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More recent work with captive chimpanzees by Bill Hopkins at the Yerkes Regional Primate Research Center in Atlanta, USA, though, has muddied the waters somewhat. Hopkins, whose work is admittedly contentious, claims that his chimpanzees exhibit a 70% right-handed preference for many tasks requiring manual dexterity, a rate that rises to almost 98% for very specific tasks, like the precise over-arm throwing of objects, although for food-related tasks (like cracking nuts or digging out honey, for example) the rate remains closer to 50-50. Hopkins has hypothesized from this that the evolutionary trigger for handedness was perhaps not speech at all but the increased need for motor coordination, and that this may therefore have occurred significantly further back in the evolutionary story, even before humanity's split with other primates.
Since the 1990s, some studies have begun to indicate some hand preference in prosimians (non-ape primates like bush-babies, lemurs, etc, which are considered more primitive in evolutionary terms than either apes or monkeys). Perhaps predictably, the right hand is usually the dominant one, the one used to grasp a branch after a leap for example, although the left is preferred for quick, small movements like catching insects. These laterality effects in prosimians were particularly notable in tasks that required the animal to reach or stand upright, and it was based on this research that Peter MacNeilage developed his postural origins theory of handedness, which suggests that handedness, and laterality in general, originally developed to enhance the stability of posture and locomotion, and the differentiation of the roles of the upper limbs developed when prosimians and primates started walking upright. Like prosimians leaping around in the tree-tops, the vast majority of modern humans tend to jump across a puddle, for example, leading with the right leg.
Michael Corballis (who generally supports Chris McManus’ genetic theory of handedness) has argued that, in evolutionary terms, the gene responsible for handedness is uniquely human and likely emerged somewhat later in our development, due to a random mutation at some point in the evolution of Homo sapiens in Africa, perhaps around 150,000 to 200,000 years ago, and that it spread quite quickly through the population thereafter. He has also suggested that the evolution of human speech implies a direct association between speech and gesture, and so our brain lateralization for speech may itself be responsible for our asymmetric hand use.
William Calvin developed another evolutionary theory of handedness in the early 1980s. His starting point was the well-documented fact that the vast majority of mothers throughout history have carried their babies on their left side, partly to free up their dominant right hand for other tasks, but also so that the baby can more easily hear the mother’s comforting heartbeat (which tends to calm them). Calvin believes that women in the early days of mankind were also involved in hunting, and that those with the quieter left-carried babies were more successful hunters and breeders, thus perpetuating the dominance of right-handedness. The theory is, however, impossible to prove, as it is purely based on conjecture, and several drawback and inconsistencies immediately jump to mind.
Robert Sainburg’s research has concentrated on further clarifying the roles played by the dominant and non-dominant hands, and he has developed a sort of evolutionary theory of handedness from this. His work has shown that, although the dominant hand is more accurate for many tasks, the non-dominant side is actually quicker and, perhaps even more importantly, responds better after an interruption or destabilization. An everyday example of this is the so-called “barman effect”, whereby a waiter can remove glasses with his dominant hand from a tray held in his non-dominant hand, automatically anticipating and compensating for the weight and balance destabilization. Sainburg has hypothesized from this that handedness may have developed in order to enable complementary tasks, and that the two sides became specialized for different functions, each being just as skillful in its own way.
The so-called Fighting Hypothesis, which is covered in more detail in the section on Handedness and Combat, can perhaps be seen as another evolutionary explanation for the continued existence of left-handedness. It argues that left-handedness confers an advantage in combat - and therefore an evolutionary advantage - because the vast majority of opponents faced by left-handers are right-handers (the majority of the population), who are less practised at dealing with the different angles, stances, etc, presented by left-handed combatants. The Fighting Hypothesis also claims to explain the greater frequency of left-handed males compared to left-handed females (which is repeatedly found in studies), on the grounds that male-male fighting is a more common occurrence than other combinations and so it is the males who stand to profit the most from the left-handed fighting advantage. However, although this theory may explain, at least to some extent, why left-handers have not died out completely in a world where the main evolutionary advantages appear to belong to right-handers, it does not provide a cogent theory for the development of handedness in the first place.