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

by Carl Sagan

Only an extragenetic learning system can possibly cope with the swiftly changing circumstances that our species faces. Thus the recent rapid evolution of human intelligence is not only the cause of but also the only conceivable solution to the many serious problems that beset us. A better understanding of the nature and evolution of human intelligence just possibly might help us to deal intelligently with our unknown and perilous future.

Evolution is adventitious and not foresighted. Only through the deaths of an immense number of slightly maladapted organisms are we, brains and all, here today.

Biology is more like history than it is like physics; the accidents and errors and lucky happenstances of the past powerfully prefigure the present.

“Mind” may be a consequence of the action of the components of the brain severally or collectively.

absence of evidence is not evidence of absence. The entire recent history of biology shows that we are, to a remarkable degree, the results of the interactions of an extremely complex array of molecules; and the aspect of biology that was once considered its holy of holies, the nature of the genetic material, has now been fundamentally understood in terms of the chemistry of its constituent nucleic acids, DNA and RNA, and their operational agents, the proteins.

in such a cosmic year the Earth does not condense out of interstellar matter until early September; dinosaurs emerge on Christmas Eve; flowers arise on December 28th; and men and women originate at 10:30 P.M. on New Year’s Eve.

All of recorded history occupies the last ten seconds of December 31; and the time from the waning of the Middle Ages to the present occupies little more than one second.

And despite the insignificance of the instant we have so far occupied in cosmic time, it is clear that what happens on and near Earth at the beginning of the second cosmic year will depend very much on the scientif...

A species is a group that can produce fertile offspring by crosses within but not outside itself.

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.

We might say that the language of heredity is written in an alphabet of only four letters.

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.

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.

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.

What counts are mutations in the gametes, the eggs and sperm cells, which are the agents of sexual reproduction.

no moth is making a conscious effort to adapt to a changed environment. The process is random and statistical.

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.

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,

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.

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.

In a way, the map of the motor cortex is an accurate portrait of our humanity.

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.

larger absolute brain size tends to produce higher intelligence.

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 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.

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.

Such experiments demonstrate that physiological changes accompany intellectual experience and show how plasticity...

This would mean that new learning corresponds to the generation of new synapses or the activation of moribund old ones,

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.

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.

Much of the history of life since the Carboniferous Period can be described as the gradual (and certainly incomplete) dominance of brains over genes.

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.

One of the most engaging views of the subsequent evolution of the brain is a story of the successive accretion and specialization of three further layers surmounting the spinal cord, hindbrain and midbrain. After each evolutionary step, the older portions of the brain still exist and must still be accommodated. But a new layer with new functions has been added.

The connection between sexual display and position in a dominance hierarchy can be found frequently among the primates. Among Japanese macaques, social class is maintained and reinforced by daily mounting: males of lower caste adopt the characteristic submissive sexual posture of the female in oestrus and are briefly and ceremonially mounted by higher-caste males. These mountings are both common and perfunctory. They seem to have little sexual content but rather serve as easily understood symbols of who is who in a complex society.

“Genital display is therefore considered the most effective social signal with respect to group hierarchy. It is ritualized and seems to acquire the meaning, ‘I am the master.’ It is most probably derived from sexual activity, but it is used for social communication and separated from reproductive activity. In other words, genital display is a ritual derived from sexual behavior but serving social and not reproductive purposes.”

The behavioral as well as neuroanatomical connections between sex, aggression and dominance are borne out in a variety of studies.

Thus the latest evolutionary change appears postpartum. The fetus may have characteristics, like the gill slits in mammals, that are entirely maladaptive after birth, but as long as they cause no serious problems for the fetus and are lost before birth, they can be retained. Our gill slits are vestiges not of ancient fish but of ancient fish embryos. Many new organ systems develop not by the addition and preservation but by the modification of older systems, as, for example, the modification of fins to legs, and legs to flippers or wings; or feet to hands to feet; or sebaceous glands to mammary glands; or gill arches to ear bones; or shark scales to shark teeth. Thus evolution by addition and the functional preservation of the preexisting structure must occur for one of two reasons—either the old function is required as well as the new one, or there is no way of bypassing the old system that is consistent with survival.

Nature could not rip out the chlorophyll and replace it with better pigments; the chlorophyll is woven too deeply into the fabric of life. Plants with accessory pigments are surely different. They are more efficient. But there, still working, although with diminished responsibilities,

“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.…

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.

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 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.

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.*

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.

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.

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