They have also been interpreted as evidence that language evolved not from primate calls, but from manual gestures –. These observations have been taken as evidence that language did not appear de novo in humans, as argued by Chomsky and others, but evolved gradually through our primate lineage. Around two-thirds of chimpanzees are right-handed, especially in gesturing and throwing, and also show left-sided enlargement in two cortical areas homologous to the main language areas in humans-namely, Broca's area and Wernicke's area (see Figure 1). A left-hemisphere dominance for vocalization has been shown in mice and frogs, and may well relate to the leftward dominance for speech-although language itself is unique to humans and is not necessarily vocal, as sign languages remind us. A right-hemisphere dominance for emotion seems to be present in all primates so far investigated, suggesting an evolutionary continuity going back at least 30 to 40 million years. A strong left-hemispheric bias for action dynamics in marine mammals and in some primates and the left-hemisphere action biases in humans, perhaps including gesture, speech, and tool use, may derive from a common precursor. Many of these asymmetries parallel those in humans, or can be seen as evolutionary precursors. Left–right asymmetries of brain and behavior are now known to be widespread among both vertebrates and invertebrates, and can arise through a number of genetic, epigenetic, or neural mechanisms. One myth that persists even in some scientific circles is that asymmetry is uniquely human. Įvolution of Brain Asymmetries, with Implications for Language Polarities of left and right brain are broadly invoked in art, business, education, literary theory, and culture, but owe more to the power of myth than to the scientific evidence. Although widely acclaimed, this book goes far beyond the neurological facts. A more recent example is Iain McGilchrist's 2009 book The Master and His Emissary, which draws on cerebral asymmetry in a sweeping account of the forces that shaped Western culture, and provocatively declares the right hemisphere to be the dominant one (“the master”). Brain imaging shows, though, that creative thought activates a widespread network, favoring neither hemisphere. One popular example is Betty Edwards' Drawing on the Right Side of the Brain, first published in 1979 but now in its fourth edition, which epitomizes the popular view that the right hemisphere is responsible for creativity. Sperry the Nobel Prize for Physiology and Medicine in 1981, but again led to speculation, most of it exaggerated or ill-founded, about the complementary functions of the two sides of the brain. Testing of each disconnected hemisphere again revealed the left to be specialized for language and the right for emotional and nonverbal functions. Interest flagged for a while, but was revived a century later, in the 1960s, with the study of patients who had undergone split-brain surgery, in which the main commissures connecting the two hemispheres were cut as a means of controlling intractable epilepsy. Nevertheless, further evidence that the right hemisphere was the more specialized for perception and emotion also led to speculation, some of it far-fetched, about the complementary roles of the two sides of the brain in maintaining psychological equilibrium. Because the left hemisphere also controls the dominant right hand, it came to be widely regarded as the dominant or major hemisphere, and the right as nondominant or minor. Since language itself is uniquely human, this reinforced the idea that brain asymmetry more generally is a distinctive mark of being human. Superstitions about left and right were compounded by the discovery, in the 1860s, that speech was based predominantly in the left hemisphere of the brain. This has often led to stigmatization of left-handed individuals, sometimes forcing them to switch hand use, occasionally with grievous consequences. The most obvious sign that our brains function asymmetrically is the near-universal preference for the right hand, which goes back at least as far as the historical record takes us, and has long been a powerful source of symbolism, with the dexterous right associated with positive values and the sinister left with negative ones.
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