Michael S. Gazzaniga - Specialized Research AI at MIT

Michael S. Gazzaniga

Humans: the party animal

By de½nition, the species Homo sapiens
is unique. Over a time course of approxi-
mately seven million years, humans have
evolved into quite a different animal from
what was the last common ancestor we
share with our closest surviving relative,
the chimpanzee. Trying to ½gure out how
we came to be what we are, and identify-
ing what aspects, both physical and behav-
ioral, we share with other animals, most
especially the chimpanzee, and those that
are uniquely human has been of ongoing
interest.1
Take a minute the next time you go

to a dinner party, barbecue, wedding
reception, or baby shower–run-of-the-
mill events for us Homo sapiens–to pon-
der the fact that such events are com-
pletely unheard of in any other species.
What other animal would plan an event,
provide food to unrelated others, and
sit together and share it without a food
½ght, all while laughing about stories
of the past and hopes and dreams of the
future? There is none. No matter how
smart your family dog may be, he would
not divvy up a prime rib roast and pass
it out to the other dogs of the neighbor-
hood with a happy little bark; neither
would our closest relatives, the chimps.
Humans are social beings, and although

there are other animal and insect spe-
cies that are social, our species takes so-
ciability to a previously unknown level.
We are party animals, and on our way
to becoming such we have evolved a
whole host of unique features–features
so unique that we humans are playing
in another ballpark.

Although many may suggest that hu-
mans act “like a bunch of animals,” and
the daily news intimates that we are end-
lessly ½ghting with one another, it is by
cooperating with and helping unrelated
others that we are unparalleled among
animal species. Something is markedly
different in our brains: we are “wired”
differently. The results of this altered
wiring allow humans to read books, or
to go to the symphony, school, or jail.
That is not to say humans are 100 per-
cent different. In fact, most of our auto-
matic processing is much the same as
in other animals.

Although all species are unique unto
themselves, all have a common origin
and are made up of the same materials.
It isn’t surprising that when Charles
Darwin ½rst proposed that humans
were descended from the great apes, he
thought that the difference between us
and our closest relatives, the chimpan-
zees, was a quantitative difference, not
a qualitative one. We were just fancier
apes with bigger brains, Darwin rea-

Dædalus Summer 2009

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Michael S.
Gazzaniga
on being
human

soned. In the mid-1960s, however,
Ralph Holloway added to Darwin’s the-
ory, concluding that brain reorganiza-
tion, rather than brain size alone, result-
ed in the evolutionary changes in cogni-
tive capacity. Evidence for Holloway’s
insight is accumulating.

What exactly is brain reorganization,
and how has it affected brain computa-
tions and the human mind? Cognitive
scientists Derek Penn, Keith Holyoak,
and Dan Povinelli, “happy to be the hos-
tage[s] of empirical fortune,”2 claim:

The profound biological continuity be-
tween human and nonhuman animals
masks an equally profound functional
discontinuity between the human and
nonhuman mind. . . . [That discontinuity]
pervades nearly every domain of cogni-
tion–from reasoning about spatial rela-
tions to deceiving conspeci½cs–and runs
much deeper than even the spectacular
scaffolding provided by language or cul-
ture alone can explain.

This “discontinuity of human cogni-
tion,” they propose, was a watershed
change that occurred after the hominid
line diverged from our last common an-
cestor with the chimp, and it resulted
in our exceptional relational ability. We
far exceed other species in our ability to
grasp analogies and to combine relations
into higher-order structures. Mindful
that there are no “unbridgeable gaps” in
evolution, ½guring out how this came to
be is the question. Regardless of whether
or not our ability to form higher-order
relations is the basis for our cognitive
differences, something very different is
going on in the human brain.

There is no question that the human

brain is big. After the hominid line di-
verged from the last common ancestor
we share with the chimps, the brain un-
derwent a huge growth spurt. In com-

parison to a chimp’s brain, which
weighs about 400 grams, an average
human brain weighs about 1,300 grams.
Homo neanderthalensis, however, had a
bigger brain than modern-day humans,
and although it is clear through fossil
evidence that their culture was more
advanced than that of the chimp, they
were not in the same league as H. sapiens.
Thus brain size is not the only variable
in human uniqueness.

In non-primate mammals, the brain’s
prefrontal cortex has two major regions
that work together to contribute to the
“emotional” aspects of decision-making.
We do, of course, make many of our de-
cisions quickly and based on our emo-
tions, and so still utilize these two evolu-
tionarily older regions. However, some
decisions are based on rational thinking.
Only primates possess a third, evolu-
tionarily newer region, the lateral pre-
frontal cortex, where the intriguing
Brodmann Area 10 is located. One hun-
dred years ago, German neurologist Kor-
binian Brodmann identi½ed ½fty-two
distinct regions of the human cerebral
cortex, based on the underlying cytoar-
chitectonics. Area 10 in humans is dis-
proportionately larger compared to the
rest of the great ape brains, and is dense-
ly interconnected with other still larg-
er regions in human brains. Area 10 is
concerned mainly with the “rational”
aspects of decision-making and is in-
volved with all sorts of abilities in which
humans excel: memory and planning,
cognitive flexibility, abstract thinking,
initiating appropriate behavior and in-
hibiting inappropriate behavior, learning
rules, and picking out relevant informa-
tion that is perceived through the senses.
The posterior parietal cortex is anoth-
er disproportionately enlarged area. One
of our hominid ancestors, Australopithe-
cus africanus, displayed the ½rst signs of
enlargement of this cortex, the part of

Dædalus Summer 2009

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

the brain active when objects are manip-
ulated and motor activities are planned.
Also disproportionately large are the
connective pathways of the temporal
lobe, indicating increased local con-
nectivity between neighboring cortical
½elds, which support the formation and
processing of declarative memory; self-
recognition; visual, auditory, and lan-
guage processing; and the detection of
biological motion.3

Neurobiologist Georg Striedter sug-
gests that the human brain has not en-
larged randomly, but that an entire cir-
cuit that has made humans more flexi-
ble and capable of ½nding novel solu-
tions to problems has enlarged. Perhaps
one of the most important abilities in-
cluded in this circuit is that of inhibit-
ing automatic response, which is noto-
riously dif½cult or impossible for other
animals, including our chimp relatives.
Only with this ability can one respond
in novel ways, and utilize the cognitive
flexibility that we uniquely possess.
(These systems are not fully developed
in adolescent humans, offering a possi-
ble explanation for their impulsive
ways.)

When brain size increases, what actu-
ally increases is the number of neurons,
their width, and their connections. In
general, the larger the area, the better
connected it is. Each neuron, however,
can connect to only a limited number
of other neurons, a number that does
not change as the overall number of
neurons increases. As absolute brain
size increases, proportional connectiv-
ity tends to decrease, and the internal
structure changes as the connectivity
pattern changes. In turn, less dense
connections force the brain to special-
ize, create local circuits, and automate.
The human brain has billions of neu-
rons that are organized into local, spe-
cialized circuits, known as modules.

Over the last several years, we have
also learned of many specialized brain
functions that are lateralized in the hu-
man brain. This means that one hemi-
sphere of the brain may perform a spe-
ci½c function that the other hemisphere
cannot perform. The ½rst hint of human
lateralization of function came in 1836,
when a French neurologist observed that
three of his patients that had lesions
of their left hemispheres had speech dis-
turbances. Twenty-½ve years later, Paul
Broca, after studying the postmortem
brains of aphasic patients, concluded
that the speech center was located in
the left hemisphere. There are some
anatomical asymmetries to be found in
the non-human primates and, more no-
tably, in the great apes; however, there
is scant evidence for lateralization of
function in other mammalian species.
The corpus callosum, the great track
of neurons that transmits information
from one hemisphere to the other, may
have provided the evolutionary innova-
tion that allowed cortical capacity to
expand. Without increasing brain size,
it allowed mutations leading to innova-
tions in one half of the brain, while pre-
serving cortical function in the other
half. There are also microscopic asym-
metries in the cellular organization of
the neocortex that have not been found
in other species and are thought to be
uniquely human.4 Clearly, our brains
are physically different. It should come
as no surprise, therefore, that they also
function differently.

Our brains and abilities evolved be-

cause our bodies evolved along with
them: changes in one happened in con-
cert with changes in the other. Between
½ve and seven million years ago (some
recent studies suggest that it may have
been more than ten million)5 we shared
our last common ancestor with the

Humans:
the party
animal

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Dædalus Summer 2009

Michael S.
Gazzaniga
on being
human

chimpanzee. For some unknown reason,
most likely climactic changes resulting
in a change in the food supply, the hom-
inid line split. The branch of the family
that produced the chimps stayed in the
tropical forest and remained much the
same. The other branch stepped out into
the woodlands, where life was quite dif-
ferent, evolving to become bipedal and,
over time, undergoing a host of other
changes as well that have led to our cur-
rent abilities.

Particularly important among the
anatomical results of bipedalism are
our elongated necks and the fact that
our tongue and pharynx dropped lower
down into the throat. Unlike the chimps
that have two separate passageways for
food and air, we developed a unique sys-
tem, in which air and food share a com-
mon pathway in the back of the throat.
We have a structure, the epiglottis, that
closes the pathway to the lungs when
we swallow and opens when we breathe.
The unique anatomy of the pharynx,
speci½cally the larynx, enables us to ut-
ter the wide variations in sound that
we can and makes speech possible. No
doubt, the survival advantage we gained
was an increased ability to communi-
cate, even though we face an increased
risk of death by choking. Bipedalism
also set our hands free, and our thumbs
became unique. Both humans and
chimps have opposable thumbs; chimps,
though, don’t have ulnar opposition:
they can’t arc their thumbs across to
their baby ½ngers. Hence, we can pick
up objects with the tips of our ½ngers,
not with just the sides, as chimps do. We
also have more sensitive ½ngertips, with
thousands of nerves per square inch that
send information to the brain.

One major physical problem presented

by bipedalism was a smaller pelvis, and
thus birth canal. (A wider pelvis would
have made bipedalism mechanically im-

possible.) Birth became more dif½cult
as brains and heads enlarged. In com-
parison to other apes, human babies are
born one year prematurely, and, unlike
chimps, their heads and brains continue
to grow for several years.

From our current viewpoint, bipedal-

ism seems only advantageous; yet the
late psychologist Leon Festinger saw the
proverbial fly in the ointment. He point-
ed out that “bipedalism, in and of itself,
must have been a nearly disastrous dis-
advantage,”6 making us slower and less
able to climb, and its evolution needed a
special explanation.

Before we leave Festinger’s bipedal
quandary, we should pause to consider
a study done by the evolutionary biolo-
gists Willem de Winter and Charles
Oxnard. Rather than looking at overall
brain size, de Winter and Oxnard sug-
gested that a brain part’s size is, to a cer-
tain extent, related to its functional rela-
tionships with other brain parts. Using
brain-part ratios from 363 species, they
ran multivariate analyses, with fascinat-
ing results. Groups emerged based on
similar lifestyles (locomotion, foraging,
and diets), rather than on phylogenetic
relationships. For instance, New World
insectivorous bats had brain-part ratios
more closely linked with Old World car-
nivorous bats, rather than with their
phylogenetically closer relatives, the
New World fruit eating bats.

The primates fell into three groups,
also based on lifestyles that cut across
phylogenetic lines: those with hind-
limb-dominant locomotion; the four-
limb dominant; and the upper-limb
dominant–those that hang from
branches while eating, reach above
their heads for fruit, and escape by
upper-limb acrobatic activities. (This
group included chimps and gorillas,
along with the phylogenetically dis-
tant spider and wooly monkeys.)

Dædalus Summer 2009

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Oxnard’s analysis revealed that the spe-
cies within a lifestyle group had similar
brain organizations: that the conver-
gence and parallels in brain relation-
ships are most likely associated with
convergences and parallels in lifestyles
that cut across phylogenetic groups.
Humans, however, the only species
of the 363 studied that have a bipedal
lifestyle, fell into a group unto them-
selves, with a highly signi½cant 22 stan-
dard deviation difference between them
and chimpanzees. Oxnard concluded,
“The nature of human brain organiza-
tion is very different from that of chim-
panzees, which are themselves scarcely
different from the other great apes and
not too different even from Old World
monkeys.”7 Something about the hu-
man bipedal lifestyle is related to our
very different brain organization.

Festinger suggested that the primary

factor allowing this “seriously handi-
capped species” to survive was an in-
ventive brain and neural system that
could ½gure out just what to do with
those appendages that would be fruit-
ful and adaptive. But perhaps it was
the disadvantage of being slower that
later resulted in so many cognitive
changes. The open woodlands were a
radically different environment. Food
sources were highly scattered, and al-
though there were more animals for
our ancestors to hunt, there were also
more animals hunting them. And just
when our ancestors were being exposed
to bigger and more dangerous preda-
tors, they could no longer run as fast
or climb as well. There are two ways to
discourage predators: either be bigger
and faster than they, or live in a larger
group. Unable to choose the former,
the early hominids banded together in
large groups not only to provide better
protection, but to make both hunting

and gathering more ef½cient, thus pro-
viding more food for the growing brain.
Over the years there have been many
suggestions as to what forces were driv-
ing the relentlessly enlarging brain. It
is coming to be accepted that, through
the process of natural and sexual selec-
tion, two factors were pushing the in-
crease in overall brain size: a diet that
provided the added calories needed to
feed the metabolically expensive big-
ger brain; and the challenges originat-
ing from living in those large groups–
“the social world”–necessary to guard
against predators.

Many proposals have been tendered
as to what diet provided the necessary
amount of calories to feed that metabol-
ic furnace of a growing brain. Richard
Wrangham, a primatologist who has
studied chimpanzees in Uganda for over
thirty years, suggests that Homo sapiens
are uniquely biologically adapted to
eat cooked food, and that cooked food,
which has more calories than raw food
and is faster to eat, drove the expansion
of the brain by increasing calories and
decreasing the amount of time and ener-
gy it takes to ingest and digest them–in
turn freeing up more time for hunting
and socializing. And, as the saying goes,
free time is the devil’s playground.

Banding together in social groups
for protection against predators pre-
sents its own set of problems, such as
competition with conspeci½cs for re-
sources (both food and prospective
mates). Thus the cognitive challenge
of surviving in increasingly larger so-
cial groups was likely the other driver
of increasing brain size, as psycholo-
gists Richard Byrne and Andrew Whiten
suggest. Their proposal, now dubbed
the Social Brain Hypothesis, states:

Most monkeys and apes live in long-last-
ing groups, so that familiar conspeci½cs

Humans:
the party
animal

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Dædalus Summer 2009

Michael S.
Gazzaniga
on being
human

are major competitors for access to re-
sources. This situation favours individ-
uals that can offset the costs of competi-
tion by using manipulative tactics, and
skillful manipulation depends on exten-
sive social knowledge. Because compe-
titive advantage operates relative to the
ability of others in the population, an
“arms race” of increasing social skill re-
sults, which is eventually brought into
equilibrium by the high metabolic cost
of brain tissue.

Successfully living in social groups

involves more than competition; it
also involves cooperation. Very recent-
ly, developmental and comparative psy-
chologists Henrike Moll and Michael
Tomasello proposed the Vygotskian In-
telligence Hypothesis. They assert that
although cognition in general was driv-
en mainly by social competition, the
unique aspects of human cognition–
the cognitive skills of shared goals, joint
attention, joint intentions, and cooper-
ative communication needed to create
such things as complex technologies,
cultural institutions, and systems of
symbols–were driven, even constitut-
ed, not by social competition, but so-
cial cooperation.8

Anthropologist and evolutionary bi-
ologist Robin Dunbar has been search-
ing for social and ecological indices that
correlate with primate brain size. He
has found ½ve aspects of social behavior
that correlate with brain size, the ½rst of
which is social group size: the bigger the
neocortex, the larger is the social group.
The great apes require a bigger neocor-
tex per given group size than other pri-
mates, indicating that the social milieu
of the great apes is more cognitively
taxing. Dunbar’s research has shown
that the factor limiting social group size
is the ability to manipulate and coordi-
nate information and social relation-

ships. The other social skills that he has
correlated with brain size are the num-
ber of individuals with whom an animal
can simultaneously maintain a cohesive
intimate relationship; how much social
skill is required in male mating strategy;
the ability to manipulate others in the
social group without the use of force;
and the frequency of social play.

While a chimp maxs out juggling a so-

cial group size of about 55 individuals,
Dunbar has calculated from the brain
size of humans that we have a social
group size of about 150. Initially this
seems rather surprising when you think
of the huge cities that many humans in-
habit; but when you look more closely,
it begins to make more sense. One hun-
dred ½fty individuals is the typical size
of hunter-gatherer clans, also the typical
number of individuals on a modern-day
Christmas card list or in military units
and businesses that can be run informal-
ly. It appears to be the maximum num-
ber of people an individual can keep
track of and for whom he would be
willing to do a favor. And the extent
to which humans do favors is unique.

Doing favors is altruistic, and Dar-

win himself could never quite ½gure
out how it occurred through natural
selection. Why would an individual do
anything that would increase the sur-
vival of another to his disadvantage?
The late William Hamilton, an evolu-
tionary biologist, realized that altruis-
tic behavior could evolve if the bene½t-
ing individuals were genetically related
to the provider, because helping one’s
close relatives survive and reproduce
also passes your genes on to the next
generation.

Humans, however, help unrelated
others all the time. We are the superla-
tive Helpful Henrys of the animal world,
and this behavior has its foundations in

Dædalus Summer 2009

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

reciprocal altruism, ½rst described by
sociobiologist Robert Trivers. If an indi-
vidual does a favor for an unrelated indi-
vidual and is sure it will be returned at a
later date, then that could provide a sur-
vival advantage. Reciprocal altruism is
very rare in the animal world, and sev-
eral problems have to be overcome in
order for it to work. First, the commit-
ment problem: how could any individ-
ual be sure (trust) that the favor would
be returned? Second, an individual has
to be able to recognize another individ-
ual speci½cally; third, he has to be able
to remember a favor was done and what
it was; fourth, he has to live in close
enough contact that predictable occa-
sions will arise for reciprocation; and
½fth, he has to be able to evaluate the
cost of his favor and make sure he gets
one back of equal value. Marc Hauser,
a professor of psychology and evolu-
tionary biology at Harvard University,
thinks that our impressive mathemat-
ical abilities evolved with the emergence
of social exchange systems. However,
because there is a time lag between the
completion of a favor and its reciproca-
tion, cheating can occur. Not surprising-
ly then, a species that practices recipro-
cal altruism has mechanisms to identi-
fy cheaters. Evolutionary psychologist
Leda Cosmides has developed a test
that indicates that the human mind has
a speci½c module that detects individu-
als who cheat in social exchange situa-
tions. She has found that cheater detec-
tion develops at an early age, operates
regardless of experience and familiar-
ity, and detects cheating, but not unin-
tentional violations.

Identifying cheaters is only half
the job. Game theory researchers have
shown that for prolonged social reci-
procity to exist, not only must cheaters
be detected, but they also must be pun-
ished; otherwise, cheaters, who invest

less but receive an equal bene½t, will
out-compete the non-cheaters and take
over. If cheaters take over, no one foots
the bill and reciprocity crumbles. Hu-
mans have evolved two abilities that are
necessary components for prolonged re-
ciprocal social exchange and are on the
short list of uniquely human capacities:
the ability to inhibit actions over time
(a.k.a. delayed grati½cation) and punish-
ment of cheaters in reciprocal exchange.

The importance of reciprocal ex-

change should not be understated. Cos-
mides observes, “As humans, we take
for granted the fact that we can help
each other by trading goods and ser-
vices. But most animals cannot engage
in this kind of behavior–they lack the
programs that make it possible. It seems
to me that this human cognitive ability
is one of the greatest engines of coop-
eration in the animal kingdom.”9 Moll
and Tomasello think that the unique as-
pects of human cognition were driven
by social cooperation.

Indeed, cooperation is rare in our
chimp relatives, and recent studies
show that it only happens in competi-
tive situations, and only under certain
circumstances with certain individu-
als.10 Brian Hare and Michael Toma-
sello suggest that the temperament of
the chimp constrains his behavior, and
that the human temperament might
be necessary for the evolution of more
complex forms of social cognition. Be-
fore hominids were able to work coop-
eratively, they had to become less ag-
gressive and competitive and more tol-
erant and friendly with one another.

Hare and Tomasello suggest that this

may have been achieved by a kind of
self-domestication process that select-
ed for systems that controlled emotion-
al reactivity, such as aggression. Perhaps
individuals in a group would either os-
tracize or kill overly aggressive or des-

Humans:
the party
animal

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Dædalus Summer 2009

Michael S.
Gazzaniga
on being
human

potic others. Dubbed the “emotional
reactivity” hypothesis, it is based on
continuing studies done by geneticist
Dmitry Balyaev, who began domesti-
cating foxes in Siberia in 1959 by select-
ing only for a single criterion: whether
they exhibited fearless and non-aggres-
sive behavior toward humans. In other
words, he selected for fear and aggres-
sion inhibition. The experimentally do-
mesticated foxes are as skilled at using
human communicative gestures–point-
ing and gazing, for example–as domes-
tic dogs.11 These results suggest that so-
ciocognitive evolution has occurred in
the experimental foxes as a correlated
by-product of selection on systems me-
diating fear and aggression. Dog domes-
tication is thought to have occurred by
a similar process. Wild dogs that were
less fearful of humans were the ones
that approached them, stuck around,
and reproduced.

Reciprocal exchange is not only an

engine of cooperation, it is one of the
driving forces behind our innate moral-
ity. Many researchers studying morals
and ethics propose that we have “ethi-
cal” modules. These modules have been
derived from the common emotions
and the behaviors they engender, which
we share with other social species and
which include being territorial; having
dominance strategies to protect territo-
ry; forming coalitions to garner food,
space, sex; and reciprocity. These mod-
ules have evolved to deal with speci½c
circumstances, common to our hunter-
gatherer ancestors, that involved what
we now consider moral or ethical issues.
An environmental trigger activates
these modules, which induce an auto-
matic judgment of approval or disap-
proval. If the trigger is strong enough,
a moral emotion is elicited. The emo-
tional state produces a moral intuition

that may motivate an individual to ac-
tion. We share many of our so-called
moral intuitions with other animals.
Humans differ by way of the reasoning
about the judgment or action that
comes afterward, as the brain seeks a
rational explanation for an automatic
unconscious reaction. This is when
the uniquely human left brain’s inter-
preter device (see below) provides an
explanation for the moral emotion, in-
tuition, and the action. Also unique to
humans, some moral emotions can evi-
dence themselves in blushing or tears,
and are dif½cult to counterfeit. As a re-
sult, these behaviors are a good adver-
tisement that an individual has a con-
science or is compassionate. Such vi-
sual proof of a moral emotion can indi-
cate that the individual would be trust-
worthy and a good partner for recipro-
cal exchange.

It appears that the moral emotions
of shame, embarrassment, guilt, dis-
gust, contempt, sympathy, and compas-
sion are also uniquely human. Moral
emotions solve the commitment prob-
lem presented by social exchange and
allow the ½rst move. Jonathan Haidt,
a social psychologist, points out that
moral emotions aren’t just for nice
guys, and this is a very important point.
Some moral emotions can also lead to
ostracism, shaming, and murderous
vengeance. Oddly enough, it may be
that those moral emotions are what ac-
tually made us nicer: moral emotions
motivate the punishment of cheaters,
which is necessary to sustain reciproc-
ity and cooperation. Perhaps they were
part of the self-domestication process
proposed by Hare and Tomasello. We
know that our relatives the chimps un-
derstand intention and are vengeful. In
experiments with humans, chimpanzees
will become more upset when a human
intentionally interrupts a feeding ses-

Dædalus Summer 2009

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

sion, than when he accidentally does12
and will retaliate against personally
harmful actions.13

But all this does not explain why peo-

ple will leave a tip in a restaurant that
they will never return to, or why people
will cooperate with unknown others or
sacri½ce for non-kin. Haidt suggests that
sociologist Emile Durkheim’s insight,
that morality binds and builds groups,
can answer this question: “Morality
constrains individuals and ties them
to each other to create groups that are
emergent entities with new properties.”
He suspects that reciprocal altruism is
supplemented with a type of “indirect
reciprocity.” Here, it pays to be virtuous
by following the morals of the commu-
nity, because such behavior enhances
your reputation or status, and rewards
you with increased future cooperation.14
Certain abilities are necessary to feel
the uniquely human moral emotions:

• In order to feel sympathy and compas-
sion, one must understand that anoth-
er has feelings, and be able to identify
them and take another’s perspective,
which requires inhibiting the default
mode of self-perspective.

• In order to feel the “self-conscious”

emotions of guilt, shame, and embar-
rassment, one also has to be self-aware
and conscious of these emotions.

• In order to cooperate with another, one
must share intentions, attention, goals,
and possess Theory of Mind, the un-
derstanding that the other has beliefs,
goals, and intentions. Once again, in
order to enact this suite of behaviors,
one has to inhibit self-perspective.

How might this all work in the brain?

Many neuroscientists think that the so-
called mirror neurons are fundamental
for the development of self-awareness,

Theory of Mind, and language, and
there are those who think the neurons
were fundamental for human conscious-
ness. Scientists studying macaque mon-
keys discovered these premotor neu-
rons, which ½red both when a monkey
observed or heard another manipulat-
ing an object with his hand or mouth
and when he himself manipulated an
object. Mirror neurons are the ½rst con-
crete evidence of a neural link between
observation and imitation of an action.
Subsequently, more extensive mirror
neuron systems have been described in
the human, where they are not restrict-
ed to just hand and mouth movements,
as in the monkey. They correspond to
movements all over the body; in fact,
the same neurons are active even when
we only imagine an action.

Mirror neurons are implicated not
only in the imitating of actions, but also
in understanding the intention of ac-
tions. Humans also appear to have mir-
ror systems in the insula involved with
understanding and experiencing the
emotions of others, mediated through
the viceromotor response. Such systems,
by unconsciously internally replicating
actions and emotions, may be the mech-
anism behind what gives us an implicit
grasp of how and what other people feel
or do, and contribute input used in our
theorizing about the reason (the why)
for the actions and emotions of others.
Giacomo Rizzoletti, who ½rst discov-
ered mirror neurons, and Michael Arbib,
director of the University of Southern
California Brain Project, suggest that
the mirror system was fundamental for
the development of speech and, before
speech, for other forms of intentional
communication, such as facial expres-
sion and hand gestures. Mirror neurons
also serve to synchronize our feelings
and movements with those of others
around us: for example, everyone in the

Humans:
the party
animal

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Dædalus Summer 2009

Michael S.
Gazzaniga
on being
human

audience claps in unison to bring the
performer back out for one more rendi-
tion of their favorite song. Mirror neu-
rons are one of the psychological mech-
anisms that create group cohesion.
But if the same neurons are active
when I observe an action as when I per-
form the action, how can I tell who has
done it? Beyond the shared neural net-
works that are active in both cases, when
we take a personal perspective, parts of
the somatosensory cortex, the part of
the brain with speci½c areas that map
to speci½c parts of the body, are also ac-
tive. When we take another’s perspec-
tive, however, we activate one area of
the temporal parietal junction that has
input and output connections with many
parts of the brain and plays a part in dif-
ferentiating self from other, as well as a
part of Area 10 in the prefrontal cortex.
The activation of Area 10 is what inhib-
its self-perspective; damage to the area
can lead to excessively egocentric behav-
ior. It has been suggested that errors in
assessing another’s perspective are a fail-
ure of suppressing one’s own. Inhibiting
our own perspective gives us flexibility
to take another’s, and although chimps
appear to be able to do it to a limited ex-
tent (even then only while in competi-
tion), humans can do it voluntarily with-
out constraints.

Neither reality nor visibility con-

strains humans. We can feel an emotion
through abstract input, such as reading,
or by merely imagining it. The emotion
of disgust activates the same brain cir-
cuitry (the operculum) whether one is
experiencing it oneself or observing or
imagining the disgust of others. The out-
put of this region, however, is connect-
ed to the rest of the brain in a modality
speci½c way, so these modalities feel dif-
ferent.15

Complex social interactions depend
on our ability to understand the mental

states of other people. Having a Theory
of Mind (ToM), also known as “intui-
tive psychology,” is our intuitive un-
derstanding that others have invisible
states–beliefs, desires, intentions, and
goals–that can cause behaviors and
events. Some cognitive psychologists
think ToM is the foundation of what
is unique about the human mind. Chil-
dren slowly develop the full suite of
ToM abilities over the ½rst ½ve years
of age, but some of the abilities are up
and running as early as nine months.
While we share some aspects of ToM
with our chimp relatives, other aspects
are uniquely human. Chimps and chil-
dren less than four can understand
what others perceive, and the perceiv-
able goals of their actions, but they can’t
understand that another may have a false
belief. This ability in children is evident
between four and ½ve years old, when
they begin to understand that what oth-
ers believe may not actually be true. A
full-blown ToM is needed for manipu-
lating others’ thinking, which is the
basis of classroom learning. Actively
teaching is a uniquely human ability.

Humans possess not only an intuitive

psychology, but also an intuitive biolo-
gy and physics, some aspects of which
are shared by other animals, and some
which are uniquely human. We humans
automatically categorize whatever we
run across as either an animate or inani-
mate object. In every society, people in-
tuitively think about animate objects–
plants and animals–in the same special
hierarchical way. This intuition is the
hardwired knowledge that animate ob-
jects have an underlying causal nature,
or essence, which is responsible for their
appearance and behavior. Harvard
researchers Alfonso Caramazza and
Jennifer Shelton claim that there are
domain-speci½c knowledge systems

Dædalus Summer 2009

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

(modules) for animate and inani-
mate categories that have distinct neu-
ral mechanisms. These domain-speci½c
knowledge systems aren’t actually the
knowledge itself, but systems that make
you pay attention to particular aspects
of situations in order to increase your
speci½c knowledge. For instance, we
intuitively understand that a large ani-
mal that has forward facing eyes and
sharp teeth and that stalks is a predator;
so, too, do other animals. We aren’t
born knowing what speci½c predator it
is. If an object meets the innate criteria
for the animate category and has bio-
logical motion, then we place it in the
animal category and we automatically
infer it has speci½c properties that all
such things have: it’s alive, it requires
food and water, it can die, it has goals,
intentions, and, inaccurate though it
may be, ToM! This automatic bestow-
al of ToM on all animals is why it is so
easy to anthropomorphize our pets,
and why it is so dif½cult to believe that
humans have a different psychology
from other animals.

This, however, is different from
how we think about inanimate ob-
jects. If something is placed in the in-
animate category, a different set of
properties are inferred, such as “is sol-
id,” or “won’t disappear.” The full ex-
tent of the intuitive physics in other an-
imals is not known, but as Marc Haus-
er suggests, along with ½ve-month-old
babies, other animals must understand
object permanence, otherwise if an an-
imal didn’t understand that the lion
that went behind the bush is still there,
there would be no prey animals left.
Daniel Povinelli and Jennifer Vonk have
reviewed what is known about the phys-
ical knowledge of non-human primates
and have concluded that even though
they can reason about the causes of ob-
served events, they do not understand

the causal forces that underlie their
observations. They appear to know by
observation that fruit will fall to the
ground, but they don’t reason that if
they are reaching for something and
drag it across a hole in a table, that it,
too, will fall into the void. Povinelli
and Vonk suggest that humans are
unique in their ability to reason about
causal forces, and this extends to the
psychological realm and is used to pre-
dict and explain events or psychologi-
cal states.

Intuitive psychology is a separate do-
main from intuitive biology and phys-
ics. A “desire” or a “belief” isn’t labeled
with physical properties such as “has
gravity” or “is solid,” or biological prop-
erties such as “walks,” “breathes,” or,
most importantly, “dies.” This separate
processing of object understanding from
psychological understanding is what
Yale psychologist Paul Bloom says gives
rise to our “duality of experience.” Hu-
mans are dualists; they act as if (and
usually believe that) a person has both a
physical body and another part–a soul,
spirit, or “essence” that de½nes that per-
son. The body, an animate object, gets
tagged by our intuitive biology as some-
thing that eats, sleeps, walks, has sex,
and dies. However, because the psycho-
logical part is not visible and does not
have an obvious physical substance, it is
subject to different inferences; “it dies”
is not one of them. Humans have an in-
tuitive belief that one’s body and one’s
essence are separate.

Because the mental separation hap-
pens automatically, it is easy to think
that either the body or the essence can
exist separately, hence the concepts of
a zombie, the body without the mind,
or the soul, spirit without the body. Hu-
mans, unsurprisingly, have been even
more creative, inventing other essences
such as ghosts, angels, demons, the dev-

Humans:
the party
animal

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Dædalus Summer 2009

Michael S.
Gazzaniga
on being
human

il, and gods or God. If Povinelli and
Vonk are correct that other animals
cannot form concepts about imper-
ceptible entities or processes, and they
do not possess a full ToM, then being a
dualist and conceiving of such entities
as spirits or God are uniquely human
qualities.

Humans endlessly generate explana-

tions and reasons for everything. My
colleagues and I noticed this tenden-
cy while studying split-brain patients.
The surgical procedure to cut the cor-
pus callosum is a last-ditch treatment
for patients with severe intractable epi-
lepsy, for whom no other treatments
have worked. Very few patients have
had this surgery, and it is done even
more rarely now because of improved
medications and other modes of treat-
ment. The treatment has been very suc-
cessful, and most patients seemed com-
pletely unaware of any changes in their
mental processes. Cutting the corpus
callosum isolates the right hemisphere
from the speech center, which usually
is in the left hemisphere, so not only
can the right hemisphere not commu-
nicate to the left hemisphere, it can’t
talk to anyone else either. With special
equipment, you can give a command to
the right hemisphere only. For example,
you could ask the right hemisphere to
pick up an apple from a bowl of fruit.
The right hemisphere controls the left
hand, so the patient would pick up the
apple with his left hand. When you ask
the patient why he picked up the apple,
his speech center, in the left hemisphere,
answers. The left hemisphere, however,
doesn’t know why the left hand picked
up an apple, because it didn’t see the
command. This is no problem for the
speech center; it will answer anyway.
It may say, “ I’m hungry,” or “I prefer
apples.” In these patients, the left hemi-

sphere will smoothly make up a reason
why an action, which was initiated by
the right brain and of which it has no
knowledge, was done.

This device, which we dubbed “the in-

terpreter,” takes all incoming informa-
tion, assembles it into a “makes sense”
explanation, and spews it out. It can on-
ly work with the information that it re-
ceives, and if there are gaps in this infor-
mation, it is of no consequence: it will
generate a story to ½t the information
it has. For example, in one experiment
with a split-brain patient, we showed
a command to the right hemisphere to
laugh, and she did. When we asked her
why she was laughing, instead of the
left brain answering that it didn’t know,
she said, “You guys are so funny!” The
speech center in the left hemisphere had
not seen the command to laugh, but cer-
tainly was receiving the input that its
person was laughing. Since that was all
the information it had, it had to come
up with a “makes sense” answer. It will
also explain emotional states. In anoth-
er experiment, we used a visual stimulus
to trigger a negative mood in the right
hemisphere. This time, although the pa-
tient denied seeing anything, she sud-
denly said that she was upset and it was
the experimenter who was upsetting
her. She felt the emotional response to
the stimulus, all of the autonomic re-
sults, but her left hemisphere had no
idea what caused them.

The interpreter is the device that puts
all the incoming information together;
it creates order out of chaos, and creates
a narrative of and explanation for our
actions, emotions, thoughts, memories,
and dreams. It is the glue that keeps us
feeling uni½ed and creates the sense that
we are rational agents. It tells our story. I
propose that the left-brain interpreter is
uniquely human. Receiving input from a
wide variety of sources–the same sourc-

Dædalus Summer 2009

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

es that are available to other animals–
it integrates that informational input
in a unique way to create our self-con-
scious self, giving humans a distinct
type of self-awareness that goes beyond
the physical self-awareness exhibited
by mirror self-recognition.

The evolutionary changes that the

hominid line has undergone have
brought us to our current state of be-
ing as a species, Homo sapiens. We share
many features with our distant relatives,
the chimpanzees, but we also have many
unique qualities, ranging from differ-
ences in our brain anatomy (on both a
macro- and microscopic level) to differ-
ences in behavior and cognition. How-
ever, the mystery remains of what ex-
actly that change was that occurred be-
tween our last common ancestor with
the chimps, and that perhaps is the foun-

dation block of our unique cognition.
But neuroscientists are not alone in try-
ing to divine ancient secrets.

There is a section in the Grand Can-
yon called the “Great Unconformity,”
which is the surface between the rock
strata called the Tapeats Sandstone,
which averages 545 million years old,
and the 1.8 billion-year-old metamor-
phic rock called Vishnu schist that it
sits upon. This unconformity repre-
sents a time gap of 1.2 billion years of
unknown geologic history–or about
25 percent of the earth’s history. I guess
if geologists can keep plugging away at
the mysteries of the 1.2 billion years
missing in the geologic record, then
we neuroscientists can keep plugging
away at the 7 million years making up
the Great Discontinuity, that unknown
record of the evolution of human cog-
nition.

Humans:
the party
animal

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

ENDNOTES
1 Michael S. Gazzaniga, Human: The Science Behind What Makes Us Unique (New York: Ecco,
2008).
2 Derek C. Penn, Keith J. Holyoak, and Dan J. Povinelli, “Darwin’s Mistake: Explaining the
Discontinuity between Human and Nonhuman Animals,” Behavioral and Brain Sciences 31
(2008): 109–130.
3 Natalie M. Schenker, Anne-Marie Desgouttes, and Katerina Semendeferi, “Neural Connec-
tivity and Cortical Substrates of Cognition in Hominoids,” Journal of Human Evolution 49
(2005): 547–569.
4 There is more neuropil, the space between cell bodies that is ½lled with dendrites, axons,
and synapses, in the left hemisphere’s speech center, in the area of the primary motor cor-
tex designated to the hand, the primary visual cortex, and extrastriate areas.
5 See discussion in Charles Oxnard, “Brain Evolution: Mammals, Primates, Chimpanzees,
and Humans,” International Journal of Primatology 25 (2004): 1127–1158.
6 Leon Festinger, The Human Legacy (New York: Columbia University Press, 1983), 4.
7 Oxnard, “Brain Evolution.”
8 Henrike Moll and Michael Tomasello, “Co-operation and Human Cognition: The Vygot-
skian Intelligence Hypothesis,” Philosophical Transactions of the Royal Society 362 (2007):
639–648.
9 Leda Cosmides, El Mercurio, October 28, 2001.
10 For a review, see Moll and Tomasello, “Co-operation and Human Cognition.”

Dædalus Summer 2009

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Michael S.
Gazzaniga
on being
human

11 Brian Hare et al., “Social Cognitive Evolution in Captive Foxes is a Correlated By-Product

of Experimental Domestication,” Current Biology 15 (2005): 226–230.

12 Michael Tomasello, Josep Call, and Brian Hare, “Chimpanzees Understand Psychological
States: The Question is Which Ones and to What Extent,” Trends in Cognitive Sciences 7
(2003): 153–156.

13 Keith Jensen, Josep Call, and Michael Tomasello, “Chimpanzees are Vengeful but not
Spiteful,” Proceedings of the National Academy of Sciences 104 (2007): 13046–13050.

14 Jonathan Haidt, “The New Synthesis in Moral Psychology,” Science 316 (2007): 998–1002.
15 Mbemba Jabbi, Jojanneke Bastiaansen, and Christian Keysers, “A Common Anterior Insula
Representation of Disgust Observation, Experience and Imagination Shows Divergent
Functional Connectivity Pathways,” PLoSONE3 (8) (2008): e2939.

D
o
w
n
o
a
d
e
d

f
r
o
m
h

t
t

:
/
/

d
i
r
e
c
t
.

i
t
.

e
d
u
d
a
e
d
a
r
t
i
c
e
–
p
d

f
/

1
3
8
3
2
1
1
8
2
9
6
8
4
d
a
e
d
2
0
0
9
1
3
8
3
2
1
p
d

b
y
g
u
e
s
t

o
n
0
8
S
e
p
e
m
b
e
r
2
0
2
3

Dædalus Summer 2009
Download pdf