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.
"If right-brainedness arises, in most cases, as a natural variant in those who lack the right-shift factor, and if in such cases each hemisphere is equally likely to serve speech, the size to the RS- proportion of the population can be inferred to be twice the proportion of right-hemisphere speakers. An estimate of this proportion is offered by the data in Table 14.4 for incidences of right-hemisphere lesion. Omitting the data of Penfield and Roberts for the reasons explained, there is a total of 647 dysphasics, of whom 9.27% suffered unilateral right-hemisphere lesions. This suggests that 18.54% of the population are RS-, and this estimate is used to infer the frequency of the RS- gene in Chapter 15. It is important to ask how much confidence can be placed in this estimate of right-braindedness."
(Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory. Lawrence Erlbaum : London p. 271)
"If the RS factor is responsible for the bias to the left hemisphere for speech, as argued in Chapter 14, it clearly does not cause human speech but merely increases the chances that speech will depend on the left hemisphere. The shortness of the evolutionary time scale, the relative paucity of genetic differences between man and chimpanzee (Chapter 1), and the limited role of the RS factor all argue for some very simple agent for the RS. Parsimony alone should urge us to look for a simple mechanism. Very small changes in genotype can have major consequences for individuals. Changes that seem to be of great evolutionary significance from the human viewpoint could well depend on very small modifications of the genetic code. This section argues that the RS could depend on a single change at one gene locus (single allele). It is hypothesized that when this allele is present on one or both chromosomes, some advantage is likely to be conferred on the left cerebral hemisphere, which tends to induce speech on that side and incidentally increases the skill of the right hand in comparison with the left hand. When the right shift allele (rs+) is absent on both chromosomes, the alternate allete(s) at that locus (rs-) are indifferent or neutral for speech and handedness."
(Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 291-292)
(Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory. Lawrence Earlbaum: London p. 263)