Dragons of Eden

by Carl Sagan

  MacLean has shown that the R-complex plays an important role in aggressive behavior, territoriality, ritual and the establishment of social hierarchies. Despite occasional welcome exceptions, this seems to me to characterize a great deal of modern human bureaucratic and political behavior. I do not mean that the neocortex is not functioning at all in an American political convention or a meeting of the Supreme Soviet; after all, a great deal of the communication at such rituals is verbal and therefore neocortical. But it is striking how much of our actual behavior—as distinguished from what we say and think about it-can be described in reptilian terms. We speak commonly of a “cold-blooded” killer. Machiavelli’s advice to his Prince was “knowingly to adopt the beast.”

  In an interesting partial anticipation of these ideas, the American philosopher Susanne Langer wrote: “Human life is shot through and through with ritual, as it is also with animalian practices. It is an intricate fabric of reason and rite, of knowledge and religion, prose and poetry, fact and dream.… Ritual, like art, is essentially the active termination of a symbolic transformation of experience. It is born in the cortex, not in the ‘old brain’; but it is born of an elementary need of that organ, once the organ has grown to human estate.” Except for the fact that the R-complex is in the “old brain,” this seems to be right on target.

  I want to be very clear about the social implications of the contention that reptilian brains influence human actions. If bureaucratic behavior is controlled at its core by the R-complex, does this mean there is no hope for the human future? In human beings, the neocortex represents about 85 percent of the brain, which is surely some index of its importance compared to the brainstem, R-complex and limbic system. Neuroanatomy, political history, and introspection all offer evidence that human beings are quite capable of resisting the urge to surrender to every impulse of the reptilian brain. There is no way, for example, in which the Bill of Rights of the U.S. Constitution could have been recorded, much less conceived, by the R-complex. It is precisely our plasticity, our long childhood, that prevents a slavish adherence to genetically preprogrammed behavior in human beings more than in any other species. But if the triune brain is an accurate model of how human beings function, it does no good whatever to ignore the reptilian component of human nature, particularly our ritualistic and hierarchical behavior. On the contrary, the model may help us to understand what human beings are about. (I wonder, for example, whether the ritual aspects of many psychotic illnesses—e.g., hebephrenic schizophrenia—could be the result of hyperactivity of some center in the R-complex, or of a failure of some neocortical site whose function is to repress or override the R-complex. I also wonder whether the frequent ritualistic behavior in young children is a consequence of the still-incomplete development of their neocortices.)

  In a curiously apt passage, G. K. Chesterton wrote: “You can free things from alien or accidental laws, but not from the laws of their own nature.… Do not go about … encouraging triangles to break out of the prison of their three sides. If a triangle breaks out of its three sides, its life comes to a lamentable end.” But not all triangles are equilateral. Some substantial adjustment of the relative role of each component of the triune brain is well within our powers.

  Opposite: Two photographs taken with an electron microscope within the third ventricle of the brain by Richard Steger of Wayne State University. Tiny waving hairs or cilia can be seen transporting small spherical brain proteins—like a crowd passing large beach balls overhead.

  2 THE LIMBIC SYSTEM

  The limbic system appears to generate strong or particularly vivid emotions. This immediately suggests an additional perspective on the reptilian mind: it is not characterized by powerful passions and wrenching contradictions but rather by a dutiful and stolid acquiescence to whatever behavior its genes and brains dictate.

  Electrical discharges in the limbic system sometimes result in symptoms similar to those of psychoses or those produced by psychedelic or hallucinogenic drugs. In fact, the sites of action of many psychotropic drugs are in the limbic system. Perhaps it controls exhilaration and awe and a variety of subtle emotions that we sometimes think of as uniquely human.

  The “master gland,” the pituitary, which influences other glands and dominates the human endocrine system, is an intimate part of the limbic region. The mood-altering qualities of endocrine imbalances give us an important hint about the connection of the limbic system with states of mind. There is a small almond-shaped inclusion in the limbic system called the amygdala which is deeply involved in both aggression and fear. Electrical stimulation of the amygdala in placid domestic animals can rouse them to almost unbelievable states of fear or frenzy. In one case, a house cat cowered in terror when presented with a small white mouse. On the other hand, naturally ferocious animals, such as the lynx, become docile and tolerate being petted and handled when their amygdalas are extirpated. Malfunctions in the limbic system can produce rage, fear or sentimentality that have no apparent cause. Natural hyperstimulation may produce the same results: those suffering from such a malady find their feelings inexplicable and inappropriate; they may be considered mad.

  At least some of the emotion-determining role of such limbic endocrine systems as the pituitary amygdala, and hypothalamus is provided by small hormonal proteins which they exude, and which affect other areas of the brain. Perhaps the best-known is the pituitary protein, ACTH (adrenocorticotropic hormone), which can affect such diverse mental functions as visual retention, anxiety and attention span. Some small hypothalamic proteins have been identified tentatively in the third ventricle of the brain, which connects the hypothalamus with the thalamus, a region also within the limbic system. The stunning pictures on this page, taken with an electron microscope, show two close-ups of action in the third ventricle. The diagram on this page may help clarify some of the brain anatomy just described.

  There are reasons to think that the beginnings of altruistic behavior are in the limbic system. Indeed, with rare exceptions (chiefly the social insects), mammals and birds are the only organisms to devote substantial attention to the care of their young—an evolutionary development that, through the long period of plasticity which it permits, takes advantage of the large information-processing capability of the mammalian and primate brains. Love seems to be an invention of the mammals.*

  An impression of the possible form of the Mesozoic reptile Lycaenops by John Germann. Such mammal-like creatures were perhaps among the first to experience a substantial evolution of the limbic system.

  Courtesy of The American Museum of Natural History

  Much in animal behavior substantiates the notion that strong emotions evolved chiefly in mammals and to a lesser extent in birds. The attachment of domestic animals to humans is, I think, beyond question. The apparently sorrowful behavior of many mammalian mothers when their young are removed is well-known. One wonders just how far such emotions go. Do horses on occasion have glimmerings of patriotic fervor? Do dogs feel for humans something akin to religious ecstasy? What other strong or subtle emotions are felt by animals that do not communicate with us?

  The oldest part of the limbic system is the olfactory cortex, which is related to smell, the haunting emotional quality of which is familiar to most humans. A major component of our ability to remember and recall is localized in the hippocampus, a structure within the limbic system. The connection is clearly shown by the profound memory impairment that results from lesions of the hippocampus. In one famous case, H. M., a patient with a long history of seizures and convulsions, was subjected to a bilateral extirpation of the entire region about the hippocampus in a successful attempt to reduce their frequency and severity. He immediately became amnesic. He retained good perceptual skills, was able to learn new motor skills and experienced some perceptual learning but essentially forgot everything more than a few hours old. His own comment was “Every day is alone in itself—whatever enjoyment I’ve had and whatever sorrow I’ve had.” He described his li
fe as a continuous extension of the feeling of disorientation many of us have upon awakening from a dream, when we have great difficulty remembering what has just happened. Remarkably enough, despite this severe impairment, his IQ improved after his hippocampectomy. He apparently could detect smells but had difficulty identifying by name the source of the smell. He also exhibited an apparent total disinterest in sexual activity.

  In another case, a young American airman was injured in a mock duel with another serviceman, when a miniature fencing foil was plunged into his right nostril, puncturing a small part of the limbic system immediately above. This resulted in a severe impairment of memory, similar to but not so severe as H. M’s; a wide range of his perceptual and intellectual abilities was unaffected. His memory impairment was particularly noticeable with verbal material. In addition, the accident seems to have rendered him both impotent and unresponsive to pain. He once walked barefoot on the sun-heated metal deck of a cruise ship, without realizing that his feet were being badly burned until his fellow passengers complained of the uncomfortable odor of charring flesh. On his own, he was aware of neither the pain nor the smell.

  From such cases, it seems apparent that so complex a mammalian activity as sex is controlled simultaneously by all three components of the triune brain—the R-complex, the limbic system and the neocortex. (We have already mentioned the involvement of the R-complex and the limbic system in sexual activity. Evidence for involvement of the neocortex can be easily obtained by introspection.)

  One segment of the old limbic system is devoted to oral and gustatory functions; another, to sexual functions. The connection of sex with smell is very ancient, and is highly developed in insects—a circumstance that offers insight into both the importance and the disadvantages of reliance on smell in our remote ancestors.

  I once witnessed an experiment in which the head of a green bottle fly was connected by a very thin wire to an oscilloscope that displayed, in a kind of graph, any electrical impulses produced by the fly’s olfactory system. (The fly’s head had only recently been severed from its body—in order to gain access to the olfactory apparatus—and was still in many respects functional.*) The experimenters wafted a wide variety of odors in front of it, including obnoxious and irritating gases such as ammonia, with no discernible effects. The line traced out on the oscilloscope screen was absolutely flat and horizontal. Then a tiny quantity of the sex attractant released by the female of the species was waved before the severed head, and an enormous vertical spike obligingly appeared on the oscilloscope screen. The bottle fly could smell almost nothing except the female sex attractant. But that molecule he could smell exceedingly well.

  Such olfactory specialization is quite common in insects. The male silkworm moth is able to detect the female’s sex attractant molecule if only about forty molecules per second reach its feathery antennae. A single female silkworm moth need release only a hundredth of a microgram of sex attractant per second to attract every male silkworm in a volume of about a cubic mile. That is why there are silkworms.

  Perhaps the most curious exploitation of the reliance on smell to find a mate and continue the species is found in a South African beetle, which burrows into the ground during the winter. In the spring, as the ground thaws, the beetles emerge, but the male beetles groggily disinter themselves a few weeks before the females do. In this same region of South Africa, a species of orchid has evolved which gives off an aroma identical to the sex attractant of the female beetle. In fact, orchid and beetle evolution have produced essentially the same molecule. The male beetles turn out to be exceedingly nearsighted; and the orchids have evolved a configuration of their petals that, to a myopic beetle, resembles the female in a receptive sexual posture. The male beetles enjoy several weeks of orgiastic ecstasy among the orchids, and when eventually the females emerge from the ground, we can imagine a great deal of wounded pride and righteous indignation. Meanwhile the orchids have been successfully cross-pollinated by the amorous male beetles, who, now properly abashed, do their best to continue the beetle species; and both organisms survive. (Incidentally, it is in the interest of the orchids not to be too consummately attractive; if the beetles fail to reproduce themselves, the orchids are in trouble.) We thus discover one limitation to purely olfactory sexual stimuli. Another is that since every female beetle produces the same sex attractant, it is not easy for a male beetle to fall in love with the lady insect of his heart’s desire. While male insects may display themselves to attract a female, or—as with stag beetles—engage in mandible-to-mandible combat with the female as the prize, the central role of the female sex attractant in mating seems to reduce the extent of sexual selection among the insects.

  Other methods of finding a mate have been developed in reptiles, birds and mammals. But the connection of sex with smell is still apparent neuro-anatomically in higher animals as well as anecdotally in human experience. I sometimes wonder if deodorants, particularly “feminine” deodorants, are an attempt to disguise sexual stimuli and keep our minds on something else.

  3 THE NEOCORTEX

  Even in fish, lesions of the forebrain destroy the traits of initiative and caution. In higher animals these traits, much elaborated, seem localized in the neocortex, the site of many of the characteristic human cognitive functions. It is frequently discussed in terms of four major regions or lobes: the frontal, parietal, temporal and occipital lobes. Early neurophysiologists held that the neocortex was primarily connected only to other places in the neocortex, but it is now known that there are many neural connections with the subcortical brain. It is, however, by no means clear that the neocortical subdivisions are actually functional units. Each certainly has many quite different functions, and some functions may be shared among or between lobes. Among other functions, the frontal lobes seem to be connected with deliberation and the regulation of action; the parietal lobes, with spatial perception and the exchange of information between the brain and the rest of the body; the temporal lobes, with a variety of complex perceptual tasks; and the occipital lobes, with vision, the dominant sense in humans and other primates.

  A schematic diagram of a side view of the human brain, dominated by the neocortex, with a smaller limbic system and brainstem or hindbrain. The R-complex is not shown.

  For many decades the prevailing view of neuro-physiologists was that the frontal lobes, behind the forehead, are the sites of anticipation and planning for the future, both characteristically human functions. But more recent work has shown that the situation is not so simple. A large number of cases of frontal lesions—largely suffered in warfare and as gunshot wounds—have been investigated by the American neurophysiologist Hans-Lukas Teuber of the Massachusetts Institute of Technology. He found that many frontal-lobe lesions have almost no obvious effects on behavior; however, in severe pathology of the frontal lobes “the patient is not altogether devoid of capacity to anticipate a course of events, but cannot picture himself in relation to those events as a potential agent.” Teuber emphasized the fact that the frontal lobe may be involved in motor as well as cognitive anticipation, particularly in estimating what the effect of voluntary movements will be. The frontal lobes also seem to be implicated in the connection between vision and erect bipedal posture.

  Thus the frontal lobes may be involved with peculiarly human functions in two different ways. If they control anticipation of the future, they must also be the sites of concern, the locales of worry. This is why transection of the frontal lobes reduces anxiety. But prefrontal lobotomy must also greatly reduce the patient’s capacity to be human. The price we pay for anticipation of the future is anxiety about it. Foretelling disaster is probably not much fun; Pollyanna was much happier than Cassandra. But the Cassandric components of our nature are necessary for survival. The doctrines for regulating the future that they produced are the origins of ethics, magic, science and legal codes. The benefit of foreseeing catastrophe is the ability to take steps to avoid it, sacrificing short-term for long-term benefi
ts. A society that is, as a result of such foresight, materially secure generates the leisure time necessary for social and technological innovation.

  The other suspected function of the frontal lobes is to make possible mankind’s bipedal posture. Our upright stance may not have been possible before the development of the frontal lobes. As we shall see later in more detail, standing on our own two feet freed our hands for manipulation, which then led to a major accretion of human cultural and physiological traits. In a very real sense, civilization may be a product of the frontal lobes.

  Visual information from the eyes arrives in the human brain chiefly in the occipital lobe, in the back of the head; auditory impressions, in the upper part of the temporal lobe, beneath the temple. There is fragmentary evidence that these components of the neocortex are substantially less well developed in blind deaf-mutes. Lesions in the occipital lobe—as produced by gunshot wounds, for example—frequently induce an impairment in the field of vision. The victim may be in all other respects normal but able to see only with peripheral vision, perceiving a solid, dark blot looming in front of him at the center of the normal field of view. In other cases, more bizarre perceptions follow, including geometrically regular, cursive floating impairments in the visual field, and “visual fits” in which (for example) objects on the floor to the patient’s lower right are momentarily perceived as floating in the air to his upper left and rotated 180 degrees through space. It may even be possible to map which parts of the occipital lobes are responsible for which visual functions by systematically calculating the impairments of vision from various occipital lesions. Permanent impairments of vision are much less likely to occur in the very young, whose brains seem able to repair themselves or transfer functions to neighboring regions very well.