Dragons of Eden

by Carl Sagan

  The hands of animals are adapted to their life styles, and vice versa. Shown are A the opossum; B the tree shrew; C the potto; D the tarsier; E the baboon (where this appendage is used partly as a hand and partly as a foot); F the orangutan, specialized for brachiation; and G humans, with a relatively long and opposable thumb.

  From Mankind in the Making, by William Howells, drawings by Janis Cirulis (Doubleday).

  A family of gracile Australopithecines five million years ago.

  Copyright © 1965, 1973 Time, Inc.

  By three million years ago, there was a variety of bipedal fellows with a wide range of cranial volumes, some considerably larger than the East African gracile Australopithecines of a few million years earlier. One of them, which L. S. B. Leakey, the Anglo-Kenyan student of early man, called Homo habilis, had a brain volume of about 700 cubic centimeters. We also have archaeological evidence that Homo habilis made tools. 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. The fact that these significant changes in behavior are accompanied by equally significant changes in brain volume does not prove that the one is caused by the other; but our previous discussion makes such a casual link appear very likely.

  The table on this page summarizes the fossil evidence, through 1976, on our most recent ancestors and collateral relatives. The two rather different kinds of Australopithecines were not of the genus Homo, not human; they were still incompletely bipedal and had brain masses only about a third the size of the average adult human brain today. Were we to meet an Australopithecine, say, on the subway, we would perhaps be struck most by the almost total absence of forehead. He was the lowest of lowbrows. There are significant differences between the two kinds of Australopithecines. The robust species was taller and heavier, with most impressive “nut-cracker” teeth and a remarkable evolutionary stability. The endocranial volume of A. robustus varies very little from specimen to specimen over millions of years of time. The gracile Australopithecines, judging again from their teeth, probably ate meat as well as vegetables. They were smaller and lither, as their name indicates. However, they are considerably older and have much more variance in endocranial volume than their robust cousins. But, most important, the gracile Australopithecine sites are associated with a clear industry: the manufacture of tools made of stone and animal bones, horns and teeth—painstakingly carved, broken, rubbed and polished to make chipping, flaking, pounding and cutting tools. No tools have been associated with A. robustus. The ratio of brain weight to body weight is almost twice as large for the gracile as for the robust Australopithecus, and it is a natural speculation to wonder whether that factor of two is the difference between tools and no tools.

  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.

  In the last sixty million years, there has been a continuous evolution of ungulates, well recorded in the fossil record, and eventually culminating in the modern horse. Eohippus, the “dawn horse” of some fifty million years ago, was about the size of an English collie, with a brain volume of about twenty-five cubic centimeters, and a ratio of brain to body weight about half that of comparable contemporary mammals. Since then, horses have experienced a dramatic evolution in both absolute and relative brain size, with major developments in the neocortex and particularly in the frontal lobes—an evolution certainly accompanied by major improvements in equine intelligence. I wonder if the parallel developments in the intelligence of horse and man might have a common cause. Did horses, for example, have to be swift of foot, acute of sense, and intelligent to elude predators which hunted primate as well as equine prey?

  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. Were we to encounter Homo habilis—dressed, let us say, in the latest fashion on the boulevards of some modern metropolis—we would probably give him only a passing glance, and that because of his relatively small stature. Associated with Homo habilis are a variety of tools of considerable sophistication. In addition, there is evidence from various circular arrangements of stones that Homo habilis may have constructed dwellings; that long before the Pleistocene Ice Ages, long before men regularly inhabited caves, H. habilis was constructing homes out-of-doors—probably of wood, wattle, grass and stone.

  The East African savannah near Olduvai Gorge a few million years ago. In right foreground are three hominids, perhaps Australopithecines, perhaps Homo habilis. The active volcano in the background is now Mt. Ngorongoro.

  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 makes Prometheus far older than many had thought.

  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 educational institutions. There must have been some sort of stonecraft guild passing on from generation to generation the precious knowledge about the fabrication and use of tools—knowledge that would eventually propel such feeble and almost defenseless primates into domination of the planet Earth. Whether the genus Homo independently invented tools or borrowed the discovery from the genus Australopithecus is not known.

  We see from the table that the ratio of body to brain weight is, within the variance of measurement, roughly the same for the gracile Australopithecines, Homo habilis, Homo erectus and modern humans. 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.

  L. S. B. Leakey emphasized that the 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. Some of these injuries may have been inflicted by leopards or hyenas; but Leakey and the South African anatomist Raymond Dart believed 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. What role killing played in that comp
etition remains an open question. 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.

  At the same time that the hominid cranial volume was undergoing its spectacular increase, there was another striking change in human anatomy; as the British anatomist Sir Wilfred Le Gros Clark of Oxford University has observed, 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. (At birth, girls already have a significantly larger pelvis and skeletal pelvic opening than do boys; another large increment in the size of the female pelvis occurs at puberty.) 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.

  So far as I know, childbirth is generally painful in only one of the millions of species on Earth: human beings. This must be a consequence of the recent and continuing increase in cranial volume. Modern men and women have braincases twice the volume of Homo habilis’. Childbirth is painful because the evolution of the human skull has been spectacularly fast and recent. The American anatomist C. Judson Herrick described the development of the neocortex in the following terms: “Its explosive growth late in phylogeny is one of the most dramatic cases of evolutionary transformation known to comparative anatomy.” The incomplete closure of the skull at birth, the fontanelle, is very likely an imperfect accommodation to this recent brain evolution.

  The connection between the evolution of intelligence and the pain of childbirth seems unexpectedly to be made in the Book of Genesis. In punishment for eating the fruit of the tree of the knowledge of good and evil, God says to Eve,* “In pain shalt thou bring forth children” (Genesis 3:16). It is interesting that it is not the getting of any sort of knowledge that God has forbidden, but, specifically, the knowledge of the difference between good and evil—that is, abstract and moral judgments, which, if they reside anywhere, reside in the neocortex. Even at the time that the Eden story was written, the development of cognitive skills was seen as endowing man with godlike powers and awesome responsibilities. 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), he must be driven out of the Garden. God places cherubim with a flaming sword east of Eden to guard the Tree of Life from the ambitions of man.†

  Perhaps the Garden of Eden is not so different from Earth as it appeared to our ancestors of some three or four million years ago, during a legendary golden age when the genus Homo was perfectly interwoven with the other beasts and vegetables. After the exile from Eden we find, in the biblical account, mankind condemned to death; hard work; clothing and modesty as preventatives of sexual stimulation; the dominance of men over women; the domestication of plants (Cain); the domestication of animals (Abel); and murder (Cain plus Abel). These all correspond reasonably well to the historical and archaeological evidence. In the Eden metaphor, there is no evidence of murder before the Fall. But those fractured skulls of bipeds not on the evolutionary line to man may be evidence that our ancestors killed, even in Eden, many manlike animals.

  Civilization develops not from Abel, but from Cain the murderer. The very word “civilization” derives from the Latin word for city. It is the leisure time, community organization and specialization of labor in the first cities that permitted the emergence of the arts and technologies we think of as the hallmarks of civilizations. The first city, according to Genesis, was constructed by Cain, the inventor of agriculture—a technology that requires a fixed abode. And it is his descendants, the sons of Lamech, who invent both “artifices in brass and iron” and musical instruments. Metallurgy and music—technology and art—are in the line from Cain. And the passions that lead to murder do not abate: Lamech says, “For I have slain a man for wounding me, and a young man for bruising me; if Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold.” The connection between murder and invention has been with us ever since. Both derive from agriculture and civilization.

  One of the earliest consequences of the anticipatory skills that accompanied the evolution of the prefrontal lobes must have been the awareness of death. Man is probably the only organism on Earth with a relatively clear view of the inevitability of his own end. Burial ceremonies that include the interment of food and artifacts along with the deceased go back at least to the times of our Neanderthal cousins, suggesting not only a widespread awareness of death but also an already developed ritual ceremony to sustain the deceased in the afterlife. 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. This may account for its popularity.* It is not so remarkable as to require us to believe in a kind of biological memory of ancient historical events, but it does seem to me close enough to risk at least raising the question. The only repository of such a biological memory is, of course, the genetic code.

  The creation of Adam: A relief on the doors of the Church of St. Peter in Bologna by Jacopo della Quercia.

  PHOTO ALINARI

  By fifty-five million years ago, in the Eocene Period, there was a great proliferation of primates, both arboreal and ground-dwelling, and the evolution of a line of descent that eventually led to Man. Some primates of those times—e.g., a prosimian called Tetonius—exhibit in their endocranial casts tiny nubs where the frontal lobes will later evolve. 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. (His approximate contemporary, Ramapithecus, is thought by some anthropologists to be ancestral to man.) Proconsul’s endocranial casts show recognizable frontal lobes but much less well developed neocortical convolutions than are displayed by apes and men today. His cranial volume was still very small. The biggest burst of evolution in cranial volume occurred in the last few million years.

  The temptation of Eve and Adam by a reptile with a remarkably human head: A relief on the doors of St. Peter in Bologna by Jacopo della Quercia.

  PHOTO ALINARI

  The expulsion from Eden: A relief on the doors of St. Peter in Bologna by Jacopo della Quercia.

  PHOTO ALINARI

  Patients who have had prefrontal lobotomies have been described as losing a “continuing sense of self”—the feeling that I am a particular individual with some control over my life and circumstances, the “me-ness” of me, the uniqueness of the individual. It is possible that lower mammals and reptiles, lacking extensive frontal lobes, also lack this sense, real or illusory, of individuality and free will, which is so characteristically human and which may first have been experienced dimly by Proconsul.

  The development of human culture and the evolution of those physiological traits we consider characteristically human most likely proceeded—almost literally—hand in hand
: 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 American anthropologist Sherwood Washburn of the University of California, a principal exponent of this view, has said: “Much of what we think of as human evolved long after the use of tools. It is probably more correct to think of much of our structure as the result of culture than it is to think of men anatomically like ourselves slowly developing culture.”

  Some students of human evolution believe that part of 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. They stress the remarkable abilities of human beings to throw projectiles accurately, to move gracefully, and—as Louis Leakey enjoyed illustrating by direct demonstration—naked, to outrun and immobilize game animals. Such sports as baseball, football, wrestling, track and field events, chess and warfare may owe their appeal—as well as their largely male following—to these prewired hunting skills, which served us so well for millions of years of human history but which find diminished practical applications today.