Children make the transition from protolanguage to language typically in the second half of the second year of life. The transition has been described in detail elsewhere, based on intensive observations of individual children (see especially Painter, 1984, 1989; Oldenburg, 1987); we may assume that in general terms they are recapitulating the phylogenetic evolution of language, although of course we can only speculate about the way that evolution took place (it is important to say explicitly that all human languages known today are equally far removed from that phase in our semiotic history). During that stage they learn to construe elements and figures, and in this way "semanticise" both the construction of experience and the enactment of interpersonal relations. In terms of the grammar, they learn to form groups and clauses, and to select systemic options simultaneously in transitivity and in mood.
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[1] To be clear, ontogenesis does not recapitulate phylogenesis, in either life or language. For example, the phylogenesis of Modern English includes stages of both Anglo-Saxon and Middle English, but the ontogenesis of Modern English does not.
A more plausible claim would be that present-day ontogenesis of language from protolanguage still broadly resembles this ontogenesis at the time that language first emerged.
[2] To be clear, 'all human languages known today are equally far removed from that phase in our semiotic history' in the same sense that all extant ape species are equally far removed from the evolutionary split from what became the monkey lineage. However, like the different ape species, different languages do not change at the same rate in phylogenetic time.