The Origin of Left-Handedness

Although individual animals such as dogs and cats display a preference for using their right or left paw, there is no 

We now know that handedness is directly associated with asymmetry in the human brain. Fine-motor control for 95% of the human population is controlled by the left hemisphere of the cerebral cortex of the human brain. Most of these people are right-handed, but 70% of left-handed individuals also use the 

FIGURE 7.3. Magnetic resonance images (MRIs) of a human brain. 

Kerr handedness. The following passage memorializing the skill of the left-handed warriors of the Kerr clan comes from a famous poem and drinking song by Walter Laidlaw from 1549 called The Reprisal [22]  

Before making any conclusions about the genetics of handedness, or the evaluation of any other model developed to explain the handedness of Homo sapiens, it is important to attempt to sort out the statistics about handedness that are generally accepted to be valid, from all the other statistics about handedness that are in dispute. The following four statements are assumed to be valid  

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The overall model for the origin of left-handedness must also explain observations 3 and 4. In this discussion we have tried to avoid the various "sinister" aspects of being left-handed that have affected 10% of the world population. Some aspects of this topic will be mentioned in the next chapter, but we should not completely omit from this discussion the less pleasant aspects of the history of left-handedness. In addition to being forced to use their right hands, left-handers have also been treated as imperfect humans in many ways. Unfortunately, this unfair categorization is still made today in many cultures. A number of researchers in the not too distant past that have said that all left-handers are pathological left-handers, that is to say, that left-handedness was a result of some disease, injury, or other abnormality. Most researchers now believe that the number of left-handed people whose handedness is due to some injury or disease of the left hemisphere is actually quite small. 

The results surveyed in the preceding two sections provide a first clue to the origin of chirality chiral patterns can emerge spontaneously in an initially uniform and isotropic medium, through a mechanism of bifurcations far from thermodynamic equilibrium (see Figs. 4 and 5). On the other hand, because of the invariance properties of the reaction-diffusion equations (1) in such a medium, chiral solutions will always appear by pairs of opposite handedness. As explained in Sections III.B and III.C this implies that in a macroscopic system symmetry will be restored in the statistical sense. We are left therefore with an open question, namely, the selection of forms of preferred chirality, encompassing a macroscopic space region and maintained over a macroscopic time interval. 

Even so, as has been pointed out, silicon may have had a part to play in the origin of life on Earth. A curious fact is that terrestrial life forms utilize exclusively right-handed carbohydrates and left-handed amino acids. One theory to account for this is that the first prebiotic carbon compounds formed in a pool of "primordial soup" on a silica surface having a certain handedness. This handedness of the silicon compound determined the preferred handedness of the carbon compounds now found in terrestrial life. An entirely different possibility is that of artificial life or intelligence with significant silicon content. 

Witelson SF, Nowakowski RS (1991) Left out axons make men right a hypothesis for the origin of handedness and functional asymmetry. Neuropsychologia 29 327-333. 

Fig. X.l. The coordinate system used in the multipole expansion, (a) Interparticle distances. The large black dots denote the origins of the two Cartesian coordinate q/stems, labelled a and b, respectively. We assume particle 1 always resides close to a. particle 2 always close to b. The figure gives a notation related to the distances considered, (b) Tvo Cartesian coordinate systems (and their polar counterparts) one associated with the centre a, the second one with centre b (the x and y axes are parallel in both systems, the x axes are collinear). Note that the two coordinate systems are not on the same footing the z axis of a points towards b, while the coordinate system b does not point to a. Sometimes in the literature we introduce an alternative coordinate system with equal footing by changing zj —zj (then the two coordinate systems point to each other), but this leads to different handedness ( right- or left-handed ) of the systems and subsequently to complications for chiral molecules. Let us stick to the non-equivalent choice .

Figure 4. TEM images of pofy-L-Glu-Bis-3 microstructures. The top images were obtained at pH 5.8 (D.I. water) showing rupture that is indicative of the origination of helices (top, left) and right-handedness of the twist (top, right). Lower images show structural transformation into nanoftbers tqmn treatment of above microstructures with pH 7.5 Tris buffer.

Coren, S., The Left-Hander Syndrome The Causes and Consequences of Left-Handedness, Vintage Books (Random House), New York, 1993. McManus, C., Right Hand, Left Hand The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures, Harvard Univ. Press, Cambridge, MA, 2004. Annett, M., Handedness and Brain Asymmetry The Right Shift Theory, Psychology Press, 2002. 

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