Aaron, P. G., & Joshi, R. M. (2006). Written Language Is as Natural as Spoken language: A Biolinguistic Perspective. Reading Psychology, 27(4), 263-311. doi:10.1080/02702710600846803
[Abstract]A commonly held belief is that language is an aspect of the biological system since the capacity to acquire language is innate and evolved along Darwinian lines. Written language, on the other hand, is thought to be an artifact and a surrogate of speech; it is, therefore, neither natural nor biological. This disparaging view of written language, even though propounded by some renowned linguists and biologists, has not gained universal acceptance. Dissenters such as linguists from the Prague circle who claim that written language is an independent system that deserves a status equivalent to that of spoken language have developed their argument along linguistic parameters. The present article also endeavors to show that written language is as natural as spoken language but does so from a biolinguistic perspective. Biolinguistics defines language as a product of biological adaptation in the Darwinian sense (Givon, 2002) and considers language to be innate and species specific (Jenkins, 2000). The present article presents evidence to show that, similar to spoken language, written language has adaptive value, evolved over time, and is relatively independent of spoken language. The Egyptian hieroglyphic writing, which has a history of about 4,000 years, is used for examining the proposition that written language evolved along Darwinian lines as much as spoken language did. It is concluded that written language is yet another manifestation of the natural endowment of the human mind and may not be treated as a proxy for speech. The educational implication is that, in literacy instruction, written language should be given as much importance in today's schools as elements of spoken language, such as phoneme awareness and phonological awareness.
[Citing Place (2000c)]  
Citing Place (2000c) in context (citations start with an asterisk *):
Section Evolutionary Nature of Language: Evidence from the Micro-Evolutionary, Neo-Darwinian Perspective
Subsection The Anatomy of Vocal Tract and Speech Perception Capabilities Are Synchronous with Language Skill
Subsubsection WRITTEN LANGUAGE
* The thumb may be considered an analog of the vocal tract when written and spoken languages are compared. It is believed that object manipulation and sound production capabilities are ontogenetically linked (Greenfield, 1991). Opposed thumb, which enables precision grip, is a hallmark of mankind, and no nonhumnan primate can duplicate it accurately (Napier, 1970). Proposing a close relationship between the presence of thumb and language, Mellars (1998) comments that it is doubtful that Neanderthals were capable of symbolic language because of the absence of a fully evolved thumb. It is possible that the neural substrate for precision thumb use might have been present in early hominids (Place, 2000) and could have been later put to exaptative use for writing.