Halliday & Matthiessen (2014: 138):
Now, in the life history of an individual child, the exchange of goods–&–services, with language as the means, comes much earlier than the exchange of information: infants typically begin to use linguistic symbols to make commands and offers at about the age of nine months, whereas it may be as much as nine months to a year after that before they really learn to make statements and questions, going through various intermediate steps along the way. It is quite likely that the same sequence of developments took place in the early evolution of language in the human race, although that is something we can never know for certain.