The human brain re-organizes itself again at 12 months into a single unit
Around one year for humans only (and earlier maybe for some chimps and gorillas), the human brain achieves yet a higher level of re-organization, and is now capable of functioning as a single unit.
It is at this point, Luria hypothesized, that symbolic behavior becomes possible. It is about this time also that children begin to use symbols to talk, albeit in one word sentences. The language bridge has begun to take shape.
In discussing the milestones of speech, books often relate these behaviors to those of general motor development. This is not to suggest, necessarily, that speech development is dependent upon motor development. Rather they are intended to serve as useful measures to obtain perspective.
For example, both speech and language have a maturational timetable for development. If one or the other alone lags, different etiologies are suggested. If both are slow in developing, yet a third range of possibilities becomes suspect.