Things that Yoram knows: left and right

"The shekel man" asks "how is it that there are right-handed and left-handed people"

The question consists of several riddles: Why would there be a favored party at all? Why is there a numerical advantage for leftists over iters and if there is indeed an advantage for leftists, how is it that the left minority has not disappeared?

Right-left is a strange axis of division: unlike the vertical axis whose direction is determined by gravity or the east-west axis that the sun draws for us, the physical world is symmetrical between right and left and these directions depend on the point of view: my right is the left of the person in front of me. It is no coincidence that the distinction between right and left is so difficult for children and the very definition of left or right relies on a different set of axes: left is the direction of the north when you look at the rising sun and accordingly "north" is the first meaning of the word left in the Bible, To the left of Damascus" (Genesis chapter 90). And yet the nervous system insists on breaking the symmetry and about XNUMX% use the right hand for tasks such as writing or brushing teeth. The source of the preference is in the brain, which is known to be divided into two lobes - each one supervises the opposite half of the body.

Who determines right-wing or left-wing?

A person with both right-handed parents has a 9% chance of being left-handed, when one parent is right-handed and the other right-handed, the chance increases to 16%, and the probability increases to 26% in the offspring of two left-handed parents. These data as well as the slightly but statistically significant increased chance of identical twins (monozygotes) with an identical dominant side compared to non-identical twins suggest a genetic component in determining the preferred side. But mapping the human genome found no genes significantly associated with left-handedness or right-handedness. This contradiction led researchers to propose an alternative hypothesis: Epigenetic inheritance of the preferred body side. Epigenetics is the hereditary transmission of traits that is not related to the sequence of the bases in the genome, but to the silencing of certain genes and the activation of others according to signals from the environment. Epigenetics plays an important role in the development of the central nervous system and in learning functions, memory and response to stress and is a bridging factor between genetics and the environment. Environmental factors that have been found to influence the chance of left-handedness include, for example, emotional distress or trauma to the mother in the last trimester of pregnancy,

An important environmental factor is the direction from which the visual stimuli come during infancy. The direction in which the baby turns his head was found to influence the choice of the preferred hand later in development. Most mothers tend to hold the baby to their left so that the right hand remains free. The baby receives, therefore more groins that stimulate him to turn his head to the right. Right-handed parents are more likely than left-handed parents to give the baby toys on the right hand and direct any activity to that hand. Left-handedness is less common in babies who were breastfed (for at least a month) compared to those who were fed from a bottle. There are those who explain this finding by increased secretion of the hormone oxytocin.

What do we get out of this?

There are important advantages to specializing specific areas in one of the lobes for different tasks. This saves the resources of the nervous system and avoids confusion that may arise if each half of the brain makes an opposite decision for the muscles in its half of the body (the right half of the body controlled by the left half of the brain and vice versa).

In humans, the brain is larger, busier and requires more specialization and differentiation of its various parts, meaning at least symmetry, already in the 60s, "Broca's area" was discovered on the left side of the brain responsible for verbal communication, and many more specific specializations for different parts of the cerebral cortex have been discovered since then. It seems that the development of high mental abilities in humans, and in particular language, required a lot of specialization of different parts of the brain, so that the great difference in the roles of the halves of the brain also results in a difference in the motor abilities of the two halves of the body. Until a few decades ago, a simplistic view ruled that it was language that created the differences in roles between the lobes and since only humans have a grammatical language, then only they have a right-handed preference in their limbs. Plenty of zoological studies have undermined this approach: it turns out that even in many animals, the left lobe takes on unique roles, such as processing local information and discerning details, while the right lobe is more busy with large-scale mapping of the environment. The emotionality of the right lobe is also not unique to humans and the left lobe curbs aggression in humans as well, also in our chicken sisters and probably in other species. As the animal requires more skill and coordination, the tendency to specialize one side of the body will increase. But even if there is an advantage in favoring one side of the body for various tasks, there is no reason for the "strong" side to be shared by everyone. In mice, for example, it is possible to find out through targeted hybridization individuals that will show a strong preference for one of the paws in various tasks: the mere hereditary side preference, but not the direction, it is impossible to produce a left- or right-handed mouse population.

Even if right-handedness is not unique to humans, we rise in the magnitude of this preference: almost all of us will brush our teeth with the same hand from the day we got our first toothbrush until we get our dentures and very few people will even try to write with their weak hand. In monkeys, the eyes are frontal, there is a large overlap in the visual fields so that each half of the brain receives a similar image and there is no strong pressure to favor a side. The chimpanzees in the wild remove termites from their nests with sticks and for this delicate operation each individual will prefer one hand, but the chimpanzees are much less decisive in their versatility and for simple and spontaneous tasks there is no side preference. Observation of bonobo monkeys - the species of chimpanzee considered the most "human-like" revealed that even in complicated tasks such as termite placentation using a branch, only about half of them show a preference for one side. Similarly, bonobo monkeys that are required to pick fruit from a tree while hanging with one hand on a branch each detail has a personal preference for the hand that holds the branch and holds the weight of the body and the picking hand, but in the population as a whole there will be no advantage for right- or left-handed people. The mutual grooming activity between the monkeys is done without side preference unlike humans where kissing couples will prefer to tilt their heads to the right.

. Since we separated from the chimpanzees, our brain has grown considerably, but the part that bridges the parts, the Corpus callosum, has grown less than the lobes it connects. Our two brain lobes are relatively isolated and are required for a large variety of unique tasks and for the specialization of more brain areas in specific tasks. This is how our ancestors were required to reinvent the division between left and right. An examination of stone tools reveals that most of us have been right-handed for several hundred thousand years. According to the direction of the sprinkling of the chips from the tool, one can learn about the producing hand and from the wear on the user's hand, and both were mostly right-handed, perhaps because it makes sense to make tools with the hand that is controlled by the half of the brain that is in any case responsible for discerning details. Most of the bone fractures that originated in face-to-face battles between our ancestors during the Ice Age were found on the left side of the body, right-hand evidence of the attacking party.

But if there are good reasons to be one-sided, why is the population divided into 90% right-wing and 10% left-wing?

It turns out that there are other animals in which the population, not just the individual, chooses a side. In an experiment in which a predator was presented to schools of fish from 16 different species, 10 species showed a clear preference for the experienced direction: 6 species of fish tended to flee to the right (i.e., they activated their left fins more aggressively) and 4 preferred to flee to the left. Sparrows gathering food near a wall will prefer to give their left eye the job of watching approaching predators and will therefore stand with their right facing the wall, while other bird species will show the opposite preference. Side preference at the population level is a puzzle for evolutionary researchers: why would most individuals choose one preferred side, and if there is indeed an advantage in doing so, why do a minority of those who choose the opposite side appear? One explanation is related to the behavior of animals at risk of predation: the safest place for a fish when a shark appears in the vicinity is in the center of the school, meaning that if most of its members are right-handed, it is better for it to have a strong right fin as well. It would seem that the population would be either completely right-handed or completely left-handed: how does the minority survive over many generations? The reason is that even a predator that feeds on right-handed prey is under strong evolutionary pressure to adopt a right-handed disposition. When predators become right-handed, an advantage is created for a potential left-handed prey and vice versa, meaning that two escape strategies are created, one of which can be chosen: to be in the center of the pack or to deviate towards the predator's weak side. Calculations based on game theory show that a stable equilibrium of a right-handed prey population with a left-handed minority or a left-handed population with a right-handed minority is possible. Humans became right-handed, as mentioned, after splitting from apes, and litters from predators were not the driving force in our evolution. Among our ancestors who were required to cooperate closely in a group, there was an advantage for those who were coordinated with their friends (even today, left-handed people are more likely to be injured in traffic and work accidents), so that a clear right-handed majority was created, but competition within the group sometimes gave an advantage to the exceptions. For example, left-handed people show an advantage in duels such as boxing or fencing. The right-handed contestant has difficulty dealing with an attack because it is unexpected, while the left-handed person has extensive experience interacting with members of the right-handed majority. It is also possible that the expression of aggressive feelings on the right side of the brain, which controls the left side of the body, makes fighting more comfortable for the left-handed person than for the right-handed person, whose fist is controlled by the left hemisphere, which is responsible for restraining aggression. Perhaps it is no coincidence that the only two references to left-handedness in the Bible are violent: the left-handed Ehud who stabs the charioteer of the king of Moab (Judges, chapter 3) and the Itir warriors of the tribe of Benjamin (Judges, chapter 20). Another advantage for lefties is in coordination: the right hand of lefties is stronger than the left hand of righties, the connection between the two hemispheres of the brain is better, and some have even found an advantage for lefties in creativity. There are clear advantages to belonging to the majority, but it turns out that there are good reasons to belong to the minority, and this balance means that a right-handed majority and a left-handed minority will be found in every human population.

Did an interesting, intriguing, strange, delusional or funny question occur to you? sent to ppgad@pucrs.br

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• A possible mechanism for the origin of chirality of biological compounds
• The cutoff limits: how deep is the phenomenon of chirality in molecules?
• Things donors know: how evolution can create emotions
• April 1: Why and how were the changes made to driving on the left and right?