[...] language is built up by children, as language evolved in the human species it began without any grammar at all; it was a two-level system, with meanings coded directly into expressions (sounds and gestures). This at least is how children’s ʻprotolanguageʼ is organized, the symbolic system they usually construct for themselves before starting on the mother tongue. This is then replaced, in the second year of life, by a three-level system in which meanings are first coded into wordings and these wordings then recoded into expressions. There were various reasons why this step had to be taken if the system was to expand; it opened up both the potential for dialogue, the dynamic exchange of meanings with other people, and the potential for combining different kinds of meaning in one utterance – using language both to think with and to act with at the same time.
- Halliday (1985), Pag. XVII-XVIII