2nd proofs e emergence of linguistic complexity Brian MacWhinney Carnegie Mellon University Linguists have oſten argued that recursion produces linguistic complexity. However, recursion relies on preexisting processes such as lexical insertion, lexical combination, memory stacks, and methods of interpretation. In the brain, recursion is an emergent property of a set of adaptations that involve at least six processing systems. Linguistic complexity arises from the interplay of all six of these systems. e complexity of this neuronal support means that the full complexity of human language could not have arisen fortuitously at some single moment in evolution. However, there is evidence that some pieces of the six systems supporting complexity have developed more recently than others. Human language is enormously complex. Each of us knows tens of thousands of words, and each of these words can specify a microcosm of meaning. When we put these words together into sentences and discourse, still further complexities arise in the form of collocations, phrases, and grammatical relations. Of the many forces that generate all this complexity, the one that has received the greatest attention is the pro- cess of recursion. Beginning with a seminal article by Bar-Hillel (1953), linguists have argued that recursion allows human beings to produce an infinite variety of possible sentences. Extending this vision, Chomsky and others have claimed that what makes human language distinct from the communication systems of all other animals is the fact that only humans have access to recursion. For decades, the characterization of recursion and its effects remained a task for linguists or computer scientists, with little connection to issues in biology. However, recently, Hauser, Chomsky, & Fitch (2002) (henceforth HC&F) hypoth- esized a link between recursion and human evolution. ey distinguished between the faculty of language broadly defined (FLB) and the faculty of language narrowly defined (FLN). Within the FLB, there are many properties that human language shares with other communication systems. For example, both birds and humans rely on a vocal apparatus to produce a broadcast transmission of sound waves that is then decoded by an auditory apparatus. In both birds and humans, all mem- bers of the species possess the mechanisms for both production and perception. More generally, even outside the FLB, humans and other mammals share com- plex systems of social organization, memory systems, and methods for learning.