Quick quiz: If a 2-meter-long shrimp asks you to shake hands, which hand should you extend?

About 90% of humans are right- handed, and only 10% are left-handed. Regardless of which hand we are, we tend to use one hand first in activities, a behavior we call "handedness."

In the past, researchers believed that this was a unique feature of humans, but more and more observations and studies have found that many vertebrates also have similar features.

Although many vertebrates do not have "hands", they have a preference for using one fin, claw, upper limb or foot. Among the more than 100 vertebrate species studied, about two-thirds of the species show "handedness" and it is a group preference.

In 2012, researchers studied handedness in 777 great apes, including chimpanzees, bonobos, gorillas, and orangutans, for bimanual coordination. They found population-level right-handedness in chimpanzees, bonobos, and African gorillas, but left-handedness in orangutans .

An orangutan using his left hand to manipulate a branch to eat. Image source: Wikipedia- William H. Calvin

In studies of other non-primates, scientists have also found that marsupials such as the eastern gray squirrel and the red kangaroo exhibit left-handedness.

Wild red-necked wallabies eating in a bipedal posture. Image source: Reference [1]

Of course, what is even more surprising is that ancient creatures dating back 500 million years ago already had a "dominant hand".

By studying Cambrian trilobite fossils, paleontologists found that two-thirds of the trilobites had bite marks on the right side of their backs from attacks by predators, while the rest had wounds on the left or both sides.

They speculated that this was caused by the Anomalocaris, the "ocean overlord" of the Cambrian period. The Anomalocaris, which could be more than 2 meters long, preyed on trilobites with its spiny and thick forelimbs. Through fossil reconstruction and analysis, it was found that the Anomalocaris used its left limb to fix trilobites and its right limb to tear them apart with its mouth. This may be the earliest example of "dominant hand".

Reconstruction of the Canadian shrimp, source: Wikipedia

01

Handedness and brain lateralization

We know that animals' actions are regulated by the brain or nervous system, so there is a theory that handedness is related to the division of labor between the left and right hemispheres of the brain.

We call the differentiation of the functions of the left and right hemispheres of the brain lateralization. Due to the different composition of the neuronal network, each hemisphere has different structures and special functions.

Taking humans as an example, the left hemisphere of the human brain is responsible for understanding and analyzing syntactic and speech information; the right hemisphere is responsible for recognizing intonation, non-verbal signals and pragmatics.

The lateralization of the brain is not a new evolutionary feature. It can be traced back to about 500 million years ago and has accompanied the evolutionary history of animals.

Research shows that brain lateralization is not acquired by animals from the same ancestor, but rather evolved in parallel. The figure below shows the evolution of brain lateralization.

The evolution of brain lateralization. Image source: physiology.org

Through observation and research on a variety of organisms, people have found that in addition to humans and orangutans, many other organisms also have lateralization, such as:

Honey bees prefer to use their right antennae during social communication, reflecting the asymmetry of smell; amphioxus has an asymmetrical mouth;

Asymmetry in the fighting response in zebrafish;

preference for the right paw in toads;

Handedness of red-necked wallabies;

Right-handedness in Asian elephants and chimpanzees in the food tube task;

Lateral vision guides foraging in pigeons;

Asymmetrical song control of canaries;

The Sulphur-crested Cockatoo prefers to use its left foot to complete the grasping action.

02

Evolutionary advantages of brain lateralization

Brain lateralization is common among animals across different ecosystems because it helps animals adapt to their environment.

Studies have found that asymmetry in the left and right hemispheres of the brain can better accomplish goals, such as chimpanzees with obvious right-handedness can catch more termites; people with obvious language lateralization in the brain have higher scores in reading and language learning; brain lateralization in domestic chickens is associated with the advanced ability to complete two tasks at the same time. The chicks must find particles in the pebbles and must be alert to birds of prey at the same time. Lateralized birds can do this well, while non-lateralized birds fail.

Brain lateralization in chickens is associated with the advanced ability to perform two tasks simultaneously. Image source: Reference [3]

Brain lateralization can improve the survival efficiency of animals. There are currently three mechanisms that are considered:

1. If an animal primarily uses one side of its limbs or one side of its sensory system to interact with its environment, the corresponding hemisphere of the brain that prefers the side will undergo more specialized perceptual or motor learning. As a result, sensory discrimination and motor efficiency will increase unilaterally;

2. Increasing training on one side of the sensory or motor system will shorten the neural reaction time. When an animal must take quick action against a predator or food, a shorter reaction time can help it survive and have a survival advantage.

3. Brain lateralization can make information processing more efficient. If two complementary pieces of information are calculated simultaneously in the two hemispheres, cognitive redundancy will be reduced. For example, the lateralization of chickens in the example just given allows chicks to feed and monitor the external environment more efficiently. Chickens with poor lateralization either cannot see predators or often mistake gravel for grain.

03

Why do most humans

Right-handed instead of left-handed?

The evolutionary advantages of brain lateralization mentioned above cannot explain why most people's lateralization is tilted in the same direction, rather than 50% left-handed and 50% right-handed. More and more studies show that this is the result of the combined effects of genes, environment and epigenetics.

1. Genes

In studies of zebrafish and other animals, humans have found genes that regulate the development of brain lateralization, so scientists are very interested in finding genes that control human dominant hand.

Early studies believed that human handedness was controlled by single gene inheritance, but recent studies have found that right-handedness is involved by multiple genes: including PCSK6, LRRTM1 and the microtubule-related gene MAP2.

However, genetic variation within these genes cannot fully explain the occurrence and distribution of handedness in the population and is weakly correlated.

One twin study showed that genetic effects could explain only 25% of the variation in handedness between twins, while the remaining 75% had to be explained by other factors.

Handedness shows complex inheritance patterns. For example, if both of a child's parents are left-handed, there is a 26% chance that the child will be left-handed. A large study of twins from 25,732 families showed that the heritability of handedness is about 24%.

A study that looked at the relationship between handedness and early life factors and genotype in more than 500,000 people showed that the heritability of left-handedness is very weak (4.35%). Birth weight, birth location, organized sexual behavior, or the occurrence of breastfeeding can all affect handedness.

2. Environment - social culture, tools

Studies have shown that among primitive humans, right-handed and left-handed individuals have coexisted for a long time.

The oldest undisputed evidence comes from Neanderthal fossils from the Middle and Early Upper Pleistocene, where researchers have shown that they used either their right or left hand to manipulate sharp tools and cut meat by holding it between their incisors and their other hand.

Studies of stone artifacts, drilling and rotation in wood, and wear marks on spoons have found that left-handedness and right-handedness also coexisted. The ratio of right-handedness (93%) to left-handedness (7%) is also close to that of modern people .

Neanderthal mandibular fossil and scratch marks on the front teeth of the mandibular mandible, showing obvious right-slanting scratches. Image source: Reference [5]

Handprints drawn by humans dating back to the late Paleolithic period confirm that the majority of humans are right-handed.

In order to draw their own handprints on the rocks of caves in Western Europe, humans hold a tube of paint in one hand and blow the paint into their other hand to draw handprints. If a person is right-handed, he will use the right hand to blow the paint into his left hand, and the handprint drawn will be the handprint of his left hand. If he is left-handed, it will be the opposite.

The left picture shows the right handprint of a primitive man, and the right picture shows the left handprint of a modern man. Image source: Reference [4]

By counting handprints, researchers found that the proportion of right-handed people at that time was about 77%, which is similar to the current 77.1% of French people who are right-handed. However, it is not clear when humans evolved to become right-handed.

In human culture, there is a prejudice against left-handed people. Many tools and programs are designed for right-handed people to use, often without realizing the difficulties left-handed people bring. In many family education, parents will force left-handed children to use their right hand to eat or write.

In schools in the 20th century, teachers would force left-handed students to use their right hands. In some countries, using the left hand to eat, write or participate in communication is considered impolite and rude. Culture and environment will affect the proportion of left-handed people in the population to a certain extent.

3. Epigenetics

We have mentioned before that genes and environment can affect handedness, but genetic factors cannot fully explain the inheritance of handedness.

And for human groups, social culture and environment do affect the development trend of human handedness. But for individuals, the impact of the environment is not significant. For example, the handedness of adoptive parents or stepparents has nothing to do with the handedness of their children. So there may be a third factor affecting handedness.

Scientists have attempted to explain this through epigenetic effects. Epigenetic mechanisms are mechanisms that alter gene expression without changing the nucleotide sequence, leading to phenotypic changes. Since epigenetic DNA modifications can be passed down to the next generation, they can shape handedness without changing the genotype.

Two investigations have examined DNA methylation, a form of epigenetic DNA modification, at the LRRTM1 and NEUROD6 genes, and a relationship has been found between handedness and the intensity of methylation at these two genes.

Furthermore, an anatomical study found profound asymmetries in DNA methylation patterns in the human embryonic spinal cord, which may be associated with the development of motor asymmetries similar to handedness.

In summary, handedness is related to the lateralization of the brain, which improves the survival efficiency of humans or animals in evolution; the fact that most humans are right-handed is the result of the combined effects of genetics, environment, and epigenetics.

References:

[1] Giljov, Andrey; (2015). Parallel Emergence of True Handedness in the Evolution of Marsupials and Placentals. Current Biology, 25(14), 1878–1884.

[2] Onur Güntürkün, Felix Ströckens, and Sebastian Ocklenburg. (2020) BRAIN LATER COMPARATIVE PERSPECTIVE. Physiological Society

[3] Rogers LJ, Zucca P, Vallortigara G. Advantages of having a lateralized brain. Proc Biol Sci 271, Suppl 6: S420 –S422, 2004.

[4] Charlotte Faurie* and Michel Raymond. Handedness frequency over more than ten thousand years. Proc. R. Soc. Lond. B (Suppl.) 271, S43–S45 (2004)

[5] Volpato, V., Macchiarelli, R., Guatelli-Steinberg, D., Fiore, I., Bondioli, L., & Frayer, DW (2012). Hand to Mouth in a Neandertal: Right-Handedness in Regourdou 1. PLoS ONE

[6] CARLES LALUEZA FOX; DAVID W. FRAYER (1997). Non-dietary Marks in the Anterior Dentition of the Krapina Neanderthals. , 7(2), 133–149.

[7] Carter-Saltzman, L (1980). Biological and sociocultural effects on handedness: comparison between biological and adoptive families. Science, 209(4462), 1263–1265.

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