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The primacy of language for Homo sapiens is demonstrated by the fact that when the main channel of language, speech (in the auditory-vocal channel), is unavailable, another channel is recruited spontaneously: the visual-manual channel. The type of language that emerges using this channel is called sign language.
Visual-manual languages have emerged independently and repeatedly throughout history in unrelated human societies around the world. There are at least two types of circumstance under which this occurs: (1) among humans with profound hearing loss (e.g., Nicaraguan Sign Language [Kegl 2004] or Martha’s Vineyard Sign Language [Groce 1985]), and (2) in cases of severe social restrictions on speech (e.g., monastic sign languages [Umiker-Sebeok & Sebeok 1987], widows in Warlpiri society in Australia [Kendon 1986]). It has also been used in some language contact situations, though the origins of these systems are unknown, and could have been influenced by existing sign languages arising from deaf populations (Indian Plains Talk , Meso-American Sign Language Complex [Fox Tree 2009]).
When a community arises with many human individuals using a visual-manual medium, the communication system can develop language-like properties within a few generations (in a school environment, a “generation” may be as little as one year). That is, the emerging language becomes highly conventionalized, fluent, minutely synchronized, abstract, ordered, and possible of conveying any conceivable concept. These languages also contains form-meaning mappings specific to the language community – there is no “universal sign language,” just as there is no universal spoken language.
It should be noted that exclusively tactile language also is used by deaf-blind humans. However, it is not known whether language in this channel has ever emerged spontaneously; all known examples of tactile language are modified from sign language.
The non-human great apes lack the vocal organ morphology that would produce human-like speech (see Tongue & Vocal Tract), but neither do they show evidence of trying to “compensate” for this by using gestures or tactile signs to communicate ideas in the same way that humans do.
Some non-human primates have been taught to use human sign language. However, there are three notable differences between the signing of humans and non-human primates. First, only a few individual non-human primates learn to use signs; this is in contrast to the universal success of human children learning sign language. Second, those non-human primates who have learned to sign succeeded only through lengthy and explicit teaching, coupled with rewards (behavioral conditioning). This is in contrast to the effortless, rapid, and automatic acquisition of sign language by human children. And third, the sign language acquired by non-human apes is a limited subset of what humans use, even in the best cases: vocabulary is much smaller and sentence structure is rudimentary, if present at all.
In general, it appears that the motivation to communicate with high levels of information-density in human populations results in the recruitment of other available communication modalities when the auditory-vocal channel is not available.
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