Sign language is language articulated in visual-spatial modality. It is a natural language as sophisticated and complex as any speech language. It has been primarily developed and used by culturally Deaf people.
Brain cannot tell the difference.
Studies show that processing ASL (or another signed language) activates the same linguistic regions of the left brain as spoken languages. That is, the brain cannot tell the difference between hands and lips, as renowned neuroscientist Dr. Petitto famously noted. That is, speech is not central to language.
Brain controls a language development.
Research studies show that the milestones of language acquisition (L1) in sign language is on the similar timeline as found in speech from babbling to two-word stage and beyond. (Petitto) This suggests that language development is maturationally controlled by the brain regardless of the modality (signlan or speech).
Sign language is not universal.
Like speech language, sign language is not universal nor international, contrary to common belief among hearing speakers. Signed languages around the world are as distinct as, for example, English and Japanese.
Sign language is not gestural nor pictorial.
Sign language is not made up of a standardized system of manual gestures. Otherwise, speech language would be said to be made up of a standardized system of vocal gestures. Language is made up of symbols (words) and sentence structure. Despite some iconicity (no different from speech which has some iconic sounds or "onomatopoeia"), ASL signs/words are as abstract as those of any spoken languages.
Lingustic studies show that sign languages have their own grammatical rules, syntax, phonology, morphology, and other linguistic features similar to that spoken languages have. Signed languages have their complexities.
Signed language is not a version of spoken language.
A signed language is its own. Ameslan (American Sign Language) and Auslan (Australian Sign Language), for example, are not signed versions of English nor are they based on English. They are the languages of their own.
Sign language is not a substitute of speech language.
Sign language and speech language are amodal, which mean language is independent from modality. Language is brain-run.