How diverse are the world's sign languages?

The answer

Contrary to common assumption, sign languages are not visual versions of spoken languages; they are fully independent natural languages with their own grammar, syntax, and vocabulary. There are over 300 documented sign languages worldwide. American Sign Language (ASL) is more closely related to French Sign Language (LSF) than to British Sign Language (BSL), because ASL was developed with French Deaf educators in the early 19th century. Nicaraguan Sign Language (ISN) emerged spontaneously in the 1980s when deaf children were first brought together in schools, providing linguists with a rare opportunity to observe a new language being born. Al-Sayyid Bedouin Sign Language in Israel developed independently in a village with a high rate of hereditary deafness. Sign languages use space, movement, facial expressions, and body posture as grammatical elements in ways that have no parallel in spoken language.