4.Late closure of the skull sutures and other marks of delayed calcification of the skeleton. Babies have a large “soft spot,” and the sutures between our skull bones do not fully close until well after adulthood. Thus, our brain can continue its pronounced postnatal expansion. (In most other mammals, the brain is nearly complete at birth and the skull is fully ossified.) A leading primate anatomist has remarked: “Though man grows in utero to larger sizes than any other primate, his skeletal maturation has progressed less at birth than in any monkey or ape for which relevant information has become available.” Only in humans are the ends of long bones and digits still entirely cartilaginous at birth.
5.Ventral pointing of the vaginal canal in women. We copulate most comfortably face to face because we are built to do it that way. The vaginal canal also points forward in mammalian embryos, but it rotates back in adults, and males mount from the rear.
6.Our strong, unrotated, nonopposable big toe. The big toe of most primates begins as ours, in conjunction with its neighbors, but it rotates to the side and opposes the others for efficient grasping. By retaining a juvenile trait to yield a stronger foot for walking, our upright posture is enhanced.
Bolk’s list was impressive (this is only a small part of it), but he tied it to a theory that doomed his observations to oblivion and gave Aldous Huxley his anti-Faustian metaphor. Bolk proposed that we evolved by an alteration in our hormonal balance that delayed development as a whole. He wrote:
If I wished to express the basic principle of my ideas in a somewhat strongly worded sentence, I would say that man, in his bodily development, is a primate fetus that has become sexually mature.
Or, to quote Aldous Huxley again:
There’s a kind of glandular equilibrium.… Then a mutation comes along and knocks it sideways. You get a new equilibrium that happens to retard the developmental rate. You grow up; but you do it so slowly that you’re dead before you’ve stopped being like your great-great-grandfather’s fetus.
Bolk did not shrink from the obvious implication. If we owe all our distinctive features to a hormonal brake on development, then that brake might be easily released: “You will note,” writes Bolk, “that a number of what we might call pithecoid features dwell within us in latent condition, waiting only for the falling away of the retarding forces to become active again.”
What a tenuous position for the crown of creation! An ape arrested in its development, holding the spark of divinity only through a chemical brake placed upon its glandular development.
Bolk’s mechanism never won much support, but it began to grow in absurdity as modern Darwinian theory became established during the 1930s. How could a simple hormonal change produce such a complicated morphological response? Not all our features are retarded (long legs, for example), and those that are display varying degrees of delay. Organs evolve separately in response to differing adaptive requirements—a concept we call mosaic evolution. Unfortunately, Bolk’s excellent observations were buried under the barrage of justified criticism for his fanciful mechanism. The theory of human neoteny is now usually relegated to a paragraph or two in anthropology textbooks. Yet I believe that it is fundamentally correct; an essential, if not dominant, theme in human evolution. But how can we rescue Bolk’s observations from his theory?
If we must base our argument upon the list of neotenic features, then we are lost. The concept of mosaic evolution dictates that organs will evolve in different ways to meet varying selective pressures. Supporters of neoteny list their features, opponents tote theirs, and a stalemate is quickly reached. Who is to say which features are “more fundamental”? For example, one recent supporter of neoteny has written: “Most animals show retardation in some features, acceleration in others.… On balance, I think that in man, as compared with other primates, the slowing down far out-weighs the speeding up.” But a detractor proclaims: “the neotenic characters … are secondary consequences of the nonneotenic key characters.” The validation of neoteny as fundamental requires more than an impressive list of retarded characters; it must be justified as an expected result of processes acting in human evolution.
The notion of neoteny achieved its initial fame as a way of opposing the theory of recapitulation, a dominant idea in late nineteenth century biology. The theory of recapitulation proclaimed that animals repeat the adult stages of their ancestors during their own embryonic and postnatal growth—ontogeny recapitulates phylogeny, in that mystical phrase we all learned in high school biology. (Recapitulationists argued that our embryonic gill slits represented the adult fish from which we descended.) If recapitulation were generally true—which it is not—then features would have to be accelerated during evolutionary history, for adult characters of ancestors can become the juvenile stages of descendants only if their development is speeded up. But neotenic characters are retarded since juvenile features of ancestors are delayed to appear in the adult stages of descendants. Thus, there is a general correspondence between accelerated development and recapitulation on the one hand and delayed development and neoteny on the other. If we can demonstrate a general delay of development in human evolution, then neoteny in key features becomes an expectation, not just an empirical tabulation.
I do not think that retardation can be denied as a basic event in human evolution. First of all, primates in general are retarded with respect to most other mammals. They live longer and mature more slowly than other mammals of comparable body size. The trend continues throughout the evolution of primates. Apes are generally larger, mature more slowly, and live longer than monkeys and prosimians. The course and tempo of our lives has slowed up even more dramatically. Our gestation period is only slightly longer than that of apes, but our babies are born much heavier—presumably because we retain our rapid fetal growth rates. I have already commented on the delay in ossification of our bones. Our teeth erupt later, we mature later, and we live longer. Many of our systems continue to grow long after comparable organs have ceased in other primates. At birth, the brain of a rhesus monkey is 65 percent of its final size, a chimpanzee’s is 40.5 percent, but we attain only 23 percent. Chimps and gorillas reach 70 percent of final brain size early in their first year; we do not attain this value until early in our third year. W. M. Krogman, our leading expert in child growth, has written: “Man has absolutely the most protracted period of infancy, childhood and juvenility of all forms of life, i.e., he is a neotenous or long-growing animal. Nearly thirty percent of his entire life-span is devoted to growing.”
This slowdown of our development does not guarantee that we will retain juvenile proportions as adults. But since neoteny and retarded development are generally linked, retardation does provide a mechanism for the easy retention of any juvenile feature that suits the adult life style of descendants. In fact, juvenile features are a storehouse of potential adaptations for descendants, and they can be utilized easily if development is strongly retarded in time (viz, the nonopposable big toe and small face of fetal primates—as discussed earlier). In our case, the “availability” of juvenile features clearly controlled the pathway to many of our distinctive adaptations.
But what is the adaptive significance of retarded development itself? The answer to this question probably lies in our social evolution. We are preeminently a learning animal. We are not particularly strong, swift, or well designed; we do not reproduce rapidly. Our advantage lies in our brain, with its remarkable capacity for learning by experience. To enhance our learning, we have lengthened our childhood by delaying sexual maturation with its adolescent yearning for independence. Our children are tied for longer periods to their parents, thus increasing their own time of learning and strengthening family ties as well.
This argument is an old one, but it wears well. John Locke (1689) praised our lengthy childhood for keeping parents together: “Wherein one cannot but admire the wisdom of the great Creator who … hath made it necessary that society of man and wife should be more lasting than that of male and female among
other creatures, that so their industry might be encouraged, and their interest better united, to make provision and lay up goods for their common issue.” But Alexander Pope (1735) said it even better, and in heroic couplets to boot:
The beast and bird their common charge attend
The mothers nurse it, and the sires defend
The young dismissed, to wander earth and air,
There stops the instinct, and there ends the care.
A longer care man’s helpless kind demands,
That longer care contracts more lasting bands.
8 | Human Babies as Embryos
MEL ALLEN, that irrepressible emcee of Yankee baseball during my youth,3 finally aroused my displeasure by overenthusiastic endorsement of his sponsors. I never balked when he referred to home runs as “Ballantine blasts,” but my patience was strained one afternoon when DiMaggio missed the left field foul pole by an inch and Allen exclaimed: “Foul by the ash on a White Owl cigar.” I hope that I won’t inspire any similar displeasure by confessing that I read and enjoy Natural History and that I even sometimes get an idea for an essay from its articles.
In the November 1975 issue, my friend Bob Martin wrote a piece on strategies of reproduction in primates. He focused upon the work of one of my favorite scientists—the idiosyncratic Swiss zoologist Adolf Portmann. In his voluminous studies, Portmann has identified two basic patterns in the reproductive strategies of mammals. Some mammals, usually designated by us as “primitive,” have brief gestations and give birth to large litters of poorly developed young (tiny, hairless, helpless, and with unopened eyes and ears). Life-spans are short, brains small (relative to body size), and social behavior not well developed. Portmann refers to this pattern as altricial. On the other hand, many “advanced” mammals have long gestations, long life-spans, big brains, complex social behavior, and give birth to a few, well-developed babies capable, at least in part, of fending for themselves at birth. These traits mark the precocial mammals. In Portmann’s vision of evolution as a process leading inexorably upward to greater spiritual development, the altricial pattern is primitive and preparatory to the higher precocial type that evolves along with enlarged brains. Most English-speaking evolutionists would reject this interpretation and link the basic patterns to immediate requirements of different modes of life. (I often exploit these essays to vent my own prejudice against equating evolution with “progress.”) The altricial pattern, Martin argues, seems to correlate with marginal, fluctuating, and unstable environments in which animals do best by making as many offspring as they possibly can—so that some can weather the harshness and uncertainty of resources. The precocial pattern fits better with stable, tropical environments. Here, with more predictable resources, animals can invest their limited energy in a few, well-developed offspring.
Whatever the explanation, no one will deny that primates are the archetypical precocial mammals. Relative to body sizes, brains are biggest and gestation times and life-spans are longest among mammals. Litter size, in most cases, has been reduced to the absolute minimum of one. Babies are well developed and capable at birth. However, although Martin doesn’t mention it, we encounter one obviously glaring and embarrassing exception—namely us. We share most of the precocial characters with our primate cousins—long life, large brains, and small litters. But our babies are as helpless and undeveloped at birth as those of most altricial mammals. In fact, Portmann himself refers to human babies as “secondarily altricial.” Why did this most precocial of all species in some traits (notably the brain) evolve a baby far less developed and more helpless than that of its primate ancestors?
I will propose an answer to this question that is bound to strike most readers as patently absurd: Human babies are born as embryos, and embryos they remain for about the first nine months of life. If women gave birth when they “should”—after a gestation of about a year and a half—our babies would share the standard precocial features of other primates. This is Portmann’s position, developed in a series of German articles during the 1940s and essentially unknown in this country. Ashley Montagu reached the same conclusion independently in a paper published in the Journal of the American Medical Association in October 1961. Oxford psychologist R. E. Passingham championed it in a piece published late in 1975 in the technical journal Brain, Behavior and Evolution. I also cast my lot with this select group in regarding the argument as basically correct.
The initial impression that such an argument can only be arrant nonsense arises from the length of human gestation. Gorillas and chimps may not be far behind, but human gestation is still the longest among primates. How then can I claim that human neonates are embryos because they are born (in some sense) too soon? The answer is that planetary days may not provide an appropriate measure of time in all biological calculations. Some questions can only be treated properly when time is measured relatively in terms of an animal’s own metabolism or developmental rate. We know, for example, that mammalian life-spans vary from a few weeks to more than a century. But is this a “real” distinction in terms of a mammal’s own perception of time and rate? Does a rat really live “less” than an elephant? Laws of scaling dictate that small, warm-blooded animals live at a faster pace than larger relatives (see essays 21 and 22). The heart beats more rapidly and metabolism proceeds at a greatly elevated rate. In fact, for several criteria of relative time, all mammals live about the same amount. All, for example, breathe about the same number of times during their lives (small, short-lived mammals breathe more rapidly than larger, slow metabolizers).
In astronomical days, human gestation is long, but relative to human developmental rates, it is truncated and abbreviated. In the previous essay, I argued that a (if not the) major feature of human evolution has been the marked slowing up of our development. Our brains grow more slowly and for a longer time than those of other primates, our bones ossify much later, and the period of our childhood is greatly extended. In fact, we never reach the levels of development attained by most primates. Human adults retain, in several important respects, the juvenile traits of ancestral primates—an evolutionary phenomenon called neoteny.
Compared with other primates, we grow and develop at a snail’s pace; yet our gestation period is but a few days longer than that of gorillas and chimpanzees. Relative to our own developmental rate, our gestation has been markedly shortened. If length of gestation had slowed down as much as the rest of our growth and development, human babies would be born anywhere from seven to eight months (Passingham’s estimate) to a year (Portmann and Ashley Montagu’s estimate) after the nine months actually spent in utero.
But am I not indulging in mere metaphor or trick of phrase in designating the human baby as “still an embryo”? I have just raised two of my own past this tender age, and have experienced all the joy and mystery of their mental and physical development—things that could never happen in a dark, confining womb. Still, I side with Portmann when I consider the data on their physical growth, for during their first year, human babies share the growth patterns of primate and mammalian fetuses, not of other primate babies. (The identification of certain growth patterns as either fetal or postnatal is not arbitrary. Postnatal development is not a mere prolongation of fetal tendencies; birth is a time of marked discontinuity in many features.) Human neonates, for example, have not yet ossified the ends of limb bones or fingers; ossification centers are usually entirely absent in the finger bones of newborn humans. This level of ossification corresponds to the eighteenth fetal week of macaque monkeys. When macaques are born at twenty-four weeks, their limb bones are ossified to an extent not reached by humans until years after birth. More crucially, our brains continue to grow at rapid, fetal rates after birth. The brains of many mammals are essentially fully formed at birth. Other primates extend brain development into early postnatal growth. The brain of a human baby is only one-fourth its final size at birth. Passingham writes: “Man’s brain does not reach the proportion found for the chimpanzee at birth until
around six months after birth. This time corresponds quite well with the time at which man would be expected to be born if his gestation period were as high a proportion of his development and life-span as it is in apes.”
A. H. Schultz, one of the greatest primate anatomists of the century, summarized his comparative study of growth in primates by stating: “It is evident that human ontogeny is not unique in regard to the duration of life in utero, but that it has become highly specialized in the striking postponement of the completion of growth and of the onset of senility.”
But why are human babies born before their time? Why has evolution extended our general development so greatly, but held our gestation time in check, thereby giving us an essentially embryonic baby? Why was gestation not equally prolonged with the rest of development? In Portmann’s spiritual view of evolution, this precocious birth must be a function of mental requirements. He argues that humans, as learning animals, need to leave the dark, unchallenging womb to gain access, as flexible embryos, to the rich extrauterine environment of sights, smells, sounds, and touches.
But I believe (along with Ashley Montagu and Passingham) that a more important reason lies in a consideration that Portmann dismisses contemptuously as coarsely mechanical and materialistic. From what I have seen (although I cannot know for sure), human birth is a joyful experience when properly rescued from arrogant male physicians who seem to want total control over a process they cannot experience. Nonetheless, I do not think it can be denied that human birth is difficult compared with that of most other mammals. To put it rather grossly, it’s a tight squeeze. We know that female primates can die in attempted childbirth when fetal heads are too large to pass through the pelvic canal. A. H. Schultz illustrates the stillborn fetus of a hamadryas baboon and the pelvic canal of its dead mother; the embryo’s head is a good deal larger than the canal. Schultz concludes that fetal size is near its limit in this species: “While selection undoubtedly tends to favor large diameters of the female pelvis, it must also act against any prolongation of gestation or at least against unduly large newborns.”
Ever Since Darwin: Reflections in Natural History Page 6