Left+Handedness

Left-handedness: a maintained polymorphism

 Left handedness is a heritable trait and an example of polymorphism – when two or more distinct phenotypes (a modern definition might say 'genotypes', after all there are polymorphisms at silent sites - e.g. those that don't change the aa sequence of protein) exist in the same population. It occurs in about 10 to 30 percent of the human population . It has maintained at a constant frequency, which several studies had shown to have been maintained over a surprisingly long time. Oldest evidence from Pleistocene Epoch show that some Homo neanderthalensis individuals printed their right hand negative on cave walls, indicating that they were left handed, as opposed to right handed individuals printing left hand negatives (using their right hand to handle the tube used to spread pigment over it). Other evidence is shown from studies of stone artefacts, and wood and wear marks on spoons. One particular study examines the proportion of left handed individuals in the Pleistocene Epoch from Gröenen, who painted negatives of their right hand compared to a group of individuals today. (Faurie, Raymond. 2004.) It concluded that the studied proportion recorded in Gröenen is similar to the one compared to from today's individuals. Therefore this suggests that the frequency of handedness has not changed since the Pleistocene Epoch. The frequency of left handedness is still maintained today, though it is variable across the planet.

Frequency-dependent selection is an evolutionary process where one phenotype's fitness is dependent on its frequency relative to other phenotypes in a population. When a phenotype has an advantage over other phenotypes and that advantage relies on its numbers  ( not really the total number, rather its relative frequency) within the population, it is known as frequency-dependent advantage. It is theorised that the polymorphism of left-handedness is an example of negative frequency-dependent selection, where left handed individuals have an advantage because it rare within the population.

 Overall, the polymorphism of this trait is not well understood and its persistence in the population since the Palaeolithic era is puzzling. If no fitness costs were involved then the frequency-dependent advantage of being left handed would lead the frequency of this trait to reach 50% at equilibrium  (only if the advantage were symmetrical between left and right handers... but what is the advantage??) , but it in fact stays near the 10% mark of the population (McManus 2002), meaning that there must be costs involved in being left-handed. Such costs involve shorter stature, reduced longevity and more accidents and deaths when it came to using tools and machinery designed for a largely right-handed population. In very traditional societies the frequency of left-handers do not surpass 30% thus implying that costs exist in non industrialized areas too (Faurie and Raymond, 2005).

Left-handedness has been associated with several fitness costs, including lower height and reduced longevity. This low selection against left-handedness is small but persistent. However these costs are not frequency dependent, and the occurrence of left-handedness maintaining polymorphism can be explained by a frequency advantage that has remained unchanged since the upper Palaeolithis. (Faurie & Raymond, 2005). This benefit has been identified from sports data in Western societies as an advantage in specific sports that involve dual confrontations. This includes sports such as fencing and tennis, in contrast to non-interactive sports such as gymnastics (Raymond et al. 1996; Grouios et al. 2000). The frequency of left-handers in these sports is best explained by a negative dependent selection mechanism; left-handers have an element of surprise when advancing towards an unprotected section of the victim’s body. This advantage increases the likelihood of survival; hence left-handedness becomes selected for and remains within the population.  Not really... you talk as if the fitness advantage comes from sport. This is not Raymond's claim. Rather he argues that this advantage in sport indicates that there would be an advantage in lethal combat. The presence of such a mechanism is supported by a statistically significant positive correlation in traditional societies between the frequency of left-handers and the levels of violence (i.e. rate of homicide). There is a 3% frequency in the most pacifistic societies, in comparison to 27% in the most violent and warlike societies (Faurie & Raymond, 2005). A higher prevalence of left-handedness in males further supports the indirect relation between left-handedness and violence (Annett, 1985).

Previous studies have shown that the frequency of left handedness in modern W.I.E.R.D humans (Western, Educated, Industrialized, Rich and Democratic (Spinney 2010)) remains relatively the same as it was in the Palaeolithic era of //Homo sapiens// (Faurie 2003). Although the experiment showed that negative right hand prints stayed at the same prevalence (no statistical significance) it also showed that a large percentage (68.3%) of the modern left-handers that made the right hand prints did not use their left hand for throwing. Conversely 0.7% (a small but notable value) of subjects that made left handed prints used their left hand for throwing. These results show that not all left-handed painters were left-handed throwers; therefore the percentage of left-handed participants effective in the frequency dependant hypothesis is roughly 32.4% (the number of left-handed throwers over both handed prints in the modern equivalent). This value may be the same throughout human evolution however it will be hard to discern whether right-hand-printing Palaeolithic spear throwers used their left hand. Studies in non-W.I.E.R.D societies may give a more accurate representation of the number of left-handed spear throwers and their frequency dependency in the Palaeolithic era. Future studies will have to describe frequency of handedness differences between combative and non-combative tasks, which will give a more accurate picture of the advantage of incongruent-handed combat (combat between right and a left-handed individuals).

Annett, M. 1985 //Left, right, hand and brain: the right shift theory//. London: LEA Publishers.

Raymond, M., Pontier, D., Dufour, A.-B. & Moller, A. P. 1996 Frequency-dependent maintenance of left handedness in humans. //Proc. R. Soc. B// 263, 1627–1633.

Grouios, G., Tsorbatzoudis, H., Alexandris, K. & Barkoukis, V. 2000 Do left-handed competitors have an innate superi- ority in sports? //Percept. Motor Skills// 90, 1273–1282.

//<span style="color: #000000; font-family: 'Times New Roman',serif; font-style: normal; font-weight: normal;">Faurie, C. & Raymond, M. 2005 Handedness, homicide and negative frequency-dependent selection // Proc.R.Soc.B //<span style="color: #000000; font-family: 'Times New Roman',serif; font-style: normal; font-weight: normal;"> 272,25–28 //

//<span style="color: #000000; font-family: 'Times New Roman',serif; font-style: normal; font-weight: normal;">McManus, I C. 2002. Right hand Left hand: The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures. Harvard University Press, Cambridge, M.A. //

<span style="color: #000000; font-family: 'Times New Roman',serif; font-weight: normal; margin-bottom: 0cm;">Faurie C, Raymond M. 2003. Handedness frequency over more than ten thousand years. //Proc. R. Soc B.// 271. S43-5.

//<span style="color: #000000; font-family: 'Times New Roman',serif; font-style: normal; font-weight: normal;">Spinney L. 2010. Who’s the oddball?. // New scientist. //<span style="color: #000000; font-family: 'Times New Roman',serif; font-style: normal; font-weight: normal;">208. 42-5. //

//<span style="color: #000000; font-family: 'Times New Roman',serif; font-style: normal; font-weight: normal;">Faurie C, Raymond M. 2004. Handedness frequency over more than ten thousand years. // Proc Biol Sci //<span style="color: #000000; font-family: 'Times New Roman',serif; font-style: normal; font-weight: normal;"> 271 3 //

Group Secretary: Carissa Chu <span style="background-attachment: initial; background-clip: initial; background-color: initial; background-origin: initial; background-position: 100% 50%; background-repeat: no-repeat no-repeat; padding-right: 10px;">bt09338@qmul.ac.uk Judit Bagi <span style="background-attachment: initial; background-clip: initial; background-color: initial; background-origin: initial; background-position: 100% 50%; background-repeat: no-repeat no-repeat; padding-right: 10px;">bt09254@qmul.ac.uk Helen Zhou <span style="background-attachment: initial; background-clip: initial; background-color: initial; background-origin: initial; background-position: 100% 50%; background-repeat: no-repeat no-repeat; padding-right: 10px;">bt09065@qmul.ac.uk Tom Gasan <span style="background-attachment: initial; background-clip: initial; background-color: initial; background-origin: initial; background-position: 100% 50%; background-repeat: no-repeat no-repeat; padding-right: 10px;">bt09351@qmul.ac.uk