Linguists dismissed sign language as mere gesture until one man proved otherwise

Before 1960, American Sign Language was widely treated as simplified pantomime rather than a real language. Linguist William Stokoe filmed and analysed ASL in detail and showed it has its own systematic phonology, syntax and grammar, entirely independent of English — a finding that took years to be accepted but eventually established sign languages as full human languages in their own right.

— William C. Stokoe, Sign Language Structure: An Outline of the Visual Communication Systems of the American Deaf — Studies in Linguistics, Occasional Papers 8, 1960