The Dragons of Eden

by Carl Sagan

  Perhaps the most striking aspect of this entire subject is that there are nonhuman primates so close to the edge of language, so willing to learn, so entirely competent in its use and inventive in its application once the language is taught. But this raises a curious question: Why are they all on the edge? Why are there no nonhuman primates with an existing complex gestural language? One possible answer, it seems to me, is that humans have systematically exterminated those other primates who displayed signs of intelligence. (This may have been particularly true of the nonhuman primates who lived in the savannahs; the forests must have offered some protection to chimpanzees and gorillas from the depredations of man.) We may have been the agent of natural selection in suppressing the intellectual competition. I think we may have pushed back the frontiers of intelligence and language ability among the nonhuman primates until their intelligence became just indiscernible. In teaching gestural language to the chimpanzees, we are beginning a belated attempt to make amends.

  * Our difficulties in understanding or effectuating communication with other animals may arise from our reluctance to grasp unfamiliar ways of dealing with the world. For example, dolphins and whales, who sense their surrounding with a quite elaborate sonar echo location technique, also communicate with each other by a rich and elaborate set of clicks, whose interpretation has so far eluded human attempts to understand it. One very clever recent suggestion, which is now being investigated, is that dolphin/dolphin communication involves a re-creation of the sonar reflection characteristics of the objects being described. In this view a dolphin does not “say” a single word for shark, but rather transmits a set of clicks corresponding to the audio reflection spectrum it would obtain on irradiating a shark with sound waves in the dolphin’s sonar mode. The basic form of dolphin/dolphin communication in this view would be a sort of aural onomatopoeia, a drawing of audio frequency pictures—in this case, caricatures of a shark. We could well imagine the extension of such a language from concrete to abstract ideas, and by the use of a kind of audio rebus—both analogous to the development in Mesopotamia and Egypt of human written languages. It would also be possible, then, for dolphins to create extraordinary audio images out of their imaginations rather than their experience.

  * Until fairly recently it was thought that humans had forty-eight chromosomes in an ordinary somatic cell. We now know that the correct number is forty-six. Chimps apparently really do have forty-eight chromosomes, and in this case a viable cross of a chimpanzee and a human would in any event be rare.

  6

  TALES

  OF

  DIM EDEN

  Very old are we men;

  Our dreams are tales

  Told in dim Eden …

  WALTER DE LA MARE

  “All That’s Past”

  “Well, at any rate it’s a great comfort,” she said as she stepped under the trees, “after being so hot to get into the—into the—into what?” she went on, rather surprised at not being able to think of the word. “I mean to get under the—under the—under this, you know!” putting her hand on the trunk of the tree. “What does it call itself, I wonder?” … And now, who am I? I will remember, if I can! I’m determined to do it!” But being determined didn’t help her much, and all she could say, after a great deal of puzzling, was “L, I know it begins with L!”

  LEWIS CARROLL

  Alice Through the Looking Glass

  Come not between the dragon and his wrath.

  WM. SHAKESPEARE

  King Lear

  … At first

  Senseless as beasts I gave men sense, possessed them of mind …

  In the beginning, seeing, they saw amiss, and hearing, heard not, but like phantoms huddled

  In dreams, the perplexed story of their days

  Confounded.

  AESCHYLUS

  Prometheus Bound

  ROMETHEUS is in a fit of righteous indignation. He has introduced civilization to a befuddled and superstitious mankind, and for his pains Zeus has chained him to a rock and set a vulture to pluck at his liver. In the passage following the above quotation, Prometheus describes the principal gifts, other than fire, that he has bestowed on mankind. They are, in order: astronomy; mathematics; writing; the domestication of animals; the invention of chariots, sailing ships and medicine; and the discovery of divination by dreams and other methods. The final gift strikes the modern ear as odd. Along with the account in Genesis of the exile from Eden, Prometheus Bound seems to be one of the major works in Western literature that presents a viable allegory of the evolution of man—although in this case concentrating much more on the “evolver” than on the evolved. “Prometheus” is Greek for “foresight,” that quality claimed to reside in the frontal lobes of the neocortex; and foresight and anxiety are both present in Aeschylus’ character portrait.

  What is the connection between dreams and the evolution of man? Aeschylus is perhaps saying that our prehuman ancestors lived their waking lives in a state akin to our dreaming lives; and that one of the principal benefits of the development of human intelligence is our ability to understand the true nature and import of dreams.

  There are, it seems, three principal states of mind in human beings: waking, sleeping and dreaming. An electroencephalograph, which detects brain waves, records quite distinct patterns of electrical activity in the brain during these three states.* Brain waves represent very small currents and voltages produced by the electrical circuitry of the brain. Typical strengths of such brain-wave signals are measured in microvolts. Typical frequencies are between 1 and about 20 Hertz (or cycles per second)—less than the familiar 60 cycles per second frequency of alternating currents in electrical outlets in North America.

  But what is sleep good for? There is no doubt that if we stay up too long the body generates neurochemicals that literally force us to go to sleep. Sleep-deprived animals generate such molecules in their cerebrospinal fluid, and the cerebrospinal fluid of sleep-deprived animals induces sleep when injected into other animals who are perfectly wide awake. There must, then, be a very powerful reason for sleep.

  The conventional answer of physiology and folk medicine alike is that sleep has a restorative effect; it is an opportunity for the body to perform mental and physical housekeeping away from the needs of daily living. But the actual evidence for this view, apart from its common-sense plausibility, seems to be sparse. Furthermore, there are some worrisome aspects about the contention. For example, an animal is exceptionally vulnerable when sleeping. Granted that most animals sleep in nests, caves, holes in trees or logs or otherwise recessed or camouflaged locations. Even so, their helplessness while asleep remains high. Our nocturnal vulnerability is very evident; the Greeks recognized Morpheus and Thanatos, the gods of sleep and death, as brothers.

  Unless there is some exceptionally strong biological necessity for sleep, natural selection would have evolved beasts that sleep not. While there are some animals—the two-toed sloth, the armadillo, the opossum, and the bat—that, at least in states of seasonal torpor, sleep nineteen and twenty hours a day, there are others—the common shrew and Dall’s porpoise—that are said to sleep very little. There are also human beings who require only one to three hours of sleep a night. They take second and third jobs, putter around at night while their spouses sink into exhaustion, and otherwise seem to lead full, alert and constructive lives. Family histories suggest that these predispositions are hereditary. In one case, both a man and his little daughter are afflicted with this blessing or curse, much to the groggy consternation of his wife, who has since divorced him for a novel incompatibility. He retained custody of the daughter. Such examples suggest that the hypothesis of the recuperative function of sleep is at best not the whole story.

  The distinctive EEG patterns of a normal human being while awake, asleep, and dreaming.

  Yet sleep is very ancient. In the electroencephalographic sense we share it with all the primates and almost all the other mammals and birds: it may extend b
ack as far as the reptiles. Temporal-lobe epilepsy and its accompanying state of unconscious automatic behavior can be induced in some people by spontaneous electrical stimulation of the amygdala, deep below the temporal lobe, at frequencies of a few cycles per second (a few Hertz). Seizures not very different from sleep have been reported when an epileptic patient is driving in an automobile near sunset or sunrise with a picket fence between him and the sun: at a certain speed the pickets intercept the sun at just the critical rate to produce a flicker at the resonant frequency for initiating such seizures. The circadian rhythm, the daily cycling of physiological function, is known to go back at least to animals as humble as mollusks. Since a state in some respects resembling dreaming can be induced by electrical stimulation of other limbic regions below the temporal lobe, as described below, centers that initiate both sleep and dreams may not be far apart in the recesses of the brain.

  There is some recent evidence that the two types of sleep, dreaming and dreamless, depend on the lifestyle of the animal. Truett Allison and Domenic Ciccheti of Yale University have found that predators are statistically much more likely to dream than prey, which are in turn much more likely to experience dreamless sleep. These studies are all of mammals and apply only to differences between, not within, species. In dream sleep, the animal is powerfully immobilized and remarkably unresponsive to external stimuli. Dreamless sleep is much shallower, and we have all witnessed cats or dogs cocking their ears to a sound when apparently fast asleep. It is also commonly held that when sleeping dogs move their legs in a kind of running pattern, they are dreaming of the hunt. The fact that deep dream sleep is rare among prey today seems clearly to be a product of natural selection. But organisms that are largely prey today may have had ancestors that were predators, and vice versa. Moreover, predators are generally organisms with larger absolute brain mass and ratio of brain to body mass than their prey. It makes sense that today, when sleep is highly evolved, the stupid animals are less frequently immobilized by deep sleep than the smart ones. But why should they sleep deeply at all? Why should a state of such deep immobilization ever have evolved?

  Perhaps one useful hint about the original function of sleep is to be found in the fact that dolphins and whales and aquatic mammals in general seem to sleep very little. There is, by and large, no place to hide in the ocean. Could it be that, rather than increasing an animal’s vulnerability, the function of sleep is to decrease it? Wilse Webb of the University of Florida and Ray Meddis of London University have suggested this to be the case. The sleeping style of each organism is exquisitely adapted to the ecology of the animal. It is conceivable that animals who are too stupid to be quiet on their own initiative are, during periods of high risk, immobilized by the implacable arm of sleep. The point seems particularly clear for the young of predatory animals; not only are baby tigers covered with a superbly effective protective coloration, they also sleep a great deal. This is an interesting notion and probably at least partly true. It does not explain everything. Why do lions, who have few natural enemies, sleep? This question is not a very damaging objection because lions may have evolved from animals that were not the king of beasts. Likewise, adolescent gorillas, who have little to fear, nevertheless construct nests each night—perhaps because they evolved from more vulnerable predecessors. Or perhaps, once, the ancestors of lions and gorillas feared still more formidable predators.

  A nest of Protoceratops eggs from the Cretaceous of the Mongolian People’s Republic.

  Courtesy of The American Museum of Natural History

  The immobilization hypothesis seems particularly apt in light of the evolution of mammals, who arose in an epoch dominated by hissing, thundering and altogether nightmarish reptiles. But nearly all reptiles are cold-blooded* and, except in the tropics, are forced into nocturnal immobility. Mammals are warm-blooded and able to function at night. The nontropical nocturnal ecological niches may have been almost untenanted in the Triassic Period, some two hundred million years ago. Indeed, Harry Jerison has suggested that the evolution of mammals was accompanied by the development of then extremely sophisticated (and now commonplace) versions of hearing and smell, senses for perceiving distances and objects at night; and that the limbic system evolved from the necessity of processing the rich array of data from these newly elaborated senses. (A great deal of the visual-information processing in reptiles is done not in the brain but in the retina; the optical processing apparatus in the neocortex was largely a later evolutionary development.)

  A reconstruction of baby Protoceratops hatching.

  Courtesy of The American Museum of Natural History

  Perhaps it was essential for the early mammals to be immobilized and hidden during the daylight hours that were ruled by predatory reptiles. I am picturing a late Mesozoic landscape in which the mammals sleep fitfully by day and the reptiles by night. But at night even humble carnivorous protomammals must have posed a real threat to the cold-immobilized reptiles, and particularly to their eggs.

  Judged by their endocranial volumes (see figure on this page), the dinosaurs were, compared to mammals, remarkably stupid. To take some “well-known” examples, Tyrannosaurus rex had a brain volume of about 200 cubic centimeters (cc); Brachiosaurus, 150 cc; Triceratops, 70 cc; Diplodocus, 50 cc; Stegosaurus, 30 cc. Not one approached a chimpanzee in absolute brain mass; Stegosaurus, which weighed two metric tons, was probably far more stupid than a rabbit. When the large body weights of the dinosaurs are taken into account, the smallness of their brains becomes even more striking: Tyrannosaurus weighed 8 metric tons; Diplodocus, 12; and Brachiosaurus, 87. The ratio of brain to body weight in Brachiosaurus was ten thousand times smaller than that of man. Just as sharks are the largest-brained fish for their body weight, the carnivorous dinosaurs such as Tyrannosaurus were relatively larger-brained than such herbivores as Diplodocus and Brachiosaurus. I am sure that Tyrannosaurus was an efficient and terrifying killing machine. But despite their awesome aspect, the dinosaurs look vulnerable to dedicated and intelligent adversaries—such as the early mammals.

  A drawing of Saurornithoides, a small intelligent dinosaur, here shown catching mammals. Specimens are known from Canada and from the Mongolian People’s Republic in the Cretaceous.

  Our Mesozoic scene has a curiously vampiric quality with the carnivorous reptiles hunting the smart sleeping mammals by day, and the carnivorous mammals hunting the stupid immobile reptiles by night. While the reptiles buried their eggs, it is unlikely that they actively protected either eggs or young. There are very few accounts of such behavior even in contemporary reptiles, and it is difficult to picture Tyrannosaurus rex brooding on a clutch of eggs. For these reasons, the mammals may have won the primordial war of the vampires; at least some paleontologists believe that the demise of the dinosaurs was accelerated by nocturnal predation on reptilian eggs by the early mammals. Two chicken eggs* for breakfast may be all—at least on the surface—that is left of this ancient mammalian cuisine.

  The most intelligent of the dinosaurs by the criterion of brain to body mass are the Saurornithoides, whose brain mass was typically about 50 grams to a body mass of about 50 kilograms, placing them near the ostrich in the figure on this page. Indeed, they resembled ostriches. It might be very illuminating to examine fossil endocasts of their braincases. They probably hunted small animals for food and used the four fingers of their handlike appendages for many different tasks. (See illustration above.)

  They are interesting beasts to speculate about. If the dinosaurs had not all been mysteriously extinguished some sixty-five million years ago, would the Saurornithoides have continued to evolve into increasingly intelligent forms? Would they have learned to hunt large mammals collectively and thus perhaps have prevented the great proliferation of mammals that followed the end of the Mesozoic Age? If it had not been for the extinction of the dinosaurs, would the dominant life forms on Earth today be descendants of Sauronithoides, writing and reading books, speculating on what would have happened had t
he mammals prevailed? Would the dominant forms think that base 8 arithmetic was quite natural, but base 10 a frill taught only in the “New Math”?

  A great deal of what we consider important about the last few tens of millions of years of Earth’s history seems to hinge on the extinction of the dinosaurs. There are literally dozens of scientific hypotheses that attempt to explain this event, which appears to have been remarkably rapid and thorough for both land and water forms. All the explanations proposed seem to be only partly satisfactory. They range from massive climatic change to mammalian predation to the extinction of a plant with apparent laxative properties, in which case the dinosaurs died of constipation.

  One of the most interesting and promising hypotheses, first suggested by I. S. Shklovskii of the Institute for Cosmic Research, Soviet Academy of Sciences, Moscow, is that the dinosaurs died because of a nearby supernova event—the explosion of a dying star some tens of light-years away, which resulted in an immense flux of high energy charged particles that entered our atmosphere, changed its properties, and, perhaps by destroying the atmospheric ozone, let in lethal quantities of solar ultraviolet radiation. Nocturnal animals, such as the mammals of the time, and deep-sea animals, such as fish, could have survived this higher ultraviolet intensity; but daytime animals that lived on land or near the surface of the waters would have been preferentially destroyed. Such a disaster would be aptly named—the word itself means “bad star.”