leftists 2 million years
First right-handed among humans seen about 1.8 million years ago, scientists from the University of Kansas founded it. The discovery was made according to the teeth of the ancestor of modern man – a talented man. According to the researchers, left-handedness may have emerged at that time, but it was not possible to determine this with certainty due to the lack of remains of ancient left-handed people.
According to various estimates, 15% of the world’s population is left-handed today. Also, in other hominids (bonobos, chimpanzees, and other great ape species), the division into right-handed and left-handed is 50% to 50%. Human ancestors also did not have a clear division into left-handed or right-handed at the beginning of species development. However, as soon as the old man mastered the conversation, he also used a pronounced right hand.
According to scientists from the University of Kansas, this effect is associated with lateralization of the brain. Asymmetry helps distribute functions between the two hemispheres. The left is responsible for the work of the right limbs, and the right is responsible for the work of the left. With the advent of speech, the left hemisphere, which is responsible for the work of the language, began to dominate the right – the same thing happened with the hands, only vice versa.
“Left-handed and right-handed people have certain features in the work of the brain, that is, in the location of the speech centers. For this reason, studies have shown that 90-95% of right-handed people have the speech center located in the left hemisphere. In left-handed people, it is found in only two-thirds of cases. His name is A.Ya. Kozhevnikova of the Sechenov University Clinical Center, neurologist Tatyana Nasonova.
The nature of the left-handed phenomenon still remains a mystery. It is believed that left-handedness develops extremely early, if not in infancy, during the embryonic development stage. Some researchers attribute this to genetics, some to hormones, and some to environmental factors.
embryonic theory
Italian science group establishedalready at the 18th week of fetal development, by the movement of the hands you can find out who will be born with 89% accuracy: right-handed or left-handed. The scientists used ultrasound to monitor the hand movements of more than 20 fetuses at 14, 18, and 22 weeks of development. Scientists were mainly concerned with delicate movements of the hand, for example, bringing it to the eyes or mouth. Nine years later, the authors learned which children were left-handed and which right-handed. As it turned out, the hand that the child uses more accurately, more actively and faster on ultrasound will be the leader.
The results of the study proved that dominance of a particular hand is an innate quality. develop this theory continued Experts from the Ruhr University. They found that the prerequisites for the emergence of right-handedness and left-handedness arose during the embryonic development of the spinal cord, even before the formation of connections between it and the motor cortex of the brain.
Observation has shown that children start sucking their right or left thumb as early as 13 weeks. But at this time the connection between the motor cortex of the brain and the spinal cord has not yet formed – it is formed at the 15th week. Therefore, scientists suggested that the spinal cord affects the dominance of one or another hand.
However, this theory has not been fully confirmed, according to Nasonova.
“There have not been enough large-scale studies to properly establish or refute the validity of the theory. The scientists didn’t do an ultrasound for 24 hours, the mother didn’t walk continuously with the sensor, so the results the group got could be just a coincidence. However, even if a person was examined three days in a row, there is no guarantee that during this time the child will suck his left finger, and the rest of his right finger, ”said the artist.
high testosterone
there is theory American neurologists say that high doses of testosterone, which affects the fetus before birth, can lead to the birth of a left-handed child. The hormone affects the prenatal growth rate of the hemispheres of the developing brain and is responsible for possible differences in brain structure in men and women (more often left-handed men).
According to the authors, high testosterone content during prenatal development slows the growth of the left hemisphere and promotes the development of the right.
Such a delay can lead to left-handedness if the process of migration of neurons to their final location in the developing brain’s left hemisphere slows down, and hence the establishment of necessary connections. This theory explains well the fact that men are more left-handed.
Excess body weight, constant stress, heavy physical work, hunger, smoking, exposure to sunlight and many other environmental factors affect the level of this hormone to some extent.
Blaming genetics?
The genetic theory of the development of left-handedness is also widespread. Experts from Oxford University clarified four gene groups associated with left-handedness.
Three of the four genetic regions they identified were associated with proteins involved in the development of the brain and its structure.
Additionally, these genetic effects have been associated with differences in brain structures in language-related white matter regions. This may explain why left-handed people have more advanced language skills.
The genetic theory is supported in part by: Statistics. That is, while the probability of having a left-handed child in right-handed parents is only 2%, this rate rises to 17% if one of the parents is left-handed. If both parents are left-handed, the chances of giving birth to another left-handed increase to 46%.
But sometimes the formation of left-handedness occurs after the birth of a child.
Congenital right-handed but left-handed
Left-handedness is not always a variant of normal individual development, sometimes the phenomenon can indicate a pathology. So, there is compulsive and pathological left-handedness.
“There are different terms used to express left-handedness. It is usually genetically determined and there is no pathology here. But sometimes left-handedness is forced and pathological. The first develops due to injuries on the right hand, then the child has to learn to use his left hand as a leader. But it also happens that children are simply re-educated by their parents,” Nasonova said.
The second type, according to the neurologist, occurs due to damage to the brain structure due to injuries or diseases that make normal control of the right hand impossible. For example, this happens due to ischemia of the right hemisphere or hypoxia during childbirth.
“In right-handers, the leading hemisphere is the left hemisphere, and if there are defects in it, then the right hemisphere and therefore the left hand will take over the leading function,” said the expert.
Some studies have found that left-handedness can also accompany neuropsychiatric disorders, delays in general, and speech development. Yes, at the University of Athens collected Statistics showing that people with autism are 2.49 times more likely to be left-handed than people without autism. in children with ADHD viewed same pattern: they are almost 30% likely to be left-handed, compared to 18% for the average person.
Now scientists advise not to retrain lefties. According to Nasonova, when a child begins to use his left hand dominantly, his brain is geared towards it. It is better to create normal conditions for him to live in a right-handed world.
“Relearning takes a lot of a child’s energy. It can also complicate the school adjustment process, causing increased fatigue, anxiety and poor performance. The working area of the child’s home should also be adjusted according to his/her characteristics. The light from the laptop window should fall to the right, not to the left. Things should be placed in such a way that the child is comfortable, ”concluded Nasonova.