2 GENES AND BRAINS
What the hammer? What the chain?
In what furnace was thy brain? What the anvil? What dread grasp
Dare its deadly terrors clasp? WM. BLAKE
“The Tyger”
Of all animals, man has the largest brain in proportion to his size.
ARISTOTLE
The Parts of Animals
BIOLOGICAL evolution has been accompanied by increasing complexity.
The most complex organisms on Earth today contain substantially more
stored information, both genetic and extragenetic, than the most
complex organisms of, say, two hundred million years ago - which is
only 5 percent of the history of life on the planet, five days ago
on the Cosmic Calendar.
The simplest organisms on Earth today have
just as much evolutionary history behind them as the most complex,
and it may well be that the internal biochemistry of contemporary
bacteria is more efficient than the internal biochemistry of the
bacteria of three billion years ago. But the amount of genetic
information in bacteria today is probably not vastly greater than
that in their ancient bacterial ancestors. It is important to
distinguish between the amount of information and the quality of
that information.
The various biological forms are called taxa (singular, taxon). The
largest taxonomic divisions distinguish between plants and animals,
or between those organisms with poorly developed nuclei in their
cells (such as bacteria and blue - green algae) and those with very
clearly demarcated and elaborately architectured nuclei (such as
protozoa or people). All organisms on the planet Earth, however,
whether they have well - defined nuclei or not, have chromosomes,
which contain the genetic material passed on from generation to
generation. In all organisms the hereditary molecules are nucleic
acids.
With a few unimportant exceptions, the hereditary nucleic
acid is always the molecule called DNA (deoxyribonucleic acid). Much
finer divisions among various sorts of plants and animals, down to
species, subspecies and races, can also be described as separate taxa.
A species is a group that can produce fertile offspring by
crosses within but not outside itself. The mating of different
breeds of dogs yields puppies which, when grown, will be
reproductively competent dogs. But crosses between species - even
species as similar as donkeys and horses - produce infertile offspring
(in this case, mules). Donkeys and horses are therefore categorized
as separate species. Viable but infertile matings of more widely
separated species - for example, lions and tigers - sometimes occur, and
if, rarely, the offspring are fertile, this indicates only that the
definition of species is a little fuzzy.
All human beings are
members of the same species, Homo sapiens, which means, in
optimistic Latin, “Man, the wise.” Our probable ancestors, Homo
erectus and Homo habilis - now extinct - are classified as of the same
genus (Homo) but of different species, although no one (at least
lately) has attempted the appropriate experiments to see if crosses
of them with us would produce fertile offspring.
In earlier times it was widely held that offspring could be produced
by crosses between extremely different organisms. The Minotaur whom
Theseus slew was said to be the result of a mating between a bull
and a woman. And the Roman historian Pliny suggested that the
ostrich, then newly discovered, was the result of a cross between a
giraffe and a gnat. (It would, I suppose, have to be a female
giraffe and a male gnat.) In practice there must be many such
crosses which have not been attempted because of a certain
understandable lack of motivation.
The chart that appears on page 26 will be referred to repeatedly in
this chapter. The solid curve on it shows the times of earliest
emergence of various major taxa. Many more taxa exist, of course,
than are shown by the few points in the figure. But the curve is
representative of the much denser array of points that would be
necessary to characterize the tens of millions of separate taxa
which have emerged during the history of life on our planet. The
major taxa, which have evolved most recently, are by and large the
most complicated.
Some notion of the complexity of an organism can be obtained merely
by considering its behavior - that is, the number of different
functions it is called upon to perform in its lifetime. But
complexity can also be judged by the minimum information content in
the organism’s genetic material.
A typical human chromosome has one
very long DNA molecule wound into coils, so that the space it
occupies is very much smaller than it would be if it were unraveled.
This DNA molecule is composed of smaller building blocks, a little
like the rungs and sides of a rope ladder. These blocks are called
nucleotides and come in four varieties. The language of life, our
hereditary information, is determined by the sequence of the four
different sorts of nucleotides. We might say that the language of
heredity is written in an alphabet of only four letters.
But the book of life is very rich; a typical chromosomal DNA
molecule in a human being is composed of about five billion pairs of
nucleotides. The genetic instructions of all the other taxa on Earth
are written in the same language, with the same code book. Indeed,
this shared genetic language is one line of evidence that all the
organisms on Earth are descended from a single ancestor, a single
instance of the origin of life some four billion years ago.
The information content of any message is usually described in units
called bits, which is short for “binary digits.” The simplest
arithmetical scheme uses not ten digits (as we do because of the
evolutionary accident that we have ten fingers) but only two, 0 and
1. Thus any sufficiently crisp question can be answered by a single
binary digit - 0 or 1, yes or no. If the genetic code were written in
a language of two letters rather than four letters, the number of
bits in a DNA molecule would equal twice the number of nucleotide
pairs.
But since there are four different kinds of nucleotides, the
number of bits of information in DNA is four times the number of
nucleotide pairs. Thus if a single chromosome has five billion (5 X
109) nucleotides, it contains twenty billion (2 X 1010) bits of
information. [A symbol such as 109 merely indicates a one followed
by a certain number of zeroes - in this case, nine of them.]
How much information is twenty billion bits? What would be its
equivalent, if it were written down in an ordinary printed book in a
modern human language? Alphabetical human languages
characteristically have twenty to forty letters plus one or two
dozen numerals and punctuation marks; thus sixty - four alternative
characters should suffice for most such languages.
Since 26 equals
64 (2 X 2 X 2 X 2 X 2 X 2), it should take no more than six bits to
specify a given character. We can think of this being done by a sort
of game of “Twenty Questions,” in which each answer corresponds to
the investment of a single bit to a yes/no question. Suppose the
character in question is the letter J. We might specify it by the
following procedure:
FIRST QUESTION: Is it a letter (0) or some other character (1)? ANSWER: A letter (0)
SECOND QUESTION: Is it in the first half (0) or the second half of
the alphabet (1)?
ANSWER: In the first half (0)
THIRD QUESTION: Of the thirteen letters in the first half of the
alphabet, is it in the first seven (0) or the second six (1)? ANSWER: In the second six (1)
FOURTH QUESTION: In the second six (H, I, J, K,L, M), is it in the
first half (0) or the second half (1)? ANSWER: In the first half (0)
FIFTH QUESTION: Of these letters H, I, J, is it
H
(0) or .is it one of I and J (1)? ANSWER: It is one of I and J
(1)
SIXTH QUESTION: Is it I (0) or J (1)?
ANSWER: It is J (1).
Specifying the letter J is therefore equivalent to the binary
message, 001011. But it required not twenty questions but six,
and it is in this sense that only six bits are required to specify a
given letter. Therefore twenty billion bits are the equivalent of
about three billion letters (2 X 1010/6 = 3 X 109).
If there are
approximately six letters in an average word, the information
content of a human chromosome corresponds to about five
hundred million words (3 X 109/6 = 5 X 108). If there are about
three hundred words on an ordinary page of printed type, this
corresponds to about two million pages (5 X 108/3 X 102 X 108). If a
typical book contains five hundred such pages, the information
content of a single human chromosome corresponds to some four
thousand volumes (2 x 108/5 x 102 = 4 x 103).
It is clear, then,
that the sequence of rungs on our DNA ladders represents an enormous
library of information. It is equally clear that so rich a library
is required to specify as exquisitely constructed and intricately
functioning an object as a human being. Simple organisms have less
complexity and less to do, and therefore require a smaller amount of
genetic information. The Viking landers that put down on Mars in
1976 each had preprogrammed instructions in their computers
amounting to a few million bits. Thus Viking had slightly more
“genetic information” than a bacterium, but significantly less than
an alga.
The chart on page 26 also shows the minimum amount of genetic
information in the DNA of various taxa. The amount shown for mammals
is less than for human beings, because most mammals have less
genetic information than human beings do. Within certain taxa - for
example, the amphibians - the amount of genetic information varies
wildly from species to species, and it is thought that much of this
DNA may be redundant or functionless. This is the reason that the
chart displays the minimum amount of DNA for a given taxon.
We see from the chart that there was a striking improvement in the
information content of organisms on Earth some three billion years
ago, and a slow increase in the amount of genetic information
thereafter. We also see that if more than some tens of billions
(several times 1010) of bits of information are necessary for human
survival, extragenetic systems will have to provide them: the rate
of development of genetic systems is so slow that no source of such
additional biological information can be sought in the DNA.
The raw materials of evolution are mutations, inheritable
changes in the particular nucleotide sequences that make up
the hereditary instructions in the DNA molecule. Mutations are
caused by radioactivity in the environment, by cosmic rays
from space, or, as often happens, randomly - by spontaneous
rearrangements of the nucleotides which statistically must occur
every now and then. Chemical bonds spontaneously break. Mutations
are also to some extent controlled by the organism itself. Organisms
have the ability to repair certain classes of structural damage done
to their DNA.
There are, for example, molecules which patrol the DNA
for damage; when a particularly egregious alteration in the DNA is
discovered, it is snipped out by a kind of molecular scissors, and
the DNA put right. But such repair is not and must not be perfectly
efficient: mutations are required for evolution. A mutation in a DNA
molecule within a chromosome of a skin cell in my index finger has
no influence on heredity. Fingers are not involved, at least
directly, in the propagation of the species. What counts are
mutations in the gametes, the eggs and sperm cells, which are the
agents of sexual reproduction.
Accidentally useful mutations provide the working material for
biological evolution - as, for example, a mutation for melanin in
certain moths, which changes their color from white to black. Such
moths commonly rest on English birch trees, where their white
coloration provides protective camouflage. Under these conditions,
the melanin mutation is not an advantage - the dark moths are starkly
visible and are eaten by birds; the mutation is selected against.
But when the Industrial Revolution began to cover the birch bark
with soot, the situation was reversed, and only moths with the
melanin mutation survived.
Then the mutation is selected for, and,
in time, almost all the moths are dark, passing this inheritable
change on to future generations. There are still occasional reverse
mutations eliminating the melanin adaptation, which would be useful
for the moths were English industrial pollution to be controlled.
Note that in all this interaction between mutation and natural
selection, no moth is making a conscious effort to adapt to a
changed environment. The process is random and statistical.
Large organisms such as human beings average about one
mutation per ten gametes - that is, there is a 10 percent chance
that any given sperm or egg cell produced will have a new and
inheritable change in the genetic instructions that determine the
makeup of the next generation. These mutations occur at random and
are almost uniformly harmful - it is rare that a precision machine is
improved by a random change in the instructions for making it.
Most of these mutations are also recessive - they do not manifest
themselves immediately. Nevertheless, there is already such a high
mutation rate that, as several biologists have suggested, a larger
complement of genetic DNA would bring about unacceptably high
mutation rates: too much would go wrong too often if we had more
genes.* If this is true, there must be a practical upper limit to
the amount of genetic information that the DNA of larger organisms
can accommodate. Thus large and complex organisms, by the mere fact
of their existence, have to have substantial resources of extragenetic information. That information is contained, in all
higher animals except Man, almost exclusively in the brain.
* To some extent the mutation rate is itself controlled by natural
selection, as in our example of a “molecular scissors.”
But there is likely to be an irreducible minimum mutation rate
(1) in order to produce enough genetic experiments for natural
selection to operate on, and (2) as an equilibrium between mutations
produced, say, by cosmic rays and the most efficient possible
cellular repair mechanisms.
What is the information content of the brain? Let us consider two
opposite and extreme poles of opinion on brain function. In one
view, the brain, or at least its outer layers, the cerebral cortex,
is equipotent: any part of it may substitute for any other part, and
there is no localization of function. In the other view, the brain
is completely hard - wired: specific cognitive functions are localized
in particular places in the brain.
Computer design suggests that the
truth lies somewhere between these two extremes. On the one hand,
any nonmystical view of brain function must connect physiology with
anatomy; particular brain functions must be tied to particular
neural patterns or other brain architecture. On the other hand, to
assure accuracy and protect against accident we would expect natural
selection to have evolved substantial redundancy in brain function.
This is also to be expected from the evolutionary path that it is
most likely the brain followed.
The redundancy of memory storage was clearly demonstrated
by Karl Lashley, a Harvard psycho-neurologist, who surgically
removed (extirpated) significant fractions of the cerebral cortex
of rats without noticeably affecting their recollection of
previously learned behavior on how to run mazes. From such
experiments it is clear that the same memory must be localized in
many different places in the brain, and we now know that some
memories are funneled between the left and right cerebral
hemispheres by a conduit called the corpus callosum.
Lashley also reported no apparent change in the general behavior of
a rat when significant fractions - say, 10 percent - of its brain were
removed. But no one asked the rat its opinion. To investigate this
question properly would require a detailed study of rat social,
foraging, and predator - evasion behavior.
There are many conceivable behavioral changes resulting from
such extirpations that might not be immediately obvious to the
casual scientist but that might be of considerable significance to
the rat - such as the amount of post-extirpation interest an
attractive rat of the opposite sex now elicits, or the degree of
disinterest now evinced by the presence of a stalking cat.*
(* Incidentally, as a test of the influence of animated cartoons
on American life, try rereading this paragraph with the word
“rat” replaced everywhere by “mouse,” and see if your sympathy for
the surgically invaded and misunderstood beast suddenly increases.)
It is sometimes argued that cuts or lesions in significant parts of
the cerebral cortex in humans - as by bilateral prefrontal lobotomy or
by an accident - have little effect on behavior. But some sorts of
human behavior are not very apparent from the outside, or even from
the inside. There are human perceptions and activities that may
occur only rarely, such as creativity. The association of ideas
involved in acts - even small ones - of creative genius seems to imply
substantial investments of brain resources. These creative acts
indeed characterize our entire civilization and mankind as a
species. Yet in many people they occur only rarely, and their
absence may be missed by neither the brain damaged subject nor the
inquiring physician.
While substantial redundancy in brain function is inevitable, the
strong equipotent hypothesis is almost certainly wrong, and most
contemporary neurophysiologists have rejected it. On the other hand,
a weaker equipotent hypothesis - holding, for example, that memory is
a function of the cerebral cortex as a whole - is not so readily
dismissible, although it is testable, as we shall see.
There is a popular contention that half or more of the brain is
unused. From an evolutionary point of view this would be quite
extraordinary: why should it have evolved if it had no function? But
actually the statement is made on very little evidence. Again, it is
deduced from the finding that many lesions of the brain, generally
of the cerebral cortex, have no apparent effect on behavior. This
view does not take into account,
(1) the possibility of redundant
function; and
(2) the fact that some human behavior is subtle.
For
example, lesions in the right hemisphere of the cerebral cortex may
lead to impairments in thought and action, but in the nonverbal
realm, which is, by definition, difficult for the patient or the
physician to describe.
There is also considerable evidence for localization of brain
function. Specific brain sites below the cerebral cortex have been
found to be concerned with appetite, balance, thermal regulation,
the circulation of the blood, precision movements and breathing. A
classic study on higher brain function is the work of the Canadian
neurosurgeon, Wilder Penfield, on the electrical stimulation of
various parts of the cerebral cortex, generally in attempts to
relieve symptoms of a disease such as psychomotor epilepsy. Patients
reported a snatch of memory, a smell from the past, a sound or color
trace - all elicited by a small electrical current at a particular
site in the brain.
In a typical case, a patient might hear an orchestral composition
in full detail when current flowed through Penfield’s electrode
to the patient’s cortex, exposed after a craniotomy. If Penfield
indicated to the patient - who typically is fully conscious during
such procedures - that he was stimulating the cortex when he was not,
invariably the patient would report no memory trace at that moment.
But when, without notice, a current would flow through the electrode
into the cortex, a memory trace would begin or continue.
A patient
might report a feeling tone, or a sense of familiarity, or a full
retrieval of an experience of many years previous playing back in
his mind, simultaneously but in no conflict with his awareness of
being in an operating room conversing with a physician. While some
patients described these flashbacks as “little dreams,” they
contained none of the characteristic symbolism of dream material.
These experiences have been reported almost exclusively by
epileptics, and it is possible, although it has by no means been
demonstrated, that non - epileptics are, under similar circumstances,
subject to comparable perceptual reminiscences.
In one case of electrical stimulation of the occipital lobe, which
is concerned with vision, the patient reported Seeing a fluttering
butterfly of such compelling reality that he stretched out his hand
from the operating table to catch it. In an identical experiment
performed on an ape, the animal peered intently, as if at an object
before him, made a swift catching motion with his right hand, and
then examined, in apparent bewilderment, his empty fist.
Painless electrical stimulation of at least some human cerebral
cortices elicits cascades of memories of particular events. But
removal of the brain tissue in contact with the electrode does not
erase the memory. It is difficult to resist the conclusion that at
least in humans memories are stored somewhere in the cerebral
cortex, waiting for the brain to retrieve them by electrical
impulses - which, of course, are ordinarily generated within the brain
itself.
If memory is a function of the cerebral cortex as a whole - a
kind of dynamic reverberation or electrical standing wave
pattern of the constituent parts, rather than stored statically in
separate brain components - this would explain the survival of
memory after significant brain damage. The evidence, however,
points in the other direction: In experiments performed by the
American neurophysiologist Ralph Gerard at the University of
Michigan, hamsters were taught to run a simple maze and then chilled
almost to the freezing point in a refrigerator, a kind of induced
hibernation.
The temperatures were so low that all detectable
electrical activity in the animals’ brains ceased. If the dynamic
view of memory were true, the experiment should have wiped out all
memory of successful maze - running. Instead, after thawing, the
hamsters remembered. Memory seems to be localized in specific sites
in the brain, and the survival of memories after massive brain
lesions must be the result of redundant storage of static memory
traces in various locales.
Penfield, extending the findings of previous researchers, also
uncovered a remarkable localization of function in the motor cortex.
Certain parts of the outer layers of our brain are responsible for
sending signals to or receiving signals from specific parts of the
body. A version of Penfield’s maps of the sensory and motor cortices
appear on pages 36 and 37. It reflects in an engaging way the
relative importance of various parts of our body.
The enormous
amount of brain area committed to the fingers - particularly the thumb
- and to the mouth and the organs of speech corresponds precisely to
what in human physiology, through human behavior, has set us apart
from most of the other animals. Our learning and our culture would
never have developed without speech; our technology and our
monuments would never have evolved without hands. In a way, the map
of the motor cortex is an accurate portrait of our humanity.
But the evidence for localization of function is now much stronger
even than this. In an elegant set of experiments, David Hubel of
Harvard Medical School discovered the existence of networks of
particular brain cells that respond selectively to lines perceived
by the eye in different orientations. There are cells for
horizontal, and cells for vertical, and cells for diagonal, each of
which is stimulated only if lines of the appropriate orientation are
perceived. At least some beginnings of abstract thought have thereby
been traced to the cells of the brain.
The existence of specific brain areas dealing with particular
cognitive, sensory or motor functions implies that there need not be
any perfect correlation between brain mass and intelligence; some
parts of the brain are clearly more important than others. Among the
most massive human brains on record are those of Oliver Cromwell,
Ivan Turgenev and Lord Byron, all of whom were smart but no
Albert Einsteins. Einstein’s brain, on the other hand, was not remarkably
large. Anatole France, who was brighter than many, had a brain half
the size of Byron’s.
The human baby is born with an exceptionally
high ratio of brain mass to body mass (about 12 percent); and the
brain, particularly the cerebral cortex, continues to grow rapidly
in the first three years of life - the period of most rapid learning.
By age six, the mass of the brain is 90 percent of its adult value.
The average mass of the brain of contemporary men is about 1,375
grams, almost three pounds. Since the density of the brain, like
that of all body tissues, is about that of water (one gram per cubic
centimeter), the volume of such a brain is 1,375 cubic centimeters,
a little under a liter and a half. (One cubic centimeter is about
the volume of an adult human navel.)
But the brain of a contemporary woman is about 150 cubic centimeters
smaller. When cultural and child - rearing biases are taken into
account, there is no clear evidence of overall differences in
intelligence between the sexes. Therefore, brain mass differences of
150 grams in humans must be unimportant. Comparable differences in
brain mass exist among adults of different human races (Orientals,
on the average, have slightly larger brains than whites); since no
differences in intelligence under similarly controlled conditions
have been demonstrated there, the same conclusion follows. And the
gap between the sizes of the brains of Lord Byron (2,200 grams) and
Anatole France (1,100 grams) suggests that, in this range,
differences of many hundreds of grams may be functionally
unimportant.
On the other hand, adult human microcephalics, who are born
with tiny brains, have vast losses in cognitive abilities; their
typical brain masses are between 450 and 900 grams. A normal
newborn child has a typical brain mass of 350 grams; a one-year- old,
about 500 grams. It is clear that, as we consider smaller and
smaller brain masses, there comes a point where the brain mass is so
tiny that its function is severely impaired, compared to normal
adult human brain function.
Moreover, there is a statistical correlation between brain mass or
size and intelligence in human beings. The relationship is not
one-to-one, as the Byron-France comparison clearly shows. We cannot
tell a person’s intelligence in any given case by measuring his or
her brain size. However, as the American evolutionary biologist
Leigh van Valen of the University of Chicago has shown, the
available data suggest a fairly good correlation, on the average,
between brain size and intelligence. Does this mean that brain size
in some sense causes intelligence?
Might it not be, for example,
that malnutrition, particularly in utero and in infancy, leads to
both small brain size and low intelligence, without the one causing
the other? Van Valen points out that the correlation between brain
size and intelligence is much better than the correlation between
intelligence and stature or adult body weight, which are known to be
influenced by malnutrition, and there is no doubt that malnutrition
can lower intelligence. Thus beyond such effects, there appears to
be an extent to which larger absolute brain size tends to produce
higher intelligence.
In exploring new intellectual territory, physicists have found it
useful to make order-of-magnitude estimates. These are rough
calculations that block out the problem and serve as guides for
future studies. They do not pretend to be highly accurate. In the
question of the connection between brain size and intelligence, it
is clearly far beyond present scientific abilities to perform a
census of the function of every cubic centimeter of the brain. But
might there not be some rough and approximate way in which to
connect brain mass with intelligence?
The difference in brain mass between the sexes is of interest in
precisely this context, because women are systematically
smaller in size and have a lower body mass than men. With less
body to control, might not a smaller brain mass be adequate? This
suggests that a better measure of intelligence than the absolute
value of the mass of a brain is the ratio of the mass of the brain
to the total mass of the organism.
The chart on page 39 shows the brain masses and body masses of
various animals. There is a remarkable separation of fish and
reptiles from birds and mammals. For a given body mass or weight,
mammals have consistently higher brain mass. The brains of mammals
are ten to one hundred times more massive than the brains of
contemporary reptiles of comparable size.
The discrepancy between
mammals and dinosaurs is even more striking. These are stunningly
large and completely systematic differences. Since we are mammals,
we probably have some prejudices about the relative intelligence of
mammals and reptiles; but I think the evidence is quite compelling
that mammals are indeed systematically much more intelligent than
reptiles.
(Also shown is an intriguing exception: a small
ostrich - like theropod class of dinosaurs from the late Cretaceous
Period, whose ratio of brain to body mass places them just within
the regional diagram otherwise restricted to large birds and the
less intelligent mammals. It would be interesting to know much more
about these creatures, which have been studied by Dale Russell,
chief of the Paleontology Division of the National Museums of
Canada.)
We also see from the chart on page 39 that the primates, a taxon that includes man, are separated, but less systematically,
from the rest of the mammals; primate brains are on the average more
massive by a factor of about two to twenty than those of nonprimate
mammals of the same body mass.
When we look more closely at this chart, isolating a number of
particular animals, we see the results on page 40. Of all the
organisms shown, the beast with the largest brain mass for its body
weight is a creature called Homo sapiens. Next in such a ranking are
the dolphins.* Again I do not think it is chauvinistic to conclude
from evidence on their behavior that humans and dolphins are at
least among the most intelligent organisms on Earth.
* By the criterion of brain mass to body mass, sharks are the
smartest of the fishes, which is consistent with their ecological
niche - predators have to be brighter than plankton browsers. Both in
their increasing ratio of brain to body mass and in the development
of coordinating centers in the three principal components of their
brains, sharks have evolved in a manner curiously parallel to the
evolution of higher vertebrates on the land.
The importance of this ratio of brain to body mass had been realized
even by Aristotle. Its principal modern exponent has been Harry Jerison, a neuro-psychiatrist at the University of California at Los
Angeles. Jerison points out that some exceptions exist to our
correlation - e. g., the European pygmy shrew has a brain mass of 100
milligrams in a 4.7 gram body, which gives it a mass ratio in the
human range. But we cannot expect the correlation of mass ratio with
intelligence to apply to the smallest animals, because the simplest
“housekeeping” functions of the brain must require some minimum
brain mass.
The brain mass of a mature sperm whale, a close relative of the
dolphin, is almost 9,000 grams, six and a half times that of the
average man. It is unusual in total brain mass, not (compare with
the figure below) in ratio of brain to body weight. Yet the largest
dinosaurs had brain weight about 1 percent that of the sperm whale.
What does the whale do with so massive a brain? Are there thoughts,
insights, arts, sciences and legends of the sperm whale?
The criterion of brain mass to body mass, which involves no
considerations of behavior, appears to provide a very useful
index of the relative intelligence of quite different animals. It is
what a physicist might describe as an acceptable first
approximation.
(Note for future reference that the
Australopithecines, who were either ancestral to man or at
least close collateral relatives, also had a large brain mass for
their body - weight; this has been determined by making casts of
fossil braincases.)
I wonder if the unaccountable general appeal
of babies and other small mammals - with relatively large heads
compared to adults of the same species - derives from our
unconscious awareness of the importance of brain to body mass
ratios.
The data so far in this discussion suggest that the evolution of
mammals from reptiles over two hundred million years ago was
accompanied by a major increase in relative brain size and
intelligence; and that the evolution of human beings from nonhuman
primates a few million years ago was accompanied by an even more
striking development of the brain.
The human brain (apart from the cerebellum, which does not seem to
be involved in cognitive functions) contains about ten billion
switching elements called neurons. (The cerebellum, which lies
beneath the cerebral cortex, toward the back of the head, contains
roughly another ten billion neurons.) The electrical currents
generated by and through the neurons or nerve cells were the means
by which the Italian anatomist Luigi Galvani discovered electricity.
Galvani had found that electrical impulses could be conducted to the
legs of frogs, which dutifully twitched; and the idea became popular
that animal motion (“animation”) was in its deepest sense caused by
electricity.
This is at best a partial truth; electrical impulses
transmitted along nerve fibers do, through neurochemical
intermediaries, initiate such movements as the articulation of
limbs, but the impulses are generated in the brain. Nevertheless,
the modern science of electricity and the electrical and electronic
industries all trace their origins to eighteenth - century experiments
on the electrical stimulation of twitches in frogs.
Only a few decades after Galvani, a group of literary
English - persons, immobilized in the Alps by inclement weather,
set themselves a competition to write a fictional work of
consummate horror. One of them, Mary Wollstonecraft Shelley,
penned the now famous tale of Dr. Frankenstein’s monster,
who is brought to life by the application of massive electrical
currents. Electrical devices have been a mainstay of gothic
novels and horror films ever since. The essential idea is Galvani’s and is fallacious, but the concept has insinuated itself
into many Western languages - as, for example, when I am galvanized
into writing this book.
Most neurobiologists believe that the neurons are the active
elements in brain function, although there is evidence that some
specific memories and other cognitive functions may be contained in
particular molecules in the brain, such as RNA or small proteins.
For every neuron in the brain there are roughly ten glial cells
(from the Greek word for glue), which provide the scaffolding for
the neuronal architecture. An average neuron in a human brain has
between 1,000 and 10,000 synapses or links with adjacent neurons.
(Many spinal - cord neurons seem to have about 10,000 synapses, and
the so - called Purkinje cells of the cerebellum may have still more.
The number of links for neurons in the cortex is probably less than
10,000.)
If each synapse responds by a single yes
- or - no answer to an
elementary question, as is true of the switching elements in
electronic computers, the maximum number of yes/no answers or bits
of information that the brain could contain is about 1010 X 103 =
1013, or 10 trillion bits (or 100 trillion = 10” bits if we had used
10* synapses per neuron).
Some of these synapses must contain the
same information as is contained in other synapses; some must be
concerned with motor and other noncognitive functions; and some may
be merely blank, a buffer waiting for the new day’s information to
flutter through.
If each human brain had only one synapse - corresponding to a
monumental stupidity - we would be capable of only two mental
states. If we had two synapses, then 22 - 4 states; three
synapses, then 23 = 8 states, and, in general, for N synapses,
2N states. But the human brain is characterized by some 1013
synapses. Thus the number of different states of a human brain
is 2 raised to this power - i.e., multiplied by itself ten trillion
times. This is an unimaginably large number, far greater, for
example, than the total number of elementary particles
(electrons and protons) in the entire universe, which is much
less than 1 raised to the power 103.
It is because of this
immense number of functionally different configurations of the
human brain that no two humans, even identical twins raised
together, can ever be really very much alike. These enormous
numbers may also explain something of the unpredictability of human
behavior and those moments when we surprise even ourselves by what
we do. Indeed, in the face of these numbers, the wonder is that
there are any regularities at all in human behavior.
The answer must
be that all possible brain states are by no means occupied; there
must be an enormous number of mental configurations that have never
been entered or even glimpsed by any human being in the history of
mankind. From this perspective, each human being is truly rare and
different and the sanctity of individual human lives is a plausible
ethical consequence.
In recent years it has become clear that there are electrical
microcircuits in the brain. In these micro - circuits the constituent
neurons are capable of a much wider range of responses than the
simple “yes” or “no” of the switching elements in electronic
computers. The microcircuits are very small in size (typical
dimensions are about 1/10,000 of a centimeter) and thus able to
process data very rapidly. They respond to about 11100th of the
voltage necessary to stimulate ordinary neurons, and are therefore
capable of much finer and subtler responses.
Such microcircuits seem
to increase in abundance in a manner consistent with our usual
notions about the complexity of an animal, reaching their greatest
proliferation in both absolute and relative terms in human beings.
They also develop late in human embryology. The existence of such
microcircuits suggests that intelligence may be the result not only
of high brain - to - body - mass ratios but also of an abundance of
specialized switching elements in the brain. Microcircuits make the
number of possible brain states even greater than we calculated in
the previous paragraph, and so enhance still farther the astonishing
uniqueness of the individual human brain.
We can approach the question of the information content of the
human brain in a quite different way - introspectively. Try to
imagine some visual memory, say from your childhood. Look at
it very closely in your mind’s eye. Imagine it is composed of a
set of fine dots like a newspaper wire-photo. Each dot has a
certain color and brightness. You must now ask how many bits
of information are necessary to characterize the color and
brightness of each dot; how many dots make up the recalled
picture; and how long it takes to recall all the details of the
picture in the eye of the mind.
In this retrospective, you focus
on a very small part of the picture at any one time; your field of
view is quite limited. When you put in all these numbers, you
come out with a rate of information processing by the brain, in
bits per second. When I do such a calculation, I come out with a
peak processing rate of about 5,000 bits per second.*
* Horizon to horizon comprises an angle of 180 degrees in a flat
place. The moon is 0.5 degrees in diameter. I know I can see detail
on it, perhaps twelve picture elements across. Thus my eye can
resolve about 0.5/12 = 0.04 degrees. Anything smaller than this is
too small for me to see. The instantaneous field of view in my
mind’s eye, as well as in my real eye, seems to be something like 2
degrees on a side. Thus the little square picture I can see at any
given moment contains about (2/0.04)2 = 2,500 picture elements,
corresponding to the wirephoto dots.
Most commonly such visual recollections concentrate on the edges of
forms and sharp changes from bright to dark, and not on the
configuration of areas of largely neutral brightness. The frog, for
example, sees with a very strong bias towards brightness gradients.
However, there is considerable evidence that detailed memory of
interiors and not just edges of forms is reasonably common. Perhaps
the most striking case is an experiment with humans on stereo
reconstruction of a three - dimensional image, using a pattern
recalled for one eye and a pattern being viewed for the other. The
fusion of images in this anaglyph requires a memory of 10,000
picture elements.
To characterize aft possible shades of gray and colors of such
dots requires about 20 bits per picture element. Thus a
description of my little picture requires 2,500 X 20 or about
50,000 bits. But the act of scanning the picture takes about 10
seconds, and thus my sensory data processing rate is probably
not much larger than 50,000/10 = 5,000 bits per second.
For
comparison, the Viking lander cameras, which also have a 0.04
degree resolution, have only six bits per picture element to
characterize brightness, and can transmit these directly to
Earth by radio at 500 bits per second. The neurons of the brain
generate about 25 watts of power, barely enough to turn on a small
incandescent light. The Viking lander transmits radio messages and
performs all its other functions with a total power of about 50
watts.
But I am not recollecting visual images all my waking hours, nor am
I continuously subjecting people and objects to intense and careful
scrutiny. I am doing that perhaps a small percent of the time. My
other information channels - auditory, tactile, olfactory and
gustatory - are involved with much lower transfer rates. I conclude
that the average rate of data processing by my brain is about
(5,000/50) = 100 bits per second. Over sixty years, that corresponds
to 2 x 1011 or 200 billion total bits committed to visual and other
memory if I have perfect recall. This is less than, but not
unreasonably less than, the number of synapses or neural connections
(since the brain has more to do than just remember) and suggests
that neurons are indeed the main switching elements in brain
function.
A remarkable series of experiments on brain changes during learning
has been performed by the American psychologist Mark Rosenzweig and
his colleagues at the University of California at Berkeley. They
maintained two different populations of laboratory rats - one in a
dull, repetitive, impoverished environment; the other in a
variegated, lively, enriched environment. The latter group displayed
a striking increase in the mass and thickness of the cerebral
cortex, as well as accompanying changes in brain chemistry.
These
increases occurred in mature as well as in young animals. Such
experiments demonstrate that physiological changes accompany
intellectual experience and show how plasticity can be controlled
anatomically. Since a more massive cerebral cortex may make future
learning easier, the importance of enriched environments in
childhood is clearly drawn.
This would mean that new learning corresponds to the
generation of new synapses or the activation of moribund old
ones, and some preliminary evidence consistent with this view
has been obtained by the American neuroanatomist William
Greenough of the University of Illinois and his coworkers. They have
found that after several weeks of learning new tasks in laboratory
contexts, rats develop the kind of new neural branches in their
cortices that form synapses. Other rats, handled similarly but given
no comparable education, exhibit no such neuro-anatomical novelties.
The construction of new synapses requires the synthesis of
protein
and RNA molecules. There is a great deal of evidence showing that
these molecules are produced in the brain during learning, and some
scientists have suggested that the learning is contained within
brain proteins or RNA. But it seems more likely that the new
information is contained in the neurons, which are in turn
constructed of proteins and RNA.
How densely packed is the information stored in the brain? A typical
information density during the operation of a modern computer is
about a million bits per cubic centimeter. This is the total
information content of the computer, divided by its volume. The
human brain contains, as we have said, about 1013 bits in a little
more than 103 cubic centimeters, for an information content of KP/IO3 = 1010, about ten billion bits per cubic centimeter; the
brain is therefore ten thousand times more densely packed with
information than is a computer, although the computer is much
larger.
Put another way, a modern computer able to process the
information in the human brain would have to be about ten thousand
times larger in volume than the human brain. On the other hand,
modern electronic computers are capable of processing information at
a rate of 1016 to 1017 bits per second, compared to a peak rate ten
billion times slower in the brain. The brain must be extraordinarily
cleverly packaged and “wired,” with such a small total information
content and so low a processing rate, to be able to do so many
significant tasks so much better than the best computer.
The number of neurons in an animal brain does not double as
the brain volume itself doubles. It increases more slowly. A
human brain with a volume of about 1,375 cubic centimeters
contains, as we have said, apart from the cerebellum about ten
billion neurons and some ten trillion bits. In a laboratory at the
National Institute of Mental Health near Bethesda, Maryland, I
recently held in my hand a rabbit brain. It had a volume of perhaps
thirty cubic centimeters, the size of an average radish,
corresponding to a few hundred million neurons and some hundred
billion bits - which controlled, among other things, the munching of
lettuce, the twitchings of noses, and the sexual dalliances of
grownup rabbits.
Since animal taxa such as mammals, reptiles or amphibians contain
members with very different brain sizes, we cannot give a reliable
estimate of the number of neurons in the brain of a typical
representative of each taxon. But we can estimate average values
which I have done in the chart on page 26. The rough estimates there
show that a human being has about a hundred times more bits of
information in his brain than a rabbit does. I do not know that it
means very much to say that a human being is a hundred times smarter
than a rabbit, but I am not certain that it is a ridiculous
contention. (It does not, of course, follow that a hundred rabbits
are as smart as one human being.)
We are now in a position to compare the gradual increase through
evolutionary time of both the amount of information contained in the
genetic material and the amount of information contained in the
brains of organisms. The two curves cross (p. 26) at a time
corresponding to a few hundred million years ago and at an
information content corresponding to a few billion bits.
Somewhere
in the steaming jungles of the Carboniferous Period there emerged an
organism that for the first time in the history of the world had
more information in its brains than in its genes. It was an early
reptile which, were we to come upon it in these sophisticated times,
we would probably not describe as exceptionally intelligent. But its
brain was a symbolic turning point in the history of life. The two
subsequent bursts of brain evolution, accompanying the emergence of
mammals and the advent of manlike primates, were still more
important advances in the evolution of intelligence.
Much of the
history of life since the Carboniferous Period can be described as
the gradual (and certainly incomplete) dominance of brains over
genes.
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