The Rise of Homo sapiens
Chapter 5
Ashley White
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Human brains = Primate brains
Much of our brain’s anatomy and the way we
think is the way it is because we are primates
Primates are the Order within the Class
Mammalia
 Encompasses
monkeys, apes (including humans),
and prosimians
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All living primates share a common ancestor
 Mammal
who lived 50 million years ago, during the
great expansion and diversification of mammals that
followed the extinction of the dinosaurs
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Most living primates are tropical
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major exceptions are humans and macaques
(Japanese snow monkeys)
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Most primates are arboreal
 Spend
some or all of their lives in trees
 This is reflected in primate anatomy, including the
anatomy of those who no longer live in trees
Grasping hands and feet for locomotion
Well-developed visual system
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Because primates have been around for over 50
million years, they have had ample
opportunity to evolve a wide variety of specific
anatomies and adaptive niches
We will review the developments, old and
recent, that are directly relevant to the
evolution of human cognition
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Primate brains are adapted to the demands of
an arboreal way of life
Expanded ventral premotor cortex, part of the motor
cortex controlling hand and finger movement
Good at directing fine motor action
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The most salient developments occurred in the
areas of the brain that process visual
information
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The center of a primate’s retina is densely
packed with photo-receptors
This allows for detailed perception in the center of the
visual field
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Perceptual information passes from the retina
though the lateral geniculate nucleus of the
thalamus to the primary visual cortex of the
occipital lobes
Here, the “raw” data is initially processed and
passed on to the parietal and temporal lobes,
where more complex analysis occurs
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Primates are particularly good at recognizing
shapes and locating themselves in space
Stereoscopic Vision:
Primate eyes face forward, with considerable overlap in
the visual fields of each eye
By comparing the two slightly different images, the
visual cortex is able to construct depth
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Advantage: depth is perceived easily when
moving about in the tops of tropical trees
Disadvantage: primates cannot see to rear
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Hemifield (half-field) vision:
 Source
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of “lateral visual field neglect” in humans
Unlike humans, most mammals have the entire
visual field of each eye processed
contralaterally
If brain damage occurs, the ability to process
information on one side of the visual field may
be lost
No one is quite sure why the primate visual
system works this way
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About 40 million years ago, the large group of
primates that includes monkeys, apes, and
humans diverged from the earlier primates
These earlier primates survive today as the prosimians
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Anthropoids were larger than the prosimians
Developed a dietary focus on fruit and leaves,
unlike the largely insectivorous prosimians
Also had larger brains, and in particular a
larger neocortex
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With the anthropoids, there were new
developments to the visual system
Trichromatic color vision results in the full range
of color perception
 enjoyed by humans, apes, and monkeys
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Required a mutation (probably a gene duplication)
that added a third variety of cone to the retina,
along with neural resources in the primary and
secondary cortices
It evolved in anthropoid ancestors as an aid to
detecting food in complex forest canopies, most
likely fruit
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Over time, anthropoids also developed a
much-heightened ability to recognize faces and
facial expressions
This resulted from an expansion of the inferotemporal visual cortex, the area of the
neocortex linked to shape recognition
This is sometimes termed the “ventral
pathway” of visual processing
Information passes from the primary visual cortex of
the occipital lobes forward to the temporal lobes, where
more sophisticated visual processing takes place
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The neurons involved in facial recognition are
linked to the amygdala, the structure that
attaches emotional value to perceptions
Anthropoids use facial expression to
communicate emotional states, which play an
essential role in anthropoid social behavior
It is social behavior that explains not just the
increase in anthropoid inferior temporal visual
cortex, but also the increase in overall brain
size and neocortex size
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Anthropoids have an EQ of about 2.1
Overall brain size is about twice as large as predicted
for a placental mammal of comparable size
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However, there is a great deal of variability
Old World monkeys ≈ 1.05
Humans ≈ 6.0
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Some of this variation in brain and neocortex
size appears related to the total amount of
visual information coming in from the eyes
This suggests that visual specializations of various
kinds may effect overall brain size
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Some of the variability also appears to result
directly from the evolutionary trade-off between
brain size and gut length
Folivory (leaf-eating) anthropoids have smaller
brains and neocortices than frugivory (fruit-eating)
anthropoids
 Leaves must be fermented in a very long gut in order to yield
digestible carbohydrates
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This requires a heavy metabolic investment in
digestion
 The balance cannot be made up by decreasing reliance on
hearts, kidneys, or livers (other metabolically expensive
tissues)
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So…brain size must be limited in folivores
This explains why folivores have smaller brains,
but why do frugivores need larger brains?
There are 2 possibilities:
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Something about foraging for fruit selected for
larger brains and neocortices
Probably “mental mapping”
The ability to locate oneself and others, and navigate in
complex forest habitats
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This spatial mapping ability is a function of the
parietal cortex (the dorsal pathway of visual
processing)
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In tropical forests, fruit is distributed in small,
concentrated patches that ripen at different times
throughout the year
 Because fruit is a high-quality nutrition, it is under great
demand
 There is serious competition from birds and other mammals
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Anthropoids have a mental map of territory
 Move efficiently between patches
 Remember where patches are and when they are likely to be
available
 “Plan” a foraging pattern that is more effective in the face of
competition
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The switch to the concentrated, easily
digestible, high-quality nutrition of fruit
released selective pressure against larger brains
Freeing them to expand in response to other selective
pressures
Probably social complexity
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Anthropoids live in a variety of social systems,
from the mostly solitary orangutan males to
large multi-male/female troops of baboons
One association of complexity is group size,
based on assumption that more individuals
produce more interactions
However, when primatologists tried to correlate brain
size with group size, they found no relationship
But, group size does reliably predict neocortex size
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Human neocortex falls about where it should
for a group size of about 150
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Brain size correlates with several life-history
factors
Age at first reproduction
Length of the juvenile period
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Several ways to explain this:
1) Both brain size and maturation reflect an
increase in body size
But, when body size is controlled, the correlation
remains
2) Natural selection may have favored delay of
reproductive age because of niche complexity
and energetic cost of reproduction, and brain
growth was a by-product
But, most brain growth occurs pre- and neo-natally, so
a simple by-product explanation seems weak
3) Selection for brain growth would require
lengthening of juvenile periods and age of first
reproduction, because there is a greater amount
of information to be learned
4) Combination of these and other factors
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What is clear: brain growth cannot have
occurred simply through selection for more
neurons in an adult individual
The process entailed changes in brain and
physical development
It was not simple, and it was
energetically expensive
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With the exception of humans, apes today are
much less numerous and successful than the
monkeys
There are only four species of great apes (chimpanzees,
bonobos, gorillas, orangutans) , all of which are
declining in number
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They were most varied and successful during
the Miocene age (23 million to 5 million years
ago)
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It is the slowness of reproduction that placed
apes at a disadvantage, compared to Old
World monkeys
Apes: long gestation, increased inter-birth interval, long
juvenile period
Monkeys: reproduce much faster, almost as clever
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What we know:
Long juvenile periods and late age of first reproduction
correlate strongly with brain size
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What we DON’T know:
Which came first?
Do apes have large brains because they have extended
developmental periods, or did their large brains require
extended development for learning?
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Unexpected conclusion:
We can suspect that large brains may be partially to
blame for the apes’ recent evolutionary decline
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If large brains no longer yield a competitive
edge, their owners will go extinct.
Large brains are metabolically expensive
Large brains have profound life-history consequences
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What edge did ape brains initially possess?
Expansion of planum temporale (area on the dorsal
surface of the superior temporal lobe)
Expansion of Brodman’s area 44 (left ventral motor
cortex)
These areas have been linked to apes’ more complex
vocalizations
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What edge did ape brains initially possess?
Lateral cerebellum of apes is 2.5x larger than monkeys
This controls many basic postural functions
Also involved in cognitive planning of complex motor
actions, sequential patterning, and procedural learning
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Some have suggested that great ape
encephalization is linked to suspensory
locomotion in forest canopies
Others propose that it may be linked to
“complex foraging” and access to embedded
foods
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Well-known as tool-users and tool-makers
Habitually use tools to access hard-to-acquire
foods such as termites, hard-shelled nuts, and
honey from well-defended nests
Solutions vary between communities, but all
rely on complex motor manipulations of
objects, sometimes requiring a series of tools
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Have been observed using tools to break open
nests
“Extractive foraging” – the use of tools to
access hidden and embedded foods
Early Hominins
Probably inherited from the common ancestor
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They rarely use tools, but do use organized,
complex manipulations to gain access to the
pith of nettles
Such manipulations require cognitive resources
similar to those required for chimpanzee tool
use
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It seems that this pattern of complex feeding is
shared by all of the great apes
This may explain the very significant difference
in the size of the lateral cerebellum, when
compared to monkeys
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The specific adaptations we have discussed do
not seem general enough to explain the
increase in overall neocortex size of all
anthropoids
Also doesn’t explain the flexible problem solving
abilities we associate with apes
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This generalizability problem led
primatologists to search for features of
anthropoid lives that might have selected for a
general problem-solving ability
Social behavior
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Nicholas Humphrey (1976)
Suggested that the complex social lives of anthropoids
selected for their problem-solving abilities
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Richard Byrne & Andrew Whiten (1988)
Pulled together various authors into a volume that
summarized evidence for the leading role of social
behavior in primate cognition
Machiavellian Intelligence
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Argues that the most complex part of a
primate’s daily life is the ever-changing nature
of polyadic social interactions
Polyadic – more than two individuals
Dyadic – how one relates to one other individual
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Primate social groups often consist of adult
females, their offspring, and adult males
Success in reproduction and access to highquality foods requires constant monitoring of
one’s social standing
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The most successful individuals monitor not
only dyadic relationships, but also polyadic
relationships
Soap opera = good anthropoid primates
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What we know:
There is a correlation between anthropoid neocortex
size and social group size, which probably reflects the
increasing complexity of polyadic interactions in larger
social groups
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What we DON’T know:
What is so mentally challenging about polyadic
interactions?
Tactical deception
Theory of mind
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Occurs when one individual misleads a second
individual in order to obtain some immediate
goal, normally available to or under control of
the second
Novel response to a specific condition
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Tomasello & Call (1997)
A female baboon, wanting to groom with a
subdominant male, slowly inched toward a boulder
while in plain view of the alpha male (who would not
approve).
When she reached a position from which the alpha
male could only see her head, she successfully groomed
with the subdominant male.
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This is tactical deception because the behavior
was tailored to the immediate circumstances
Many organisms use deception, but not tactical
(camouflage, or feigned injury by mother birds)
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Tactical responses are impressive solutions to
problem-solving
We cannot over-interpret them
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Almost all examples of tactical deception
among anthropoids can be explained by
learning
Humans, however, regularly rely on another
ability…
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Knowing that other individuals have minds and
beliefs, and these beliefs may differ from one’s
own beliefs
“mind reading”
Usually evaluated through the “false belief test”
 A child is shown a situation in which a doll watches the
experimenter place a piece of candy in one of two opaque
boxes. The doll is then removed and placed somewhere out
of view of the table. The experimenter then moves the candy
into the second box (the child is watching all of this). The doll
is brought back, and the child is asked where the doll will
look for the candy.
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PASS: choose the first box
The child knows the doll had not seen the candy move,
and therefore had a false belief
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Human 3-year-olds generally fail the test
5-year olds almost always pass
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What about other anthropoids?
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Comparative psychologists have devised a number
of experimental procedures that require the subject
to construct what another individual knows
Tomasello & Call (1997)
 Four chimpanzees, all with extended human interactions,
witness an experimenter put food into one of four cups, all
behind a partition (so the chimpanzees could not tell which
cup contained the food). Another human remained outside
the room, so he did not know which cup contained the food
either. After the naïve human entered the room, the
chimpanzee subjects could choose either the naïve or
knowledgeable human to inform them of the location of the
food.
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Tomasello & Call (1997)
3 of the 4 achieved a 70% success rate
BUT, only after 100-150 trials
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Conclusions:
Human 5-year-olds would almost certainly do better
Chimpanzees may be learning to solve the test over
time by other means (unintended cueing)
The experimenter may become a conditioned stimulus
for food
Chimpanzees generally FAIL tests of false beliefs
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Do apes/monkeys have a concept of self?
This is a prerequisite for theory of mind, but a bit more
basic than false belief
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Mirror recognition test
Subject is briefly anesthetized, and a spot of paint is
placed above its eye. When presented with a mirror, if
it immediately touches its face, it demonstrates that it
knows the mirror image is of itself. It has therefore
passed the mirror recognition test.
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Mirror recognition test
Monkeys and gorillas invariably FAIL
Chimpanzees sometimes pass/sometimes fail
Chimpanzees that are raised in human conditions are the
most likely to PASS
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Conclusions:
Experience with human objects plays a role
Under the right circumstances, chimpanzees are likely
to pass, so they almost certainly have some concept of
self
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Theory of mind is evident in humans, but there
are no clear evolutionary antecedents
There is a correlation between theory of mind
and general problem-solving ability
Suggests ToM is not an encapsulated, domain-specific
cognitive ability that evolved to solve a narrow
adaptive problem
Instead, it arose from a need for flexible responses to
complex but generic problems
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Anthropoids have expressive faces, along with
a neural network sensitive to differences in
faces and facial expressions
This network is linked to the centers of emotional
processing via the amygdala
So, anthropoids communicate their mood through
facial expression
Anthropoid vocalizations are tied into the same
emotive network
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It is very hard for an anthropoid to inhibit
these emotionally-based facial responses
because they are involuntary
This may explain their poor performance on
many false-belief tests
They may find it impossible to select the object of false
belief because their desire for the reward is very strong
and cannot be inhibited, even if they “know” that it is
the wrong choice for the test
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Anthropoids do use tactical deception, but
without theory of mind they are relatively poor
liars
Liars must have the ability to inhibit their emotive
response, one of the key components of executive
functions, which ToM is related to
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Conclusions:
Sometime after the evolutionary split from
chimpanzees, our ancestors acquired the ability to
inhibit natural vocal and facial responses
We learned to hide our emotions
The primate evidence suggests that the selective agent
may have been success in tactical deception
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Apes’ general inability to suppress automatic
responses leads one to expect an equally poor
performance on tests of working memory
Methodological problems:
Impossible to test recall of lexical items with apes
Most working memory tests require considerable
cooperation by the subject
Apes are usually poor at maintaining attention on one
goal when distracted by another, a clear part of most
working memory tests
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Kawai & Matsuzawa (2000)
Tested chimpanzee “Ai” and her ability to remember
the location, on a screen, of a set of sequential integers.
She had already learned to recognize the integers 0-9,
and ordered the series while all the integers were
visible. This was done so only her spatial working
memory would be tested. The memory portion began
after she selected the first integer in the series, at which
point the remaining integers were masked.
They found that a chimpanzee’s working memory
capacity was 5 items
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Kawai & Matsuzawa challenge the “magic
number seven” for human spatial memory
Suggest that chimpanzees are closer to us than many
think
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Chimpanzees have a shorter short-term
memory capacity than humans – but, perhaps
not by much