William H. Calvin
"The Emergence of Intelligence", Scientific American Presents 9(4):44-51 (November 1998).
See also http://WilliamCalvin.com/1990s/1998SciAmer.htm
It is a 1998 revision of what appeared in
Scientific American 271(4):100-107, October 1994.
is also available in audiotape
(no, that's not my voice), Human Evolution : Selections from Scientific
American Magazine (and in nice company, too: Stephen Jay Gould, Yves Coppens,
Ian Tattersall, & Luca Cavalli-Sforza).
Webbed Reprint Collection
William H. Calvin
University of Washington
Seattle WA 98195-1800 USA
The Emergence of Intelligence
Language, foresight, musical skills and other hallmarks of intelligence may all be linked to the
human ability to create rapid movements.
by William H. Calvin
To most observers, the essence of intelligence is cleverness, a versatility in solving novel
problems. Foresight is also said to be an essential aspect of intelligence -- particularly after an
encounter with one of those terminally clever people who are all tactics and no strategy. Other
observers will add creativity to the list. Personally, I like the way neurobiologist Horace Barlow
of the University of Cambridge frames the issue. He says intelligence is all about making a guess
that discovers some new underlying order. This idea neatly covers a lot of ground: finding the
solution to a problem or the logic of an argument, happening on an appropriate analogy, creating
a pleasing harmony or guessing what's likely to happen next. Indeed, we all routinely predict
what comes next, even when passively listening to a narrative or a melody. That's why a joke's
punch line or a P. D. Q. Bach musical parody brings you up short -- you were subconsciously
predicting something else and were surprised by the mismatch.
We will never agree on a universal definition of intelligence because it is an open-ended
word, like consciousness. Both intelligence and consciousness concern the high end of our
mental life, but they are frequently confused with more elementary mental processes, such as
ones we use to recognize a friend or tie a shoelace. Of course, such simple neural mechanisms
are probably the foundations from which our abilities to handle logic and metaphor evolved. But
how did that occur? That's both an evolutionary question and a neurophysiological one. Both
kinds of answers are needed to understand our own intelligence. They might even help explain
how an artificial or an exotic intelligence could evolve.
Did our intelligence arise from having more of what other animals have? The
two-millimeter-thick cerebral cortex is the part of the brain most involved with making novel
associations. Ours is extensively wrinkled but, were it flattened out, it would occupy four sheets
of typing paper. A chimpanzee's cortex would fit on one sheet, a monkey's on a postcard, a rat's
on a stamp. But a purely quantitative explanation seems incomplete. I will argue that our
intelligence arose primarily through the refinement of some brain specialization, such as that for
language. This specialization allowed a quantum leap in cleverness and foresight during the
evolution of humans from apes. If, as I suspect, the specialization involved a core facility
common to language, the planning of hand movements, music and dance, it has even greater
A particularly intelligent person often seems "quick" and capable of juggling many ideas at
once. Indeed, the two strongest influences on your IQ score are how many novel questions you
can answer in a fixed length of time, and how good you are at simultaneously manipulating a half
dozen mental images -- as in those analogy questions: A is to B as C is to (D, E or F).
Versatility is another characteristic of intelligence. Most animals are narrow specialists,
especially in matters of diet: the mountain gorilla consumes 50 pounds of green leaves each and
every day. In comparison, a chimpanzee switches around a lot -- it will eat fruit, termites, leaves
and even a small monkey or piglet if it is lucky enough to catch one. Omnivores have more basic
moves in their general behavior because their ancestors had to switch between many different
food sources. They need more sensory templates, too -- mental search images of things such as
foods and predators for which they are "on the lookout." Their behavior emerges through the
matching of these sensory templates to responsive movements.
Sometimes animals try out a new combination of search image and movement during play,
and find a use for it later. Many animals are only playful as juveniles; being an adult is a serious
business (they have all those young mouths to feed). Having a long juvenile period, as apes and
humans do, surely aids intelligence. A long life further promotes versatility by affording more
opportunities to discover new behaviors.
A social life also gives individuals the chance to mimic the useful discoveries of others.
Researchers have seen a troop of monkeys in Japan copy one inventive female's techniques for
washing sand off food. Moreover, a social life is full of interpersonal problems to solve, such as
those created by pecking orders, that go well beyond the usual environmental challenges to
survival and reproduction.
Yet versatility is not always a virtue, and more of it is not always better. When the
chimpanzees of Uganda arrive at a grove of fruit trees, they often discover that the efficient local
monkeys are already speedily stripping the trees of edible fruit. The chimps can turn to termite
fishing, or perhaps catch a monkey and eat it, but in practice their population is severely limited
by that competition, despite a brain twice the size of their specialist rivals.
The Impact of Abrupt Climate Change
Versatility becomes advantageous, however, when the weather changes abruptly. The
fourfold expansion of the hominid brain started 2.5 million years ago, when the ice ages began.
Ice cores from Greenland show that warming and cooling episodes occurred every several
thousand years, superimposed on the slower advances and retreats of the northern ice sheets. The
vast rearrangements in ocean currents lasted for centuries, with sudden transitions that took less
than a decade.
Ice core data of Dansgaard et al Nature 1993. Younger Dryas shown in red.
Note the two episodes during the warm period 130,000 years ago.
The abrupt coolings likely devastated the ecosystems on which our ancestors depended.
Because of lower temperatures and less rainfall, the forests in Africa dried up and animal
populations began to crash. Lightning strikes ignited giant forest fires, denuding large areas even
in the tropics. There was very little food after the fires. Once the grasses got started on the burnt
landscape, however, the surviving grazing animals had a boom time. Within several centuries, a
succession of forests came back in many places, featuring species more appropriate to the cooler
Cool, crash and burn. Our ancestors lived through hundreds of such episodes -- but each
was a population bottleneck that eliminated most of their relatives. Had the cooling taken a few
centuries to happen, so that the forests could have gradually shifted, our ancestors would not
have been treated so badly. The higher-elevation plant species would have slowly marched down
the hillsides to occupy the valley floors. Each hominid generation could have made their living in
the way their parents taught them, culturally adapting to the new milieu. But when the cooling
and drought was abrupt, it was one unlucky generation that suddenly had to improvise amidst
crashing populations and burning ecosystems.We are the improbable descendants of those who
survived -- probably because they had ways of coping with these episodes that the other great
apes did not exploit.
Improvising meant learning to eat grass -- or managing to regularly eat animals that eat
grass. The trouble is, such animals are fast and wary, whether rabbit or antelope. Small or big,
they're best tackled by cooperative groups. But sharing a rabbit leaves everyone hungry, so the
hunters would have tried for the bigger animals that cluster in herds. And that had an interesting
consequence. If a single hunter killed a big animal, it was too much to eat; best to give most of
the meat away and count on reciprocity when someone else succeeded. Sharing food also meant
fewer fights and more time available to seek out scarce food.
Each population bottleneck temporarily exaggerated the importance of such traits as
cooperation, altruism and hunting abilities. Even if each episode changed the inborn predilections
of the hominids by only a small amount, the hundreds of repetitions of this scenario may explain
some of the differences between human abilities and those of our closest relatives among the
great apes. It is tempting to say that the abrupt coolings pumped up brain size, but what makes
for better survival is something much more specific: hunting abilities and perhaps altruism. What
might they have to do with intelligence?
Syntax and Structured Thought
One of the improvements that occurred during the ice ages was the capacity for human
language. In most of us, the brain area critical to language is located just above our left ear.
Monkeys lack this left lateral language area: their vocalizations (and simple emotional utterances
in humans) employ a more primitive language area near the corpus callosum, the band of fibers
connecting the cerebral hemispheres.
Language is the most defining feature of human intelligence: without syntax -- the orderly
arrangement of verbal ideas -- we would be little more clever than a chimpanzee. For a glimpse
of life without syntax, we can look to the case of Joseph, an 11-year-old deaf boy. Because he
could not hear spoken language and had never been exposed to fluent sign language, Joseph did
not have the opportunity to learn syntax during the critical years of early childhood. As
neurologist Oliver Sacks described him: "Joseph saw, distinguished, categorized, used; he had
no problems with perceptual categorization or generalization, but he could not, it seemed, go
much beyond this, hold abstract ideas in mind, reflect, play, plan. He seemed completely literal --
unable to juggle images or hypotheses or possibilities, unable to enter an imaginative or
figurative realm....He seemed, like an animal, or an infant, to be stuck in the present, to be
confined to literal and immediate perception, though made aware of this by a consciousness that
no infant could have."
To understand why humans are so intelligent, we need to understand how our ancestors
remodeled the apes' symbolic repertoire and enhanced it by inventing syntax. Wild chimpanzees
use about three dozen different vocalizations to convey about three dozen different meanings.
They may repeat a sound to intensify its meaning, but they don't string together three sounds to
add a new word to their vocabulary. We humans also use about three dozen vocalizations, called
phonemes. Yet only their combinations have content: we string together meaningless sounds to
make meaningful words. Furthermore, human language uses strings of strings, such as the word
phrases that make up this sentence.
Our closest animal cousins, the common chimpanzee and the bonobo (pygmy chimpanzee),
can achieve surprising levels of language comprehension when motivated by skilled teachers.
Kanzi, the most accomplished bonobo, can interpret sentences he has never heard before, such as
"Go to the office and bring back the red ball," about as well as a 2.5-year-old child. Neither
Kanzi nor the child constructs such sentences independently, but they can demonstrate by their
actions that they understand them.
With a year's experience in comprehension, the child starts constructing sentences that nest
one word phrase inside another. The rhyme about the house that Jack built ("This is the farmer
sowing the corn/That kept the cock that crowed in the morn/...That lay in the house that Jack
built") is an example of such a sentence. Syntax has treelike rules of reference that enable us to
communicate quickly -- sometimes with fewer than a hundred sounds strung together -- who did
what to whom, where, when, why and how. Even children of low intelligence seem to acquire
syntax effortlessly, although intelligent deaf children like Joseph may miss out.
Something very close to syntax also seems to contribute to another outstanding feature of
human intelligence, the ability to plan ahead. Aside from hormonally triggered preparations for
winter, animals exhibit surprisingly little evidence of advance planning. For instance, some
chimpanzees use long twigs to pull termites from their nests. Yet as Jacob Bronowski observed,
none of the termite-fishing chimps "spends the evening going round and tearing off a nice tidy
supply of a dozen probes for tomorrow."
Human planning abilities may stem from our talent for building narratives. We can borrow
the mental structures for syntax to judge combinations of possible actions. To some extent, we do
this by talking silently to ourselves, making narratives out of what might happen next and then
applying syntax-like rules of combination to rate a scenario as unlikely, possible or likely.
Narratives are also a major foundation for ethical choices: we imagine a course of action and its
effects on others, then decide whether or not to do it. But our thinking is not limited to
languagelike constructs. Indeed, we may shout "Eureka!" when feeling a set of mental
relationships click into place, yet have trouble expressing them verbally.
Ballistic Movements and their Relatives
Language and intelligence are so powerful that we might think evolution would naturally favor their increase. But as the Harvard evolutionary biologist Ernst Mayr once said, most species are not intelligent, which suggests "that high intelligence is not at all favored by natural selection" -- or that it's very hard to achieve. So we must consider indirect ways of achieving it, rather than general principles.
Evolution often follows indirect routes rather than "progressing" via adaptations. To account
for the breadth of our higher intellectual functions (syntax, planning, logic, games with rules,
music), we need to look at improvements in common-core facilities. We humans certainly have a
passion for stringing things together: words into sentences, notes into melodies, steps into
dances, narratives into games with rules of procedure. Might stringing things together be a core
facility of the brain?
As improbable as the idea initially seems, the brain's planning of ballistic movements may
have once promoted language, music and intelligence. Ballistic movements are extremely rapid
actions of the limbs, that, once initiated, cannot be modified. Striking a nail with a hammer is an
example. Apes have only elementary forms of the ballistic arm movements at which humans are
expert -- hammering, clubbing and throwing. Perhaps it is no coincidence that these movements
are important to the manufacture and use of tools and hunting weapons: in some settings such as
cool-crash-and-burn, hunting and toolmaking were important additions to hominids' basic
Compared to most movements, ballistic ones require a surprising amount of planning. Slow
movements leave time for improvisation: when raising a cup to your lips, if the cup is lighter
than you remembered, you can correct its trajectory before it hits your nose. Thus, a complete
advance plan isn't needed. You start in the right general direction and then correct your path. For
sudden limb movements lasting less than one fifth of a second, feedback corrections are largely
ineffective because reaction times are too long. The brain has to plan every detail of the
movement. Hammering, for example, requires planning the exact sequence of activation for
dozens of muscles.
The problem of throwing is compounded by the briefness of the launch window -- the range
of time in which a projectile can be released to hit a target. Because the human sense of timing is
inevitably jittery, when the distance to a target doubles, the launch window becomes eight times
narrower. To shrink the timing jitter enough requires a chorus of independent timing
mechanisms, about 64 times as many neurons "singing" the same "plainchant" in unison.
If mouth movements rely on the same core facility for sequencing that ballistic hand
movements do, then improvements in dexterity might improve language, and vice versa.
Accurate throwing abilities, rewarded by surviving the cool-crash-and-burn episodes in the
tropics, also open up some options, such as the possibility of eating meat regularly, or of being
able to survive winter in a temperate zone. The gift of speech would be an incidental benefit -- a
free lunch, as it were, because of the linkage.
There certainly seems to be a sequencer common to both hands and language. Much of the
brain's coordination of movement occurs at a subcortical level in the basal ganglia or the
cerebellum, but novel movements tend to depend on the premotor and prefrontal cortex. Two
major lines of evidence point to cortical specialization for sequencing, and both of them suggest
that the lateral language area has a lot to do with it. Doreen Kimura of the University of Western
Ontario has found that stroke patients with language problems (aphasia) resulting from damage
to left lateral brain areas also have considerable difficulty executing novel sequences of hand and
arm movements (apraxia). By electrically stimulating the brains of patients being operated on for
epilepsy, George A. Ojemann of the University of Washington has also shown that at the center
of the left lateral areas specialized for language lies a region involved in listening to sound
sequences. This perisylvian region seems equally involved in producing oral-facial movement
sequences -- even nonlanguage ones.
These discoveries reveal that the "language cortex," as people sometimes think of it, serves
a far more generalized function than had been suspected. It is concerned with novel sequences of
various kinds: both sensations and movements, for both the hands and the mouth. The big
problem with creating new sequences and producing original behaviors is safety. Even simple
reversals in order can be dangerous, as in "Look after you leap." Our capacity to make analogies
and mental models gives us a measure of protection, however. We humans can simulate future
courses of action and weed out the nonsense off-line; as philosopher Karl Popper said, this
"permits our hypotheses to die in our stead." Creativity -- indeed, the whole high end of
intelligence and consciousness -- involves playing mental games that shape up quality before
acting. What sort of mental machinery might it take to do something like that?
Natural Selection in the Brain
By 1874, just 15 years after Darwin published The Origin of Species, American psychologist
William James was talking about mental processes operating in a Darwinian manner. In effect,
he suggested, ideas might somehow "compete" with one another in the brain, leaving only the
best or "fittest." Just as Darwinian evolution shaped a better brain in two million years, a similar
Darwinian process operating within the brain might shape intelligent solutions to problems on
the time scale of thought and action.
Researchers have demonstrated that a Darwinian process operating on a time scale of days
governs the immune system. Through a series of cellular generations spanning several weeks, the
immune system produces defensive antibody molecules that are better and better "fits" against
invaders. By abstracting the essential features of a Darwinian process from what is known about
species evolution and immune responses, we can see that any "Darwin machine" must have six
First, it must operate on patterns of some type; in genetics, they are strings of DNA bases, but
the patterns of brain activity associated with a thought might qualify. Second, copies are
somehow made of these patterns. Third, patterns must occasionally vary, either through mutations, copying errors, or superimposed patterns. Fourth, variant patterns must compete to
occupy some limited space (as when bluegrass and crabgrass compete for my backyard). Fifth,
the relative reproductive success of the variants is influenced by their environment; this result is
what Darwin called natural selection. And finally, the make-up of the next generation of patterns
depends on which variants survive to be copied. The patterns of the next generation will be
variations based on the more successful patterns of the current generation. Many of the new
variants will be less successful than their parents, but some may be more so.
Let us consider how these principles might apply to the evolution of an intelligent guess
inside the brain. Thoughts are combinations of sensations and memories -- in a way, they are
movements that haven't happened yet (and maybe never will). They take the form of cerebral
codes, which are spatiotemporal activity patterns in the brain that each represent an object, an
action or an abstraction. I estimate that a single code minimally involves a few hundred cortical
neurons within a millimeter of one another, either keeping quiet or firing in a musical pattern.
Evoking a memory is simply a matter of reconstituting such an activity pattern, according to
the cell-assembly hypothesis of psychologist Donald O. Hebb [see "The Mind and Donald O.
Hebb," by Peter M. Milner; Scientific American, January 1993]. Long-term memories are frozen
patterns waiting for signals of near resonance to reawaken them, like ruts in a washboarded road
waiting for a passing car to recreate a bouncing spatiotemporal pattern.
Some "cerebral ruts" are permanent, while others are short-lived. Short-term memories are
just temporary alterations in the strengths of synaptic connections between neurons, left behind
by the last spatiotemporal pattern to occupy a patch of cortex; they fade in a matter of minutes.
The transition from short-term to long-term memory is not well understood, but it appears to
involve structural alterations in which the synaptic connections between neurons are made strong
and permanent, hardwiring the pattern of neural activity into the brain.
A Darwinian model of mind suggests that an activated memory can compete with others for
"workspace" in the cortex. Both the perceptions of the thinker's current environment and the
memories of past environments may bias that competition and shape an emerging thought. An
active cerebral code moves from one part of the brain to another by making a copy of itself,
much as a fax machine re-creates a pattern on a distant sheet of paper. The cerebral cortex also
has circuitry for copying spatiotemporal patterns in an adjacent region less than a millimeter
away, though our present imaging techniques lack enough resolution to see it in progress.
Repeated copying of the minimal pattern could colonize a region, rather the way that a crystal
grows or wallpaper repeats an elementary pattern.
The picture that emerges from these theoretical considerations is one of a quilt, some
patches of which enlarge at the expense of their neighbors as one code copies more successfully
than another. As you try to decide whether to pick an apple or a banana from the fruit bowl, so
my theory goes, the cerebral code for "apple" may be having a cloning competition with the one
for "banana." When one code has enough active copies to trip the action circuits, you might reach
for the apple. But the banana codes need not vanish: they could linger in the background as
subconscious thoughts. Our conscious thought may be only the currently dominant pattern in the
copying competition, with many other variants competing for dominance, one of which will win
a moment later when your thoughts seem to shift focus.
It may be that Darwinian processes are only the frosting on the cognitive cake, that much of
our thinking is routine or rule-bound. But we often deal with novel situations in creative ways, as
when you decide what to fix for dinner tonight. You survey what's already in the refrigerator and
on the kitchen shelves. You think about a few alternatives, keeping track of what else you might
have to fetch from the grocery store. All of this can flash though your mind within seconds -- and
that's probably a Darwinian process at work.
In both its phylogeny and ontogeny, human intelligence first solves movement problems
and only later graduates to ponder more abstract ones. An artificial or extraterrestrial intelligence
freed of the necessity of finding food and avoiding predators might not need to move -- and so
might lack the what-happens-next orientation of human intelligence. It is difficult to estimate
how often high intelligence might emerge, given how little we know about the demands of
long-term species survival and the courses evolution can follow. We can, however, evaluate the
prospects of a species by asking how many elements of intelligence each has amassed. Chimps
and bonobos may be missing a few of the elements -- the ability to construct nested sentences,
for example -- but they're doing better than the present generation of artificial intelligence
Why aren't there more species with such complex mental states? There might be a hump to
get over: a little intelligence can be a dangerous thing. A beyond-the-apes intelligence must
constantly navigate between the twin hazards of dangerous innovation and a conservatism that
ignores what the Red Queen explained to Alice in Through the Looking Glass: "...it takes all the
running you can do, to keep in the same place." Foresight is our special form of running,
essential for the intelligent stewardship that Stephen Jay Gould of Harvard University warns is
needed for longer-term survival: "We have become, by the power of a glorious evolutionary
accident called intelligence, the stewards of life's continuity on earth. We did not ask for this
role, but we cannot abjure it. We may not be suited to it, but here we are."
also an audiotape
(no, that's not my voice), Human Evolution : Selections from Scientific
American Magazine (and in nice company, too: Stephen Jay Gould, Yves Coppens,
Ian Tattersall, & Luca Cavalli-Sforza).
About the author
WILLIAM H. CALVIN's career has taken a Darwinian course: his scientific interests have evolved significantly over the past four decades. He studied physics as an undergraduate at Northwestern University, but devoted his spare time to a research project exploring how the brain processes color vision. This project led to graduate work in neuroscience at M.I.T. and Harvard Medical School, then to a Ph.D. in physiology and biophysics from the University of Washington in 1966. His early research focused on neuron firing mechanisms. "I wiretapped neurons, trying to figure out how they transformed information," he says. But in the 1980s he took on a bigger question -- how the human brain evolved -- and his interests broadened to include anthropology, zoology and psychology. He has written several acclaimed books, including The Cerebral Code, How Brains Think and (with George Ojemann) Conversations with Neil's Brain. "The puzzles I'm trying to solve require information from many different fields," he says. He is currently a theoretical neurophysiologist on the faculty of the University of Washington School of Medicine.
Derek Bickerton, Language and Species (University of Chicago Press, 1990). More.... amazon.com
Derek Bickerton, Language and Human Behavior (University of Washington Press, 1995). More.... amazon.com
William H. Calvin, Derek Bickerton, Lingua ex machina: Reconciling Darwin and Chomsky with the Human Brain (MIT Press, forthcoming in 1999). You can currently read a draft of the manuscript on the web.
William H. Calvin, The Cerebral Code: Thinking a Thought in the Mosaics of the Mind (MIT Press, 1996). More.... amazon.com
William H. Calvin, How Brains Think: Evolving Intelligence, Then and Now (Science Masters, BasicBooks, 1996). More.... amazon.com
William H. Calvin and George A. Ojemann, Conversations with Neils Brain: The Neural Nature of Thought and Language (Addison-Wesley, 1994). More.... amazon.com
Kathleen R. Gibson and Tim Ingold (Editors), Tools, Language and Cognition in Human Evolution (Cambridge University Press, 1993). More.... amazon.com
E. Sue Savage-Rumbaugh, Stuart Shanker, Talbot J. Taylor, Apes, Language, and the Human Mind (Oxford University Press, May 1998).
Terrence Deacon, The Symbolic Species: The Co-Evolution of Language and the Brain (W. W. Norton, August 1997). More.... amazon.com
Daniel C. Dennett, Kinds of Minds: Toward an Understanding of Consciousness (Science Masters, BasicBooks, 1996). More.... amazon.com
Steven Pinker, The Language Instinct (Morrow, 1994). More.... amazon.com
Oliver Sacks, Seeing Voices (University of California Press, 1989). More.... amazon.com
Sue Savage-Rumbaugh and Roger Lewin, Kanzi (Wiley, 1994). More.... amazon.com