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 system 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 "Wirdfam ad-Hova asher 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, as early as the 60s, "Broca's area" was discovered on the left side of the brain responsible for verbal communication, and many other specific specializations for different parts of the cerebral cortex have been discovered since then. It seems that the development of high mental abilities in a person 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, 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 preferring one side of the body for different tasks, there is no reason for the "strong" side to be common to 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 strength 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 laterality and in 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, a preference for one side appears only in about half of them. Similarly, bonobo monkeys who 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 to right-handed 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 where the population and 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 run to the right (meaning they activated their left fins more vigorously) and 4 preferred to run to the left. Sparrows that gather food near a wall will prefer to let their left eye watch for approaching predators and will therefore stand with their right facing the wall, while other bird species will show the opposite preference. The side preference at the population level is a puzzle for evolutionary researchers: why would the majority of individuals choose one preferred side and if there is indeed an advantage in doing so, why does the minority vote on the opposite side. One of the explanations is related to the behavior of animals at risk of predation: the safest place to fish when a shark appears in the vicinity is in the center of the pack, meaning that if most of its friends are right-handed, it should also have a strong right fin. Apparently, one should expect the population to be without preference or entirely right-wing or entirely left-wing: how does the minority survive over many generations? The reason is that even on a predator that makes a living from right-handed prey, evolution exerts a strong pressure to train itself right-handed. When the predators turn right-handed, an advantage is created for a potential left-handed prey and vice versa, that is, two escape strategies are created that can be chosen from one of them: find yourself in the center of the pack or deviate in the weak direction of the predator. Calculations based on game theory show that it is possible to have a stable equilibrium of a right-wing population with a left-wing minority or a left-wing population with a right-wing minority. Man became right-handed, as mentioned, after splitting from the monkeys and crazy litters was not the driving force in our evolution. With our ancestors, who were required to cooperate closely in the group, there was an advantage for those who were coordinated with their friends (even today, left-handed people are more injured in car accidents and at work) so that a clear right-wing majority was created, but competitiveness within the group sometimes gives an advantage to the outliers. For example, left-handed people show superiority in duels such as boxing or fencing. The right-wing contender has difficulty dealing with an attack from an unexpected source, while the left-wing has a lot of experience in interacting with the members of the right-wing majority. It is also possible that the expression of feelings of aggression on the right side of the brain, which controls the left body, makes the fight more comfortable for the left-hander than for the right-hander, whose fist is controlled by the left lobe whose function is to restrain aggression. Perhaps it is no coincidence that the only two mentions of left-handedness in the Bible are violent: Ehud the left-handed stabbing Egalon the king of Moab (Judges chapter XNUMX) and the Itari warriors of the tribe of Benjamin (Judges chapter XNUMX). Another advantage for left-handed people is in coordination: the right hand in left-handed people is stronger than the left hand in right-handed people, the connection between the two halves of the brain is better, and some have even found an advantage for left-handed people in creativity. There are clear advantages in belonging to the majority, but it turns out that there are good reasons to belong to the minority, and this balance results in a right-wing majority and a left-wing minority being found in every human population.

Did an interesting, intriguing, strange, delusional or funny question occur to you? sent to ysorek@gmail.com

More of the topic in Hayadan:

• Snails and snakes
• 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?