Halliday & Matthiessen (2014: 7):
Since language evolved as speech, in the life of the human species, all writing systems are in origin parasitic on spoken language (cf. Halliday, 1985a; Matthiessen, 2006b); and since language develops as speech, in the life of every hearing individual, this dependency is constantly being re-enacted. Even with the deaf, whose first language uses the visual channel, this is not writing; Sign is more closely analogous to spoken than to written language, signs being in a sense visible forms of articulation and facial expressions visible prosodies. But as writing systems evolve, and as they are mastered and put into practice by the growing child, they take on a life of their own, reaching directly into the wording of the language rather than accessing the wording via the sound; and this effect is reinforced by the functional complementarity between speech and writing. Writing evolved in its own distinct functional contexts of book keeping and administration as ‘civilisations’ first evolved – it never was just ‘speech written down’; and (at least until very recent advances in technology) the two have continued to occupy complementary domains.