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With almost complete certainty, I can predict that you, dear reader, are right-handed. If I were a betting man, I’d put money on it. I’d make the same bet if you were reading this in India or Iowa, Kansas or Kathmandu. And a hundred years from now, I’d make the same bet again.
I can be so sure of myself not because I am some prodigious prognosticator, but because about 90 percent of humans are right-handed. That phenotypic ratio—nine right-handed people for every lefty—is relatively stable, not just across cultures and geographic regions, but perhaps across the span of human evolution. The archaeological record suggests that hominins were predominantly right-handed as far back as 2 million years ago,1 and a 2010 study of the wear patterns on 32,000-year-old Neanderthal teeth found that this extinct cousin of Homo sapiens was likely about 88 percent right-handed.2
It’s a ratio that has baffled scientists for more than a century, despite the fact that handedness is immediately familiar to everyone who uses their limbs. “We understand handedness, because it’s easy to say which hand you use for writing,” says Silvia Paracchini, a geneticist at the University of St. Andrews in the U.K. who is searching for genes that underpin human handedness. “But the reality is that we know very little about it. We cannot explain it with data.” Scientists are unsure how or why humans are so strongly biased toward right-handedness. What are the biological underpinnings of the behavioral asymmetry? Is this overwhelming dominance of righties reinforced by culture, or is it strictly a product of genetics?
Recently, some of the most interesting findings have come not from studying handedness in humans, but from observing the behaviors and brains of other animals. Once considered to be a uniquely human trait, handedness may exist on the individual level in other species, and may be common in primates. At the population level, some primate groups even appear to show biases toward one side or the other. A 2011 study involving nearly 800 great apes conducted by Emory University neuroscientist Bill Hopkins and colleagues found that for a particular bimanual task—which necessitates the use of both hands, one dominant and one supportive—gorillas, chimps, and bonobos tend to be right-handed, while orangutans showed a population-wide preference for using the left hand.3 And in 2012, Hopkins and collaborators in China found that a population of wild Sichuan snub-nosed monkeys (Rhinopithecus roxellana) is predominantly left-handed (about 70/30) for the same bimanual task.4
The more we look at the evolution of vertebrates, the more important, widespread, and deeply evolved functional asymmetries such as handedness are. If it’s that ubiquitous it ought to be important.—Peter MacNeilage,
University of Texas at Austin
By far the most biased species is Homo sapiens, and some investigators hold firm that a consistent and overwhelming handedness bias to the right is a uniquely human attribute, much like language and other higher cognitive functions. Handedness, they say, likely stems from consistently reinforced cultural norms, and any reported biases toward one side of the body in nonhuman animals are primarily the result of experimental or observational artifacts or statistical error.
But in the past couple of decades, Hopkins and his collaborators have provided strong evidence that chimps do favor one side of their bodies over the other. The captive chimp colonies that Hopkins studies are 60 percent to 70 percent right-handed, regardless of the proportion of individuals in each colony that were human-reared,5 and recent scans of the chimps’ brains reveal corresponding brain asymmetries—hallmarks linked to hand preferences in the highly complex human brain. “I think the chimps offer an interesting comparison, because—biologically, genetically, cognitively, and anatomically—they show a lot of homologies to humans,” says Hopkins.
Our closest living relatives may serve as good models for the evolutionary roots of human handedness because, presumably, the apes’ behavior is less affected by culture and social learning and more a function of basic biology. “We don’t have social/cultural systems that are going to influence their handedness,” Hopkins adds. “So, in some ways the chimps are an easier way for us to think about or examine the role that these biological systems might have on individual variation in handedness.”
COURTESY OF BILL HOPKINS“There’s Rowena. She’s a lefty; you can see that,” Hopkins says from our perch on an observation tower at the Yerkes National Primate Research Center outside Atlanta. In an enclosure 30 feet below us, the 25-year-old chimpanzee is using her left index finger to extract peanut butter from a length of PVC pipe, the so-called “tube task.” Hopkins studies Rowena and the 11 other chimps in her social group to determine if the primates have a hand preference and, if so, whether the animals more commonly depend on the right, like humans.
But no sooner has Hopkins spoken than, as if on cue, Rowena switches hands and starts scraping the inside of the pipe with her right index finger. “Oop. Now Rowena switched to the right, so that’s actually fairly unusual.”
Unusual, perhaps. But Rowena’s behavior illustrates what a thorny question handedness in nonhuman animals has become. Rowena also tends to favor her left hand in other tasks aimed at assessing chimp handedness, including a simulated termite-fishing task that approximates tool use in wild chimps, and manual gesturing, which is relatively common among captive chimps. But is the chimp really showing an intrinsic preference for her left hand over her right?
University of Alberta evolutionary biologist Rich Palmer argues that Hopkins’s observations are simply a testament to the animals’ ability to ape their human handlers. Primates are famous for their ability to mimic and learn from other organisms. Findings of population-level right handedness in captive chimps may be the “by-product of being reared in an environment where the captors are handed,” says Palmer, who also questions the statistical validity of some of Hopkins’s early results.6
But Hopkins is confident that his results are valid—that the captive chimps he studies really do show human-like right handedness. If mimicry were the driver of the bias, he says, the proportion would more closely approach the human condition of 90 percent righties. “All of [the chimps] should be predominantly right-handed and strongly right-handed if it’s just about the influence of human experiences and human environments,” he says. “But we don’t find that at all.”
Hopkins and other researchers have also reported hand preferences for tool use in wild chimps, where population-level biases vary by task. Left-handedness predominates for termite fishing in some populations,7 for example, while other wild chimps seem to prefer using their right hands for ant dipping and leaf sponging. Hopkins acknowledges inconsistencies in conclusions gleaned from wild primates, but suggests that small sample sizes and experimental setups that use insufficiently bimanual tasks or those that fail to elicit consistent hand preferences in each individual may be muddling the data.
For captive chimps, however, which do show more consistent population-level handedness, technologies such as MRI are allowing Hopkins to study brain structure and function in individual apes with established hand preferences. About a decade ago, Hopkins showed that right- and left-handed Yerkes chimps differed with respect to lateralization in the area of the cerebral cortex called the precentral gyrus. Specifically, chimps that preferred to do the tube task right-handed had a significantly more pronounced motor-hand area referred to as the “knob” in their left hemisphere, while those who did it left-handed had a bigger knob in the right hemisphere.8
More recently, Hopkins discovered that chimps that preferentially engage in communicative gestures, such as clapping or motioning to their keepers for food, with one hand or the other have characteristic asymmetries in the inferior frontal gyri on either side of the brain, areas homologous to the human brain’s Broca’s area, the seat of speech production.9 “Chimps that gesture with their right hand predominantly have a larger left inferior frontal gyrus,” he says, while chimps that gesture mainly with their left hands have a more pronounced gyrus in the right cerebral hemisphere.
Hopkins suggests that handedness might be a by-product of brain lateralization, which became exaggerated as primates evolved. As the primate brain grew bigger, the connectivity between its right and left halves became weaker. Specifically, the size of the corpus callosum, a bundle of nerve fibers that connects the two hemispheres, did not keep pace with growing brain size. Relatively less communication between bigger, busier cerebral hemispheres necessitated divvying up tasks to be processed mostly in one half of the brain or the other. At the same time, Hopkins argues, hand preference became more pronounced. “As the brain got larger and larger . . . there could have been some kind of qualitative change, where you had some emergence of duality of function in the brain,” Hopkins says. “And that is what would have resulted in the emergence of something like handedness.”
Researchers generally agree that the human brain is unique in the extent to which its two halves differ. Language functions are typically lateralized to the left cerebral hemisphere, while cognitive functions, such as spatial manipulation and facial perception, more commonly show right hemisphere dominance. But Hopkins’s work shows that our closest living relatives might represent an intermediate level of lateralization, indicative of what the brains of our primate ancestors may have looked like.
We understand handedness, because it’s easy to say which hand you use for writing. But the reality is that we know very little about it. We cannot explain it with data.—Silvia Paracchini,
University of St. Andrews
“A whole lot of brain-imaging studies on chimps are showing that the hemispheric differences in humans are also there in chimps,” says Peter MacNeilage, an emeritus professor and evolutionary psychologist at the University of Texas at Austin. “That’s an interesting new development.”
On the other paw . . .
If handedness is an intrinsic primate trait that harks back to an evolutionary past before Homo sapiens arrived on the scene, it would most likely be written in our genes and those of our primate kin. Hopkins’s findings of chimp brain asymmetry indicate that genes may underlie handedness in this species, and probably in humans as well. “There’s some pretty good evidence now showing that individual variation in handedness is likely attributable to genetic factors,” Hopkins says. “What those specific genes are, we don’t really know.”
Additional evidence for a genetic basis of handedness comes from studies of human development. In a preliminary study using ultrasound, Peter Hepper, a psychologist at Queen’s University in Belfast, found that human fetuses display a preference for moving the right arm instead of the left by 12 weeks gestation10 and for sucking the thumb of the right hand as early as 15 weeks.11 “At this age, movement is not under brain control,” Hepper says. “It’s genetic, but where that’s being controlled from we just don’t know.”
© FIONA ROGERS/SHAH ROGERS PHOTOGRAPHY/CORBIS; BRAIN SCAN COURTESY OF BILL HOPKINS
But pinning down the exact genes that might control handedness has been hampered by oversimplified experimental setups, says Paracchini of the University of St. Andrews. Case-control approaches that just compare southpaws to righties lump people with mixed hand preferences or varying acuity with either hand into those two categories, she says. Last year, Paracchini and her team devised an alternative approach to quantifying handedness, testing the ability of more than 700 subjects to use either hand in a task that measures hand speed and accuracy, then placing each participant on a continuum from more dexterous with the left hand to more dexterous with the right. The genetic variant most strongly associated with this measure of handedness was a gene called PCSK6.12 Early in development, PCSK6 codes for an enzyme that cleaves a protein called NODAL, which is central to a signaling cascade that triggers the formation of mesoderm and endoderm. Importantly, this cascade also determines directional organization of organs and other structures in the body.
“This is the same biology that we know controls left/right body asymmetry,” Paracchini says. “It contributes to neural development, brain asymmetry if you like, and behavioral laterality, which we study through handedness.” Paracchini adds that this is likely only one gene in a network that controls handedness in humans. A good place to look may be pathways that mediate other lopsided aspects of physiology, such as having the heart positioned to the left side of the body and the liver to the right. “I believe many of these genes will be involved in controlling structural asymmetry as well.”
Still, Palmer and others maintain that handedness may not be directly controlled by genes. If that’s the case, what could be driving individual or population-level biases toward one side over the other?
Fred Biddle, a geneticist at the University of Calgary, has come up with an intriguing explanation. Working with laboratory mice—which show individual handedness but typically no population-level preferences, with a given population usually 50/50, righty/lefty—Biddle has found that he can train the rodent populations to become predominantly left- or right-handed using specially designed feeding chambers that force the animals to use a particular paw to procure food. Mouse strains with gene variants that make them better learners and have a slower rate of memory loss are more apt to develop a bias in either direction, left or right, given a biased environment.13 These findings, Biddle says, indicate that it could be the genes behind learning that really determine population-level handedness biases, not a direct genetic input that programs animals to use one hand predominantly over the other.
“Hand preference may be simply an unintended consequence” of an environment that causes an animal to use one hand more than the other, he says. Such an antecedent bias, reinforced by people’s high capacity to learn and remember, along with social norms that encourage children to write with their right hands, could, Biddle argues, morph into the extreme 90/10 bias that we see in the human population.
Although questions of why humans tend to favor one hand over the other and what drives this preference are far from settled, the idea that handedness has evolutionary roots that extend deeper than the relatively short history of Homo sapiens seems poised for general acceptance among scientists. Researchers are reporting individual and population-level handedness biases in a suite of animals (see “Paw Preference” below), and functional and anatomical brain asymmetries, coupled with behavioral asymmetries, in our closest primate relatives lend support to that hypothesis. This mounting evidence is propelling scientific work that seeks to ferret out the molecular roots of both human handedness and what appear to be similar, if less robust, biases in other animals.
“The more we look at the evolution of vertebrates, the more important, widespread, and deeply evolved functional asymmetries [such as handedness] are,” says MacNeilage. “If it’s that ubiquitous, it ought to be important.”
For years researchers have been uncovering hand, foot, fin, or paw preferences across the animal kingdom. Although some debate the existence of population-level handedness in nonhuman animals, examples of species that show individual and population-wide biases toward the right or left abound.
WHIT WELLES/WIKIMEDIA COMMONS
- N.T. Uomini, “The prehistory of handedness: Archeological data and comparative ethology,” J Hum Evol, 57:411-19, 2009.
- D.W. Frayer et al., “Right handed Neanderthals: Vindija and beyond,” J Anthro Sciences, 88:113-27, 2010.
- W.D. Hopkins et al., “Hand preferences for coordinated bimanual actions in 777 great apes: Implications for the evolution of handedness in hominins,” J Hum Evol, 60:605-11, 2011.
- D. Zhao et al., “Handedness in nature: first evidence on manual laterality on bimanual coordinated tube task in wild primates,” Am J Phys Anthropol, 148:36-44, 2012.
- W.D. Hopkins et al., “Chimpanzees (Pan troglodytes) are predominantly right-handed: replication in three populations of apes,” Behav Neurosci, 118:659-63, 2004.
- A.R. Palmer, “Chimpanzee right-handedness reconsidered: evaluating the evidence with funnel plots,” Am J Phys Anthropol, 118:191-99, 2002.
- S.L. Bogart et al., “Termite fishing laterality in the Fongoli savanna chimpanzees (Pan troglodytes verus): further evidence of a left hand preference,” Am J Phys Anthropol, 149:591-98, 2012.
- W.D. Hopkins, C. Cantalupo, “Handedness in chimpanzees (Pan troglodytes) is associated with asymmetries of the primary motor cortex but not with homologous language areas,” Behav Neurosci, 118:1176-83, 2004.
- J. Taglialatela et al., “Gesture handedness predicts asymmetries in the chimpanzee inferior frontal gyrus,” NeuroReport, 17:923-27, 2006.
- G. McCartney, P.G. Hepper, “Development of lateralized behaviour in the human fetus from 12 to 27 weeks’ gestation,” Dev Med Child Neurol, 41:83-86, 1999.
- P.G. Hepper et al., “Handedness in the human fetus,” Neuropsychologia, 29:1107-11, 1991.
- W.M. Brandler et al., “Common variants in left/right asymmetry genes and pathways are associated with relative hand skill,” PLOS Genetics, 9:e1003751, 2013.
- F.G. Biddle, B.A. Eales, “Hand-preference training in the mouse reveals key elements of its learning and memory process and resolves the phenotypic complexity in the behaviour,” Genome, 49:666-77, 2006.