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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Pygmies: Heterochronic Patterns


"Primary endocrine control of somatic growth is established by the secretion and circulation of growth hormone (GH), a large polypeptide molecule with multiple effects on cell division and metabolism. GH is synthesized and secreted by the somatotropic cells of the anterior pituitary gland. This secretion is regulated by the interaction of two hypthalamic hormones, growth hormone-releasing factor (GHRF) and growth hormone inhibiting factor (somatostatin), which in turn are regulated by a variety of humoral and central nervous system feedback controls. GH travels form the pituitary to all cells of the body, where it binds to specific growth horone receptors (GHR), which are widely distributed. After GH binds to the receptor on the cell membrane, it circulates with a carrier protein known as high-affinity growth hormone-binding protein (GH-BP), which appears to be the extracellular portion of the GHR dissociated from the cell. Serum levels of high-affinity GH-BP can be assayed, and are thought to provide an indirect measure of the number of GHR present and active (Baumann et al., 1989)." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 313)

"Whatever the precise hormonal mechanisms ultimately determined to underlie the reduced body size in pygmies, their effects appear to be exhibited throughout the entire span of postnatal growth (Bailey, 1991a) and not simply during or just prior to puberty, as previously claimed by Merimee et al. (1987). This is just what would be expected of an alteration involving IGF-l levels and/or distal subresponsiveness, since the predominant effects of this important mitogen are seen postnatally (e.g. Rechler et al., 1987)." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 314)

"For the morphologist dealing with terminal phenotypes, this means that complex and highly integrated patterns of proportion changes during growth and size shifts are determined by rather simple underlying controls. Consequently, a shift in terminal size will automatically yield a cascade of coordinated allometric proportions. These underlying genetic and developmental controls, and their gross phenotypic expression, must be fully understood when analyzing individuals, sexes, groups, or species which differ in body size. In such a model, globel allometric transformations will occur with body size changes through these genetic and developmental pathways, regardless of whether natural selection has driven the size change or not." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 325)

"The global ontogenetic scaling of allometries and reduced rates of body weight growth-in-time are indicative of the heterochronic transformation termed rate hypomorphosis (Shea, 1983b). Neoteny and time hypomorphosis (Shea, 1983b, or the progenesis of Gould, 1977, and other authors) will also yield a juvenilized or paedomorphic morphology. But in the first case, allometric patterns will not be coincident )or aontogentically scales) in the two groups, and in the second case, the duration of growth (and not merely the rate of growth-in -time) will be truncated, so that the transformed group will cease growth and become sexually mature at a significantly earlier age. Neither of these profiles fit the African pygmy..."(Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 326)

"The argument that the small overall body size and/or specific body proportions of human pygmies are the result of strong and sustained selection for thermoregulatory efficiency in hot and humid tropical rain forest environments is both long-standing and physiologically reasonable. However, this proposition clearly needs to be treated as a preliminary hypothesis." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 327)

"In this scenerio, small body size was specifically selected for in the pygmies in order to increase the amount of surface area available for heat loss via convection cooling. The unlikelihood of losing sufficient heat by this mechanism in the hot, humid rainforest has resulted in a shift toward the view that small size was selectively advantageous due to the absolutely (though not relatively) lower levels of heat production (e.g., Cavalli-Sforza, 1986; see also Lewin, 1991)." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 332)

"Bailey et al. (1989), Headland (1987), and Bailey and Headland (1991) have suggested that the tropical rain forest offers a very limited supply of calories, particurly in the form of carbohydrates and fat, for human foragers." (Shea, B.T. & Bailey, R.C. (1996) Allometry and adaptation of body proportions and stature in African pygmies. American Journal of Physical Anthropology 100(3): pp. 335)


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