Ontogeny of Language
Studies show that manual babbling exists and occurs in deaf infants who are exposed to sign languages from birth. The similarities between manual and vocal babbling suggest evidence for the ontogeny of language, whether it is spoken or signed. This suggests that the "speech modality is not critical in babbling." Longitudinal studies also show the similarities of developmental patterns of language and communication in deaf and hearing --
from manual/vocal babbling
to manual/vocal gesture (prelinguistic communication)
to early lexical development to full-fledged language.
Regardless of the modality of language, this suggests that babbling is "determined by an amodal brain-based language capacity." (Petitto and Marentette).
Petitto, Laura Ann and Marentette, Paula. "Babbling in the Manual Mode: Evidence for the Ontogeny of Language." Science Reports. Vol 251, 22 March 1991. Pages 1493-1496.
Petitto, Dr. Laura Ann. Untitled. Montreal: McGill University media. October 28, 1997.
Corballis, Michael C. From Hand to Mouth: the Origins of Language. Princeton University Press, 2002.
Bauman, H-Dirksen. Open Your Eyes: Deaf Studies Talking. "On the Disconstruction of (Sign) Language in the Western Tradition: A Deaf Reading of Plato's Cratylus." pp 127-145.