Those born deaf and not taught sign language will invent their own sign language. There are no human cultures without language.

Spoken (or signed) language is natural for humans, and developed as a form of telepathy. I can get a thought that's in my head into your head over a great distance transmitted over noisy media in a short time with a few words. But the thought in my head isn't in the form of an extremely ambiguous language like English or ASL or whatever I happened to learn as a child. It's the other way around: the thoughts come first.

Check out The Language Instinct by Steven Pinker for a nice overview on the scientific understanding around this.