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August 18, 2009

The Origins of Handedness

About 85% of humans worldwide are right-handed, at least for most tasks they perform. Most of the rest are left-handed; true ambidexterity is very rare. Yet researchers are not sure just when in hominin evolution a tendency to use one hand over the other evolved—evidence for handedness in other animals is inconsistent and controversial—nor why handedness might have been favored by natural selection. (Why right-handed people dominate rather than left-handed people is a whole other question.) Chimpanzees, gorillas, and other nonhuman primates do not show a  consistent bias for one hand or the other, which means that our handedness probably arose sometime after the chimp and human lines went their evolutionary ways about 5 million to 7 million years ago. Many researchers have suggested that there is a link between handedness and the evolution of language, because both involve asymmetries—or “lateralization”—in the brain. (Most people, including most who are left-handed, do their speech processing on the left side of their brains.)

One researcher hot on the trail of these issues is Natalie Uomini, an archaeologist at the University of Liverpool in the United Kingdom. In a new paper in press at the Journal of Human Evolution (JHE), Uomini reviews the evidence for the prehistory of handedness, including studies in nonhuman primates, and even offers a brief report on her own observations of human children and adults. Uomini also makes some interesting suggestions about why handedness evolved on the human line. The paper is part of a special JHE issue titled “Paleoanthropology Meets Primatology,” which is edited by researchers William McGrew and Robert Foley of the University of Cambridge.

Citing earlier work by other researchers, Uomini points out that handedness does not mean that one hand is “dominant” over the other. Rather, she writes, “both hands have different but equally important roles.” In right-handed people, for example, the right hand might be used for tasks requiring greater manual dexterity whereas the left hand might perform the more mundane but nevertheless crucial role of supporting an object. (Imagine eating dinner with just a knife but no fork, for example.) In human children, this kind of handedness begins to emerge between 7 and 13 months of age and is well-established by age 3. In nonhuman primates, however, there is no evidence of species-wide bias for one hand or the other, although some individual primate populations do show such trends. Yet even those trends are not consistent: Although researchers studying the captive chimpanzees at the Yerkes National Primate Research Center in Atlanta have found that they tend to be right-handed for many tasks, wild chimps at the Gombe Stream National Park in Tanzania tend to use their left hands to fish termites out of trees with sticks. These are trends only, Uomini points out, and they never reach the “extreme degree of consistency seen in humans.”

Figuring out when such consistency arose in hominins is not an easy task. In a small number of cases, it is possible to detect signs of handedness in early hominin fossils, such as the shoulder and arm bones of the Nariokotome Boy, a nearly complete 1.6-million-year-old Homo ergaster skeleton found in Kenya by Richard Leakey’s research team. The Nariokotome Boy was clearly right-handed, as indicated by the deeper bone insertions of his deltoid muscles in his clavicle (collar bone) and the greater length of his ulna. Yet Uomini notes that knowing the hand bias of one individual tells us nothing about the handedness of the population he belonged to, let alone that of his entire species. And she critiques the seminal 1980s studies by Nicholas Toth (now at the Stone Age Institute in Bloomington, Indiana) and other researchers who claimed to have detected a trend toward right-handedness in early hominin tools by looking at the direction in which flakes were struck off of a larger stone core. Uomini argues that such studies are limited by their assumption that the trends seen in an assemblage of flakes or tools directly represent the hominins who produced them. Because many of the tools might have been produced by one expert right-handed toolmaker, “it is not appropriate to treat each flake … as one data point,” she writes.

Uomini concludes that although hand bias may be hard to detect among early hominins, there is clear evidence from the large number of available Neandertal skeletons that our evolutionary cousins tended to be right-handed. Their right arms and shoulders show greater robusticity, possibly from throwing spears as they hunted wild animals. And studies of the teeth of both Neandertals and H. heidelbergensis, the presumed common ancestor of Neandertals and modern humans, show that the direction of their striations is consistent with biting on food held in the right hand.

If the trend toward right-handedness was indeed well-established by the era of H. heidelbergensis, which thrived in Europe and Africa about 500,000 years ago, what was its evolutionary advantage? To help get at that question, Uomini observed the spontaneous hand use of children and adults who visited the Lejre Historical Archaeological Research Center in

Handedness studyDenmark, using techniques similar to those employed by scientists with nonhuman primates. Visitors were invited to try two different “prehistoric games,” one that involved cracking walnuts using anvils and stone hammers (right) and the other a puzzle that required the subjects to refit flakes back into a flint core from which they were struck. The visitors were also asked to sign their names so their writing hands could be determined. Uomini performed statistical analyses on 48 children and adults ranging in age from 3 to 49 years who performed at least six bouts of nut-cracking or puzzle-solving and found that although the great majority of nut-crackers held the hammer stone in their right hands (which corresponded to their writing hand), most of the puzzle-solvers used both hands equally.

Uomini argues that these findings are similar to those found in studies of nonhuman primates, in which the degree of handedness varies depending on the complexity of the task they are facing and the level of skill required to perform it. The nut-cracking task required five basic actions, Uomini says—“grasp nut, place nut on anvil, grasp hammer, hit nut, and eat nut”—whereas the puzzle required at most three actions (grasp flake, grasp core, fit flake to core.) And along with other researchers who have offered a similar hypothesis, she suggests that the increasing sophistication of hominin toolmaking technologies over time may have selected for a greater degree of handedness. Indeed, Uomini says, the “technology-dense lifestyles” of early hominins might have required our ancestors to more or less make up their minds about what hands they were going to use to perform complex tasks. Moreover, such hand bias could have aided the learning process as hominins taught each other toolmaking and other skills; a number of studies have shown that people learn manually difficult tasks, such as knot-tying, more easily when they use the same left- and right-hand movements as their teachers.

If the hypothesis that handedness was selected for because it made hominins better toolmakers is correct, Uomini concludes, it should be testable in the archaeological record: The more complex the tool, the more evidence there should be for handedness in its use. Yet a number of questions remain, including whether there are selective advantages for right over left handedness that are strong enough to explain why almost the entire human species skews to the right, and is such advantages exist, why 15% of humans  remain stubbornly left-handed. The answer, Uomini writes, may come with ongoing research into the genetics of brain asymmetries.

—Michael Balter

Photo credit: Lejre Land of Legends

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