The Dragons of Eden: Speculations on the Evolution of Human Intelligence

by Carl Sagan

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?

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.

(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.

Could abstract thought be a matter not of kind but of degree? Could other animals be capable of abstract thought but more rarely or less deeply than humans?

do we simply equate the absence of our style of expression of intelligence with the absence of intelligence?

first serious study of the behavior of simians—including their behavior in the wild—was made in Indonesia by Alfred Russel Wallace, the co-discoverer of evolution by natural selection. Wallace concluded that a baby orangutan he studied behaved “exactly like a human child in similar circumstances.” In fact, “orangutan” is a Malay phrase meaning not ape but “man of the woods.”

two chimpanzees were observed maltreating a chicken: One would extend some food to the fowl, encouraging it to approach; whereupon the other would thrust at it with a piece of wire it had concealed behind its back. The chicken would retreat but soon allow itself to approach once again—and be beaten once again. Here is a fine combination of behavior sometimes thought to be uniquely human: cooperation, planning a future course of action, deception and cruelty.

The exploitation of organ systems with other functions for communication in humans is also indicative of the comparatively recent evolution of our linguistic abilities.

In fact, Rensberger was conversing with a member of another species in his native “hand.” And it is just this transition from tongue to hand that has permitted humans to regain the ability—lost, according to Josephus, since Eden—to communicate with the animals.

a community of chimps initially competent in gestural language could pass down the language to subsequent generations. Where such communication is essential for survival, there is already some evidence that apes transmit extragenetic or cultural information.

Differences in group behavior—something that it is very tempting to call cultural differences—have been reported among chimpanzees, baboons, macaques and many other primates.

In recent years it has become clear that the brains of nonhuman primates are similarly prepared, although probably not quite to the same degree, for the introduction of language.

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.)

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.

“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.

There are, it seems, three principal states of mind in human beings: waking, sleeping and dreaming.

The circadian rhythm, the daily cycling of physiological function, is known to go back at least to animals as humble as mollusks.

There is some recent evidence that the two types of sleep, dreaming and dreamless, depend on the lifestyle of the animal.

predators are statistically much more likely to dream than prey, which are in turn much more likely to experience dreamless sleep.

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 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.

Judged by their endocranial volumes (see figure on this page), the dinosaurs were, compared to mammals, remarkably stupid.

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.

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.

In the metaphor of the triune brain, dreams are partially a function of the R-complex and the limbic cortex, but not of the rational part of the neocortex.

Experiments suggest that as the night wears on our dreams engage increasingly earlier material from our past, reaching back to childhood and infancy.

The dream state is accompanied by rapid eye movements (REM), which can be detected by electrodes taped lightly over the eyelids in sleep, and by a particular brain wave pattern on the EEG.

On awakening, an individual in the midst of REM sleep will usually remember his dream.

Dreams seem to be primarily a mammalian function.

dream sleep is most vigorously engaged in by human beings in the early postnatal period.

There is another connection between infancy and dreams: both are followed by amnesia. When we emerge from either state, we have great difficulty remembering what we have experienced. In both cases, I would suggest, the left hemisphere of the neocortex, which is responsible for analytic recollection, has been functioning ineffectively.

The survival of the early mammals depended on intelligence, daytime unobtrusiveness, and devotion to the young.

We are descended from reptiles and mammals both. In the daytime repression of the R-complex and in the nighttime stirring of the dream dragons, we may each of us be replaying the hundred-million-year-old warfare between the reptiles and the mammals.

The olfactory recognition of individual molecules is apparently accomplished by individual nasal receptors sensitive to particular functional groups, or parts, of organic molecules. One receptor, for example, may be sensitive to COOH, another to NH2, and so on. (C stands for carbon, H for hydrogen, O for oxygen and N for nitrogen.)

The sense of smell is very ancient, and indeed, much of the early evolution above the level of the neural chassis may have been spurred by selection pressure for such molecular detection: the distinctive olfactory bulbs in the brain (see figure on this page) are among the first components of the neocortex to have developed in the history of life.

sight is the primary information channel in humans. We are capable of visual sensitivity and discrimination at least as impressive as the olfactory abilities of the bloodhound. For example, we are able to discriminate among faces.