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

by Carl Sagan

the central role of the female sex attractant in mating seems to reduce the extent of sexual selection among the insects.

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.

The benefit of foreseeing catastrophe is the ability to take steps to avoid it, sacrificing short-term for long-term benefits.

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,...

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.

There is in the neocortex a striking separation of function, which is contrary to such common-sense notions as that reading and writing, or recognizing words and numbers, are very similar activities.

A wide range of experiments shows that lesions in the right temporal lobe produce amnesia for certain types of nonverbal material, while lesions in the left temporal lobe produce a characteristic loss of memory for language.

A lesion of the angular gyrus of the neocortex, in the parietal lobe, results in alexia, the inability to recognize the printed word. The parietal lobe appears to be involved in all human symbolic language and, of all the brain lesions, a lesion in the parietal lobe causes the greatest decline in intelligence as measured by activities in everyday life.

While most olfactory processing is in the limbic system, some occurs in the neocortex. The same division of function seems to apply to memory. A principal part of the limbic system, other than the olfactory cortex, is, as we have mentioned, the hippocampal cortex.

The original function of the hippocampus may have been exclusively the short-term memory of smell, useful in, for example, tracking prey or finding the opposite sex. But a bilateral hippocampal lesion in humans results, as in the case of H. M., in a profound impairment of all varieties of short-term memory.

both hippocampus and frontal lobes are involved in human short-term memory.

we have great difficulty in accessing new material into the long-term memory. Penfield believed that this lost accessing ability arises from an inadequate blood supply to the hippocampus in old age—because of arteriosclerosis or other physical disabilities. Thus elderly people—and ones not so elderly—may have serious impairments in accessing short-term memory while being otherwise perfectly alert and intellectually keen.

The mechanics of recall can be complex. A common experience is that we know something is in our long-term memory—a word, a name, a face, an experience—but find ourselves unable to call it up. No matter how hard we try, the memory resists retrieval. But if we think sideways at it, recalling some slightly related or peripheral item, it often follows unbidden.

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 neocortex, shared to some extent with the higher primates and such cetaceans as dolphins and whales.

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.

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.

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.

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.

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.

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

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 Austral-opithecine 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 e...

Whether the genus Homo independently invented tools or borrowed the discovery from the genus Australopithecus is not known.

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.

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.

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.

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.

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.

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.

Childbirth is painful because the evolution of the human skull has been spectacularly fast and recent.

The connection between the evolution of intelligence and the pain of childbirth seems unexpectedly to be made in the Book of Genesis.

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),

Man is probably the only organism on Earth with a relatively clear view of the inevitability of his own end.

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.

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 Dryopithecus 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.

Patients who have had prefrontal lobotomies have been described as losing a “continuing sense of self”—the

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 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.

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.

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:

Extensive gestural languages were employed by many human hunting communities—for

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.

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.

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.

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.

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.

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;