct hominids (Holloway et al.,

2004).

The archeological record of these later Homo species shows that these

groups had migrated around the Old World, used complexly designed tools, and

lived in larger, more complex groups. Large, complex neocortices with greater

association capacity were clearly positive adaptations for these hominids. In fact,

the size of the neocortex is highly correlated with social group size and complexity

(Dunbar, 1998). Cooperation and social interaction puts a premium on efficient

symbolic communication, which in turn, contributes to higher reproductive success

in even the harshest environments.

Homo floresiensis

In October 2004 Peter Brown and his colleagues introduced to the world a

new species of hominid called Homo floresiensis that survived until around 13,000

years ago, making it the latest surviving species in the genus Homo other than our

own. The bones of H. floresiensis indicate an extremely small stature for amember of the genus Homo (about 3 feet tall) and an equally small cranial capacity

(417 cm3) (Falk et al., 2005). The skeletal remains were also associated with a

diverse group of well-made stone tools (see Chapter 14).

Fortunately, among the skeletal remains was a very well-preserved cranium

that enabled paleoanthropologists to create a virtual endocast using 3-dimensional

CT. The endocast of the specimen called Liang Bua 1 (LB1) was compared to a



36

series of ape, human, and fossil hominid virtual endocasts. The results of this

study showed that although the endocranial volume of LB1 is similar to the much

more ancient australopithecines, the shape is most similar to that of Homo erectus

from Asia. Like H. erectus, the endocast is wider at the back - and lower portions

are long and low in profile (Falk et al., 2005). Furthermore, the pattern of sulci on

the frontal, parietal, and occipital lobes appear to be humanlike in configuration.

The position of the lunate sulcus is similar to that in humans, and the distinctive

apelike fronto-orbital sulcus is absent. Other than its extremely small size, the

endocast of LB1 is clearly quite derived. There has also been some suggestion that

this individual suffered from a genetic disorder called microcephaly that leads to

abnormally small brain size. Based on comparative evidence, this suggestion

doesn’t seem to be the best explanation for the morphology that is reproduced on

the LB1 endocast. It appears normal in shape, just small in size, although debate

continues (Falk et al., 2006; Falk et al., 2007; Jacob et al., 2006; Martin et al.,

2006; Weber et al., 2005).This fascinating find extends the range of Homo cranial capacity from the

1600 cm3

high of Neandertals to the low 417 cm3

of H. floresiensis (Figure 3).

What does this range say about the intellectual capacity of hominids from one end

of the spectrum to the other? Did the “hobbits” from Flores use symbolic

communication like modern humans? Did they use their brains to manipulate the

social environment to “get ahead”? How much brain tissue does a species need to

be considered human? The questions are easier to pose than the answers are to

find – or harder yet – to get scientists to agree about.

SUMMARYEvolution has acted on the brains of primates to mold a highly flexible,

intelligent, and visually-oriented organism. Anthropoids (monkeys, apes, and

humans) tend to diverge more than prosimians from the less-derived brain

morphology of the common ancestor of primates. The brains of anthropoids reflect

a clear reduction in the neural tissue allocated to olfaction, an increase in visual

processing cortex, and a trend toward more complicated association areas in the

neocortex. These trends are maximized in the hominid lineage. Small advances in

neocortical association areas and brain wiring enabled hominids to plan ahead,

work in groups, and communicate more effectively. Brainpower began to replacebrute strength, and the energy demands of successful adaptive responses shifted.

The uncertain and dangerous environment of early hominids created an important

selective pressure on the brains of these terrestrial bipeds. Those individuals that

were able to eke out a living by using tools, planning migration routes,

remembering the location and season of fruiting trees, and hunting wild game were



37

more likely to pass on their genes to the next generation. Their most powerful tool

was their brain.

The brain is a powerful tool not only for competing with other species in an

ecological niche but also for competing with members of one’s own group. Social

intelligence or the ability to navigate a rich and complex network of family,

friends, and foes developed along with brain size during hominid evolution. When

it comes to keeping track of social relationships and large brains, Homo sapiens is

the species par excellence.

Today, the neurocentric age is in full swing (Zimmer, 2004). We learn in

leaps and bounds. Details of how we store memories, how we process thoughts,

how we form sentences, and how we solve problems will all become clearer in

time. New technologies will allow us to delve more deeply into the thinking brain

and to look more closely at intraspecific diversity (Holloway et al., 2004). It is a

very exciting time, indeed.

GLOSSARY AND KEY WORDS

Absolute brain size Total weight or volume of all brain structures

Action potentials Electrochemical impulses more commonly known as

nerve impulses

Alexia Word blindness

Ancestral Retaining the same traits as the common ancestor

Association areas Zones of integration of several modes of external

information

Auditory agnosia Word deafness

Axon Long tail-like extension from the body of the neuron

Brain stem The midbrain, the pons, and the medulla oblongata

Caudal Toward the tail or rear

Central nervous system The brain and the spinal cord

Chemical synapse Release of neurotransmitters across the synaptic cleft

that initiates an action potential

Coronolateral sulcus Sulcus seen in prosimians and early primates but not



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humans

Cortical microanatomy The pattern and types of cells in the cortex

Cytoarchitecture The cell composition of the neocortex

Derived Possessing different traits than the common ancestor

Diencephalon Brain region that contains the thalamus and

hypothalamus

Direct method Studying the inside of the cranium of fossils through

endocasts

Encephalized Having a larger brain than predicted by body size

Endocasts A cast (plaster, latex, silicon, or virtual) of the insideof the cranium

Exteroception Perceiving the external environment

Frontal pole The most anterior (front) part of the cerebral

hemispheres

Gyri (sing. Gyrus) The hills in the complicated neocortical pattern

Gyrification The pattern of hills and valleys that make up theneocortex

Homeostasis The body’s ability to maintain internal equilibrium

even as the environment changes

Indirect method Comparative method of studying brains where humans

are compared to other primates

Lateralization One side functions differently or has greater facility

with tasks than the other side of the body. Example:

handedness.

Lesions Pathological alterations in the brain caused by loss of

blood flow or physical damage

Lunate sulcus Furrow or valley that delineates the primary visual



39

cortex

Myelin Insulation around nerve cell axons

Neocortex Meaning “new cortex,” is the structure that contains

six layers of cells and controls the body’s higherfunctions like thinking

Neuroglia (glia) Support cells in the nervous system that provide

structure and insulation to neurons

Neurons Cell that is the basic functional unit of the nervous

system

Neurotransmitters Special signaling chemicals released by neurons

Nuclei Bundles of neurons

Occipital lobe Rear part of the brain that contains the primary visual

cortex

Occipital pole The most posterior (back) part of the cerebral

hemispheres

Ontogeny The study of growth and development

Parietal lobe Top rear part of the brain that contains essentialassociation areas

Peripheral nervous

system

Nerve fibers that extend out from the brain and spinal

cord to the rest of the body

Petalia The asymmetrical shape pattern of human brains. One

side usually projects forward more than the other and

is wider.

Planum temporale Area in the temporal lobe that is associated withlanguage in humans

Primary areas Early developing areas that have control over one

major modality (vision, hearing, motion, sensation)

Proprioception Perceiving your own body in its environment



40

Prosopagnosia The inability to recognize faces

Relative brain size Brain size divided by body size

Rostral Toward the front

Sulci (sing. Sulcus) The valleys in the complicated neocortical pattern

Superior Above in anatomical terms

Synaptic cleft The space between the terminal ends of one neuron

and another

Temporal lobe Lower lateral portion of the brain that contains the

primary auditory cortex

Venous sinuses System of veins that drains blood from the brain and

head

Wernicke’s area Area in the temporal lobe (mainly) that is involved in

the comprehension of language in humans

QUESTIONS FOR REVIEW

1. What is the difference between relative and absolute brain size and what can

these measurements tell us about the life and behavior of an animal?2. What are the major components of the human brain and what basic functions

are they responsible for?

3. How can studying brain development in human children help us better

understand the evolution of fossil hominid brains?

4. How is language linked to brain lateralization?

5. What are some of the basic differences between male and female brains?

6. What methods do scientists use to study brain evolution?

7. In what areas do human and prosimian brains differ most?

8. What are the major adaptive shifts in primate evolution that impact thebrain?

9. Why is the Taung child significant to scientists who study the brain?

10. Why is the discovery of Homo floresiensis significant for anthropologists

who study the brain?



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Falk D (1980) A reanalysis of the South African australopithecine natural

endocasts. Am J Phys Anthropol 53:525-39.

Falk D (1982) A reconsideration of the endocast of Proconsul africanus. In R

Ciochan and R Corruccini (eds.): New interpretations of ape and human

ancestry. New York: Plenum Publishing Company, pp. 239-248.

Falk D (1987) Brain lateralization in primates and its evolution in hominids.

Yearbook of Physical Anthropology 30:107-125.

Falk D (1992) Evolution of the brain and cognition in hominids: Sixty-Second

James Arthur Lecture on th evolution of the human brain. American

Museum of Natural History, New York.

Falk D (2000) Hominid brain evolution and the origins of music. In N Wallin, B

Merker and S Brown (eds.): The origins of Music: MIT, pp. 197-216.

Falk D (2001) The evolution of sex differences in primate brains. In KR Gibson

and D Falk (eds.): Evolutionary anatomy of the primate cerebral cortex.

Cambridge: Cambridge University Press, pp. 98 - 112.

Falk D (2004a) Braindance. Gainesville: University Press of Florida.

Falk D (2004b) Hominin brain evolution--new century, new directions. Coll

Antropol 28 Suppl 2:59-64.

Falk D (2006) Evolution of the Primate Brain. In W Henke, H Rothe and I

Tattersall (eds.): Handbook of Paleoanthropology. Amsterdam: Spring-

Verlag.

Falk D, Hildebolt C, Smith K, Morwood MJ, Sutikna T, Brown P, Jatmiko,

Saptomo EW, Brunsden B, and Prior F (2005) The brain of LB1, Homo

floresiensis. Science 308:242-5.

Falk D, Hildebolt C, Smith K, Morwood MJ, Sutikna T, Jatmiko, Saptomo EW,

Brunsden B, and Prior F (2006) Response to Comment on "The Brain of

LB1, Homo floresiensis". Science 312:999c.

Falk D, Hildebolt C, Smith K, Morwood MJ, Sutikna T, Jatmiko, Saptomo EW,

Imhof H, Seidler H, and Prior F (2007) Brain shape in human

microcephalics and Homo floresiensis. PNAS %R

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Rakic P, and Kornack D (2001) Neocortical expansion and elaboration during

primate evolution: a view from neuroembryology. In KR Gibson and D Falk

(eds.): Evolutionary anatomy of the primate cerebral cortex. Cambridge:

Cambridge University Press, pp. 30-56.

Semendeferi K (2001) Advances in the study of hominoid brain evolution:magnetic resonance imaging (MRI) and 3-D reconstruction. In D Falk and

KR Gibson (eds.): Evolutionary anatomy of the primate cerebral cortex.

Cambridge: Cambridge University Press, pp. 257-289.

Semendeferi K, Armstrong E, Schleicher A, Zilles K, and Van Hoesen GW (2001)

Prefrontal cortex in humans and apes: a comparative study of area 10. Am J

Phys Anthropol 114:224-41.

Semendeferi K, and Damasio H (2000) The brain and its main anatomical

subdivisions in living hominoids using magnetic resonance imaging. J HumEvol 38:317-32.

Shaw P, Greenstein D, Lerch J, Clasen L, Lenroot R, Gogtay N, Evans A,

Rapoport J, and Giedd J (2006) Intellectual ability and cortical development

in children and adolescents. Nature 440:676-9.

Stephan H, Baron G, and Frahm H (1988) Comparative size of brains and brain

components: Comparative primate biology.

Stephan H, Frahm HD, and Baron G (1981) New and revised data on volumes of brain structures in insectivores and primates. Folia Primatol (Basel) 35:1-29.

Weber J, Czarnetzki A, and Pusch CM (2005) Comment on "The brain of LB1,

Homo floresiensis". Science 310:236; author reply 236.

White DD, and Falk D (1999) A quantitative and qualitative reanalysis of the

endocast from the juvenile Paranthropus specimen l338y-6 from Omo,

Ethiopia. Am J Phys Anthropol 110:399-406.

Zilles K (2005) Evolution of the human brain and comparative cyto- and receptorarchitecture. In S Dehaene, J Duhamel, M Hauser and G Rizzolatti (eds.):

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FIGURE TITLES AND CREDITS

Figure 1. Brain volume measurements from Stephan et al. (1981) data set.

(modified after Rilling, 2006).

Box 1 Figure. Scaling: Isometry and Allometry

Figure 2. Double logarithmic scale comparing body weight in grams to brain

weight in milligrams of extant primates.

Figure 3. Cranial capacity (a measure of brain size) of fossil hominid specimens

derived from Holloway et al. (2004) and Falk et al. (2005).

Figure 4. The basic components of a motor neuron.

Figure 5. The human brain in occipital view. Modified from the Comparative

Mammalian Brain Collections website http://brainmuseum.org from the University

of Wisconsin-Madison Brain Collection.

Figure 6. Brodmann’s 1909 cytoarchitectonic map of the human brain. Modified

after Garey (2006:110).

Figure 7. The major subdivisions of the human brain.

Figure 8. Major subdivision of the cerebrum.

Figure 9. Primary areas and regions of interest in primate and hominid brain

evolution.

Figure 10. Human endocast in superior view.

Figure 11. A rubber copy of the human brain and an endocast of the internal

cranium of the same individual. Replicas by Bone Clones Inc.

Figure 12. Lateral view of a human endocast.

Figure 13. The brains of A. Bushbaby B. Macaque C. Chimpanzee drawn to

approximately the same size to eliminate the effects of body size. Modified from



the Comparative Mammalian Brain Collections website http://brainmuseum.org

from the University of Wisconsin-Madison Brain Collection

Figure 14. Percentage of brain volume dedicated to olfactory bulbs, visual cortex,

and neocortex from the Stephan et al. (1988) data set.

Figure 15. A. Prosimian brain showing the direction of the coronolateral sulcus; B.

New World monkey brain showing the central sulcus separating the primary

sensory and primary motor areas; C. Old World monkey brain.

Figure 16. A. Human brain B. Chimpanzee brain.

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