As a feature of life on earth, language is one of science's great remaining mysteries. A central difficulty is that it appears so radically incommensurate with nonhuman systems of communication as to cast doubt on standard neo-Darwinian accounts of its evolution by natural selection. Yet scientific (as opposed to religious or philosophical) arguments for a discontinuity between human and animal communication have come into prominence only over the past 40 years. As long as behaviourism dominated anglophone psychology and linguistics, the transition from animal calls to human speech seemed to offer no particular difficulty (see, for example, Mowrer 1960; Skinner 1957). But the generative revolution in linguistics, begun with the publication of Noam Chomsky's Syntactic Structures in 1957 and developed in many subsequent works (e.g. Chomsky 1965, 1966, 1972, 1975, 1986; Chomsky and Halle 1968) radically altered our conception of language, and posed a challenge to evolutionary theory that we are still striving to meet.

The central goal of Chomsky's work has been to formalise, with mathematical rigour and precision, the properties of a successful grammar, that is, of a device for producing all possible sentences, and no impossible sentences, of a particular language.