August 26, 2009 | Leave a Comment
“The existence of mammary ridges on the embryo concording with ancient synapsids suggests that those ancient animals also had nutrient-supplying ridges on their bodies for which there is no paleontological evidence. On the human embryo, the mammary ridges gradually coalesce and finally resolve into discrete nipples on day 58. This event concords almost exactly with the lowermost Triassic, where the fossils of Cynognathus are found. Discrete mammary glands and a fused secondary palate in the embryo coincide with a fused secondary palate in the fossil record.” (Swan, Lawrence W. (1990) The concordance of ontogeny with phylogeny. Bioscience 40: 380)
Because male humans differentiate from the foundation female at six weeks after conception, might this reflect an ancient emergence of testosterone after estrogen? Might the Pre-Cambrian explosion have had something to do with there being no testosterone to call an end to the party?
“No one, least of all Williams and Kafatos, expect the eventual story to be so simple. But it does seem likely that normal development is controlled by gradually decreasing concentration of a hormone acting primarily at high levels of the regulatory system. This is also an ideal mechanism for the simple and rapid production of heterochronic effects. Any acceleration of adult characters by reduction in the titer of juvenile hormone, or extension of juvenile traits by maintenance of a high titer, represents heterochrony. Since minor alterations in the concentration of a hormone can lead to substantial changes in morphology, heterochrony may play an important role in geographic variation (secretion of juvenile hormone is influenced by temperature and photoperiod, for example), polymorphism (including sex, caste, and phase) and speciation itself.” (Gould, S. J. (1977) Ontogeny and Phylogeny. Cambridge: Belknap Press, pp. 295-6)
Why can’t it be simple? Why not hormones acting at the level of the regulatory system influencing evolution quickly and profoundly? Particularly if those hormone levels are easily influenced by the environment?
“According to this theory [right-shift], the benefits of left hemisphere specialization for speech are induced by a gene (rs+) which impairs right hemisphere function at some sensitive period of cerebral growth. Those carrying one copy of the gene (rs+ -) heterozygotes, about 49% of the population) enjoy the advantages of lateralization of speech to the left hemisphere, with minimal risk to the right hemisphere, while those having two copies (rs+ +) homozygotes, about 32% of the population) risk significant loss of right hemisphere power. Those with no copy of the gene (rs – - homozygotes) are at no risk of hemisphere impairment (right or left), but risk developmental delays of speech and associated language skills due to the inherent difficulty of programming a large brain to serve speech…” (Annett, M. and Manning, M. (1990) Arithmetic and laterality. Neuropsychologia 28 (1): pp. 61-62)
The work of Annett and her colleagues makes clear that there may be a direct connection between Right Shift theory and the cause of autism. There exists a population with a tendency to display no particular handedness. That population produces more maturationally delayed people. Much of the autistic population comes from that group of the maturationally delayed.
“Schacter reported that women exposed in utero to the synthetic estrogen diethylstilbestrol had a handedness distribution on the Edinburgh Handedness Inventory (EHI) that was shifted away from strong right-handedness. Nass et al. found that females with congenital adrenal hyperplasia (CAH), a disorder that results in increased androgen production during gestation, displayed a lesser degree of right-hand preference than unaffected sibling controls on the EHI. However, males with CAH displayed a trend in the opposite direction. More recently, Helleday et al. reported that females with CAH did not differ from controls in either degree of right-hand preference or in dichotic listening asymmetry.” (Moffat, S. D. and Hampson, E. (1996) Salivary testosterone levels in left- and right-handed adults. Neuropsychologia 34 (3): pp. 225)
If sexual hormone levels affect handedness, and if handedness is associated with degrees of cerebral lateralization, with degrees of cerebral lateralization suggesting variations in human evolution with increased lateralization over time, then are hormone levels influencing evolution?
“The finding from these three tests of behavioral laterality suggest that as one side of the brain assumes control of the behavior in these tasks, a smaller CC [corpus callosum] favours increased control by the specialized hemisphere, whereas the larger CC distributes this role more equitably between the two sides. The magnitude of the resulting asymmetry, with better performance for the tasks lateralized to different hemispheres as expected, did not correlate significantly with the CC area. However, the amount of dual task interference was strongly inversely correlated with the CC area in both within-hemisphere (right hand) and between-hemispheres (left hand) conditions. Left hand slowing was significantly higher than in previously reported results, reflecting the increased demands and complexity of the task we used. As the CC area became smaller, the left hand (right hemisphere) performance was more interfered with by the verbal (left hemisphere) activity. This between-hemispheres relationship might reflect activation of systems distributed through the whole cerebrum rather than activation of a single hemisphere with increased task demands.” (Yazgan, M. Y., Wexler, B. E., Kinsbourne, M., Peterson, F., Leckman, J. F. (1995) Functional significance of individual variations in callosal area. Neuropsychologia 33: 775-6)
It seems to me that the corpus callosum is hugely important as regards speech production. A smaller CC may be encouraging speech. Does CC size reflect different degrees of self awareness? How does CC size interface with degrees of cerebral lateralization? For example, in what ways is a person with two cerebral hemispheres the same size with a large CC different from a person that is highly lateralized (right-handed with a far smaller right hemisphere) with a small CC?
“It is satisfying to consider embryos and adults as merely different parts of the slope of a curve subject to natural selection. If biologists cannot agree to Haeckel’s concept, ‘ontogeny recapitulates phylogeny,’ there may be room for a less ringing slogan, ‘ontogeny concords with phylogeny.’” (Swan, Lawrence W. (1990) The concordance of ontogeny with phylogeny. Bioscience 40: 384)
How about we just return to seriously considering that ontogeny and evolution are related? If we note sexual hormones influencing ontogeny, what is so difficult in believing sexual hormones might influence biological evolution? For humans, social structure seems an obvious place to start.
“This broad category [developmental learning disorders] principally includes developmental dyslexia, stuttering, delayed speech, childhood autism, and hyperactivity (CL, p. 83), and Giles de la Tourette syndrome should probably also be included (CL, p. 83). These conditions are linked by having an excess of males, a ‘rather similar pattern of inheritance’ (CL, p. 84), and increased personal and familial left-handedness (CL, p. 84; Bishop, 1983; Boucher, 1977; Colby & Parkinson, 1977; Parac & Coren, 1981).” (McManus, I. C. and Bryden, M. P. (1991) Geschwind’s theory of cerebral lateralization: developing a formal, causal model. Psychological Bulletin 110 (2): 242)
Those conditions that exhibit an excess of males might suggest testosterone (and/or an absence of estrogen) as integral to causation. Add an evolutionary theory that suggests sexual hormones as integral to understanding transformation and we might be getting somewhere.
“Tan (1990a, b, c, 1991a, b, c) has investigated the relation between serum testosterone levels and hand performance extensively. In a 1990 (1990c) study, he reported that serum testosterone levels correlated with right-hand skill on a modified version of the Annett pegboard: right-handed men showed a positive correlation between serum testosterone level and right-hand skill, while right-handed women showed a negative correlation. This would suggest that high testosterone levels are associated with increased right-hand skill in men, but with decreased right-hand skill in women. In a further study, Tan (1990b) found that right-hand superiority on the Tapley and Bryden (1985) dot-filling task increased with increasing serum testosterone level in males, but was unaffected by testosterone in women. Next, Tan (1991a) showed that high testosterone levels in right-handed women were associated with poorer peg-moving performance and less improvement with practice, generally replicating his 1990 (1990a) study. Subsequently, Tan (1991b) found the reverse pattern in male subjects. These findings generally show that increased serum testosterone is associated with increased right-handed performance in men, but not in women. However, Tan (1991c) has also reported that testosterone levels are significantly higher in both women with AD than in those with standard dominance, when the AD group includes left-handers, weak right-handers, and right-handers with a history of familial sinistrality. This latter finding is in general agreement with Tan’s (1990a) report that degree of hand preference is negatively correlated with testosterone level in right-handed women and in right-handed men without a history of familial sinistrality. However, Tan’s studies of hand skill (1990b,c, 1991a,b) suggest that increased levels of testosterone are associated with stronger right-handedness, at least in men. Given the relatively small sample sizes employed in these studies (usually about 45 men and 20 women), the large number of potential confounding factors (eye dominance, footedness, and intellectual ability, to name a few), and the apparent inconsistencies in the effects on preference and on skill, it is unclear how best to interpret these data.” (Bryden M. P., McManus, I. C., Bulman-Fleming, M. B. (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 151)
How about we interpret the data from an evolutionary perspective with complementary opposites as the norm?
“However, Moffat and Hampson (1993) have found that salivary testosterone levels are significantly lower in left-handers than in right-handers. While circulating testosterone levels in adults may not correlate well with fetal exposure to testosterone, these data provide suggestive evidence against the Geschwind hypothesis — one would expect higher, rather than lower, levels of testosterone in left-handers.” (Bryden, M. P., McManus, I. C., Bulman-Fleming, M. B. (1994) Evaluating the Empirical Support for the Geschwind-Behan-Balaburda Model of Cerebral Lateralization. Brain and Cognition 26: pp. 151)
I would expect male lefties to be low testosterone, female lefties to be high testosterone (relative to females). Discussing these issues without regard to sex makes no sense.
“Bryden mentions only studies in his review of research directly linking T and cognitive abilities, namely, a study by Hassler (1991) reporting decreased levels of salivary T in male musical composers and increased levels of T in female musical composers, and a study by Christiansen and Knussman (1987) showing that, in college-age men, T levels correlated positively with spatial relations and negatively with performance on verbal-sequential tasks. The latter study is unusual in that there have been several studies showing better spatial performance in higher-androgen young women and lower-androgen young men, compared to their same sex counterparts (Gouchie & Kimura, 1991; Moffat & Hampson, in preparation; Shute et al., 1983). One interpretation of these findings is that, across sexes, there may be a nonmonotonic relationship between T (or its metabolites, which have not been measured directly in any of these studies) and spatial performance, with optimal performance occurring in the middle range of T values, closer to the lower end of the normal range of T for young adult males.” (Hampson, E. and Moffat, S. D. (1994) Is testosterone related to spatial cognition and hand preference in humans? Brain and Cognition 26: 257)
Another interpretation of these findings might be to explore handedness and sexual hormone levels from an evolutionary perspective with high-testosterone females and low-testosterone males as the musical composing forebears of current society.