Why Are Most People Naturally Right-Handed and Rare to be Left-Handed?

Why Are Most People Right-Handed?

Previously, researchers thought this was a trait unique to humans, but more and more observations and studies have found that animals also share similar traits. About two-thirds of the more than 100 vertebrate species studied exhibit "handedness," and that's group preference.

In 2012, scientists studied two-handed coordination in 777 primates, including chimpanzees, chimpanzees, gorillas, orangutans. They found that group-level right-handedness is common in chimpanzees, chimpanzees, and African gorillas, and left-handedness is more common in orangutans.

In studies of other non-primate animals, scientists have also found that marsupials such as eastern squirrels and red kangaroos are left-handed.

Of course, what's even more surprising is that ancient creatures 500 million years ago also had dominant hands. By studying Cambrian trilobite fossils, paleontologists discovered that two-thirds of trilobites had bites on the right side after being attacked from behind by an enemy, while the rest have left or two side wounds. They speculate that this was caused by the Cambrian ocean lord Anomalocaris.

1. Dominant hand behavior and brain evolution

We know that the actions of animals are regulated by the brain or nervous system, so it is hypothesized that handedness is related to the "division of labor" between the left and right hemispheres of the brain. This is called the functional distinction of the left and right hemispheres of the brain. Each hemisphere has a special structure and function that is different due to the different components of the neural network. Taking humans as an example, the left hemisphere of the human brain is responsible for understanding and analyzing syntactic and phonological information; The hemisphere is responsible for recognizing intonation, non-verbal cues, and pragmatics.

The brain is not a new "feature" in evolution, it dates back about 500 million years and has appeared in animals throughout their evolutionary history. Studies have shown that late brain evolution is not due to animals sharing a common ancestor, but rather to parallel evolution between species.

2. Evolution advantage

Brain lateralization is common in many animals of different ecosystems because it helps animals adapt to their environment. Studies have found that the asymmetry of the left and right hemispheres confers certain survival advantages. For example, chimpanzees with obvious right-handed behavior can catch more termites; people with clear brain language can score higher in reading and linguistics; The process of brain chemistry in domestic chickens is related to the ability to enhance senses to perform two tasks simultaneously, chickens must find food underground, and must be wary of birds of prey above.

Brain chemotaxis can improve the survival efficiency of animals. If an animal primarily uses one limb or one side of its sensory system to interact with its environment, the corresponding cerebral hemisphere on the preferred side undergoes specialized motor or cognitive learning. more distinct. As a result, sensory discrimination and motor efficiency will increase unilaterally. Increased training of the sensory or motor systems will shorten the neural response time when the animal must act quickly against predators or foraging, shorter reaction times can help it survive. exist and have a survival advantage.

3. Why are most people right-handed?

Photo: verywellhealth

The aforementioned evolutionary advantage of evolution doesn't explain why most people are right-handed instead of 50% left- and right-handed. An increasing number of studies suggest that this is the result of a combination of genes, environment, and epigenetics.

1.Gen

Humans have found genes related to brain development in the study of seahorses and other animals, so scientists are very interested in finding the gene that controls handedness in humans. . Previous studies suggested that human handedness is controlled by a single gene, but recent studies have found that right-handedness is associated with multiple genes: including PCSK6, LRRTM1 and related genes to MAP2 microtubules.

However, genetic variation in these genes does not fully explain the occurrence and distribution of handedness in the population and is weakly correlated. Another study of twins showed that genetic influences only explained 25% of the difference in the odds of twin-handedness, while other factors dictated the remaining 75%. Skillful hands show complex genetic patterns. For example, if the parent of a child is left-handed, there is a 26% chance that the child will be left-handed. A large study by researchers of twins in 25,732 families found that handedness is about 24% inherited.

An analysis that examined the relationship between handedness and early life factors and genotypes in more than half a million people found the heritability of left-handed people to be very weak (4.35%).

2.Environment

Studies have shown that in hominids (the family of humans), right- and left-handed people coexist for a long time. The oldest unconfirmed evidence comes from Neanderthal fossils from the Middle Pleistocene (about 425,000–180,000 years ago) and the Early Pleistocene (180,000–10,000 years ago).

By studying the markings on their fossilized front teeth, the researchers discovered that they used sharp tools with either their right or left hand to cut the flesh, sandwiching the meat between the incisors and the hand. that. Left-handedness and right-handedness also exist from stone studies. The ratio of right and left-handedness is also close to that in modern humans.

Late Paleolithic (about 35,000-10,000 years ago), handprints left by prehistoric humans can confirm that most humans were right-handed.

On the cave paintings in Western Europe, to draw their own handprints, people hold a paint tube in one hand, and the other hand will put it on the cliff to draw handprints. If a person is right-handed, they will get used to blowing paint with a pipe from right hand to left hand, and the handprint drawn is the left handprint, if left-handed, vice versa.

The researchers found that the percentage of people who were right-handed at the time was about 77%. But the exact timing of how humans evolved into a right-handed majority remains unclear.

In human culture, there is also a prejudice against left-handed people. Many tools are designed for the convenience of right-handed people. In many countries, left-handed children still have to use their right hand to eat or write – in some countries it is considered impolite and rude to use the left hand to eat, write or engage in communication. Culture and environment can influence the proportion of left-handed people in the population to a certain extent.

3.Epigenetic

As mentioned earlier, genes and the environment can influence handedness. However, genetic factors cannot fully explain the inheritance of handedness. And for groups of people, social culture and environment influence human development trends. But for individuals, the environmental impact is negligible. For example, the dominant hand of the adoptive or stepparent has nothing to do with the child's dominant hand. Then there may be a third factor that affects the forehand.

Scientists have tried to explain it through epigenetic effects. An epigenetic mechanism is a change in gene expression without altering the nucleotide sequence, resulting in a phenotypic change. Because epigenetic DNA modifications can be passed on to the next generation, they can alter handedness without altering the genotype.

Two studies that investigated DNA methylation of the genes LRRTM1 and NEUROD6 revealed that an epigenetic DNA variation has a relationship between handedness and methylation intensity of these two genes. In addition, an anatomical study revealed severe asymmetries in DNA methylation patterns in the spinal cord of human embryos, possibly related to the development of asymmetrical movements similar to handedness. .

In summary, handedness behavior is related to brain development, which may improve the survival efficiency of humans or animals in the process of evolution; Most humans are right-handed, the result of a combination of genetics, environment, and epigenetics.

Why is Left-Handedness so Rare?

Sure, lefties make up about 10 percent of the population -- but, frankly, it seems like society has forgotten about them

About 10% of the world's population is left-handed. As a rule of thumb, "what is rare is precious", but unfortunately left-handed people are not like that.

They have always suffered a lot of injustice in history: from inconvenience in daily life, to discrimination, forced to switch to the right hand... Even, there are places where left-handed people are considered a sign of the devil. evil.

The answer has nothing to do with the brain

In the early '80s, scientists realized that you can be determined from the 8th week of pregnancy - based on the ultrasound tool. By week 13, the fetus begins to tend to suck the thumb of the dominant hand.

Many previous studies have suggested that the brain is what determines which hand you are. More specifically, genetic differences between the two hemispheres of the brain will determine this. However, according to research from the end of 2017, things suddenly turned in a different direction: the cause turned out to be in the spine.

This is a study by experts from Bochum University (Germany), in conjunction with a number of researchers in the Netherlands and South Africa. Accordingly, the gene activity in the spine seems to be unbalanced during pregnancy, and depending on which side it is deviated, it is decided to the dominant hand of the child later.

Why doesn't the brain play a role in this?

The brain that controls the hands and shoulders relies on the movement cortex. It sends a signal to the spine, and from here turns it into motion.

However, studies show that the spinal cord and the motor cortex do not connect until 15 weeks of gestation. Meanwhile, the dominant hand was determined long before that. This means that there must be something else in the brain that decides this, and that thing is the spine.

To find out more, experts analyzed gene activity in the spine from 8 to 12 weeks of pregnancy. They found that there is a genetic difference between the two sides of the spine that control our arms and legs.

This disparity was concluded to be due to a process called "epigenetics" - epigenetics. This is the process by which genes are expressed differently, forming different organs. That difference affects gene activity on both sides of the spine, creating the so-called dominant and contrarian hands.

But if it's just genetics, why are left-handed people so rare?

This is a question scientists have taken a long time to answer.

In 2012, experts from Northwestern University (USA) developed a mathematical model, to prove that left-handed people are the product of evolution through training. They suggest that there will be social reasons behind this story, rather than purely genetic factors.

In other words, from an evolutionary perspective, this process has favored right-handed people. In fact, the most recent study also shows that the gene distribution process for the left hand is only 25% of the cases.

However, even the latest research still cannot fully explain why society favors the right hand. Just know that the difference in neural activity between the two types of dominant hand is almost non-existent. And even left-handed people sometimes prove to be superior.

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