By Haoyu Li
For many years, scientists have always believed that right-handedness is more likely to be a phenomenon unique to humans (Balter, 2009). Humans’ preference for the right hand began millions of years ago: In a study published on Evolutionary Anthropology, Lozano et al (2017) analyzed the fossil of Homo habilis and found that the ratio of right-handed to left-handed individuals is as high as 9:1 for these ancestors of ours who lived 1.8 million years ago. That said, more and more studies have found that many animals are also more inclined to use a certain palm/claw/tentacle, and this preference is probably related to the brain.
The technical term used to describe which hand a creature prefers to use is biological chirality. The main manifestation of chirality is left-right asymmetry. This asymmetry is either due to physiological structures, for example, the left hand is a little larger than the right hand; or it is behavioral, such as the tendency to work with the right hand.
Chirality may be at the individual level or at the group level. In recent years, researchers have discovered this chirality in many different species: most primates have dexterity (Tarlach, 2018), walruses have webs that they are more accustomed to using (Learn, 2021), and octopuses have more preferred tentacles (Bryne et al, 2006) (Some studies believe that this preference may be related to eye-arm coordination: the preferred tentacles are often on the same side as the octopus’s dominant eye. However, this conclusion has not been sufficiently supported by research). Even the brainless single-celled organism Physarum polycephalum (a type of acellular slime mold) tends to grow to the right.
There is still no clear answer as to why humans are mostly right-handed. One theory is that the human brain’s left hemisphere is generally related to logical thinking and language, and it controls our right-side body. As the language ability of human ancestors gradually developed, they became more inclined to use the left hemisphere, which led to the phenomenon that modern humans are mostly right-handed. But this hypothesis is now rejected by many researchers. For example, William Hopkins, director and chairman of the Killing Center for Comparative Medicine at the University of Texas, said that chimpanzees who do not have language skills are also right-handed: the behavioral measurements of chimpanzees show that when they are throwing things. 65% to 70% of attempts utilizes the right hand. In addition, experiments have shown that gorillas and bonobos generally tend to use their right hands (Grant, 2014).
The evolutionary theory holds that in the past primates were mostly left-handed, but with evolution, the chirality of these primates has changed. Stephanie Poindexter, a biological anthropologist from the State University of New York at Buffalo in the United States, said that species more distantly related to humans tend to use the left hand. Researchers verified this conjecture in some Lemuridae, Lorisidae and Galago species (Poindexter, 2018). However, there are still many blind spots in our understanding of primates, and this conjecture needs further verification. For example, the same study by Poindexter (2018) showed that several types of slow lorises (Nycticebus spp.) are more inclined to grab food with their right hands.
Therefore, not all results of animal chiral studies are consistent with each other. Part of the reason for this is that we generally explore the chirality of animals through behavioral observations, but each animal has a different degree of dependence on different behaviors. To give an example: we know that right-handed people may be more inclined to write with their right hand and use their left hand to turn over reference materials. But if a right-handed person were a tree, which hand would he be more inclined to use to grasp the trunk and which one to pick the fruit? For activities that we spend less time on, the tendency to use a certain hand is more difficult to predict. It is thus difficult for us to directly compare different species, because each species spends different amounts of time on different activities.
So, what is the reason that humans are mostly right-handed? Ruth Byrne, a biologist who studies the chirality of octopuses, said that biological chirality may have started as the brain evolved into two-sided structures. “Since people have left and right brain hemispheres, they must work together.” After the brain is divided into two sides, different tasks are assigned to the hands on each side. As for why the same species gradually showed the same preferences, Hopkins believes that this may be because (for these animals) certain tasks are more suitable for one side of the hand to complete. For instance, the heart is to the left of the body, so while the left hand is more restricted to protecting the heart, the right hand can be used to do more extended activities.
When primates moved from trees to the ground, the importance of the activities of each hand changed accordingly. For example, for primates in the early days, the hand used to hold the tree may be more important than the hand that grabs food, because falling from a tree is fatal. After starting to live on land, primates may still use the tree-supporting hand for manual labor, such as supporting their body, while the other hand is used for more delicate activities, such as making tools. The hand that makes the tool becomes more important at this time, so the tendency to use the right hand for finer activities remains. Hopkins stated that some studies have indeed found evidence that left-handed preference has transformed into right-handed.
Regardless of what caused this shift in the beginning, chiral features have been preserved in evolution and have now become a very significant bias in the population. Hopkins said that biology and evolution may have led to the emergence of chirality, and cultural factors ensure that right-handed tendencies are more popular among the population: cultural customs in some countries are very unfriendly to left-handers, such as in Africa and in the Middle East. In these places, it is impolite to touch food or shake hands with others using your left hand.
References:
Balter M. (2009). The origins of handedness.
https://blogs.sciencemag.org/origins/2009/08/the-origins-of-handedness.html
accessed 28/02/21.
Byrne R. et al (2006). Does Octopus vulgaris have preferred arms? Journal of Comparative Psychology. 120(3), 198–204.
Grant B. On the Other Hand.
https://blogs.sciencemag.org/origins/2009/08/the-origins-of-handedness.html
accessed 28/02/21.
Learn J. (2021) Can Animals Be Right- or Left-Pawed?
https://www.discovermagazine.com/planet-earth/the-animal-kingdom-is-full-of-righties-and-lefties
accessed 28/02/21.
Lozano M. (2017). Right-handed fossil humans. Evol. Anthropol. 2017; 26: 313– 324.
Tarlach G. (2018). Right or Left: Human Handedness Is An Ancient Trait
https://www.discovermagazine.com/the-sciences/right-or-left-human-handedness-is-an-ancient-trait
accessed 28/02/21.
Poindexter A. et al (2018). Slow lorises (Nycticebus spp.) display evidence of handedness in the wild and in captivity. Laterality, 23:6, 705-721.
Imperial Bioscience Review 