Quotes From Stephen J. Gould and Others on the Influence of Heterochrony on Human EvolutionThis page contains a collection of excerpts from sources used to support Shift Theory, an alternative theory of human evolution. Click here for an introduction to this new and unique theory of evolution.
"Lynn and Wachowski (1951), Brunst (1955), and Kollross (1961) reviewed earlier work on successful induction to metamorphosis by thyroxin and various organic iodines in facultative paedomorphs of several Ambystoma species. Neotenic A. tigrinum populations from cold Rocky Mountain lakes can be induced to transform simply by increased temperature in the laboratory (Jenkin, 1970). The correlation holds well in nature, since populations form warmer lakes usually do metamorphose; moreover, in Pleistocene A. tigrinum fromthe Kansas-Oklahoma area, glacial specimens are giant and neotenic, whereas interglacial forms are normal and metamorphosing (Tihen, 1955). Snyder (1956) induced metamorphosis in a neotenic A. gracile from cold ponds around Mount Rainier. In all these cases, as for the axolotl of A. mexicanum, metamorphosis is not suppressed by any deficiency of the thyroid gland itself; for it is normal in morphology and potentially capable of secreting its hormone in amounts sufficient for transformation (Dent, 1968). The thyroid gland can function autonomously at a low level, but higher levels require activation by thyrotropin, (also called thyroid-stimulating horone, or TSH) produced by anterior lobe of the pituitary (Etkin, 1968). TSH is itself regulated by thyrotropin releasing factor (TRF) produced by the hypothalamus. Jenkin (1970) suggests tht the failure of A. tigrinum to transform at low temperatures involves a retardation in production of TRF and TSH; these hormones only reach their threshold for action when the temperature is raised. Prahlad and DeLanney (1965) and Norris et al. (1973) showed that axolotls (A. mexicanum) do not release sufficient TSH to activate their own fully potent thyroid hormones." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 299-300)
"In most species [Ambystoma or amphibian] with facultative paedomorphosis, the ease and frequency of transformation can be correlated with geographic, climatic, and ecological gradients. These correlations provide a key for determining the adaptive significance of paedomorphosis and for ascertaining whether it represents a progenetic truncation by precocious maturation or a neotenic retardation of somatic development." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 319)
"Such harsh and unstable environements should favor r selection and progenesis for rapid maturation. Margalef (1949) discusses several brackish-water progenetic subspecies of the marine amphipod Gammarua locusta. He regards the quicker generation time of these progenetic forms as the primary determinant of their ability to colonize such ephemeral waters." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 325)
"organs last to differentiate in normal ontegeny are the first to disappear in these progenetics" (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 330)
"Gene duplication has been granted an important role int he origin of evolutionary novelties (Britten and Davidson, 1971; Markert et al., 1975) because it provides "extra" genetic material freed form the need to function in only one way and therefore available for experimental change. An analogous process must occur in progenesis when genes formerly expressed only in the ancestral adult stage become, in de Beer's term, "unemployed." by precocious maturation of a larva. Thus, into the laboratory mix of unbound morphology and novel combinations, we must throw a set of unusually transformable genes. Stir these three ingrediants together often enough in the history of life and they may congeal every once in a great while to yield a new higher taxon. Darwinian theory has been overly burdened by a rigid insistence upon very slow, continuous, adaptive transformations---an unwarrented extrapolation from directional selection upon single loci in local populations to the origin of new designs. Progenesis is a perfectly orthodox (though unfamiliar) mechanism that permits rapid transition for very little initial genetic input, and that frees morphology to experiment not by releasing selective control altogether (an abandoning Darwinism), but by directing it elsewhere." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 339)
"Bonner (1974, p. 27) has emphasized the usual association of increased size and delayed reproduction. Sexual maturation usually marks the termination (or at least the pronounced slowdown) of both size increase and differentiation." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 341)
"We do, however, note the basic features of delayed maturation associated with K selection: stable, crowded environments, populations near carrying capacity, and intense intraspecific competition. In mammals, this association is often accompanied by neoteny." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 346)
"The general juvenilization of behavior permits a more gregarious society; Geist regards his bighorn rams as more "cowardly" than other sheep and argues that they recognize a heirarchy better in declining more often to fight equals: "It appears that neotenization of sheep leads to a simplification and reduction of 'ritual' in mountain sheep. The mature individual resembles behaviorally the juvenile of the original parent population and not the adult. It appears then that neotenization produces a more 'immature' behavior, although the individuals are more highly evolved" (p.345). Neotenization is clearly linked, as my general hypothesis requires, with intense intraspecific competition and relatively stable environments, both external and internal: "The social behavior of sheep appears to be an adaptation to create and maintain a predictable social environment....Thus rams do not fight for females, but for dominance or, conversely, for a reduction of uncertainty in social status" (p.351)." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 348)
"Fagan has linked the occurence of play in mammals to large brains, slow maturation, and K selection. He argues that r habitats with catastrophic environmental variability require increasingly rapid development and eliminate the protracted juvenile period "whose existence appears to be a necessary condition for play" (1974, p. 855). Among rodents and dasyurids, small, rapidly maturing species do not play, while larger, more slowly maturing species do. Eayrs (1964) has demonstrated that artificially accelerated neural maturation leads to impaired intelligence in adult rats. The extent of play in early ontogeny correlates with levels of sociality in canids: "A striking relationship emerges from comparative developmental studies on canids, namely that the more social canids fight less and play more very early in life than do the less social canids. The delay in the appearance of rank-related aggression may be responsible for the development of a coordinated social group" (Bekoff, in press; see also Bekoff, 1972, p. 424.)" (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 351)
To support the argument that we evolved by retaining juvenile features of our ancestors, Bolk provided lists of similarities between adult humans and juvenile apes: "Our essential somatic properties, i.e. those which distinguish the human body form from that of other Primates, have all one feature in common, viz they are fetal conditions that have become permanent. What is a transitional stage in the ontogensis of other Primates has become a terminal stage in man" (1926a, p. 468). In his most extensive work Bolk (1926c, p. 6) provided an abbreviated list in the following order:
1. Our "flat faced" orthognathy (a phenomenon of complex cause related both to facial reduction and to the retention of juvenile flexure, reflected, for example, in the failure of the sphenoethmoidal angle to open out during ontogeny).
2. Reduction of lack of body hair.
3. Loss of pigmentation in skin, eyes, and hair (Bolk argues that black peoples are born with relatively light skin, while ancestral primates are as dark at birth as ever).
4. The form of the external ear.
5. The epicanthic (or Mongolian) eyefold.
6. The central position of the foramen magnum (it migrates backward during the ontogeny of primates).
7. High relative brain weight.
8. Persistence of the cranial sutures to an advanced age.
9. The labia majora of women.
10. The structure of the hand and foot.
11. The form of the pelvis.
12. The ventrally directed position of the sexual canal in women.
13. Certain variations of the tooth row and cranial sutures.
To this basic list, Bolk added many additional features; other compendia are presented by Montagu (1962), de Beer (1948, 1958), and Keith (1949). The following items follow Montagu's order (pp. 326-327) with some deletions and additions:
14. Absence of brow ridges.
15. Absence of cranial crests.
16. Thinness of skull bones.
17. Position of orbits under cranial cavity.
19. Small teeth.
20. Late eruption of teeth.
21. No rotation of the big toe.
22. Prolonged period of infantile dependency.
23. Prolonged period of growth.
24. Long life span.
25. Large body size (related by Bolk, 1926c, p. 39, to retardation of ossification and retention of fetal growth rates).
These lists from Bolk and Montagu display the extreme variation in type and importance of the basic data presented by leading supporters of human neoteny." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 356-58)
"Standard allometric plots of gestation period, age at sexual maturity, and lifespan versus body size (Fig 62, for example) express this retardation. Primates live longer and mature more slowly than other mammals of comparable body size (Sacher, 1959, p. 128). We reach puberty at about 60 percent of our final body weight, chimpanzees at slightly less than 60 percent. Most laboratory and farm animals reach puberty at about 30 percent of final weight (Bryden, 1968). Some recent studies indicate that retardation begins early in human development and increases continually throughout embryogenesis, at least by comparison with nonprimate mammals. Otis and Brent (1954) compared the appearance of 147 stage marks in the prenatal development of mouse and human. The sequential order is essentially the same in both species, but early stages take two to four times as long to develop in humans while later stages take five to fifteen times as long." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 366)
"In a particularly forceful statement, Zuckerkandl (in press) advances the case for regulatory change as the essential ingredient of evolution: 'If it were possible to take judiciously chosen structural genes and put them together in the right relationship with regulatory elements, it should be possible to make any primate, with some small variations, out of human genes.... Likewise it should be possible to make any crustacean out of the genes of higher Crustacea. This fairy tale may be conservative. It is told only to emphasize that structural genes are building stones which can be used over again for achieving different styles of architecture, and that evolution is mostly the reutilization of essentially constituted genomes. We may not generally have been tempted to take such a view, impressed as we were by the constant structural divergence of genes throughtout evolution. Yet, by and large, this divergence resembles Brownian motion. With notable, all important exceptions, the essential properties of the protein are not changed.' " (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press.pp. 408)
"But such paedomorphic trend could be explained if we could somehow show that it was part and parcel of the overall process of female choice for hunting skills. In fact, this is easier than it seems, because there are good reasons for believing that it would have been young, rather than mature, male hominids who first began to hunt. Experiments in which a colony of monkeys living near a beach were fed first with potatoes and then with rice left on the sand showed that it was a young member of the group who first discovered how to wash the food (as it happens, a female) and that the readiness to adopt this practice varied directly with age, younger individuals being more ready to take it up than older one. In the second place, it seems likely that younger, unmated males, probably associating in loose "all-male groups." would have been much better placed to undertake what must have been cooperative hunts than were older males encumbered with females and young who could not be left unguarded while their "owner" ran off chasing game. If hominid females with a taste for meat were prepared to reward younger, meat-giving hunters with matings, the reproductive success of such younger males would rise relative to that of older ones. This in itself could favor paedomorphosis by way of selection for youth, but it would also have set the scene for the other inevitable consequence of a meat-eating economy, in which greatly increased male parental investment could enable the gradual evolution of more retarded, paedomorphic infants." (Badcock, C. (1991) Evolution and Individual Behavior: An Introduction to Human Sociobiology Oxford: Blackwell.pp. 186)
"Compared with the adults of most other carnivores, adult felids have large eyes, short muzzels, and smooth, round, wide skulls that rest in a relatively erect position on the vertebral column. In all these respects felids resemble juvenile carnivores of other families and are therefore said to be paedomorphic, or juvenile-like in morphology (Fagen &Wiley 1978). These cranial-facial features are best explained as adaptations related to felid predatory tactics *Carmill 1972, 1974, Fagen & Wiley 1978)." (Fagen, Robert (1981) Animal Play Behavior: New York: Viking. pp. 160)
"Attempts to integrate the natural history of play with life-history theory at the level of gross life-history patterns are suggested at best (Fagen 1977, Gould1977), especially since life-history theory as such has little to say about the degree to which environmental variation within the life of the individual should modify that individual's development. Indeed, two opposite speculations exist. Lorenz (1956), Morris (1964), and Geist (1978a) claim that play is most likely in morphologically unspecialized, ecologically opportunistic organisms, "specialists for non-specialization." In life-history terms, these organisms would be r-selected. Fagen (1974a, 1977) and Gould (1977) see play as a behavior or K-selected species, species expected to have large brains, slow development, and intense parental care. The latter point of view is also implicit in Happold's (1976a,b) comparison of play in conilurine rodents whose life-history patterns differ." (Fagen, Robert (1981) Animal Play Behavior: New York: Viking. pp. 256)
"A seeming paradox in the study of form is that certain species lose highly developed adult characteristics of their evolutionary ancestors and exhibit a unique, relatively undifferentiated morphology that actually resembles that of an immature animal. In these so-called paedomorphic species, the head and brain are large relative to the body, the eyes are large, and the muzzel is short, giving the organism a child-like phenotype (Lorenz's Kinderschema).The physical appearance of paedomorphic species is striking, and fully adult members of these species are often mistaken for juveniles at first sight." (Fagen, Robert (1981) Animal Play Behavior: New York: Viking. pp. 257)
"A seeming paradox in the study of form is that certain species lose highly developmed adult characteristics of their evolutionary ancestors and exhibit a unique, relatively undifferentiated morphology that actually resembles that of an immature animal. .... By the principle of allocation, an animal or a plant whose rate of sexual maturation is accelerated by evolution puts its resources into reproduction and accordingly diverts these resources into reproduction and accordingly diverst these resources from growth and differentiation of other body components, whose development then slows down or ceases completely. This process of acceleration of sexual maturation relative tothe development of the rest of the body, known as progenesis, yields a paedomorphic adult adapted to rapid reproduction. It is an opportunist, designed to exploit localized and transient resource patches. It fecundity is high, and its ability to withstand environmental induced stresses, such as extreme temperatures or predation, is correspondingly low. A prediction of life-history theory well supported by available data (Gould 1977) is that such organisms will evolve in severely fluctuating environments in which even adults cannot withstand these perturbations. Here age and size at first reproduction are expected to be relatively "lower and smaller, reproductive effort higher, size of young smaller, and number of young per brood higher, than in constant environments, where the opposite trends should hold" (Stearns 1976 p.42)." (Fagen, Robert (1981) Animal Play Behavior: New York: Viking. pp. 258)
"Cartmill (a972, 1974, 1975) presents a second reason for the paedomorphic morphology of both primates and other mammals (e.g., felids) that rely on both vision and use of the forelimbs for feeding. In these animals muzzel length is reduced because the hands or forepaws take over many functions served by the mouth in other species. Skilled forelimb-eye coordination is crucial, and special perceptual and motor skills are required. The result, again, is paedomorphosis, certainly as a consequence of muzzel (rostral) reduction and possibly also through selection for increased brain size relative to the size of the body." (Fagen, Robert (1981) Animal Play Behavior: New York: Viking. pp. 259)
"An important recent result of theoretical life-history analysis is that not one, but several fundamentally different, evolutionarily stable patterns of development may be selected in the same species and in the same environment (Fagen 1977, Schaffer & Rosenzweig 1977). Does this result mean that only historical factors can explain life-history evolution? If so, life-history theory is in big trouble as a predictive science. Shaffer & Rosenzweig (1977) snatch victory from the jaws of defeat by ingeniously linking life-history theory to a theory of optimal modifiability that had grown up independently of it. They argue that if organisms could switch life-history patterns in response to environmental cues, it would always be possible to pick the life-history peak in the adaptive landscape that had the highest fitness, in effect performing global rather than local optimization." (Fagen, Robert (1981) Animal Play Behavior: New York: Viking. pp. 259-60)
"As usual, Karl Groos (1898) was among the first troublemakers, as illustrated by his much-quoted remark "Animals do not play because they are young, but they have their youth because they must play." (Fagen, Robert (1981) Animal Play Behavior: New York: Viking. pp. 270)
"Morris (1985, p. 53) claims that "the male eye is very slightly bigger than the female, while the female eye shows a higher proportion of white than the male". This suggests that the male eye may have enlarged as an allometric correlate of larger body and head size, but the female eye white may have evolved through slightly more intense sexual selection through male mate choice. Across all human cultures, the eyes are used conspicuously in sexual flirtation, with the alternation between nervous eye contact and coy glancing-away being used to signal sexual interest (Eibl-Eibesfelt, 1989)." (Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 171)
"Humans seem particularly gesticulative and expressive during courtship, and hand gestures play an important role in the dances of many cultures. (particularly in India, South-East Asia, and the Pacific Islands), so again sexual selection could have been important in the evolution of manual gesticulation. The human ability to use mouth and hands in playing musical instruments may represent a modification of motor skills originally used in courtship and foreplay." (Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 181)
"Neotenous Physical Traits in Humans Cranial flexure, head situated over top of spine, forward position of foramen magnum, forward position of occipital condyles, lack of heavy brow ridges, orbits under cranial cavity, flatness of face (orthognathy), contact between sphenoid and ethmoid bones in anterior cranial cavity, retarded closure of cranial sutures, large size of brain, round-headedness (fetal head index 72-82) small jaws, small face, large braincase, small teeth, late eruption of teeth, prominent nose, absence of cranial crests, thiness of skull bones, gracile skeleton, thin nails, nonrotion of big toe, relative hairlessness of body, lack of pigment in some groups, curvature of pelvic axis, lack of pronounced physical differences, anterior position of vagina, downward direction of vagina, persistence of labia majora, persistence of hymen, persistence of penile prepuce." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 23)
"But to return to the "baby face": With the preference for that understandable liking often went a preference for the baby voice. "Betty Boop" epitomized the type, and today there are several well-known show-biz ladies with all the appropriate and much admired fetal endowments --- and more---plus the baby voice." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 37)
"Mongoloid women accordingly tend to be more paedomorphic than women of other groups. Not only do women of Mongoloid origin present more prominent and rounded foreheads, but the bones of the whole skull, and, indeed, the whole skeleton, are more delicately made. Mongoloids generally tend to be shorter, and have larger heads, including larger brains --- 150 cc by volume greater, on the average, than Caucasoids. The face is flatter, the jaws and palate smaller, the nose smaller and flatter at the root (the miscalled "bridge"), and the slight fold of skin over the median part of the eye (the epicanthic fold) is preserved. The body is less hirsute, and there are fetal traits. One result of this is the high frequency of beauty among mongoloid males and females, a beauty of great delicacy (see Table I, page 21). The differential action of neoteny has produced some peculiar effects. For example, among the highly neotenized Japanese the males upper and lower jaws have been reduced in size while the teeth have not. The result has created a disharmony in many males in the form of extreme crowding and malocclusion of the teeth." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 40)
"Despite the fact that the absolute dimensions of the body and brain are smaller in girls than in boys, their intrauterine rate of growth is the same as that of boys, and the weight and size of the brain is about the same. However, in the female, brain weight in adolescent and adult is 2 1/2 percent of body weight as compared with 2 percent in the male. So in relative brain weight the female is neotenously ahead of the male." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 44)
"Bolk drew attention to the findings of Muller that the Malayans of Java have much smaller thyroid glands than do Causasoids, and that the pituitary gland is heavier is Caucasoids than in Malaysians. A similar difference exists between Chinese and Causasoids. Anatomist J. Shellshear also found that the thymus gland, situated at the front of the neck and passing into the upper part of the chest, persists among the Chinese, sometimes into old age. Shellshear regarded the persistence of the thymus as an expression of retarded development, a more general evidence of which he saw in the neotenous "childlike" appearance of the Chinese---a view which both Keith and Bolk were in full agreement. In recent years investigations of the thymus gland have revealed that it secretes a number of hormones, among them a growth-promoting substance called promine. Dr. Albert Szent-Gyorgi has shown that the thymus reaches its peak when growth of the body is fastest. In his studies on bovines, Szent-Gyorgi found that the thymus is somehow connected with youth, and that extracts injected into old animals make them behave as young ones. In this respect, he stated, promine is similar in its action to the "juvenile hormone," also known as the "Peter Pan hormone," found in insects that undergo metamorphosis into butterflies. In addition to producing a large number of lymphocytes, the thymus also produces a variety of hormones and plays an important role in the development of immunologic competence in fetus and child. There is also good evidence that maturation of fetal liver and splenic cells is dependent on an intact thymus. Removal of the thymus in newborn mice stunts their growth; thymus-derived serum (gamma globulin) injected into such animals causes them to grow faster, to maintain a higher weight average, and survive almost twice as long as thymectomized controls who have been given a neutral serum." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 47-8)
"Brachycephaly---that is, broadheadedness---in contrast to dolichocephaly (longheadedness) also appears to be a fetal trait. Up to the sixth fetal month the head tends to be long. The "brachycephalic races," Bolk suggested, are the offspring of longheaded ones. In the course of time, he argued, the transforming processes in some groups of humanity were retarded and, growing weaker, "the initial fetal form of the skull was increasingly retained," until, finally, brachycephaly was established as the persisting form of the head. After the sixth month the fetal cephalic (head) index varies from mesocephalic 75.9 to 79.9, to brachycephalic 80.0 to 84.9. (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 51)
"Humans sweat from almost every part of their body surfaces. Caucasoids and American blacks have about 750 sweat glands to each square inch of skin. African blacks have more sweat glands than other peoples. Mongoloids have about 450 sweat glands per square inch. Since fetus and infant have fewer sweat glands than adults, Mongoloids are more neotenous in this respect than other peoples." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 56)
"It was the hunting way of life that almost certainly resulted in the development in humans of the largest number of sweat glands to be found in any animal, a system of glands capable of producing two quarts of sweat per hour over the body surface!" [what about dance] (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 57)
"The reduction of body hair and number of sweat glands has gone furthest in that major group of humans in which in every other respect the most advanced morphological developments have occured, to wit, in the Mongoloid major group. Both in the density of hair folicles and in the actual number to hairs per square centimeter there has been an appreciable reduction." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 57)
"Humans have come a long way by increasing not only their power to think soundly, but also their power to feel (emotionally) soundly. Sound thinking alone is not enough; sound feeling is as least as necessary. Indeed, emotions are perhaps even more anciently the product of neoteny than is reason." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 69)
"Myelinization---that is, completion ofthe fatty sheaths ofnerve fibers---is not achieved until the fifteenth postnatal month, nor is completion of the tracts associated with the development of many muscular movements---the pyramidal tracts, for example, which pass from brain to spinal cord." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 83)
"Professor J.Z. Young, distinguished English Biologist, has suggested that with the long, extended period of childhood before puberty, the young would not only be more readily restrainable and teachable by their elders, to the cululative benefit of the community, but that some of the physical determinants of aggressiveness and noncooperation might well be eliminated from the population altogether by neoteny. These are important points, for nonaggresive, cooperative behavior is characteristic of fetus, infant, and child. In the evolution of humankind there is little doubt that nonaggressive behavior has been at the highest selective premium. Haldane was among the first to point out that insofar as cooperative behavior makes for the survival of one's descendants and relatives, it is a kind of Darwinian fitness, and may be expected to spread as a result of natural selection. As Professor Young has emphasized, the equipment humans have received from their past history is especially designed to ensure communication and cooperation." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 87)
"Slow developers have often been made miserable in school because they were not doing the work expected at ther age, even though they often later turned out to be much abler than the fast developers. The slow learner may be marching to the beat of a very different drummer. Neoteny may be working rather more to the advantage of slow developer than the fast developer." (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 98)
"The neotenous traits of the child 1. The need for love 2. Friendship 3. Sensitivity 4. The need to think soundly 5. The need to know. 6. The need to learn 7. The need to work 8. The need to organize 9. Curiosity 10. The sense of wonder 11. Playfulness 12. Imagination 13. Creativity 14. Openmindedness 15. Flexibility 16. Experimental-mindedness 17. Explorativeness 18. Resiliency 19. The sense of humor 20. Joyfulness 21. Laughter and tears 22. Optimism 23. Honesty and trust 24. Compassionate intelligence 25. Dance 26. Song" (Montagu, Ashley (1989) Growing Young N.Y.: McGraw Hill pp. 131)
"These formal causes of morphology are as much a determinant of evolutionary pathways as any efficient cause of shaping by natural selection. I did work out correlations of heterochronic modes with ecological contexts, but I also came to understand how coherences of form, expressed in heterochrony, delineated the channels along which organisms push back upon forces of selection to produce evolutionary change." (Gould, Stephen J. (1988) The uses of Heterochrony in: Heterochrony in Evolution: A Multidisciplinary Approach (M.L. McKinney, ed.) pp. 1-13, Plenum Press, New York. p.11)
"Thus, we see that allometric plots alone cannot with certainty distinguish any of the three major heterochronic processes of offset timing, onset timing, or rate change, because they cannot divorce size from age. The heart of the problem is that we are observing trait change as a function of size instead of time when in fact size itself is also a variable function of time." (McKinney, Michael L. (1988) classifying heterochrony: allometry, Size, and time in The uses of Heterochrony in: Heterochrony in Evolution: A Multidisciplinary Approach (M.L. McKinney, ed.) pp. 17-34, Plenum Press, New York. p. 22)
"In summary, heterochrony produces two forms of morphological expression: paedomorphosis, the retention of ancestral juvenile characters by later ontogenetic stages of descendants (Gould, 1977, p. 484), and peramorphosis, new descendant characters produced by additions to the ancestral ontogeny (Alberch et al., 1979). McNamara (1986) had defined three processes for each form of expression as follows. The paedomorphic processes are (1) progenesis, precocious sexual maturation, (2) neoteny, reduced rate of morphological development, and (3) postdisplacement, delayed onset of growth. The peramorphic processes are (1) hypermorphosis, delayed sexual maturation, (2) acceleration, increased rate of morphological development, and (3) predisplacement, earlier onset of growth (see also McKinney, this volume)." (Lindberg, David R. (1988) Heterochrony in gastropods: a neontoglogical view in: Heterochrony in Evolution: A Multidisciplinary Approach (M.L. McKinney, ed.) pp. 17-34, Plenum Press, New York. p. 198)
"More recently, Atchley and colleagues (Riska and Atchley, 1985; Atchley et al., 1984) have further generalized aspects of these arguments and suggested a genetic and hormonal basis for slope variations in brain/body allometry at different taxonomic levels. They emphasize that the hormonal factors controlling early growth (such as insulinlike growth factor II (Hinkz, 1985)] reduce correlated growth in both brain and body size, while other factors [such as insulinlike growth factor I (Hall et al., 1981)] underlie later growth in overall size and do not affect brain growth directly. Additional research on the covariation of patterns of size, shape, and age will be central to our understanding of heterochrony (see Section 10). Atchley (1987) has provided some important theoretical and methodological beginnings for such a project of research." Shea, Brian T. (1988) Heterochrony in Primates in: Heterochrony in Evolution: A Multidisciplinary Approach (M.L. McKinney, ed.) pp. 237-266, Plenum Press, New York. p. 254)
"A significant problem with a neotenic theory of human origins (or later anagenetic transformations within hominid lineages), is that many of our characteristic features are simply not paedomorphic, as Schultz (1969) and others have shown. Among these are almost the entire suite of features associated with bipedal adaptations, perhaps the defining attribute of the hominid clade. .... There is considerable evidence that the adult human skullbase and upper respiratory tract exhibit a number of specializations related to the development of our speech-producing apparatus (e.g. Laitman et al., 1978: Laitman and Crelin, 1980; Laitman and Heimbuch, 1982) [see Lieberman (1984) for discussion]. In this area, human infants resemble adult apes, rather than the converse as predicted by neoteny." Shea, Brian T. (1988) Heterochrony in Primates in: Heterochrony in Evolution: A Multidisciplinary Approach (M.L. McKinney, ed.) pp. 237-266, Plenum Press, New York. p. 259)
Alberch et al. (1979) showed that between ancestor and descendant, development can either be reduced (resulting in paedomorphosis) or increased (resulting in what they termed peramorphosis). Each could be produced by three processes, involving: developmental rate change, change in onset time of development, or change in its offset time. In the case of paedomorphosis, reduced rate is neoteny; delayed onset time is postdisplacement; and earlier offset is progenesis. For the opposing case of peramorphosis, increased rate of acceleration; earlier onset predisplacement; and delayed offset hypermorphoses. These six processes could therefore describe all heterochronic processes. (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 11)
The heterochronic changes just outlined do not have to affect the entire organism (in fact they usually do not). The length of an organ such as horn may grow at a faster rate in the descendant (i.e., be accelerated) while shoulder height might not. This is an example of dissociated heterochrony, whereby different organs (or growth fields) can undergo heterochronies (or remain uneffected) independent of what is going on elsewhere.This process had long been known as "mosaic evolution"... (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 12)
"The 100 trillion or so cells in the human body contain about the same 200 cell types found in frogs, reptiles, and other mammals (Wolpert, 1978), making the main differences ones of cell number and configuation." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 55)
"In mammals, the most important overall mechanism for control of postnatal body growth is somatotropin or growth hormone (GH). This polypeptide is produced by the pituitary gland which is in turn stimulated by hypothalamic production of the neurotransmitter growth hormone releasing factor (GHRF), and inhibited by the production of somatostatin (Harrison et al., 1988). Circulating through the body, GH stimulates the liver and other cells (e.g. bone epiphyses) to produce the insulinlike growth factor I (IGF-I) which acts as a mitogen in a variety of tissues (Nilsson el al., 1986). Interestingly, analyzed in transgenic mice (Borrelli el al., 1989), and is probably quite similar to that of our own. Clearly there is much roomin this chain of events for alteration of growth rate or timing by changing the rate or time of activity of GH and the factors it stimulates. Een more, another potentially changeable link in the chain has recently been discovered in that retinoic acid now appears to control GH production in pituitary cells by acting on GH gene expression (Bedo et al., 1989). This is yet another remarkable case of evolutionary parsimony in that retinoic acid, noted above, is the first identified morphogen, acting in chick limb growth. Rate or timing change in the production sequence of one or more of these peptides is not the only way to produce heterochronies among the tissues. Changes in threshold responses (in target tissue) to the same amount of stimulus would have the same effect. Two well documented examples of body growth rate changes, in rodents and primates, have been discussed by Brian Shea. In the first case (Shea el al., in pres), Snell dward mice growth was compared to that of transgenic giant mice. The dwarf mice produce almost no GH due to a pituitary malfunction, while the transgenic mice (created by microinjection of fusion genes coding for increased production of rat GH and IGF-I) produce abnormally high amounts. Shea found that while the duration of much faster (body acceleration), reaching a much larger adult size (Fig. 3-3). Also of note in Fig. 3-3 is that, at least for long bones, the change truly was "global" with allometric "extension" (allometric hypermorphosis of Chapter 2). In the primate case, Shea (1983,1988) showed that gorillas do not grow for a longer time than chimpanzees, but grow faster to attain their roughly threefold greater adult body size...This is also true for the two chimpanzee species; the pygmy chimpanzee simplys grows slower than the larger form. Much of the size (and size-determined allometric) polymorphism exhibited by dogs (see Wayne, 1986, for definitive study) seems to be due to a similar process. There is a strong correlation between adult size and levels of circulating IGF-I in various dog breeds (Eigenmann et al., 1984).(McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 56)
"IGF-I has little effect on brain growth although it can greatly affect body growth, as discussed above. In contrast, IGF-II which acts earlier, during fetal development, can increase mitosis in both brain and body because brain tissue is responsive to it then. Later, brain responsiveness to it is lost." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 59)
"Probably the most familiar metamorphic heterochronies are those of salamanders, especially the axolotl. In one of the simplest changes, (indefinitely) delayed secretion of the thyroid hormone thyroxine will result in delayed metamorphosis. The result is that larval somatic traits are never lost although sexual maturation and large size will be attained (see Raff and Kaufman, 1983, for review). Similar delays are common in frogs (e.g. Emerson, 1986) and insects (Matsuda, 1987). (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 63)
"At least numerically, the most sucessful group to exploit this type of heterochrony has been arthropods, especially the insects (see French, 1983, for review in developmental context). This success is especially impressive given the relative simplicity of the mechanisms involved. Very generally, two hormones ae the proximate motivators of change, ecdysone and juvenile hormone, produced by the prothoracic glands and corpora allata, respectively. As long as both hormones are circulating, the larval stage is maintained and the organism feeds and grows in that form. When the juvenile hormone is no longer secreted, metamorphosis begins. Ecdysone, acting alone, functions to promote formation of "adult" tissues. Prolonged secretion of the juvenile hormone leads to smaller size and "underdevelopment." A particularly obvious sign of the latter is wing reduction. Depending on degree of prematurity, this can cause changes ranging from microptery through aptery." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 64)
"Insulinlike growth factors (IGF) are small single-chain polypeptides that bear a remarkable amino acid sequence resemblance to human proinsulin (review in Blundel and Humbel, 1980). There are two major forms, I and II, which are closely homologous. As noted earlier, they are especially important in mammalian development. IGF-II is mainly a fetal mitogen active in embryonic tissues and declining during gestation. However, high levels are found in the adult mammalian brain where is apparently continue to stimulate axon outgrowth of existing neurons (Haselbacher et al., 1985) IGF-I shows an opposite overall pattern. Instead of decreasing within a few weeks after birth, as does IGF-II, IGF-I shows increasing levels of tissue concentration. Both forms may operate over large areas, circulating throughout the organism bound to protein carriers, and mammalian brain-body allometries discussed elsewhere. Heterochronic changes in IGF-II would be expected to have a more global effect, on both brain and body, since they would generally act earlier. Also, IGF-II seems especially active in neural tissue. Late-acting IGF-I would affect mainly body size alone because most brain growth is finished early on." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 74)
"Because of the enormous spatial connectance and serial (temporal) continency of the system, minor changes at one scale (the molecular level) can cascade to a variety of amplifications to higher levels, from minor to profound. Thus changes in rate or timing at molecular levels can result in simple, "linear" extrapolations in rate or timing at the tissue level (i.e., allometric), can have saltatory effects, or no effect at all." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 93-4)
"Many cases of environmentally induced phenotypic variation show that it is the organism's developmental program that is affected, induing heterochronies. The balance of effects of intrinsic and extrinsic agents determines the evolutionary potential of heteochrony. When extrinsic agents (e.g., temperture perturbations) affect the developmental program of an organism, the heterochronic changes producing phenotypic variation may not often have great potential for being the springboard for the evolution of a new species. However, it is this very lability, the power of the organism to be externally "manipulated," that may the important target of selection. By responding morphologically or behaviorally to extrinsic agents in a "positive" manner, the species thrives and prospers; it moves with the external changes; it sways in the environmental breeze. If its developmental system was not able to respond in such a way, then the species would snap off the evolutionary tree, as readily as a rigid branch in a gale. As Tomlinson (1987) has observed in plants, "plasticity of organization rather than initial architecture may be the more significant adaptive mechanism." However, from an evolutionary viewpoint, internally generated phenotypic variation, with its basis in heterochrony, is far more potent in the generation of evolutionary novelties." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 102) [use this quote to summarize the novelty of human polymorphism]
"However, other environmental factors can produce dramatic phenotypic effects. Bernays (1986) demostrated how differences n diet in the grass-feeding caterpillar Pseudoaletia unipuncta can have a pronounced effect on head size, changes in diet inducing changes in head allometry. Individuals reared on hard grass developed heads with twice the mass of those fed on soft, artificial diet, even though body masses were the same. Individuals fed on an intermediate diet (soft wheat seedlings) had intermediate head masses (Fig. 4-2). Bernays attributed these allometric differences to an increase in muscular development, which resulted in a significant morphogenetic effect on head size. Size differences, with correlated differences in mandibular strength, directly affect the insect's abiltity to cope with foods of different hardnesses; those with large heads are adaptively more suited to dealing with hard grasses. " (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 105)
"Experimental work has shown that removal of the corpora allata (the endocrine glands that secrete juvenile hormones) of immature male desert locusts, which prevents them from reaching maturity, not only inhibits production of the accelerting pheromone, and thus the potential for induction of maturity in other males, but seems also to result in a pronounced delay in the onset of normal maturity in others (Norris and Pener, 1965). It has been suggested (Butler, 1967) that similar accelerating pheromones may also be present in ladybirds (Coccinellidae), coming into effect when immature adults congregate. The pheromone induces synchronous sexual maturity, allowing mating followed by dispersal." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 110)
"Higher water temperatures in shallow water might have caused premature maturation of the pelagic larvae of the ancestral species of Olenellus. [trilobites] (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 118)
"Consequently, by virtue of earlier maturation induced by higher water temperatures, successive paedomorphs may have been restricted to progressively more oxygenated, shallower waters. Thus, the effect of temperature was not only to induce variable timing of maturation, but also, as a by-product, effective niche partitioning. In the Early Cambrian seas, vacant niche space is likely to have been common, enabling effective ecological isolation, restriction in gene flow, and subsequent allopatric speciation. Extrinsic perturbing factors, particularly temparature, therefore were particularly crucial in initiating heterochronic changes early in the Phanerozoic." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 118)
"As we have already stressed, the evolutionary significance of polymorphisms arising from perturbations to the developmental system may not always lie with the evolutionary potential of such polymorphisms, but from the fact that the plymorphisms themselves are very often the traits that have been selected for. As a consequence, they provide improved fitness for the species." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 125)
"To establish polarity in cases of interspecific heterochrony, ancestor-descendant relationships can only be recognized indirectly. This has proved to be one of the major stumbling blocks in studies of heterochrony for over a century. Indeed Hyatt's neglect in determining polarity with any degree of rigor (see Chapter 1) was largely instrumental in initiating the demise of the Biogenetic Law." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 135)
"Cold temperture, as we have discussed above, is positively correlated (Bergmann's rule) with increases in body size (Davis, 1981; Koch, 1986).(McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 262)
"The evolution of smaller- or larger-sized traits may vary dimorphically and result in a reduction in competition for resources between sexes. Dissociation of head growth from body growth has been recorded in species of North American garter snakes, such as Thamnophis sirtalis parietalis (Shine and Crews, 1988). Because of the action of testicular androgens early in development, inhibition in growth of the head occurs in males, resulting in smaller heads (shorter jaws) than in females. Body sizes are similar in males and females. Androgen administration occurs early in ontogeny and initiates different growth rates between males and females." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 267)
"Also of interest is the phylogenetic information found in Irwin's ontogenetic comparisons. As shown in Fig. 7-2, song development is characterized by a definite sequence of changes. By comparing the ontogenies of certain species to such a sequence, she found, for example, some groups that are clearly paedomorphic, in singing continuous songs and other traits (ratain memicry and have many syllable types). Even the type of heterochrony leading to the paedomorphosis was discernable in some cases. The mimids are apparently neotenic for song development. They never reach the adult stage of crystallized song. Instead the song develops slowly throughout life, ever-changing until they die. They are like perpetual juveniles for song development, continually learning new syllables and dropping old one. In contrast, the reed warblers appear to be progenitic. Their songs crystallize at a relatively earlier stage than most and no new one are learned thereafter." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 281-2)
"The proximate cause of this general hypermorphic progression seems traceabe to simple change in timing of events in the [human] hypothalamus, which initiates the adolescent growth spurt. (Harrison et al., 1988). (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 296)
"Riska and Stchley (1985) have shown that selection acting to increase size in early ontogeny will effect both body and brain while that acting on later ontogeny will affect mainly body size. As discussed in Chapter 3, there is hormonal basis for this in the IGF (insulin growth factor)-II is active during fetal growth and is an effective mitogen on both brain and body tissue (Hintz, 1985). IGF-I is a major mitogen in later ontogeny and is not effective on brain tissue. While much of this is genetically dtermined, Leamy (1988) shows that the prenatal maternal environment is also a key determinant of brain size." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 301)
chart with squirrel monkey on it showing unusually large brain to body weight (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 300)
"A good example of both local enlargement and changed connectivity is Broca's area in the human brain, which is intimately involved in speech and language. Located in the prefrontal cortex, it is greatly enlarged compared to its homologues in other primates. Functionally, it is involved with the face and mouth movements during feeding in apes. Vocalizations are under limbic control. In contrast, our own vocalizations are integrated into neocortical circuitry including of course the enlarged Broca's area (see Lieberman, 1984, for review). (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 304)
"In humans the first (sensorimotor) period occurs during the first 2 years or so of life. The mental learning achievements at this time can be subdivided into six series (see Parker and Gibson, 1979, for fuller discussion): sensorimotor intelligence, space, time, casuality, imitation, and object concept. Each series in turn undergoes a six-stage sequence of development. Of extreme interest to the heterochronic perspective is that when we observe other primates, we find that they go through each of the same six behavioral series, but generally stop short in the development of that series, at an earlier stage than us. Because the truncation is associated with earlier cessation of mental development, they are "progenetic" to us, or more correctly in phylogenetic terms, we are hypermorphic to them. Moreover, the more "advanced" the primate, themore hypermorphic it generally is in serial development (see Fig. 7-7). Thus, studies on prosimians (Jolly, 1964; Parker and Gibson, 1979) showed that lorises and lemurs have reflex and grasping and simple manipulations typical of the first two stages of sensorimotor intelligence but show no evidence ofthe fourth and fifth stages of object concept. Nor in fact did they show any sign to object manipulation abilities appropriate for the last three in these stages of any of the other sensorimotor series (Box, 1984). Monkeys are more advanced in these behaviors but not as much as the apes. The stump-tailed macaque, unlike the prosimians, completed the last stage of the object concept series (an object is shown and then hidden) and got up to the fourth stage of the sensorimotor series (e.g., pulling things apart). However, they did not show tool-use or imitative behavior characteristic of the most advanced stages. For example, they never reached the last two stages on the spatial and causality series such as placing objects in other objects. Nor did they "experiment" with new objects characteristic of the fifth stage of sensorimotor intelligence. Chimpanzees and gorillas also complete the object concept series but go further in the space, causality, and imitation serier (Parker, 1977a,b). As is well known, chimpanzees use tools." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 306-7)
"Here we encounter the truly crucial distinction between neoteny and hypermorphosis: the former is a process of paedomorphosis such that the descendant adult (us) never attains behaviors possessed by the ancestor. In stark contrast, hypermorphosis is a process of peramorphosis such that the descendant adult goes beyond behaviors of the ancestor." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 309)
"For instance, maturation of Piagetian stages occurs much sooner in children with higher IQ's. In turn, mental age shows developmental stages that agree with those found in the brain itself. Considering the delayed Piagetian pattern of our young juveniles compared to ape juveniles (shown above) and all the evidence for hypermorphosis of brain growth is general, it seems reasonable to state that humans are peramorphic in behavior, not paedomorphic." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 311)
"In humans and other primates, many intersexual limb proportions are simple allometric correlates of smaller female body size (Wood, 1986). For example, males have longer legs relative to trunk length simply because males spend a longer time in the prepubertal growth phase, when legs grow relatively faster than the trunk (Harrison et al., 1988). Many other female traits, the more gracile skeleton, narrower joints, and so on, are also simple correlates." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 322)
"Clutton-Brock and Harvey (1980) showed that primate relative brain size decreases as the proportion of leaves in the diet increases. They suggested that a larger memory is needed to exploit fruits, which are relatively more scattered and patchy in occurence than leaves. A similar pattern has recently been found in myomorph rodents (Mann et al., 1988). Folivorous groups average only about two-thirds the brain size of granivorous, insectivorous, or generalist groups of the same body weight. Hence, we may infer that nonfolivory played a role in our ancestors reaching the threshold. An important additional factor has been extensively discussed by Gibson (1986 and references therein) who has shown that primates regularly eating foods that need to be extracted from the environment have relatively larger brains. Such "extractive foods" include such diverse diets as nut-meat, pod seeds, termites, snails, and many others. While they are more trouble to extract, they are generally higher in nutrition and more available throughout the year compared to more readily eaten foods such as berries or leaves. Omnivorous diets of extractive foods lead to the largest relative brain sizes of all, as opposed to those primates which specialize on just one of a few types. Hence, it may be that dietary habits of extractive omnivory and nonfolivory played a major role in the development of brains large enough to reach the threshold." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 324)
"Evidence that complex ontogenies actually do show more rapid change had been found by Stanley (1979, 1990): more "advanced" taxa (e.g., mammals, birds, trilobites, ammonoids) have higher rates of evolution than "simpler" forms (foraminifera, corals, bivalve mollusks). He also found that animals with narrower niche breadths have higher rates." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 343) [Note: this could be evidence of additional selection processes]
"Simple "rate" or "timing" genes are not always involved in many heterochronies. Many gene changes can act to change rate or timing of ontogentic processes at a fine scale to produce the changes just described. For example, alteration of rate or timing of morphogen transmission or reception may occur from change in genes regulating morphogen production, or changes in the "structural" genes determining the physicochemical properties of matrix (or cell membrane) across with the signal is transmitted." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 366)
"Clearly, recapitulation is not the whole story but our view is that evolutionists have overreacted to Haeckel's oversimplifications. Gould (1977) has also noted this, applying the doubly apt metaphor that they have thrown out the baby with the bathwater on this count. The most recent concurrence with this view is that of Swan (1990) who proposes that we use the phrase ontogeny "concords" with phylogeny. This removes the connotations that burden the term "recapitulation." In addition, it conveys the idea that the relationship is not one of exact repetition by ontogeny of phylogeny, but a less precise one of "agreement." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 381)
Neoteny boils down to the ability of a given "species" to sexually reproduce during its infantile period- mating before full development. The famous illustrative species is the Axolotl, but it's true of humans too. Larry Niven wrote a science fiction story based around- what would happen if we lived long enough to turn into whatever it is we're the juvenile form of?
Annunaki, Pak, King Kong... Or something else? Alien hybrid?
Cool story, bro.