Who invented sign language?

Answer 1

No one invented sign language. Languages develop among a community over time. If you asked the same question about Spanish or French you would get the same answer. No one in particular invented these languages. The users of the language with a shared need to communicate with one another organically develop the language as they unconsciously agree that certain vocal sounds will mean a particular thing. It works the same way for sign languages.

Answer 2

There are many, many forms of sign language, and it varies all over the world. No single person came up with it. Many people misperceive American Sign Language as the universal sign language - that is not true. There is ASL, along with British Sign Language, French Sign Language, and so many more. There are indigenous cultures who use sign language regardless of their hearing status. There are also English-based forms of sign language that differ from ASL. Much of ASL was derived from French Sign Language (LSF), but it has evolved over time, and it continues to. Just like any other language, visual languages change as culture changes.