We have known since australopithecines were discovered in the 1920s that these hominids had relatively small brains and fully erect posture. (A. africanus has a brain about one-third the volume of ours and a completely upright gait. A correction for its small body size does not remove the large discrepancy between its brain and ours.) This “anomaly” of small brain and upright posture has been a major issue in the literature for decades and wins a prominent place in all important texts.
Thus, the designation of A. afarensis does not establish the historical primacy of upright posture over large brains. But it does, in conjunction with two other ideas, suggest something very novel and exciting, something curiously missing from the press reports or buried amidst misinformation about the primacy of upright posture. A. afarensis is important because it teaches us that perfected upright gait had already been achieved nearly four million years ago. Lucy’s pelvic structure indicates bipedal posture for the Afar remains, while the remarkable footprints just discovered at Laetoli provide even more direct evidence. The later south and east African australopithecines do not extend back much further than two and a half million years. We have thus added nearly one and a half million years to the history of fully upright posture.
To explain why this addition is so important, I must break the narrative and move to the opposite end of biology—from fossils of whole animals to molecules. During the past fifteen years, students of molecular evolution have accumulated a storehouse of data on the amino acid sequences of similar enzymes and proteins in a wide variety of organisms. This information has generated a surprising result. If we take pairs of species with securely dated times of divergence from a common ancestor in the fossil record, we find that the number of amino acid differences correlates remarkably well with time since the split—the longer that two lineages have been separate, the more the molecular difference. This regularity has led to the establishment of a molecular clock to predict times of divergence for pairs of species without good fossil evidence of ancestry. To be sure, the clock does not beat with the regularity of an expensive watch—it has been called a “sloppy clock” by one of its leading supporters—but it has rarely gone completely haywire.
Darwinians were generally surprised by the clock’s regularity because natural selection should work at markedly varying rates in different lineages at different times: very rapidly in complex forms adapting to rapidly changing environments, very slowly in stable, well-adapted populations. If natural selection is the primary cause of evolution in populations, then we should not expect a good correlation between genetic change and time unless rates of selection remain fairly constant—as they should not by the argument stated above. Darwinians have escaped this anomaly by arguing that irregularities in the rate of selection smooth out over long periods of time. Selection might be intense for a few generations and virtually absent for a time thereafter, but the net change averaged over long periods could still be regular. But Darwinians have also been forced to face the possibility that regularity of the molecular clock reflects an evolutionary process not mediated by natural selection, the random fixation of neutral mutations. (I must defer this “hot” topic to another time and more space.)
In any case, the measurement of amino acid differences between humans and living African great apes (gorillas and chimpanzees) led to the most surprising result of all. We are virtually identical for genes that have been studied, despite our pronounced morphological divergence. The average difference in amino acid sequences between humans and African apes is less than one percent (0.8 percent to be precise)—corresponding to a mere five million years since divergence from a common ancestor on the molecular clock. Allowing for the slop, Allan Wilson and Vincent Sarich, the Berkeley scientists who uncovered this anomaly, will accept six million years, but not much more. In short, if the clock is valid, A. afarensis is pushing very hard at the theoretical limit of hominid ancestry.
Until recently, anthropologists tended to dismiss the clock, arguing that hominids provided a genuine exception to an admitted rule. They based their skepticism about the molecular clock upon an animal called Ramapithecus, an African and Asian fossil known mainly from jaw fragments and ranging back to fourteen million years in age. Many anthropologists claimed that Ramapithecus could be placed on our side of the ape-human split—that, in other words, the divergence between hominids and apes occurred more than fourteen million years ago. But this view, based on a series of technical arguments about teeth and their proportions, has been weakening of late. Some of the strongest supporters of Ramapithecus as a hominid are now prepared to reassess it as an ape or as a creature near to the common ancestry of ape and human but still before the actual split. The molecular clock has been right too often to cast it aside for some tentative arguments about fragments of jaws. (I now expect to lose a $10 bet I made with Allan Wilson a few years back. He generously gave me seven million years as a maximum for the oldest ape-human common ancestor, but I held out for more. And while I’m not shelling out yet, I don’t really expect to collect.*)
We may now put together three points to suggest a major reorientation in views about human evolution: the age and upright posture of A. afarensis, the ape-human split on the molecular clock, and the dethroning of Ramapithecus as a hominid.
We have never been able to get away from a brain-centered view of human evolution, although it has never represented more than a powerful cultural prejudice imposed upon nature. Early evolutionists argued that enlargement of the brain must have preceded any major alteration of our bodily frame. (See views of G. E. Smith in essay 10. Smith based his pro-Piltdown conviction upon an almost fanatical belief in cerebral primacy.) But A. africanus, upright and small brained, ended that conceit in the 1920s, as predicted by a number of astute evolutionists and philosophers, from Ernst Haeckel to Friedrich Engels. Nevertheless, “cerebral primacy,” as I like to call it, still held on in altered form. Evolutionists granted the historical primacy of upright posture but conjectured that it arose at a leisurely pace and that the real discontinuity—the leap that made us fully human—occurred much later when, in an unprecedented burst of evolutionary speed, our brains tripled in size within a million years or so.
Consider the following, written ten years ago by a leading expert: “The great leap in cephalization of genus Homo took place within the past two million years, after some ten million years of preparatory evolution toward bipedalism, the tool-using hand, etc.” Arthur Koestler has carried this view of a cerebral leap toward humanity to an unexcelled height of invalid speculation in his latest book, Janus. Our brain grew so fast, he argues, that the outer cerebral cortex, seat of smarts and rationality, lost control over emotive, animal centers deep within our brains. This primitive bestiality surfaces in war, murder, and other forms of mayhem.
I believe that we must reassess fundamentally the relative importance we have assigned to upright posture and increase in brain size as determinants of human evolution. We have viewed upright posture as an easily accomplished, gradual trend and increase in brain size as a surprisingly rapid discontinuity—something special both in its evolutionary mode and the magnitude of its effect. I wish to suggest a diametrically opposite view. Upright posture is the surprise, the difficult event, the rapid and fundamental reconstruction of our anatomy. The subsequent enlargement of our brain is, in anatomical terms, a secondary epiphenomenon, an easy transformation embedded in a general pattern of human evolution.
Six million years ago at most, if the molecular clock runs true (and Wilson and Sarich would prefer five), we shared our last common ancestor with gorillas and chimps. Presumably, this creature walked primarily on all fours, although it may have moved about on two legs as well, as apes and many monkeys do today. Little more than a million years later, our ancestors were as bipedal as you or I. This, not later enlargement of the brain, was the great punctuation in human evolution.
Bipedalism is no easy accomplishment. It requires a fundamental reconstruction of our anatomy,
particularly of the foot and pelvis. Moreover, it represents an anatomical reconstruction outside the general pattern of human evolution. As I argue in essay 9, through the agency of Mickey Mouse, humans are neotenic—we have evolved by retaining juvenile features of our ancestors. Our large brains, small jaws, and a host of other features, ranging from distribution of bodily hair to ventral pointing of the vaginal canal, are consequences of eternal youth. But upright posture is a different phenomenon. It cannot be achieved by the “easy” route of retaining a feature already present in juvenile stages. For a baby’s legs are relatively small and weak, while bipedal posture demands enlargement and strengthening of the legs.
By the time we became upright as A. afarensis, the game was largely over, the major alteration of architecture accomplished, the trigger of future change already set. The later enlargement of our brain was anatomically easy. We read our larger brain out of the program of our own growth, by prolonging rapid rates of fetal growth to later times and preserving, as adults, the characteristic proportions of a juvenile primate skull. And we evolved this brain in concert with a host of other neotenic features, all part of a general pattern.
Yet I must end by pulling back and avoiding a fallacy of reasoning—the false equation between magnitude of effect and intensity of cause. As a pure problem in architectural reconstruction, upright posture is far-reaching and fundamental, an enlarged brain superficial and secondary. But the effect of our large brain has far outstripped the relative ease of its construction. Perhaps the most amazing thing of all is a general property of complex systems, our brain prominent among them—their capacity to translate merely quantitative changes in structure into wondrously different qualities of function.
It is now two in the morning and I’m finished. I think I’ll walk over to the refrigerator and get a beer; then I’ll go to sleep. Culture-bound creature that I am, the dream I will have in an hour or so when I’m supine astounds me ever so much more than the stroll I will now perform perpendicular to the floor.
12 | In the Midst of Life…
GREAT STORYTELLERS OFTEN insert bits of humor to relieve the pace of intense drama. Thus, the gravediggers of Hamlet or the courtiers Ping, Pong and Pang of Puccini’s Turandot prepare us for torture and death to follow. Sometimes, however, episodes that now inspire smiles or laughter were not so designed; the passage of time has obliterated their context and invested the words themselves with an unintended humor in our altered world. Such a passage appears in the midst of geology’s most celebrated and serious document—Charles Lyell’s Principles of Geology, published in three volumes between 1830 and 1833. In it, Lyell argues that the great beasts of yore will return to grace our earth anew:
Then might those genera of animals return, of which the memorials are preserved in the ancient rocks of our continents. The huge iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyl might flit again through the umbrageous groves of tree-ferns.
Lyell’s choice of image is striking, but his argument is essential to the major theme of his great work. Lyell wrote the Principles to advance his concept of uniformity, his belief that the earth, after “settling down” from the effects of its initial formation, had stayed pretty much the same—no global catastrophes, no steady progress towards any higher state. The extinction of dinosaurs seemed to pose a challenge to Lyell’s uniformity. Had they not, after all, been replaced by superior mammals? And didn’t this indicate that life’s history had a direction? Lyell responded that the replacement of dinosaurs by mammals was part of a grand, recurring cycle—the “great year”—not a step up the ladder of perfection. Climates cycle and life matches climates. Thus, when the summer of the great year came round again, the cold-blooded reptiles would reappear to rule once more.
In a satirical cartoon drawn by one of Lyell’s colleagues in response to the cited passage about returning ichthyosaurs and pterodactyls, the future Prof. Ichthyosaurus lectures to students on the skull of a strange creature of the last creation.
And yet, for all the fervor of his uniformitarian conviction, Lyell did allow one rather important exception to his vision of an earth marching resolutely in place—the origin of Homo sapiens at the latest instant of geological time. Our arrival, he argued, must be viewed as a discontinuity in the history of our planet: “To pretend that such a step, or rather leap, can be part of a regular series of changes in the animal world, is to strain analogy beyond all reasonable bounds.” To be sure, Lyell tried to soften the blow he had administered to his own system. He argued that the discontinuity reflected an event in the moral sphere alone—an addition to another realm, not a disruption of the continuing steady-state of the purely material world. The human body, after all, could not be viewed as a Rolls Royce among mammals:
When it is said that the human race is of far higher dignity than were any pre-existing beings on the earth, it is the intellectual and moral attributes only of our race, not the animal, which are considered; and it is by no means clear, that the organization of man is such as would confer a decided pre-eminence upon him, if, in place of his reasoning powers, he was merely provided with such instincts as are possessed by the lower animals.
Nonetheless, Lyell’s argument is a premier example of an all too common tendency among natural historians—the erection of a picket fence around their own species. The fence sports a sign: “so far, but no farther.” Again and again, we encounter sweeping visions, encompassing everything from the primordial dust cloud to the chimpanzee. Then, at the very threshold of a comprehensive system, traditional pride and prejudice intervene to secure an exceptional status for one peculiar primate. I discuss another example of the same failure in essay 4—Alfred Russel Wallace’s argument for the special creation of human intelligence, the only imposition by divine power upon an organic world constructed entirely by natural selection. The specific form of the argument varies, but its intent is ever the same—to separate man from nature. Below its main sign, Lyell’s fence proclaims: “the moral order begins here” Wallace’s reads: “natural selection no longer at work.”
Darwin, on the other hand, extended his revolution in thought consistently throughout the entire animal kingdom. Moreover, he explicitly advanced it into the most sensitive areas of human life. Evolution of the human body was upsetting enough, but at least it left the mind potentially inviolate. But Darwin went on. He wrote an entire book to assert that the most refined expressions of human emotion had an animal origin. And if feelings had evolved, could thoughts be far behind?
The picket fence around Homo sapiens rests on several supports; the most important posts embody claims for preparation and transcendence. Humans have not only transcended the ordinary forces of nature, but all that came before was, in some important sense, a preparation for our eventual appearance. Of these two arguments, I regard preparation as by far the more dubious and more expressive of enduring prejudices that we should strive to shed.
Transcendence, in modern guise, states that the history of our peculiar species has been directed by processes that had not operated before on earth. As I argue in essay 7, cultural evolution is our primary innovation. It works by the transmission of skills, knowledge and behavior through learning—a cultural inheritance of acquired characters. This nonbiological process operates in the rapid “Lamarckian” mode, while biological change must plod along by Darwinian steps, glacially slow in comparison. I do not regard this unleashing of Lamarckian processes as a transcendence in the usual sense of overcoming. Biological evolution is neither cancelled nor outmaneuvered. It continues as before and it constrains patterns of culture; but it is too slow to have much impact on the frenetic pace of our changing civilizations.
Preparation, on the other hand, is hubris of a much deeper kind. Transcendence does not compel us to view four billion years of antecedent history as any foreshadowing of our special skills. We may be here by unpredictable good fortune and still embody something new and powerful. But preparation leads
us to trace the germ of our later arrival into all previous ages of an immensely long and complicated history. For a species that has been on earth for about 1/100, 000 of its existence (fifty thousand of nearly five billion years), this is unwarranted self-inflation of the highest order.
Lyell and Wallace both preached a form of preparation; virtually all builders of picket fences have done so. Lyell depicted an earth in steady-state waiting, indeed almost yearning, for the arrival of a conscious being that could understand and appreciate its sublime and uniform design. Wallace, who turned to spiritualism later in life, advanced the more common claim that physical evolution had occurred in order, ultimately, to link pre-existing mind with a body capable of using it:
We, who accept the existence of a spiritual world, can look upon the universe as a grand consistent whole adapted in all its parts to the development of spiritual beings capable of indefinite life and perfectibility. To us, the whole purpose, the only raison d’être of the world—with all its complexities of physical structure, with its grand geological progress, the slow evolution of the vegetable and animal kingdoms, and the ultimate appearance of man—was the development of the human spirit in association with the human body.
I think that all evolutionists would now reject Wallace’s version of the argument for preparation—the foreordination of man in the literal sense. But can there be a legitimate and modern form of the general claim? I believe that such an argument can be constructed, and I also believe that it is the wrong way to view the history of life.
The Panda's Thumb: More Reflections in Natural History Page 12