Language Emergence.

Language emergence describes moments in historical time when nonlinguistic systems become linguistic. Because language can be invented de novo in the manual modality, this offers insight into the emergence of language in ways that the oral modality cannot. Here we focus on homesign, gestures developed by deaf individuals who cannot acquire spoken language and have not been exposed to sign language. We contrast homesign with (a) gestures that hearing individuals produce when they speak, as these cospeech gestures are a potential source of input to homesigners, and (b) established sign languages, as these codified systems display the linguistic structure that homesign has the potential to assume. We find that the manual modality takes on linguistic properties, even in the hands of a child not exposed to a language model. But it grows into full-blown language only with the support of a community that transmits the system to the next generation.