Although most humans are right-handed, other animals don’t seem to show a similar motoric asymmetry. As Corballis mentions in his 2003 BBS article, even the great apes – our closest relatives in the animal kingdom – tend not to show a right-hand preference unless raised in captivity, suggesting handedness is learned through imitation of caregivers. So why should humans be the only species to show clear manual asymmetry, 9:1 in favor of righties?
While manual asymmetry may be unique to humans, cerebral asymmetry is not. Corballis reviews how animals as diverse as frogs, birds, mice, rats, gerbils and marmosets tend to show more reliance on their left hemispheres in producing and recognizing their species’ vocalizations. Even in humans, right-handed adults are more likely to show greater activity in their left cerebral hemispheres on language tasks. And the only situation in which chimpanzees show clear handedness is during pointing – when chimpanzees point, they tend to do so (around 2/3 of the time) with their right hands (some chimps have even been observed to point spontaneously in the wild, suggesting this behavior is not due merely to imitating humans.)
To explain handedness, Corballis constructs an elaborate argument about the evolution of language. According to his theory, communicative (as opposed to merely expressive) behavior began with bipedalism, freeing the hands for use in communication. Corballis argues that this communicative behavior may have slowly shifted towards facial movements, and that the final stages in the evolution of spoken language involved the addition of voicing to those “facial gestures.” Handedness emerged as vocalization was incorporated into manual gesture, extending the left-hemispheric dominance for communication into manual control.
Corballis argues that lateralization of speech may have emerged simply because the benefits of symmetrical neuronal organization are minimal when the output of the neural system is patterned in time rather than space – a hypothesis which relates to other theories of hemispheric lateralization, suggesting that the left hemisphere may be specialized for more rapid processing of temporally-patterned stimuli.
If you’re just interested in why animals don’t show handedness, there’s no need to read further – the rest of this post is dedicated to examining Corballis’s argument about the evolution of language, which I found surprising. Corballis uses six points to support his claims:
1) Chimps do not have control over their vocal systems (and presumably, nor did early humans)
Observation of monkeys in the wild suggests that their vocalization are more “emotionally driven” and “undirected” as opposed to being clearly communicative. Consistent with this claim, destruction of the monkey anterior cingulate – a limbic region associated with emotion – completely disrupts their ability to vocalize. In contrast, destruction of a monkey’s Broca’s area (err, it’s homologue) does nothing to affect their vocalization, even though this region has been famously associated with speech production in humans.
2) Chimps have much more control over their hands (and presumably, so did early humans)
Although chimps have not been successfully trained to speak, they have shown surprising ability to communicate by pointing to abstract signs and symbols. Even more fascinating is the observation that several populations of monkeys (both wild and captive) have been observed to invent gestures and use them to communicate with other monkeys!
3) Broca’s area contains mirror neurons
Mirror neurons are a class of cells which seem to be involved not only in the production of certain manual gestures, but also in the perception of those same manual gestures being produced by others (thus they “mirror” gestures, whether produced or perceived). The region in which these cells reside is critically involved in human language (Broca’s area) and similarly involved in monkey’s manual gestures.
4) Bipedalism may enable language
Corballis argues that language is gestural in origin, and that bipedalism allows for the hands to be used more freely for communicative purposes.
5) Anatomy for vocalized speech may have evolved later than gestural communication
Only homo sapiens (and perhaps also Neanderthals) show an enlargement of the hypoglossal nerve, which controls the muscles in the tongue necessary for speech. It therefore appears that speech emerged rather late, according to Corballis, suggesting a potential role for gesture in the origins of speech.
6) Gesture is integral to modern language
Corballis reviews evidence that isolated deaf children have invented their own sign languages, complete with their own forms of grammar, suggesting that there is a clear pathway from manual gesture to language. Furthermore, babies can learn to sign long before they learn to speak, and people themselves will adopt grammatical structure in their own gestures when asked to communicate through gesture alone. This evidence suggests a tight linkage between gesture and language, in some ways more close than that between speech and language.
As reviewed briefly above, Corballis proposes the following timeline for the evolution of language:
1) Bipedalism emerges 6 million years ago, and some primitive manual communication may have been enabled
2) The emergence of stone tools 2.5 million years ago, and the migration to open savanna from forested environments occurred 2 million years ago. These developments imply a shift away from manual communication (since the arms would be involved in carrying things during migration or using tools) and towards facial communication. This potentially explains why humans are the sole species with white sclera surrounding the iris: direction of gaze may have been enhanced as a communicative tool by this adaptation.
3) The third step was to add voicing to facial gestures, reflected in the lowering of the larynx to cause both swallowing and breathing to share the same passage (in which we are unique among primates, suggesting there must have been a large advantage to this change to offset the obvious cost of choking). Note that some believe this change occurred as recently as 170,000 years ago.
4) Corballis argues the final step was a cultural rather than evolutionary invention – early homo sapiens may have learned to uncouple speech from facial gesture so that speech was communicative on its own. Corballis notes that facial gestures still assume dominance over vocalizations among modern humans (see the McGurk effect), again suggesting our cultural heritage from gestural communication.
In summary, Corballis claims that handedness emerged only after speech, which was itself lateralized due to preexisting dominance of the left-hemisphere for communicative behaviors.