Is there manual babbling in sign language? Yes. Does signed language function in the same left linguistic regions of the brain as spoken language? Yes. Is a it (e.g. ASL, Auslan, etc) a real human language? Yes. Are language developmental milestones in signed language the same as spoken language? Yes. Does it have linguistics? Yes. Research studies in the fields from neuroscience to linguistics can confirm these.
Documented language acquisition of the child Juli's language development, literacy, and phonological acquisition in full-fledged American Sign Language (ASL) as a first language from birth proves to be the similar development and linguistic process as a spoken language.
In addition, this sequential bilingual child acquires English as a second language (typically at age two) shows the similar bilingual developmental milestones as those sequential bilinguals in both spoken languages.
This longitudinal documentation of the language acquistion in sign language illustrates a normal language development and literacy in sign language among Deaf children and codas who acquire ASL as a first language from birth or early childhood (e.g. deaf children of hearing parents) into adulthood.
Codas (hearing children of Deaf parents) usually have no problem acquiring English on par with English monolinguals by the preschool time earlier or later, just like spoken bilinguals. Also, ASL-speaking Deaf children of Deaf parents can acquire written English as fluently as hearing English speakers.