What role does language play in our lives? I'm sure we can agree it is fundamental to much of what we do --some say it's what made us "human."
Investigation of language functions is important on its own terms; in addition it is impossible to make progress on the topic of language evolution without a clear sense -- however speculative-- on the adaptive aspect of human language. Charles Darwin, for example, suggested the following:
"As the voice was used more and more, the vocal organs would have been strengthened and perfected through the principle of the inherited effects of use...but the relation between the continued use of language and development of the brain, has no doubt been far more important....we may confidently believe that the continued use and advancement of this power would have reacted on the mind itself, by enabling it and encouraging it to carry on long trains of thought." (Origins, 1871)
While Darwin believed language evolved from the calls and cries of animals, it is clear in the above passage that Darwin suggests it was the enabling of "thought" that drove the evolution of language.
We know that all animals engage in some form of social communication. Humans, as well as other primates, share in these basic functions in addition to whatever advantages human language itself provides. Jane Goodall notes in the "Language" video that human language is the single most important difference between humans and chimpanzees. That chimpanzees have been as successful as they have been for several million years indicates human language is hardly necessary for creatures sharing many of our human characteristics.
In addition to the role of language in interpersonal communication, we can look to its role within individual humans and also its function within human society. These functions, of course, will not be independent of one another, but by focussing on individuals and societies, we may get away from overemphasizing communication.
Language, in addition to nonverbal sources, communicates emotional states, sex of speakers, and even their geographical origins. To a large extent this reflects our mammalian heritage.
Beginning with prelinguistic communication between mother and infant, intentional linguistic communication becomes the dominant mode of human communication. This includes "small talk" and gossip forming and maintaining social relationships, as well as the explicit linguistic speech acts of stating, questioning, commanding, promising, etc.
Here language supports thought, rationalization and attribution, creativity, memory, self-direction, self-expression, humor, and perhaps even aspects of consciousness.
Building upon both interpersonal and individual functions, a language in a sense defines a society. Speakers of the same dialect share an immediate sense of recognition. In addition the traditions, myths and religions of every human society are encoded in their language. Could they even exist without language? Finally, science itself --human's ultimate adaptive tool -- is based on human language.