Despite the intriguing localization of function in the triune brain model, it is, I stress again, an oversimplification to insist upon perfect separation of function. Human ritual and emotional behavior are certainly influenced strongly by neocortical abstract reasoning; analytical demonstrations of the validity of purely religious beliefs have been proffered, and there are philosophical justifications for hierarchical behavior, such as Thomas Hobbes' "demonstration" of the divine right of kings. Likewise, animals that are not human-and in fact even some animals that are not primates-seem to show glimmerings of analytical abilities. I certainly have such an impression about dolphins, as I described in my book The Cosmic Connection.
Nevertheless, while bearing these caveats in mind, it seems a useful first approximation to consider the ritualistic and hierarchical aspects of our lives to be influenced strongly by the R-complex and shared with our reptilian forebears; the altruistic, emotional and religious aspects of our lives to be localized to a significant extent in the limbic system and shared with our nonprimate mammalian forebears (and perhaps the birds); and reason to be a function of the neo-cortex, shared to some extent with the higher primates and such cetaceans as dolphins and whales. While ritual, emotion and reasoning are all significant aspects of human nature, the most nearly unique human characteristic is the ability to associate abstractly and to reason. Curiosity and the urge to solve problems are the emotional hallmarks of our species; and the most characteristically human activities are mathematics, science, technology, music and the arts-a somewhat broader range of subjects than is usually included under the "humanities." Indeed, in its common usage this very word seems to reflect a peculiar narrowness of vision about what is human. Mathematics is as much a "humanity" as poetry. Whales and elephants may be as "humane" as humans.
The triune-brain model derives from studies of comparative neuroanatomy and behavior. But honest introspection is not unknown in the human species, and if the triune-brain model is correct, we would expect some hint of it in the history of human self-knowledge. The most widely known hypothesis that is at least reminiscent of the triune brain is Sigmund Freud's division of the human psyche into id, ego and superego. The aggressive and sexual aspects of the R-complex correspond satisfyingly to the Freudian description of the id (Latin for "it"-i. e., the beast-like aspect of our natures); but, so far as I know, Freud did not in his description of the id lay great stress on the ritual or social-hierarchy aspects of the R-complex. He did describe emotions as an ego function-in particular the "oceanic experience," which is the Freudian equivalent of the religious epiphany. However, the superego is not depicted primarily as the site of abstract reasoning but rather as the internalizer of societal and parental strictures, which in the triune brain we might suspect to be more a function of the R-complex. Thus I would have to describe the psychoanalytic tripartite mind as only weakly in accord with the triune-brain model.
Perhaps a better metaphor is Freud's division of the mind into the conscious; the preconscious, which is latent but capable of being tapped; and the unconscious, which is repressed or otherwise unavailable. It was the tension that exists among the components of the psyche that Freud had in mind when he said of man that "his capacity for neurosis would merely be the obverse of his capacity for cultural development." He called the unconscious functions "primary processes."
A superior agreement is found in the metaphor for the human psyche in the Platonic dialogue Phaedrus. Socrates likens the human soul to a chariot drawn by two horses-one black, one white-pulling in different directions and weakly controlled by a charioteer. The metaphor of the chariot itself is remarkably similar to MacLean's neural chassis; the two horses, to the R-complex and the limbic cortex; and the charioteer barely in control of the careening chariot and horses, to the neocortex. In yet another metaphor, Freud described the ego as the rider of an unruly horse. Both the Freudian and the Platonic metaphors emphasize the considerable independence of and tension among the constituent parts of the psyche, a point that characterizes the human condition and to which we will return. Because of the neuroanatomical connections between the three components, the triune brain must itself, like the Phaedrus chariot, be a metaphor; but it may prove to be a metaphor of great utility and depth.
4 EDEN AS A METAPHOR: THE EVOLUTION OF MAN
Then wilt thou not be loth
To leave this Paradise, but shalt possess
A Paradise within thee, happier far ...
They hand in hand with wandering steps and slow
Through Eden took their solitary way.
JOHN MILTON Paradise Lost
Why didst thou leave the trodden paths of men Too soon, and with weak hands though mighty heart
Dare the unpastured dragon in his den? Defenseless as thou wert, oh, where was then Wisdom, the mirrored shield . . . ?
FOR THEIR surface area, insects weigh very little. A beetle, falling from a high altitude, quickly achieves terminal velocity: air resistance prevents it from falling very fast, and, after alighting on the ground, it will walk away, apparently none the worse for the experience. The same is true of small mammals-squirrels, say. A mouse can be dropped down a thousand-foot mine shaft and, if the ground is soft, will arrive dazed but essentially unhurt. In contrast, human beings are characteristically maimed or killed by any fall of more than a few dozen feet: because of our size, we weigh too much for our surface area. Therefore our arboreal ancestors had to pay attention. Any error in brachiating from branch to branch could be fatal. Every leap was an opportunity for evolution. Powerful selective forces were at work to evolve organisms with grace and agility, accurate binocular vision, versatile manipulative abilities, superb eye-hand coordination, and an intuitive grasp of Newtonian gravitation. But each of these skills required significant advances in the evolution of the brains and particularly the neocortices of our ancestors. Human intelligence is fundamentally indebted to the millions of years our ancestors spent aloft in the trees.
And after we returned to the savannahs and abandoned the trees, did we long for those great graceful leaps and ecstatic moments of weightlessness in the shafts of sunlight of the forest roof? Is the startle reflex of human infants today to prevent falling from the treetops? Are our nighttime dreams of flying and our daytime passion for flight, as exemplified in the lives of Leonardo da Vinci or Konstantin Tsiolkovskii, nostalgic reminiscences of those days gone by in the branches of the high forest? *
Other mammals, even other nonprimate and non-cetacean mammals, have neocortices. But in the evolutionary line leading to man, when was the first large-scale development of the neocortex? While none of our simian ancestors are still around, this question can nevertheless be answered or at least approached: we can examine fossil skulls. In humans, in apes and monkeys, and in other mammals, the brain volume almost fills the skull. This is not true, for example, in fish. Thus by taking a cast of a skull, we can determine what is called the endocranial volume of our immediate ancestors and collateral relatives and can make some rough estimates of their brain volumes.
The question of who was and who was not an ancestor of man is still being hotly debated by the paleontologists, and hardly a year goes by without the discovery of some fossil of remarkably human aspect -much-older- than anyone had previously thought possible. What seems certain is that about five million years ago, there was an abundance of apelike animals, the gracile Australopithecines, who walked on two feet and had brain volumes of about 500 cubic centimeters, some 100 cc more than the brain of a modern chimpanzee. With this evidence, paleontologists have deduced that "bipedalism preceded encephalization," by which they mean that our ancestors walked on two legs before they evolved big brains.
* Modern rocket technology and space exploration owes an incalculable debt to Dr. Robert H. Goddard, who through many decades of devoted and lonely research was single-handedly responsible for the development of essentially all important aspects of the modern rocket. Goddard's interest in this subject originated in a magic moment. In the New England autumn of 1899, Goddard was a seventeen-year-old high school sophomore who had climbed a cherry tree and, while idly looking down at the ground around him, experienced a kind of epiphanal vision of a vehicle that would transport human beings to the planet Mars. He resolved to devote himself to the task. Exactly one year later, he climbed the tree again, and on every October 19th for the rest of his life, made a special point of recollecting that moment. Can it be an accident that this vision of voyages to the planets, which has led directly to its own historical fulfillment, was glimpsed in the limbs of a tree?
By three million years ago, there was a variety of bipedal fellows with a wide range of cranial volumes, some considerably larger than the East African gracile Australopithecines of a few million years earlier. One of them, which L. S. B. Leakey, the Anglo-Kenyan student of early man, called Homo habilis, had a brain volume of about 700 cubic centimeters. We also have archaeological evidence that Homo habilis made tools. The idea that tools are both the cause and the effect of walking on two legs, which frees the hands, was first advanced by Charles Darwin. The fact that these significant changes in behavior are accompanied by equally significant changes in brain volume does not prove that the one is caused by the other; but our previous discussion makes such a casual link appear very likely.
The table on page 92 summarizes the fossil evidence, through 1976, on our most recent ancestors and collateral relatives. The two rather different kinds of Australopithecines were not of the genus Homo, not human; they were still incompletely bipedal and had brain masses only about a third the size of the average adult human brain today. Were we to meet an Australopithecine, say, on the subway, we would perhaps be struck most by the almost total absence of forehead. He was the lowest of lowbrows. There are significant differences between the two kinds of Australopithecines. The robust species was taller and heavier, with most impressive "nut-cracker" teeth and a remarkable evolutionary stability. The endocranial volume of A. robustus varies very little from specimen to specimen over millions of years of time. The gracile Australopithecines, judging again from their teeth, probably ate meat as well as vegetables. They were smaller and lither, as their name indicates. However, they are considerably older and have much more variance in endocranial volume than their robust cousins. But, most important, the gracile Australopithecine sites are associated with a clear industry: the manufacture of tools made of stone and animal bones, horns and teeth-painstakingly carved, broken, rubbed and polished to make chipping, flaking, pounding and cutting tools. No tools have been associated with A. robustus. The ratio of brain weight to body weight is almost twice as large for the gracile as for the robust Australopithecus, and it is a natural speculation to wonder whether that factor of two is the difference between tools and no tools.
At apparently the same epoch as the emergence of Australopithecus robustus, there arose a new animal, Homo habilis, the first true man. He was larger, both in body and in brain weight, than either of the Australopithecines, and had a ratio of brain to body weight about the same as that of the gracile Australopithecines. He emerged at a time when, for climatic reasons, the forests were receding. Homo habilis inhabited the vast African savannahs, an extremely challenging environment filled with an enormous variety of predators and prey. On these plains of low grass appeared both the first modern man and the first modern horse. They were almost exact contemporaries.
In the last sixty million years, there has been a continuous evolution of ungulates, well recorded in the fossil record, and eventually culminating in the modern horse. Eohippus, the "dawn horse" of some fifty million years ago, was about the size of an English collie, with a brain volume of about twenty-five cubic centimeters, and a ratio of brain to body weight about half that of comparable contemporary mammals. Since then, horses have experienced a dramatic evolution in both absolute and relative brain size, with major developments in the neocortex and particularly in the frontal lobes-an evolution certainly accompanied by major improvements in equine intelligence. I wonder if the parallel developments in the intelligence of horse and man might have a common cause. Did horses, for example, have to be swift of foot, acute of sense, and intelligent to elude predators which hunted primate as well as equine prey?
H. habilis had a high forehead, suggesting a significant development of the neocortical areas in the frontal and temporal lobes as well as the regions in the brain, to be discussed later, that seem to be connected with the power of speech. Were we to encounter Homo habilis-dressed, let us say, in the latest fashion on the boulevards of some modern metropolis-we would probably give him only a passing glance, and that because of his relatively small stature. Associated with Homo habilis are a variety of tools of considerable sophistication. In addition, there is evidence from various circular arrangements of stones that Homo habilis may have constructed dwellings; that long before the Pleistocene Ice Ages, long before men regularly inhabited caves, H. habilis was constructing homes out-of-doors-probably of wood, wattle, grass and stone.
Since H. habilis and A. robustus emerged at the same time, it is very unlikely that one was the ancestor of the other. The gracile Australopithecines were also contemporaries of Homo habilis but much more ancient. It is therefore possible-although by no means certain-that both H. habilis, with a promising evolutionary future, and A. robustus, an evolutionary dead end, arose from the gracile A. africanus, who survived long enough to be their contemporary.
The first man whose endocranial volume overlaps that of modern humans is Homo erectus. For many years the principal specimens of H. erectus were known from China and thought to be about half a million years old. But in 1976 Richard Leakey of the National Museums of Kenya reported a nearly complete skull of Homo erectus found in geological strata one and a half million years old. Since the Chinese specimens of Homo erectus are clearly associated with the remains of campfires, it is possible that our ancestors domesticated fire much more than one half million years ago -which makes Prometheus far older than many had thought.
Perhaps the most striking aspect of the archaeological record concerning tools is that as soon as they appear at all they appear in enormous abundance. It looks very much as though an inspired gracile Australopithecine discovered for the first time the use of tools and immediately taught the toolmaking skill to his relatives and friends. There is no way to explain the discontinuous appearance of stone tools unless the Australopithecines had educational institutions. There must have been some sort of stonecraft guild passing on from generation to generation the precious knowledge about the fabrication and use of tools-knowledge that would eventually propel such feeble and almost defenseless primates into domination of the planet Earth. Whether the genus Homo independently invented tools or borrowed the discovery from the genus Australopithecus is not known.
We see from the table that the ratio of body to brain weight is, within the variance of measurement, roughly the same for the gracile Australopithecines, Homo habilis, Homo erectus and modern humans. The advances we have made in the last few million years cannot therefore be explained by the ratio of brain to body mass, but rather by increasing total brain mass, improved specialization of new function and complexity within the brain, and-especially-extrasomatic learning.
L. S. B. Leakey emphasized that the fossil record of a few million years ago is replete with a great variety of manlike forms, an interesting number of which are found with holes or fractures in their skulls. Some of these injuries may have been inflicted by leopards or hyenas; but Leakey and the South African anatomist Raymond Dart believed that many of them were inflicted by our ancestors. In Pliocene/Pleistocene times there was almost certainly a vigorous competition among many manlike forms, of which only one line survived-the tool experts, the line that led to us. What role killing played in that competition remains an open question. The gracile Australopithecines were erect, agile, fleet and three and a half feet tall: "little people." I sometimes wonder whether our myths about gnomes, trolls, giants and dwarfs could possibly be a genetic or cultural memory of those times.
At the same time that the hominid cranial volume was undergoing its spectacular increase, there was another striking change in human anatomy; as the British anatomist Sir Wilfred Le Gros Clark of Oxford University has observed, there was a wholesale reshaping of the human pelvis. This was very likely an adaptation to permit the live birth of the latest model large-brained babies. Today, it is unlikely that any further substantial enlargement of the pelvic girdle in the region of the birth canal is possible without severely impairing the ability of women to walk efficiently. (At birth, girls already have a significantly larger pelvis and skeletal pelvic opening than do boys; another large increment in the size of the female pelvis occurs at puberty.) The parallel emergence of these two evolutionary events illustrates nicely how natural selection works. Those mothers with hereditary large pelvises were able to bear large-brained babies who because of their superior intelligence were able to compete successfully in adulthood with the smaller-brained offspring of mothers with smaller pelvises. He who had a stone axe was more likely to win a vigorous difference of opinion in Pleistocene times. More important, he was a more successful hunter. But the invention and continued manufacture of stone axes required larger brain volumes.
So far as I know, childbirth is generally painful in only one of the millions of species on Earth: human beings. This must be a consequence of the recent and continuing increase in cranial volume. Modem men and women have braincases twice the volume of Homo habilis'. Childbirth is painful because the evolution of the human skull has been spectacularly fast and recent. The American anatomist C. Judson Herrick described the development of the neocortex in the following terms: "Its explosive growth late in phylogeny is one of the most dramatic cases of evolutionary transformation known to comparative anatomy." The incomplete closure of the skull at birth, the fontanelle, is very likely an imperfect accommodation to this recent brain evolution.
The connection between the evolution of intelligence and the pain of childbirth seems unexpectedly to be made in the Book of Genesis. In punishment for eating the fruit of the tree of the knowledge of good and evil, God says to Eve,* "In pain shalt thou bring forth children" (Genesis 3:16). It is interesting that it is not the getting of any sort of knowledge that God has forbidden, but, specifically, the knowledge of the difference between good and evil-that is, abstract and moral judgments, which, if they reside anywhere, reside in the neocortex. Even at the time that the Eden story was written, the development of cognitive skills was seen as endowing man with godlike powers and awesome responsibilities. God says: "Behold, the man is become as one of us, to know good and evil; and now, lest he put forth his hand, and take also of the Tree of Life, and eat, and live forever" (Genesis 3:22), he must be driven out of the Garden. God places cherubim with a flaming sword east of Eden to guard the Tree of Life from the ambitions of man.+
* God's judgment on the serpent is that henceforth "upon thy belly shalt thou go"-implying that previously reptiles traveled by an alternative mode of locomotion. This is, of course, precisely true: snakes have evolved from four-legged reptilian ancestors resembling dragons. Many snakes still retain anatomical vestiges of the limbs of their ancestors. + Cherubim is plural; Genesis 3:24 specifies one flaming sword. Presumably flaming swords were in short supply.
Perhaps the Garden of Eden is not so different from Earth as it appeared to our ancestors of some three or four million years ago, during a legendary golden age when the genus Homo was perfectly interwoven with the other beasts and vegetables. After the exile from Eden we find, in the biblical account, mankind condemned to death; hard work; clothing and modesty as preventatives of sexual stimulation; the dominance of men over women; the domestication of plants (Cain); the domestication of animals (Abel); and murder (Cain plus Abel). These all correspond reasonably well to the historical and archaeological evidence. In the Eden metaphor, there is no evidence of murder before the Fall. But those fractured skulls of bipeds not on the evolutionary line to man may be evidence that our ancestors killed, even in Eden, many manlike animals.
Civilization develops not from Abel, but from Cain the murderer. The very word "civilization" derives from the Latin word for city. It is the leisure time, community organization and specialization of labor in the first cities that permitted the emergence of the arts and technologies we think of as the hallmarks of civilizations. The first city, according to Genesis, was constructed by Cain, the inventor of agriculture-a technology that requires a fixed abode. And it is his descendants, the sons of Lamech, who invent both "artifices in brass and iron" and musical instruments. Metallurgy and music-technology and art-are in the line from Cain. And the passions that lead to murder do not abate: Lamech says, "For I have slain a man for wounding me, and a young man for bruising me; if Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold." The connection between murder and invention has been with us ever since. Both derive from agriculture and civilization.
One of the earliest consequences of the anticipatory skills that accompanied the evolution of the prefrontal lobes must have been the awareness of death. Man is probably the only organism on Earth with a relatively clear view of the inevitability of his own end. Burial ceremonies that include the interment of food and artifacts along with the deceased go back at least to the times of our Neanderthal cousins, suggesting not only a widespread awareness of death but also an already developed ritual ceremony to sustain the deceased in the afterlife. It is not that death was absent before the spectacular growth of the neocortex, before the exile from Eden; it is only that, until then, no one had ever noticed that death would be his destiny.
The fall from Eden seems to be an appropriate metaphor for some of the major biological events in recent human evolution. This may account for its popularity.* It is not so remarkable as to require us to believe in a kind of biological memory of ancient historical events, but it does seem to me close enough to risk at least raising the question. The only repository of such a biological memory is, of course, the genetic code.
* In the West. There are, of course, many insightful and profound myths on the origin of mankind in other human cultures.
By fifty-five million years ago, in the Eocene Period, there was a great proliferation of primates, both arboreal and ground-dwelling, and the evolution of a line of descent that eventually led to Man. Some primates of those times-e. g., a prosimian called Tetonius-exhibit in their endocranial casts tiny nubs where the frontal lobes will later evolve. The first fossil evidence of a brain of even vaguely human aspects dates back to eighteen million years to the Miocene Period, when an anthropoid ape which we call Proconsul or Dryo-pithecus appeared. Proconsul was quadrupedal and arboreal, probably ancestral to the present great apes and possibly to Homo sapiens as well. He is roughly what we might expect for a common ancestor of apes and men. (His approximate contemporary, Ramapithecus, is thought by some anthropologists to be ancestral to man.) Proconsul's endocranial casts show recognizable frontal lobes but much less well developed neocortical convolutions than are displayed by apes and men today. His cranial volume was still very small.
The biggest burst of evolution in cranial volume occurred in the last few million years.
Patients who have had prefrontal lobotomies have been described as losing a "continuing sense of self"-the feeling that I am a particular individual with some control over my life and circumstances, the "me-ness" of me, the uniqueness of the individual. It is possible that lower mammals and reptiles, lacking extensive frontal lobes, also lack this sense, real or illusory, of individuality and free will, which is so characteristically human and which may first have been experienced dimly by Proconsul.
The development of human culture and the evolution of those physiological traits we consider characteristically human most likely proceeded-almost literally-hand in hand: the better our genetic predispositions for running, communicating and manipulating, the more likely we were to develop effective tools and hunting strategies; the more adaptive our tools and hunting strategies, the more likely it was that our characteristic genetic endowments would survive. The American anthropologist Sherwood Wash-burn of the University of California, a principal exponent of this view, has said: "Much of what we think of as human evolved long after the use of tools. It is probably more correct to think of much of our structure as the result of culture than it is to think of men anatomically like ourselves slowly developing culture."
Some students of human evolution believe that part of the selection pressure behind this enormous burst in brain evolution was in the motor cortex and not at first in the neocortical regions responsible for cognitive processes. They stress the remarkable abilities of human beings to throw projectiles accurately, to move gracefully, and-as Louis Leakey enjoyed illustrating by direct demonstration-naked, to outrun and immobilize game animals. Such sports as baseball, football, wrestling, track and field events, chess and warfare may owe their appeal-as well as their largely male following-to these prewired hunting skills, which served us so well for millions of years of human history but which find diminished practical applications today.
Effective defense against predators and the hunting of game were both necessarily cooperative ventures. The environment in which man evolved-in Africa in Pliocene and Pleistocene times-was inhabited by a variety of terrifying mammalian carnivores, perhaps the most awesome of which were packs of large hyenas. It was very difficult to defend oneself alone against such a pack. Stalking large animals, either solitary beasts or herds, is dangerous; some gestural communication among the hunters is necessary. We know, for example, that shortly after man entered North America, via the Bering Straits in the Pleistocene Period, there were massive and spectacular kills of large game animals, often by driving them over cliffs. In order to stalk a single wildebeest or stampede a herd of antelope to their deaths, hunters must share at least a minimal symbolic language. Adam's first act was linguistic-long before the Fall and even before the creation of Eve: he named the animals of Eden.
Some forms of gestural symbolic language, of course, originated much earlier than the primates; canines and many other mammals who form dominance hierarchies may indicate submission by averting the eyes or baring the neck. We have mentioned other submissive rituals in primates such as macaques. The human greetings of bow, nod and curtsy may have a similar origin. Many animals seem to signal friendship by biting, but not hard enough to hurt, as if to say, "I am able to bite you but choose not to do so." The raising of the right hand as a symbol of greeting among humans has precisely the same significance: "I could attack you with a weapon but choose not to wield one." *
* The upraised and open right hand is sometimes described as a "universal" symbol of good will. It at least runs the gamut from Praetorian Guards to Sioux scouts. Since those wielding weapons are, in human history, characteristically male, it should be and is a characteristically male greeting. For these reasons, among others, the plaque aboard the Pioneer 10
spacecraft-the first artifact of mankind to leave the solar system-included a drawing of a naked man and woman, the man's hand raised, palm out, in greeting (see illustration on p. 246). In The Cosmic Connection I describe the humans on the plaque as the most obscure part of the message. Nevertheless, I wonder. Could the significance of the man's gesture be deduced by beings with very different biologies?
Extensive gestural languages were employed by many human hunting communities-for example, among the Plains Indians, who also used smoke signals. According to Homer, the victory of the Hellenes at Troy was conveyed from Ilium to Greece, a distance of some hundred miles, by a series of signal fires. The date was about 1100 B.C. However, both the repertoire of ideas and the speed with which ideas can be communicated in gestural or sign languages is limited. Darwin pointed out that gestural languages cannot usefully be employed while our hands are otherwise occupied, or at night, or when our view of the hands is obstructed. One can imagine gestural languages being gradually supplemented and then supplanted by verbal languages -which originally may have been onomatopoeic (that is, imitative in sound of the object or action being described). Children call dogs "bow-wows." In almost all human languages the child's word for "mother" seems imitative of the sound made inadvertently while feeding at the breast. But all of this could not have occurred without a restructuring of the brain.
We know from skeletal remains associated with early man that our ancestors were hunters. We know enough about the hunting of large animals to realize that some language is required for cooperative stalking. But ideas on the antiquity of language have received a measure of unexpected support from detailed studies of fossil endocasts made by the American anthropologist Ralph L. Holloway of Columbia University. Holloway's casts of fossil skulls are made of rubber latex, and he has attempted to deduce something of the detailed morphology of the brain from the shape of the skull. The activity is a kind of phrenology, but on the inside rather than on the outside and much more soundly based. Holloway believes that a region of the brain known as Broca's area, one of several centers required for speech, can be detected in fossil endocasts; and that he has found evidence for Broca's area in a Homo habilis fossil more than two million years old. The development of language, tools and culture may have occurred roughly simultaneously.
There were, incidentally, manlike creatures who lived only a few tens of thousands of years ago-the Neanderthals and the Cro-Magnons--who had average brain volumes of about 1,500 cubic centimeters; that is, more than a hundred cubic centimeters larger than ours. Most anthropologists guess that we are not descended from Neanderthals and may not be from Cro-Magnons either. But their existence raises the question: Who were those fellows? What were their accomplishments? Cro-Magnon was also very large: some specimens were well over six feet tall. We have seen that a difference in brain volume of 100 cubic centimeters does not seem to be significant, and perhaps they were no smarter than we or our immediate ancestors; or perhaps they had other, still unknown, physical impediments. Neanderthal was a lowbrow, but his head was long, front to back; in contrast, our heads are not so deep, but they are taller: we can certainly be described as highbrows. Might the brain growth exhibited by Neanderthal man have been in the parietal and occipital lobes, and the major brain growth of our ancestors in the frontal and temporal lobes? Is it possible that the Neanderthals developed quite a different mentality than ours, and that our superior linguistic and anticipatory skills enabled us to destroy utterly our husky and intelligent cousins?
So far as we know, nothing like human intelligence appeared on Earth before a few million, or at least a few tens of millions of years ago. But that is a few tenths of a percent of the age of Earth, very late in December in the Cosmic Calendar. Why did it appear so late? The answer clearly seems to be that some particular property of higher primate and cetacean brains did not evolve until recently. But what is that property? I can suggest at least four possibilities, all of which have already been mentioned, either explicitly or implicitly: (1) Never before was there a brain so massive; (2) Never before was there a brain with so large a ratio of brain to body mass; (3) Never before was there a brain with certain functional units (large frontal and temporal lobes, for example); (4) Never before was there a brain with so many neural connections or synapses. (There seems to be some evidence that along with the evolution of the human brain there may have been an increase in the number of connections of each neuron with its neighbor, and in the number of microcircuits.) Explanations 1, 2 and 4 argue that a quantitative change produced a qualitative change. It does not seem to me that a crisp choice among these four alternatives can be made at the present time, and I suspect that the truth will actually embrace most or all of these possibilities.
The British student of human evolution Sir Arthur Keith proposed what he called a "Rubicon" in the evolution of the human brain. He thought that at the brain volume of Homo erectus-about 750 cubic centimeters, roughly the engine displacement of a fast motorcycle -the uniquely human qualities begin to emerge. The "Rubicon" might, of course, have been more qualitative than quantitative. Perhaps the difference was not so much an additional 200 cubic centimeters as some specific developments in the frontal, temporal and parietal lobes which provided us with analytical ability, foresight and anxiety.
While we can debate what the "Rubicon" corresponds to, the idea of some sort of Rubicon is not without value. But if there is a Rubicon anywhere near 750 cubic centimeters, while differences of the order of 100 or 200 cubic centimeters do not-at any rate to us-seem to be compelling determinants of intelligence, might not the apes be intelligent in some recognizably human sense? A typical chimpanzee brain volume is 400 cubic centimeters; a lowland gorilla's, 500 cc. This is the range of brain volumes among the tool-using gracile Australopithecines.
The Jewish historian Josephus added to the list of penalties and tribulations that accompanied Mankind's exile from Eden the loss of our ability to communicate with the animals. Chimpanzees have large brains; they have well-developed neocortices; they, too, have long childhoods and extended periods of plasticity. Are they capable of abstract thought? If they're smart, why don't they talk?
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