Conclusion: The development of conceptualization of a biological basis of language during the 20th century has come about, in part, through the appreciation of the central nervous system's ability to utilize varied sensory inputs, and particularly vision, to develop language.
Objective: Sign language has been a part of the linguistic experience from prehistory to the present day. Data suggest that human language may have originated as a visual language and became primarily auditory with the later development of our voice/speech tract. Sign language may be categorized into two types. The first is used by individuals who have auditory/oral language and the signs are used for special situations, such as communication in a monastery in which there is a vow of silence. The second is used by those who do not have access to auditory/oral language, namely the deaf.
Material and methods: The history of the two forms of sign language and the development of the concept of the biological basis of language are reviewed from the fourth century BC to the present day.
Results: Sign languages of the deaf have been recognized since at least the fourth century BC. The codification of a monastic sign language occurred in the seventh to eighth centuries AD. Probable synergy between the two forms of sign language occurred in the 16th century. Among other developments, the Abbey de L'Epée introduced, in the 18th century, an oral syntax, French, into a sign language based upon indigenous signs of the deaf and newly created signs. During the 19th century, the concept of a "critical" period for the acquisition of language developed; this was an important stimulus for the exploration of the biological basis of language. The introduction of techniques, e.g. evoked potentials and functional MRI, during the 20th century allowed study of the brain functions associated with language.