Why are humans right-handed?

"Why are humans right-handed?" is a question that scientists have been discussing for a long time, and it is also considered one of the most challenging scientific frontier issues at present.

About 90% of humans are right-handed, and this behavior of using one hand preferentially in activities is called "handedness." In the past, researchers believed that this was a unique characteristic of humans, but more and more observations and studies have found that animals also have similar characteristics. Although vertebrates do not have hands, they have a preference for using one side of the 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 branches 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 500 million years ago already had a "dominant hand". Paleontologists studied the fossils of Cambrian trilobites and found that two-thirds of the trilobites had bite marks on the right side of their backs after being attacked by predators, while the rest had wounds on the left or both sides. They speculated that this was caused by the Cambrian marine overlord Anomalocaris, which could reach more than 2 meters in length and preyed on trilobites with its spiny and sturdy forelimbs. Through fossil reconstruction and analysis, Anomalocaris used to fix trilobites with its left limbs and tear them with its right limbs and mouth. This may be the earliest example of a "dominant hand".

Part 1

Handedness and brain lateralization

We know that the actions of animals 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 the understanding and analysis of syntactic and speech information; the right hemisphere is responsible for the recognition of intonation, non-verbal signals and pragmatics.

The division of labor between the left and right brains of humans is not the same. Image source: pixabay

The lateralization of the brain is not a new evolutionary feature. It can be traced back to about 500 million years ago and has been with the evolution of animals. Studies have shown that brain lateralization is not acquired by animals from the same ancestor, but evolved in parallel. The figure below shows the evolution of brain lateralization.

The evolution of brain lateralization Image source: physiology.org

From left to right in the picture:

Lateralization in the stillwater snail Lymnaeastagnalis;

Cuttlefish have an asymmetric vision, with a preference for using the tentacle on the same side as the dominant eye;

The nematode Caenorhabditis elegans has an asymmetric nervous system; honey bees prefer to use their right antennae during social communication, reflecting an asymmetric sense of smell; and the amphioxus (Branchiostoma lanceolatum) has an asymmetric mouth;

Lateral predator avoidance in the Australian lungfish ( Neocratodus forsteri );

Asymmetry in the fighting response of zebrafish ( Danio rerio );

right-paw preference in toads (BufoBufo);

Handedness in red-necked wallabies (Notacrospus rufogriesus)

Right-handedness in Asian elephants ( Elephasmaximus ) and chimpanzees ( Pan troglodytes ) in the food tube task;

visual lateralization in dolphins ( Tursiopstruncatus ) and lateralization in monitoring and escape behaviors in geckos ( Podarcis muralis );

Asymmetric visual responses to mirrors in turtles ( Testudohermanni );

Lateral vision-guided foraging in pigeons ( Columbalivia );

Asymmetrical song control in canaries (Serinus canaria);

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

Part 2

Evolutionary advantages of brain lateralization

Brain lateralization is common in animals of 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 score higher 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 a pellet among 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 the environment, the corresponding hemisphere of the brain that prefers the side will undergo more specialized perceptual or motor learning. As a result, sensory discrimination ability and motor efficiency will increase unilaterally;

2. Increasing the training of one side’s perception or motor system will shorten the neural reaction time. When an animal must act quickly 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 eat and monitor the external environment more efficiently. Chickens with poor lateralization either cannot see predators or often mistake gravel for grains.

Part 3

Why are most humans right-handed rather than 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) Gene

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 hand dominance. Early studies believed that human hand dominance was controlled by single gene inheritance, but recent studies have found that right-handedness is involved by multiple genes: including PCSK6, LRRTM1 and 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 can only explain 25% of the variation in handedness between twins, while the remaining 75% must be explained by other factors. Handedness exhibits complex inheritance patterns. For example, if a child's parents are both 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 influence handedness.

(2) Environment: social culture, tools

Studies have shown that right-handed and left-handed individuals have coexisted for a long time among primitive humans. The oldest undisputed evidence comes from Neanderthal fossils from the Middle Pleistocene (about 425,000–180,000 years ago) and the early Upper Pleistocene (180,000–10,000 years ago in the Upper Pleistocene). By studying the marks on their incisor fossils, the researchers found that they used their right or left hands to operate sharp tools, and when cutting meat, they clamped the meat between their incisors and the other hand. Studies on stone products, drilling rotation in wood, and wear marks on spoons have found that left-handedness and right-handedness also coexist. The ratio of right-handedness to left-handedness is also close to that of modern people, which is 27:2 (93%:7%)

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

Handprints drawn by humans dating back to the late Paleolithic period (about 35,000-10,000 years ago) 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 is the right handprint of a primitive man, and the right picture is the left handprint sprayed by a modern man. Image source: Reference [4]

By counting handprints, the 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 the majority right-handed.

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

As mentioned above, genes and environment can affect handedness. However, genetic factors cannot fully explain the inheritance of handedness. And for human groups, social culture and environment 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 studies investigated DNA methylation, a form of epigenetic DNA modification, at the LRRTM1 and NEUROD6 genes, and found a relationship between handedness and the intensity of methylation at these two genes. In addition, an anatomical study found a profound asymmetry 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 Marsupialsand Placentals. Current Biology, 25(14), 1878–1884.

[2]. Onur Güntürkün, Felix Ströckens, and Sebastian Ocklenburg. (2020) BRAIN LATER COMPARATIVEPERSPECTIVE. 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]. CharlotteFaurie* and Michel Raymond. Handednessfrequency 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 inRegourdou 1. PLoS ONE

[6]. CARLESLALUEZA FOX; DAVID W. FRAYER (1997). Non-dietary Marks in the AnteriorDentition 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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