The rules of syntax and intonation and words matured over time into the system we have today because they were progressively refined by use and the forge of survival and reproduction - not because the brain got big and complicated for some other reason, and all of a sudden we discovered we could now manipulate symbols as well.

[I]t is almost impossible to talk about space without gesturing. Gesture is spontaneous, and is integral to individual expression as it is to communication. Even though you probably won't gesture as much if you are talking on the phone, you will still wave your arms about. Blind people gesture when they speak in the same way that seeing people do.

In our evolutionary history some individuals must have been born with a greater inclination and ability to collaborate than our common ancestor with chimpanzees. These individuals were more successful and bred more offspring with those characteristics [...]. What we have evolved into now is a species for whom an experience means little if it's not shared.

At its most fundamental, language is an act of shared attention, and without the fundamentally human willingness to listen to what another person is saying, language would not work.