The evolution of human intelligence is closely
tied to the evolution of the human brain and
to the origin of language.
The timeline of human evolution spans approximately
7 million years, from the separation of the
genus Pan until the emergence of behavioral
modernity by 50,000 years ago.
The first 3 million years of this timeline
concern Sahelanthropus, the following 2 million
concern Australopithecus and the final 2 million
span the history of the genus Homo in the
Paleolithic era.
Many traits of human intelligence, such as
empathy, theory of mind, mourning, ritual,
and the use of symbols and tools, are apparent
in great apes although in less sophisticated
forms than found in humans, such as great
ape language.
== History ==
=== 
Hominidae ===
The great apes (hominidae) show considerable
cognitive and empathic abilities.
Chimpanzees can make tools and use them to
acquire foods and for social displays; they
have sophisticated hunting strategies requiring
cooperation, influence and rank; they are
status conscious, manipulative and capable
of deception; they can learn to use symbols
and understand aspects of human language including
some relational syntax, concepts of number
and numerical sequence.
=== Homininae ===
Around 10 million years ago, the Earth's climate
entered a cooler and drier phase, which led
eventually to the Quaternary glaciation beginning
some 2.6 million years ago.
One consequence of this was that the north
African tropical forest began to retreat,
being replaced first by open grasslands and
eventually by desert (the modern Sahara).
As their environment changed from continuous
forest to patches of forest separated by expanses
of grassland, some primates adapted to a partly
or fully ground-dwelling life.
Here they were exposed to predators, such
as the big cats, from whom they had previously
been safe.
These environmental pressures caused selection
to favor bipedalism: walking on hind legs.
This gave the Homininae's eyes greater elevation,
the ability to see approaching danger further
off, and a more efficient means of locomotion.
It also freed the arms from the task of walking
and made the hands available for tasks such
as gathering food.
At some point the bipedal primates developed
handedness, giving them the ability to pick
up sticks, bones and stones and use them as
weapons, or as tools for tasks such as killing
smaller animals, cracking nuts, or cutting
up carcasses.
In other words, these primates developed the
use of primitive technology.
Bipedal tool-using primates form the Hominina
subtribe, of which the earliest species, such
as Sahelanthropus tchadensis, date to about
7 to 5 million years ago.
From about 5 million years ago, the hominin
brain began to develop rapidly in both size
and differentiation of function.
There has been a gradual increase in brain
volume as humans progressed along the timeline
of evolution (see Homininae), starting from
about 600 cm3 in Homo habilis up to 1500 cm3
in Homo neanderthalensis.
Thus, in general there's a correlation between
brain volume and intelligence.
However, modern Homo sapiens have a brain
volume slightly smaller (1250 cm3) than neanderthals,
and the Flores hominids (Homo floresiensis),
nicknamed hobbits, had a cranial capacity
of about 380 cm3 (considered small for a chimpanzee)
about a third of that of H. erectus.
It is proposed that they evolved from H. erectus
as a case of insular dwarfism.
With their three times smaller brain the Flores
hominids apparently used fire and made tools
as sophisticated as those of their ancestor
H.erectus.
In this case, it seems that for intelligence,
the structure of the brain is more important
than its volume.
=== Homo ===
Roughly 2.4 million years ago Homo habilis
had appeared in East Africa: the first known
human species, and the first known to make
stone tools, yet the disputed findings of
signs of tool use from even earlier ages and
from the vicinity as multiple Australopithecus
fossils may put this to question its "greater
intelligence when compared to earlier and
more primitive Australopithecus genus".
The use of tools conferred a crucial evolutionary
advantage, and required a larger and more
sophisticated brain to co-ordinate the fine
hand movements required for this task.
Our knowledge of the complexity of behaviour
of Homo habilis is not limited to stone culture,
they also had habitual therapic use of toothpicks.
The evolution of a larger brain created a
problem for early humans, however.
A larger brain requires a larger skull, and
thus requires the female to have a wider birth
canal for the newborn's larger skull to pass
through.
But if the female's birth canal grew too wide,
her pelvis would be so wide that she would
lose the ability to run, which was a necessary
skill 2 million years ago.The solution to
this was to give birth at an early stage of
fetal development, before the skull grew too
large to pass through the birth canal.
This adaptation enabled the human brain to
continue to grow, but it imposed a new discipline.
The need to care for helpless infants for
long periods of time forced humans to become
less mobile.
Human bands increasingly stayed in one place
for long periods, so that females could care
for infants, while males hunted food and fought
with other bands that competed for food sources.
As a result, humans became even more dependent
on tool-making to compete with other animals
and other humans, and relied less on body
size and strength.
About 200,000 years ago Europe and the Middle
East were colonized by Neanderthal man, extinct
by 39,000 years ago following the appearance
of modern humans in the region from 40,000–45,000
years ago.
=== Homo sapiens ===
Dates approximate, consult articles for details
(From 2000000 BC till 2013 AD in (partial)
exponential notation)
See also: Java Man (−1.75e+06), Yuanmou
Man (−1.75e+06 : -0.73e+06),
Lantian Man (−1.7e+06), Nanjing Man (- 0.6e+06),
Tautavel Man (- 0.5e+06),
Peking Man (- 0.4e+06), Solo Man (- 0.4e+06),
and Peștera cu Oase (- 0.378e+05)
==== Homo sapiens intelligence ====
Around 200,000 years ago, Homo sapiens first
appeared in East Africa.
It is unclear to what extent these early modern
humans had developed language, music, religion
etc.
They spread throughout Africa over the following
approximately 50,000 years.According to proponents
of the Toba catastrophe theory, the climate
in non-tropical regions of the earth experienced
a sudden freezing about 70,000 years ago,
because of a huge explosion of the Toba volcano
that filled the atmosphere with volcanic ash
for several years.
This reduced the human population to less
than 10,000 breeding pairs in equatorial Africa,
from which all modern humans are descended.
Being unprepared for the sudden change in
climate, the survivors were those intelligent
enough to invent new tools and ways of keeping
warm and finding new sources of food (for
example, adapting to ocean fishing based on
prior fishing skills used in lakes and streams
that became frozen).Around 80,000–100,000
years ago, three main lines of Homo sapiens
diverged, bearers of mitochondrial haplogroup
L1 (mtDNA) / A (Y-DNA) colonizing Southern
Africa (the ancestors of the Khoisan/Capoid
peoples), bearers of haplogroup L2 (mtDNA)
/ B (Y-DNA) settling Central and West Africa
(the ancestors of Niger–Congo and Nilo-Saharan
speaking peoples), while the bearers of haplogroup
L3 remained in East Africa.The "Great Leap
Forward" leading to full behavioral modernity
sets in only after this separation.
Rapidly increasing sophistication in tool-making
and behaviour is apparent from about 80,000
years ago, and the migration out of Africa
follows towards the very end of the Middle
Paleolithic, some 60,000 years ago.
Fully modern behaviour, including figurative
art, music, self-ornamentation, trade, burial
rites etc. is evident by 30,000 years ago.
The oldest unequivocal examples of prehistoric
art date to this period, the Aurignacian and
the Gravettian periods of prehistoric Europe,
such as the Venus figurines and cave painting
(Chauvet Cave) and the earliest musical instruments
(the bone pipe of Geissenklösterle, Germany,
dated to about 36,000 years ago).
== Models ==
=== 
Social brain hypothesis ===
The social brain hypothesis was proposed by
British anthropologist Robin Dunbar, who argues
that human intelligence did not evolve primarily
as a means to solve ecological problems, but
rather as a means of surviving and reproducing
in large and complex social groups.
Some of the behaviors associated with living
in large groups include reciprocal altruism,
deception and coalition formation.
These group dynamics relate to Theory of Mind
or the ability to understand the thoughts
and emotions of others, though Dunbar himself
admits in the same book that it is not the
flocking itself that causes intelligence to
evolve (as shown by ruminants).Dunbar argues
that when the size of a social group increases,
the number of different relationships in the
group may increase by orders of magnitude.
Chimpanzees live in groups of about 50 individuals
whereas humans typically have a social circle
of about 150 people, which is also the typical
size of social communities in small societies
and personal social networks; this number
is now referred to as Dunbar's number.
In addition, there is evidence to suggest
that the success of groups is dependent on
their size at foundation, with groupings of
around 150 being particularly successful,
potentially reflecting the fact that communities
of this size strike a balance between the
minimum size of effective functionality and
the maximum size for creating a sense of commitment
to the community.
According to the social brain hypothesis,
when hominids started living in large groups,
selection favored greater intelligence.
As evidence, Dunbar cites a relationship between
neocortex size and group size of various mammals.
==== Criticism ====
Phylogenetic studies of brain sizes in primates
show that while diet predicts primate brain
size, sociality does not predict brain size
when corrections are made for cases in which
diet affects both brain size and sociality.
The exceptions to the predictions of the social
intelligence hypothesis, which that hypothesis
has no predictive model for, are successfully
predicted by diets that are either nutritious
but scarce or abundant but poor in nutrients.Meerkats
have far more social relationships than their
small brain capacity would suggest.
Another hypothesis is that it is actually
intelligence that causes social relationships
to become more complex, because intelligent
individuals are more difficult to learn to
know.There are also studies that show that
Dunbar's number is not the upper limit of
the number of social relationships in humans
either.The hypothesis that it is brain capacity
that sets the upper limit for the number of
social relationships is also contradicted
by computer simulations that show simple unintelligent
reactions to be sufficient to emulate "ape
politics" and by the fact that some social
insects such as the paper wasp do have hierarchies
in which each individual has its place (as
opposed to herding without social structure)
and maintains their hierarchies in groups
of approximately 80 individuals with their
brains smaller than that of any mammal.
=== Reduction in aggression ===
Another theory that tries to explain the growth
of human intelligence is the reduced aggression
theory (aka self-domestication theory).
According to this strand of thought what led
to the evolution of advanced intelligence
in Homo sapiens was a drastic reduction of
the aggressive drive.
This change separated us from other species
of monkeys and primates, where this aggressivity
is still in plain sight, and eventually lead
to the development of quintessential human
traits such as empathy, social cognition and
culture.
This theory has received strong support from
studies of animal domestication where selective
breeding for tameness has, in only a few generations,
led to the emergence of impressive “humanlike”
abilities.
Tamed foxes, for example, exhibit advanced
forms of social communication (following pointing
gestures), pedomorphic physical features (childlike
faces, floppy ears) and even rudimentary forms
of theory of mind (eye contact seeking, gaze
following).
Evidence also comes from the field of ethology
where it has been found that animals with
a gentle and relaxed manner of interacting
with each other – like for example stumptailed
macaques, orangutans and bonobos – have
more advanced socio-cognitive abilities than
those found among the more aggressive chimpanzees
and baboons.
It is hypothesized that these abilities derive
from a selection against aggression.On a mechanistic
level these changes are believed to be the
result of a systemic downregulation of the
sympathetic nervous system (the fight-or-flight
reflex).
Hence, tamed foxes show a reduced adrenal
gland size and have an up to fivefold reduction
in both basal and stress-induced blood cortisol
levels.
Similarly, domesticated rats and guinea pigs
have both reduced adrenal gland size and reduced
blood corticosterone levels.
It seems as though the neoteny of domesticated
animals significantly prolongs the immaturity
of their hypothalamic-pituitary-adrenal system
(which is otherwise only immature for a short
period when they are pups/kittens) and this
opens up a larger ”socialization window”
during which they can learn to interact with
their caretakers in a more relaxed way.
This downregulation of sympathetic nervous
system reactivity is also believed to be accompanied
by a compensatory increase in a number of
opposing organs and systems.
Although these are not as well specified various
candidates for such “organs” have been
proposed: the parasympathetic system as a
whole, the septal area over the amygdala,
the oxytocin system, the endogenous opioids
and various forms of quiescent immobilization
which antagonize the fight-or-flight reflex.
=== Social exchange theory ===
Other studies suggest that social exchange
between individuals is a vital adaptation
to the human brain, going as far to say that
the human mind could be equipped with a neurocognitive
system specialized for reasoning about social
change.
Social Exchange is a vital adaptation that
evolved in social species and has become exceptionally
specialized in humans.This adaption will develop
by natural selection when two parties can
make themselves better off than they were
before by exchanging things one party values
less for things the other party values for
more.
However, selection will only pressure social
exchange when both parties are receiving mutual
benefits from their relative situation; if
one party cheats the other by receiving a
benefit while the other is harmed, then selection
will stop.
Consequently, the existence of cheaters—those
who fail to deliver fair benefits—threatens
the evolution of exchange.
Using evolutionary game theory, it has been
shown that adaptations for social exchange
can be favored and stably maintained by natural
selection, but only if they include design
features that enable them to detect cheaters,
and cause them to channel future exchanges
to reciprocators and away from cheaters.
Thus, humans use social contracts to lay the
benefits and losses each party will be receiving
(if you accept benefit B from me, then you
must satisfy my requirement R).
Humans have evolved an advanced cheater detection
system, equipped with proprietary problem-solving
strategies that evolved to match the recurrent
features of their corresponding problem domains.
Not only do humans need to determine that
the contract was violated, but also if the
violation was intentionally done.
Therefore, systems are specialized to detect
contract violations that imply intentional
cheating.One problem with the hypothesis that
specific punishment for intentional deception
could coevolve with intelligence is the fact
that selective punishment of individuals with
certain characteristics selects against the
characteristics in question.
For example, if only individuals capable of
remembering what they had agreed to were punished
for breaking agreements, evolution would have
selected against the ability to remember what
one had agreed to.
Though this becomes a superficial argument
after considering the balancing positive selection
for the ability to successfully ‘make ones
case’.
Intelligence predicts the number of arguments
one can make when taking either side of a
debate.
Humans who could get away with behaviours
that exploited within and without-group cooperation,
getting more while giving less, would overcome
this.
=== Sexual selection ===
This model, which invokes sexual selection,
is proposed by Geoffrey Miller who argues
that human intelligence is unnecessarily sophisticated
for the needs of hunter-gatherers to survive.
He argues that the manifestations of intelligence
such as language, music and art did not evolve
because of their utilitarian value to the
survival of ancient hominids.
Rather, intelligence may have been a fitness
indicator.
Hominids would have been chosen for greater
intelligence as an indicator of healthy genes
and a Fisherian runaway positive feedback
loop of sexual selection would have led to
the evolution of human intelligence in a relatively
short period.In many species, only males have
impressive secondary sexual characteristics
such as ornaments and show-off behavior, but
sexual selection is also thought to be able
to act on females as well in at least partially
monogamous species.
With complete monogamy, there is assortative
mating for sexually selected traits.
This means that less attractive individuals
will find other less attractive individuals
to mate with.
If attractive traits are good fitness indicators,
this means that sexual selection increases
the genetic load of the offspring of unattractive
individuals.
Without sexual selection, an unattractive
individual might find a superior mate with
few deleterious mutations, and have healthy
children that are likely to survive.
With sexual selection, an unattractive individual
is more likely to have access only to an inferior
mate who is likely to pass on many deleterious
mutations to their joint offspring, who are
then less likely to survive.Sexual selection
is often thought to be a likely explanation
for other female-specific human traits, for
example breasts and buttocks far larger in
proportion to total body size than those found
in related species of ape.
It is often assumed that if breasts and buttocks
of such large size were necessary for functions
such as suckling infants, they would be found
in other species.
That human female breasts (typical mammalian
breast tissue is small) are found sexually
attractive by many men is in agreement with
sexual selection acting on human females secondary
sexual characteristics.
Sexual selection for intelligence and judging
ability can act on indicators of success,
such as highly visible displays of wealth.
Growing human brains require more nutrition
than brains of related species of ape.
It is possible that for females to successfully
judge male intelligence, they must be intelligent
themselves.
This could explain why despite the absence
of clear differences in intelligence between
males and females on average, there are clear
differences between male and female propensities
to display their intelligence in ostentatious
forms.This absence of difference is now known
to exist at the middle of distributions.
Average intelligence doesn’t differ much
between genders, but because female selection
is restricted more towards males at the top
end of male-male hierarchies or those increasingly
above average in physical attractiveness,
male trait distributions often have longer
tails; that is to say the lowest and highest
intelligences (and many more traits) in male
populations extend further out into the lowest
and highest values of the distribution than
for female traits.
This is because it paid to be a highly variable
male, as average males would have consistently
low opportunity, but variable males had a
chance of falling on the preferred side of
the trait distribution.
==== Critique ====
The sexual selection by the disability principle/fitness
display model of the evolution of human intelligence
is criticized by certain researchers for issues
of timing of the costs relative to reproductive
age.
While sexually selected ornaments such as
peacock feathers and moose antlers develop
either during or after puberty, timing their
costs to a sexually mature age, human brains
expend large amounts of nutrients building
myelin and other brain mechanisms for efficient
communication between the neurons early in
life.
These costs early in life build facilitators
that reduce the cost of neuron firing later
in life, and as a result the peaks of the
brain's costs and the peak of the brain's
performance are timed on opposite sides of
puberty with the costs peaking at a sexually
immature age while performance peaks at a
sexually mature age.
Critical researchers argue that this means
that the costs that intelligence is a signal
of reduce the chances of surviving to reproductive
age, does not signal fitness of sexually mature
individuals and, since the disability principle
is about selection for disabilities in sexually
immature individuals that evolutionarily increase
the offspring's chance of surviving to reproductive
age, would be selected against and not for
by its mechanisms.
These critics argue that human intelligence
evolved by natural selection citing that unlike
sexual selection, natural selection have produced
many traits that cost the most nutrients before
puberty including immune systems and accumulation
and modification for increased toxicity of
poisons in the body as a protective measure
against predators.
=== Intelligence as a disease-resistance sign
===
A 2008 study argues that human cleverness
is simply selected within the context of sexual
selection as an honest signal of genetic resistance
against parasites and pathogens.
The number of people with severe cognitive
impairment caused by childhood viral infections
like meningitis, protists like Toxoplasma
and Plasmodium, and animal parasites like
intestinal worms and schistosomes is estimated
to be in the hundreds of millions.
Even more people live with moderate mental
damages, such as inability to complete difficult
tasks, that are not classified as ‘diseases’
by medical standards, may still be considered
as inferior mates by potential sexual partners.
Thus, widespread, virulent, and archaic infections
are greatly involved in natural selection
for cognitive abilities.
People infected with parasites may have brain
damage and obvious maladaptive behavior in
addition to visible signs of disease.
Smarter people can more skillfully learn to
distinguish safe non-polluted water and food
from unsafe kinds and learn to distinguish
mosquito infested areas from safe areas.
Smarter people can more skillfully find and
develop safe food sources and living environments.
Given this situation, preference for smarter
child-bearing/rearing partners increases the
chance that their descendants will inherit
the best resistance alleles, not only for
immune system resistance to disease, but also
smarter brains for learning skills in avoiding
disease and selecting nutritious food.
When people search for mates based on their
success, wealth, reputation, disease-free
body appearance, or psychological traits such
as benevolence or confidence; the effect is
to select for superior intelligence that results
in superior disease resistance.
=== Ecological dominance-social competition
model ===
A predominant model describing the evolution
of human intelligence is ecological dominance-social
competition (EDSC), explained by Mark V. Flinn,
David C. Geary and Carol V. Ward based mainly
on work by Richard D. Alexander.
According to the model, human intelligence
was able to evolve to significant levels because
of the combination of increasing domination
over habitat and increasing importance of
social interactions.
As a result, the primary selective pressure
for increasing human intelligence shifted
from learning to master the natural world
to competition for dominance among members
or groups of its own species.
As advancement, survival and reproduction
within an increasing complex social structure
favored ever more advanced social skills,
communication of concepts through increasingly
complex language patterns ensued.
Since competition had shifted bit by bit from
controlling "nature" to influencing other
humans, it became of relevance to outmaneuver
other members of the group seeking leadership
or acceptance, by means of more advanced social
skills.
A more social and communicative person would
be more easily selected.
=== Intelligence dependent on brain size ===
Human intelligence is developed to an extreme
level that is not necessarily adaptive in
an evolutionary sense.
Firstly, larger-headed babies are more difficult
to give birth to and large brains are costly
in terms of nutrient and oxygen requirements.
Thus the direct adaptive benefit of human
intelligence is questionable at least in modern
societies, while it is difficult to study
in prehistoric societies.
Since 2005, scientists have been evaluating
genomic data on gene variants thought to influence
head size, and have found no evidence that
those genes are under strong selective pressure
in current human populations.
The trait of head size has become generally
fixed in modern human beings.While decreased
brain size has strong correlation with lower
intelligence in humans, some modern humans
have brain sizes as small as Homo Erectus
but normal intelligence (based on IQ tests)
for modern humans.
Increased brain size in humans may allow for
greater capacity for specialized expertise.
=== Group selection ===
Group selection theory contends that organism
characteristics that provide benefits to a
group (clan, tribe, or larger population)
can evolve despite individual disadvantages
such as those cited above.
The group benefits of intelligence (including
language, the ability to communicate between
individuals, the ability to teach others,
and other cooperative aspects) have apparent
utility in increasing the survival potential
of a group.
=== Nutritional status ===
Higher cognitive functioning develops better
in an environment with adequate nutrition,
and diets deficient in iron, zinc, protein,
iodine, B vitamins, omega 3 fatty acids, magnesium
and other nutrients can result in lower intelligence
either in the mother during pregnancy or in
the child during development.
While these inputs did not have an effect
on the evolution of intelligence they do govern
its expression.
A higher intelligence could be a signal that
an individual comes from and lives in a physical
and social environment where nutrition levels
are high, whereas a lower intelligence could
imply a child, its mother, or both, come from
a physical and social environment where nutritional
levels are low.
Previc emphasizes the contribution of nutritional
factors, especially meat and shellfish consumption,
to elevations of dopaminergic activity in
the brain, which may have been responsible
for the evolution of human intelligence since
dopamine is crucial to working memory, cognitive
shifting, abstract, distant concepts, and
other hallmarks of advanced intelligence.
== See also ==
Behavioral modernity
Brain size
Encephalization quotient
Fisherian runaway
Human evolution
Noogenesis
Primate cognition
Race and intelligence
