Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.

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Heterochrony

heterochrony: bibliographical excerpts


'If ontogeny includes a change in form sufficently abrupt and substantial to warrant the term metamorphosis, then heterochronic effects can be easily diagnosed either by an alteration in timing of metamorphosis itself or by the differential acceleration and retardation of morphological traits with repect to metamorphosis. Moreover, the hormonal basis of several metamorphoses has been established (Jenkin, 1970) and heterochronies can be produced and replicated experimentally (though biologists have rarely discussed this experimental research in the context of relationships between ontogeny and phylogeny). Although Willis (1974, p. 98) reminds us that "much of this scheme is still subject to controversy." the classical interpretation of hormonal control of metamorphosis in holometabolous insects involves the interaction of two substances. Molting is regulated my ecdysone (molting hormone), secreted by the prothoracic glands. The morphological results of any molt, however, are determined by the juvenile hormone produced by the corpus allatum, an endocrine organ lying just behind the brain. This hormone has been synthesized and found to be relatively nonspecific in its effects; juvenile hormone from one insect is generally effective in other species, as are several chemical analogs and mimics. Its chemistry is reasonably well understood, and it has been discussed extensively as an ecologically benign method of insect control. It engenders as many review articles each year as some popular subjects inspire in the primary literature (see Schneiderman, 1972; Truman and Riddiford, 1974; and Willis, 1974, for example). Metamorphosis depends upon the concentration of juvenile hormone. In the presence of a high titer of juvenile hormone, ecdysone will produce a larval molt; with low titers, the larval-pupal transformation is initiated, while an absence of juvenile hormons allows the pupa to molt into an adult. Thus, juvenile hormone allows the pupa to molt into an adult. Thus, juvenile hormone is not an antagonist to the ecdysones, as once believed, but acts with them in normal development. Schneiderman has contrasted the hormonal control of maturation invertebrates and insects: "Maturation in man and other higher vertebrates is promoted by the secretion of maturation hormones, the gonadotropins of the pituitary. The juvenile condidition in man hinges upon the absence of these maturing hormones. The situation in insects depends upon the continued presence of the juvenile hormones, which act on the cells themselves, and prevent them from maturing" (1972, pp. 10-11)." (Gould, S.J. (1977) Ontegeny and Phylogeny. Cambridge: Belknap Press. pp. 294-5)

"A final piece of evidence linking non-right-handedness with immaturity comes from Coren et al. (1986), who obtained questionnaire data on handedness and retrospective reports of age at puberty form a sample of 1180 university students. There was a significant association between left-handedness and late puberty in both sexes." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 152)

"There will also be nongenetic effects. Thus, when the mother is anomalously dominant, she will often be hormonally anomalus in such a way as to favor the production of children with similar dominance patterns. the anomalous hormonal pattern of the mother may reflect her own genetic pattern, but when the responsible genes are not shared with the fetus, then the effects on the fetus will be independent to a great extent of its own genetic endowment. There will be other cases in which the mother was herself exposed to an anomalous hormonal environment, as a result of her own genetic endowment or as a result of nongenetic effects, for example, hormones controlled by maternal genes that she did not share or exogenous stimuli that altered the hormonal atmosphere, such as sex steroids, other drugs, and even the season of birth." ( Geschwind, N. & Galaburda, A.M. (1987) Cerebral Laterization. MIT Press: Cambridge p. 177)

“Nonright-handedness (NRH) has been attributed to hypoxia-induced brain changes in the fetus and associated pregnancy and birth complications (PBCs). Maternal smoking during pregnancy is known to produce prenatal hypoxia for the fetus, which may result in low birth weight and other PBCs. It was hypothesized that maternal smoking during pregnancy results in a leftward shift of handedness in the offspring. This study compared the distribution of handedness in the offspring of mothers who did and did not smoke cigarettes during pregnancy. Information on maternal smoking, handedness, and PBCs was analyzed for 803 university students. There was a significant shift to the left in the distribution of handedness scores for the offspring of smoking mothers (N = 216), as compared to those of nonsmoking mothers (N = 587). Offspring of smoking mothers also reported significantly more PBCs. Results are consistent with the hypothesis that NRH is associated with pathological neurodevelopment.” (Bakan P (1991) Handedness and maternal smoking during pregnancy. Int J Neurosci 56 (1-4): 161)

"Yalom et al. (1973) studied 20 16-year old boys of diabetic mothers, who had received estrogen or progesterone during pregnancy. These boys showed less heterosexuality and less masculinity than 20 control boys. Netley and Rovet (1982) showed that among 33 males with 47,XXY syndrome, 24% were nonrighthanded, compared to 10% of a control group. ... In the present study, as well as in Lindesay (1987), only homosexual men were studied. In Rosenstein and Bigler (1987) and McCormick et al. (1990), both men and women were studied, and in the latter study, a significant increase in lefthandedness (or rather nonrighthandedness) was obtained for women. This was assumed to be related to higher-than-normal levels of prenatal testosterone levels. In their results, the increase in lefthandedness in homosexual women (which have lower occurrence than men in the general population) is much larger than that of homosexual men. It is, therefore, fair to assume that the increase in testosterone, believed to cause both lefthandedness and homosexuality in women, will give a more pronounced effect in women than in men." (Gotestam, K.O., Coates,T.J., Ekstrand, M. (1992) Handedness, dyslexia and twinning inhomosexual men. International Journal Neuroscience 63 (3-4): 184)

“Musical composers, instrumentalists, and painters were compared with nonmusicians from a student and from an nonstudent population on testosterone levels in saliva. This steroid served as a marker for physiological androgyny. The ANOVA showed a significant group by sex interaction. Male composers attained significantly lower mean testosterone values than male instrumentalists and male nonmusicians; female composers had significantly higher mean testosterone values than female instrumentalists and female nonmusicians. Painters of both sexes did not differ significantly from controls. Spatial ability was assessed in the five groups. Significant differences on spatial test performance were not reflected in differences on salivary testosterone. Our results showed that musical composers of both sexes were physiologically highly androgynous. Creative musical behavior was associated with testosterone levels that minimized sex differences.” (Hassler M (1991)Testosterone and artistic talents. Int J Neurosci 56 (1-4): 25)

"Our data obtained from adults allow for the hypothesis that an optimal T range may exist for the expression of creative musical behavior. This range may be at the bottom of the normal male T range and at the top of the normal female T range. T may be one component of a complex biological system contributing to musical creativity. ... The limitation given by the measurement of only one hormone must be overcome in future research in order to get more information about the hormone/behavior relationship in adolescence. For instance, E2 surges in girls occurring at age 13 should be taken into account. In women, creative musical capacities may emerge, or emerge again, in adulthood, when female hormone levels have reached the adult state. Though we have no indication of a reemergence of musical creativity in our adolescent girls, who have not yet reached the adulthood, there are clues from our study with adults (Hassler et al., 1990). Half our adult female composers began composing after puberty. General musical ability as measured with the Wing test showed some fluctuation during the course of adolescence. In 1987, when children had reached a mean age of 15.5 yr, the only significant correlation between T and Wing test scores was in girls and was positive. No other clue was found to indicate that general musical ability was related to T levels." (Hassler, M. (1992) Creative musical behavior and sex hormones: musical talent and spatial ability in the two sexes. Psychoneuroendocrinology 17 (1): pp. 66)

"Although numerous researchers have hypothesized a biological factor in the etiology of homosexuality, there is a lack of empirical evidence. Previous investigations did not focus on behavioral functions of the brain. Using neuropsychological testing, we found an increased incidence of left-hand preference (defined as non-consistent right-hand preference) in a group of 32 homosexual women. A trend in the same direction was found in a group of 38 homosexual men. These results suggest that homosexual orientation has a neurobiological component possibly related to hemispheric functional asymmetry. The results are consistent with previous reports that (1) prenatal neuroendocrine events are a factor in the development of human sexual orientation and functional brain asymmetries, and (2) the mechanisms associated with homosexual orientation and related neuropsychological characteristics are different between the sexes, i.e. elevated levels of prenatal sex hormones in women and decreased levels in men." (McCormick CM, Witelson SF, Kingstone E (1990) Left-handedness in homosexual men and women. Neuroendocrine implications. Psychoneuroendocrinology 15: 69)

"The rate of left-handedness in male patients was higher than that for female patients, and also 1.6 times higher than the corresponding rate for male control subjects, although this difference was not significant .... The rate of left-handedness in female patients was 2.2 times greater than the corresponding rate in female control subjects, and this difference was significant. (x2=5.31, d.f.=1, P<0.05). .... The number of left-handers in certain categories of patients is shown in Table 3. There were thus 313 patients attending the clinic, and among this group there were 47 (15.02%) left-handers. ... Although the rate of left-handedness was 1.8 times as great in the former as in the latter, this result was not significant.....However, the rate of left-handedness was 2.1 times as great in the allergic patients as in the controls, and this result was highly significant. (x2= 10.20, d.f.=1, P<0.005)." (Smith, J. (1987) Left-handedness: Its association with allergic disease. Neuropsychologia 25: 668-70

"Specifically, there are reduced proportions of strong right-handedness among alcoholics (Bakan, 1973; London, 1986; Nasrallah, Keelor, & McCalley-Whitter, 1983). In some reports the effects are quite large." (Coren, S. & Halpern, D.F. (1991) Left-handedness: A marker for decreased survival fitness. Psychological Bulletin 109: 102)

"The fundamental pattern of the brain thus appears to be asymmetrical, with the same pattern of asymmetries found in most adults. There are, however, influences in pregnancy that tend to diminish the extent of left-sided predominance, at least in the regions involved in handedness and language, and thus secondarily to result in larger regions on the right side. As noted earlier, our hypothesis is that some factor related to male sex, perhaps testosterone or some closely related factor, is the most likely candidate. The net effect of these intrauterine influences is to produce a shift from left predominance to symmetry, and in a smaller number of cases to modest right predominance." ( Geschwind, N. & Galaburda, A.M. (1987) Cerebral Laterization. MIT Press: Cambridge p. 46)

"Furthermore, in the neonate male rat, concentration of estrogen receptors is greater in the left than in the right cortex (Diamond, 1991). Testosterone has also been implicated in influencing the size of specific areas of the corpus callosum (Fitch, Berrebi, Cowell, Schrott, & Denenberg, 1990). More recently, Witelson (1991) has shown that lower levels of androgens lead to less axon elimination in specific areas of the brain, apparently resulting in specific patterns of functional asymmetry. She found that left-handed men had a larger isthmus of the corpus callosum than right-handed men." (Forget, H. & Cohen, H. (1994) Life after Birth: The Influence of Steroid Hormones on Cerebral Structure and Function is Not Fixed Prenatally. Brain and Cognition 26: 244)

"Environmental factors can be an important source of nongenetic influences on laterality. Since the effect of a gene is to play a role in some form of chemical reaction, it is not surprising that genetic determination is not absolute. Every chemical reaction can be modified by alterations in pressure, temperature, pH, light, the presence of other substances, the availability of chemical precursors, and the rate at which products are removed. With growing sophistication of molecular genetics, it has become increasingly clear that nongenetic effects can play a powerful role; methylation, for example, has been shown to suppress expression of many genes. We will now consider some of the random effects that might modify lateralization. One implication of our hypothesis is that even if the genetic endowment of any particular fetus were known precisely, it would not be possible to make predictions concerning the distribution in a population basis. One of the reasons for this relative freedom from genetic determination is that if hormones do play a role in determining laterality, then the effects of testosterone or related substances on the developing brain will be modified by factors not under the control of the fetal genes. Androgens are produced not only by fetal testes and the placenta but also by the maternal ovaries, adrenals, and nonglandular tissues. The fetus can be influenced by the actions of many of the unshared maternal genes. It is reasonable to expect that if a fertilized ovum were transplanted into the uterus of an unrelated female, the final pattern of the brain would be quite different, because the brain would develop in an environment of hormones and other substances that would certainly differ in many respects. It might therefore be reasonable to take a different approach than usual to the genetics of many condiditons. One should perhaps consider, not the genes carried by the offspring alone, but rather the genes of that organism existing or active only for the nine months of pregnancy; in other words, one should consider the mother and the fetus as a unit. This unit contains three groups of different genes: one paternal set present in the fetus, one maternal set present in the mother, and another maternal set present both in the mother and in the fetus. The situation is even more complex when dizygotic twins are involved, since the maternal-fetal unit will contain another group of paternal genes. The effects of substances produced by the mother will, however, be diminished by the capacity of the placenta to act as a barrier to some maternal hormones. The fetus is protected to a great extent form maternal testosterone, which is converted to estradiol by placental aromatase. Dihydrotestosterone, which is not aromatized and therefore crosses the placenta, is, however, usually present in the mother at much lower levels than testosterone. The protection from maternal testosterone is not complete, since offspring do show signs of masculinization when mothers are exposed to this hormone. In addition, progesterone administered to the mother may masculinize female fetuses. It is clear that the placental barrier is far from complete. Furthermore, it is likely that there are individual variations in the aromatizing capacity of the placenta. It is conceivable that some maternal genes not shared by the offspring have greater effects on females fetuses. Thus, the testosterone to which female fetuses are exposed comes predominantly from maternal tissues, whereas males produce it themselves in high quantities. In the study of Nichols and Chen (1981) sex hormones given to mothers were associated with a higher rate of hyperactivity in female offspring than in males." ( Geschwind, N. & Galaburda, A.M. (1987) Cerebral Laterization. MIT Press: Cambridge pp. 133-134)

"It is undisputed that in the fetal environment, testosterone can have profound effects on neonatal brain development. In animal studies, when hormonal levels have been controlled experimentally, many developmental processes are affected by exposure to testosterone, and thus lead to anatomical differences between males and females (i.e., sexual dimorphism.). Most of these are mediated by estrogen, from which testosterone is converted by normal enzymatic operations involving aromatase, a catalytic enzyme found within the brain and expressed quite early in development (McEwen, Lieberburg, Chaptal, & Krey, 1977). Ironically, the role of the female hormone estrogen in this process occurs to a significant extent only in males and not females, since the estrogen secreted by the developing ovaries never reaches the brain. High levels of alpha fetoprotein in the neonatal serum bind these estrogens and prevent their access to the central nervous system. By contrast, estrogen has an effect on brain development in males, since conversion of testosterone to estrogen takes place within the developing brain itself." (Small S.L., Hoffman G.E. (1994) Neuroanatomical lateralization of language: sexual dimorphism and the ethology of neural computation. Brain and Cognition 26: 307-8)

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