This paper is more about the questions for a theory of language evolution than about the answers. I'd like to ask what there is for a theory of the evolution of language to explain, and I want to show how this depends on what you think language is.
So, what is language? Everybody recognizes that language is partly culturally dependent: there is a huge variety of disparate languages in the world, passed down through cultural transmission. If that's all there is to language, a theory of the evolution of language has nothing at all to explain. We need only explain the cultural evolution of languages: English, Dutch, Mandarin, Hausa, etc. are products of cultural history.
However, most readers of the present volume probably subscribe to the contemporary scientific view of language, which goes beneath the cultural differences among languages. It focuses on individual language users and asks:
(Structure and acquisition of language competence) What is the structure of the knowledge that individual language users store in their brains, and how did they come to acquire this knowledge?
The question of acquisition leads to an important corollary question:
(Structure of capacity to learn language) What is the structure of the knowledge/ability in the child that makes language acquisition possible?
This latter knowledge is independent of what language the child actually learns in response to the environment. It is closer to what is generally called the language capacity (or the language instinct or universal grammar or the language acquisition device).