Download the 2010 edition of Langauge and Thinking. 

From the Introduction:

"Humans resemble other animals in their ability to communicate. Communications involve chemical senses, sounds, body language, and visual signals. Communication is all about community, sharing information, sending warning signals and fulfilling the needs of the group. Human languages combine many different expressions of communication in a complex manner. Ideas about written language tend to dominate scholarly investigations, but sounds and gestures have been more important in the evolution of communication systems.

Humans speak naturally and spontaneously and learn the language spoken around them. Babies start to say words about 12 months. In the second year, a child develops vocabulary of about 250 words and makes simple statements.  Children use correct sentence structures by the age of three. Vocabulary increases to about 2600 words at the age of six. Learning language is a variable and creative process. While there are sounds such as ma ma that all human babies tend to say, infants bable and coo, often inventing sounds and syntax with individual content. Babies spontaneously make non-verbal sounds that with brain maturation and practice gradually form some sounds into recognizable words.

Speaking is a spontaneous feature of the brain, and all normal children will speak if they hear a language spoken; any language will do. Older infants imitate words they hear spoken and if adults engage them in conversation, will expand their vocabularies and start to make meaningful statements.  Adults spontaneously speak “baby talk” to infants using high pitched, somewhat melodic and non-verbal sounds, exaggerated facial expressions and hand gestures. Babies like the entertainment and babble and coo in response. This mimetic exchange marks the beginning of human conversation. Human conversations always retain an infrastructure of nonverbal sound communication.

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