Post Graduate Student (Biotechnology)
DR. D.Y Patil Vidyapeeth
Handedness is commonly described as the inclination to use one hand over the other. Around 88% of individuals are right-handed, 10% being left-handed and with around 2% being ambidextrous i.e. having the ability to use both hands. The question as to why predominantly people are right-handed have fascinated researchers for a protracted time. Preference towards using one’s hand is initially observed during gestation when single arm movement is revealed by the embryos. Consistent use of one hand during the life-time of an individual results in lasting asymmetries in the form of bone density and form that occur through the macro- and microstructural changes in the bone. Various areas of research have been structured and supported by human evolution to cognitive neuroscience due to the periodicity of left-handedness. However, satisfactory geographical population distribution data is still insufficient. Archaeological pieces of evidence including ancient tools and artefacts has exhibited the use of the left hand for as long as 500,00 years ago. This implies that evolution has produced a very small population of lefties with their maintenance over the course of millennia.
Various theories are persisting regarding handedness, among them a widely known hypothesis is that of the twin and family models. These models resulted in the suggestion that around 25% of the variation in handedness is an outcome of genetic effects and a greater amount of 75% of the variation is justified by factors governing one’s environment.
Identical twins are not often seen using the same hands and individuals having same-handed parents have also been seen having children with a different hand preference as having occurred in my hand inclination, which supports the fact that inheritance does not seem to accompany Mendelian laws.
The first largely known genetic model was one-locus model, which was however replaced by evidence showing proof of the contribution of multi genes. One such recent study through genome-wide analysis published in 2020, performed in the University of Queensland recognized genome-wide significance attained by 41 left-handedness and 7 ambidexterity loci which were hugely contradictory to the dextral- chance hypothesis which said that out of two alleles of a gene responsible for handedness, one gene was responsible for the inheritance of right-handedness and the other hand preference associated with the second gene was determined stochastically. They noticed a correlation among left-handedness and the 17q21.31 locus. Koolen de Vries syndrome, a disorder distinguished by neurological abnormalities of the corpus callosum, hippocampi and ventricle occur as a result of a deletion in 17q21.31 locus. Left-handedness was not associated with any other 1,349 complex traits tested, such as which side of the body is the heart present on. However, ambidexterity was associated with injury as a result of which led to the use of the other hand.
Microtubule proteins have been seen playing a very crucial role in the development and migration of neurons in the context of handedness. The attribute of variants and multi-genes have also been indicated by the association of handedness and variation of microtubule genes.
Another novel mathematical model presented in 2012, suggested a balance between competitive and cooperative pressures on human evolution. Elite athletes were chosen as test candidates for this evolutionary model which represented that 50% of top players were left-handed in competitive sports like baseball as this included practice against mostly right-handed players which as a result gave an unpredictable benefit to the left-handed hitters. This fighting hypothesis where there is an imbalance in the population in absence of other selection stresses is an example of negative frequency-dependent selection which is a major implementation of upkeeping polymorphisms. But according to the principles of evolution, natural selection would lead to a substantial increase in the population of lefties in such a way that it would be at par with the right-handed population thus eliminating the uniqueness of left-handedness.
However, human evolution has also been shaped by cooperative pressure as much as the competition. As we can observe in sports where performance doesn’t depend on the opponent, shows that cooperative pressure thrusts handedness distribution in the opposing direction. Many of the important tools and instruments were designed for the right-handed majority, as a result of which lefties would face higher cases of accidents and be eliminated from a highly cooperative world. This model therefore accumulated data of the distribution of left-handed people from the general population also, matching data from different sports indicating that the endurance of left-handed people as a stable and small minority comes from competitive and cooperative effects thus forming an equilibrium.
These genetic, cognitive and evolutionary shreds of evidence have greatly broadened our view as to how handedness is not merely a simple event, but there are complex processes underlying which work in coordination and contrast to develop this trait. Further detailed analysis of wider population density will open new horizons for this area of research.