Members of left-hander groups on Facebook frequently post lists of so-called facts about left-handedness. It is unclear who compiles these lists and many of the items mentioned have little scientific support. A recent such post featured this entry, Science does not know what causes left-handedness. Left-handedness does not emerge from a single cause. For example, a skin rash may be traced to a single source, such as an allergic reaction. Handedness, however, is a behavior not a disorder. Many factors contribute to a person’s display of handedness. Science does know a lot about handedness but not by looking for one cause. Rather researchers explore various aspects of handedness. Their goal is to increase our understanding of this complex behavior.
Researchers have challenged themselves to explain the extreme asymmetry between the prevalence of right- versus left-handedness in human populations (85-90% versus 10-15%, respectively). Why are humans predominantly right-handed? Does this large population asymmetry in handedness type occur in other species? If so, what can this tell us about the origins of handedness? Scientists have turned to our closest evolutionary primate relatives, the great apes (chimpanzees, bonobos, orangutans and gorillas), to try and answer these questions. Is there a continuity of handedness evolution from these primates to human populations?
The left hemisphere of the brain controls the movements of the right hand. The left hemisphere also contains the centers of the brain that control speech in humans. The speech areas of the left hemisphere are typically enlarged relative to analogous areas in the right hemisphere. Neural imaging studies indicate that the enlarged areas of the left hemisphere associated with speech in humans are also enlarged in the brains of great apes. The brains of great apes and humans appear to be organized in a similar fashion with a structural advantage for the left hemisphere despite the fact that great apes do not have speech. They vocalize but do not have human-like speech capabilities.
William Hopkins has studied the handedness of great apes for decades. The structural similarities in the left hemispheres of humans and great apes led him to propose that handedness, defined as the preferential use of the same hand to complete one-handed activities, could have evolved from primate tool use. For example, primates display handedness when they use one hand consistently to extract a piece of food from a long, narrow tube. Chimpanzees are one species of great apes that are active tool users. Hopkins found that chimpanzees are significantly more right-handed than great ape species who do not use tools. Chimpanzees also show a population bias toward right-handedness. The bias is not as large as found in humans…3 right-handers for every 1 left-hander among chimpanzees as compared to 8-9 right-handers for every 1-2 left-handers among humans…but the asymmetry trends in the right direction. Right-handedness is more common in both chimpanzees and humans.
This explanation of the origins of handedness and the population bias toward the right hand stems from the idea that the left hemisphere has a structural advantage over the right hemisphere for movement control. This advantage became apparent when primate species started to use tools that required one-handed manipulation. Over the millennia of evolution the left hemisphere superiority for movement control led to an increased population prevalence of right-handedness in our primate ancestor, the chimpanzee, and ultimately in humans. Both species are active tool users. How did the left hemisphere advantage for movement control and the subsequent predominance of right-handedness evolve? For the answer to this question one must turn to genetics which is the topic of the my next blog post.