Neuropsychology and Evolutionary Theory
Marian Annett (Annett & Manning, 1990; Annett & Kilshaw, 1984) has hypothesized a balanced polymorphism in dyslexia that neatly fits with our theory of biological and cultural evolution. Shift theory predicts a specific structure of health and disease in humans. Heterochronic theory's descriptions of the operation of relative rate and timing changes of development and maturation are directly transposable to Annett's (1985) right-shift theory. It fact, superimposing Gould's (1977) clock model of heterochronic evolution directly over Annett's (1985) right-shift graph reveals the relationship between evolutionary time, the etiology of cerebral asymmetry, and neurological disease.
|Right-shift theory (Annett, 1985) states that there is a gene (+) that predisposes most people for language facility. Annett noted that there is a difference in the distribution of handedness between human and animal populations characterized by a right-shift in human beings. This right-shift makes clear that not all humans are equally well disposed to language use. People with a (- -) genotype (18-19 % of the population) evidence no predilection to specific handedness or cerebral asymmetry and so achieve a left- or right-handedness close to random. People with (+ +) (32%), or a strong predilection to right handedness and asymmetrical lateralization, are highly disposed to language usage, but at the expense of right hemispheric strengths. Annett believes the mixture of both genetic propensities, (- +), offers the advantages evidenced by 49% of the population belonging in this category. She characterizes these advantages as a balanced polymorphism (Annett 1984, 1990) when applied to overall strength in language facility. It is important to understand that changes from population to population are gradual, not clearly demarcated and that movements across this arc or spectrum from (- -) to (+ +) are incremental.
Heterochronic principles describe the effects of relative rates of development and maturation on species evolution. We believe these concepts, when paired with zygote selection and uterine selection, can be used to describe specific developmental trajectories in individuals vulnerable to neurological conditions. We believe that transitional model 1.0 is a solid start in understanding the etiology of neurological disease. We are accumulating evidence from many studies which lead us to believe that other diseases can be explained with these principles. Geschwind and Galaburda's (1987) observations form the foundation for the patterns we have discerned. They noted the connections between handedness; immune and auto-immune disorders; and conditions associated with developmental delay. The following patterns have been particularly noteworthy.
1) High testosterone (T) females (the older genotype) are at the (- -) end of the developmental spectrum and are developmentally accelerated compared to the low T females (+ +) at the developmentally delayed end of the spectrum. Females at the right end are markedly more neotenous and/or hypermorphic than left end females. At the left end, relative to the females at the right end, the females are more left-handed and ambidextrous.
2) Low T males (the older genotype) are at the (- -) end of the developmental spectrum and are developmentally delayed compared to the high T males at the (+ +) other end. Males, perhaps, exhibit more variation than females (Darwin, 1871) in the arc from (- -) to (+ +). At the left end, relative to the males at the right end, the males have bigger brains (Annett, 1991), more symmetrical cerebral hemispheres, larger corpus callosums (Witelson, 1991a, 1991b,1989, 1985), lower T (Tan, 1990) slower metabolic rates (Badcock, 1991), a less acute sense of the passing of time, increased left handedness and ambidextrousness, and increased speed (Annett, 1984), agility and coordination. Males at the left end are markedly more neotenous (Coren, 1991) and/or hypermorphic than males at the right end.
3) Females with high T give birth to females with high T and males with low T. Males with low T tend to sire progeny characterized by females with high T and males with low T. Older females, females with higher T, have more left-handed progeny, not because of increased birth trauma, but because females program the developmental rate of their progeny based on the sex of their progeny and the mother's T level (Geschwind & Galaburda, 1987). Low T females and high T males create low T females and high T males.
4) The eight environmental variables influencing T; light (Geschwind & Galaburda, 1987), diet (Schmidt , 1997), body fat (Ross, 1986; Glass, 1977), alcohol and drugs (Castilla-Garcia, 1987; Ahluwalia, 1992), tobacco (estrogen levels) (MacMahon, 1982; Barrett-Connor, 1987), touch, physical activity (MacConnie, 1986; Morville, 1979), and stress (James, 1986), often do not affect the two sexes the same way. For example, increased body fat raises female T and lowers male T (Pasquali, 1991).
5) These eight specific environmental variables impact the distance and direction progeny can slide along the (- -) to (+ +) developmental arc. Moving left and right across the arc moves people backwards and forwards in genetic time. Impact points include the somatic environment of the parents at zygote creation and the uterine environment. Along with sexual selection, zygote selection and uterine selection (Geschwind & Galaburda, 1987) have the greatest influence on evolution in humans. Particularly vulnerable to neurological disease are those children whose parents are genetically already at either the left (- -) or right (+ +) ends who are exposed to these environmental variables. It is by the increasing or decreasing of the parents' testosterone (and possibly estrogen) levels that these variables further impact the developmental maturation rates of these vulnerable genotypes. For example the raising and lowering of the mother's T levels directly influences the developmental rates of the children during gestation (Geschwind & Galaburda, 1987).
6) Left spectrum individuals retain the older genotype, evidencing skill clusters highly valued before the advent of the (+) gene for a decrease in corpus callosum size and a reduction in portions of the right cerebral hemisphere which increased cerebral asymmetry. The highly selected (sexual selection being the primary selection force) character of the (+) gene proffers a heightened sense of passing of time (Marshack, 1972), increased split consciousness (Thompson, 1981), with a resulting ability to use language linearly; to plan (Annett, 1985). The (+) does not increase language facility directly, it creates an increased time dissociation evolving symbol to sign, through a disassociation of the cerebral hemispheres.
7) Dyslexia is not the only disease that has confounded studies by masking its roots at both the left and right ends of the developmental spectrum (Annett et. al., 1996). We believe that schizophrenia, Tourette's, diabetes and several other diseases may be split according to the same principles. By using peg tests (Annett, 1985); comparisons of brain size, planum temporale (Annett, 1992) and corpus callosum (Witelson, 1985); T levels; metabolic rates; developmental stage markers; and family histories; we can sort out the (- -) from the (+ +) from the pathological cases. Pathologically developmentally delayed and accelerated individuals can now be identified and treated separately from the genetic/environmental cases. The post-natal influences of the eight environmental variables mentioned above can then be assessed, because in addition to influencing a child's developmental rates before birth, these same variables can exacerbate and alleviate existent conditions and diseases by their ability to raise and lower T. Raised testosterone can have profoundly negative effects on the immune and autoimmune systems (Wingfield et.al., 1997). By assessing where a person naturally belongs on the left-right scale, a person's natural T level can be calculated. Once a person's natural T level is known, the same eight variables can be used to change T, bringing that person in line with his or her natural immune and autoimmune threshold. It is vital to note that the influence of these eight variables mask the natural T levels existent in each individual, throwing off studies, confusing the patterns.
8) The timing of the onset of puberty, the heterochronic principle of progenesis (Gould, 1977), has powerful correlations with neurological and cognitive variation. Diet, percentage of body fat, and physical activity are primary variables responsible for pubertal timing. There are studies (Saugstad, 1989) that suggest that specific forms of schizophrenia and bi-polar disorder are directly related to the timing of the onset of puberty. The relationship between pubertal timing and an individual's location on the developmental arc may reveal in greater detail the etiology of specific diseases. Depression may be directly related to the worldwide curtailment of the final stage of cognitive development, abstract thinking, caused by an earlier onset of puberty. There has been a drop in the age of puberty by three to four years over the last 100 years in urban cultures worldwide (Eveleth & Tanner, 1976) caused primarily by changes in diet. These dietary changes signal our bodies that increased fat, carbohydrate, and protein resources are available for an increase in birth rate, accomplished by lowering the age of procreation; a naturally selected response.
9) Tracking the distribution of neurological conditions at the left end along the (- -) to (+ +) spectrum is tracking the sequence of our genetic heritage and cultural history. At the far left end is autism representing anatomically modern humans maybe 100 M years ago when we had bigger brains (Wiercinski, 1979; Lainhart, 1997), ambidextrousness (Soper, 1986), and no dominant hemisphere. We hypothesize that the absence of constant touch as infants (Witelson, 1991a), in a genotype as old as autism that requires constant touch for full functioning, is responsible for many autistic syndrome complications over and above expected developmental delays. Phonetic dyslexics (Annett, 1990); stutterers (Corballis, 1981; Bryden, 1994); many Tourette's sufferers (Shapiro et. al., 1972); many homosexuals and lesbians (McCormick et. al., 1990); many gifted athletes, mathematicians, artists, musicians (Deutsch, 1978; Hassler, 1991b; Hasler & Gupta, 1993), and composers (Hassler, 1992); many schizophrenics (Crow et. al., 1996); specific alcoholic types (London, 1985) and many obese women are left spectrum, old genotype individuals who can be located along specific places on the left end of the (- -) to (+ +) arc. Several other conditions congregate at the right end of the left-right arc. We believe that the human species moves through time inside a (- -) to (+ +) developmental arc, its character determined by the effects of sexual selection, zygote selection, uterine selection, and natural selection on the rate and timing of development and maturation, creating the balanced polymorphism revealed by right-shift theory.
The process of evolution, the rules of species transformation, evolves. The rules change. Different rules apply to different species, the rules becoming more intricate and subtle with an increase in complexity in hormonal systems. Ancient species are still evolving exclusively according to random variation unlike more recently evolved complex species with longer ontogenetic histories. Each species needs to be examined for its signature methods of transcending the random variation barrier. Each human individual can be explored for his or her signature response to environmental messages mediated through a genetic history revealing a map of the responses of our past.
For a detailed explanation to how this thesis unfolds as it explains the origins of autism, click here.