Language evolution: consensus and controversies Morten H. Christiansen 1 and Simon Kirby 2 1 Department of Psychology, Uris Hall, Cornell University, Ithaca, NY 14853, USA 2 School of Philosophy, Psychology and Language Sciences, 40, George Square, University of Edinburgh, Edinburgh EH8 9LL, UK Why is language the way it is? How did language come to be this way? And why is our species alone in having complex language? These are old unsolved questions that have seen a renaissance in the dramatic recent growth in research being published on the origins and evolution of human language. This review provides a broad overview of some of the important current work in this area. We highlight new methodologies (such as computational modeling), emerging points of consen- sus (such as the importance of pre-adaptation), and the major remaining controversies (such as gestural origins of language). We also discuss why language evolution is such a difficult problem, and suggest probable direc- tions research may take in the near future. Language is one of the hallmarks of the human species – an important part of what makes us human. Yet, despite a staggering growth in our scientific knowledge about the origin of life, the universe and (almost) everything else that we have seen fit to ponder, we know comparatively little about how our unique ability for language originated and evolved into the complex linguistic systems we use today. Why might this be? When Charles Darwin published his book, The Origin of Species, in 1859 there was already a great interest in the origin and evolution of language. A plethora of ideas and conjectures flourished but with few hard constraints to limit the realm of possibility, the theorizing became plagued by outlandish speculations. By 1866 this situation had deteriorated to such an extent that the influential Socie´te´ de Linguistique de Paris imposed a ban on all discussions of the topic and effectively excluded all theorizing about language evolution from scientific dis- course for more than a century. Fueled by theoretical constraints derived from advances in the brain and cognitive sciences, the field of language evolution finally emerged from its long hiatus as a legitimate area of scientific enquiry during the last decade of the twentieth century. Considerable progress has been made since then, but the picture that is emerging is highly complex (see Box 1). Understanding language evolution poses many challenges for contemporary science. Here we provide a broad overview of the current state of the art, focusing on major points of consensus as well as the remaining controversies. Major points of consensus The necessity of interdisciplinary collaborations One might expect linguists to contribute the most to research on language evolution, but this is not the case. In fact, most language evolution researchers do not have a background in linguistics, but instead come from one of many other disciplines within the cognitive sciences and elsewhere. Although this may be a legitimate cause for concern among linguists [1], it is perhaps better seen as a testament to the cross-disciplinary nature of the field of language evolution (see Fig. 1). Indeed, possibly the strongest point of consensus among researchers is that to fully understand language evolution, it must be approached simultaneously from many disciplines [1–5]. We must understand how our brains work; how language is structured and what it is used for; how early language and modern language differ from each other and from other communication systems; in what ways the biology of hominids have changed; how we manage to acquire language during development; and how learning, culture and evolution interact. Thus, language evolution research must necessarily be cross-disciplinary in order to provide sufficient constraints on theorizing to make it a legitimate scientific enquiry. Nevertheless, most researchers in language evolution only cover parts of the relevant data, perhaps for the reason that it is nearly impossible to be a specialist in all the relevant fields. Still, as a whole, the field appears to be moving in the direction of becoming more interdisciplin- ary. Collaborations between researchers in different fields with a stake in language evolution (such as [5,6]) are likely to become increasingly more important. Exploring language evolution through computational modeling Another emergent area of consensus is the growing interest in using computational modeling to explore issues relevant for understanding the origin and evolution of language (see Box 2). Many researchers across a variety of different disciplines now either conduct language evol- ution simulations or refer to such work as evidence for particular theoretical perspectives. For example, modeling work has been used to inform high-level theories about BIOLOGICALADAPTATIONS (see Glossary) for grammar [7–10] Corresponding author: Morten H. Christiansen (mhc27@cornell.edu). Review TRENDS in Cognitive Sciences Vol.7 No.7 July 2003 300 http://tics.trends.com 1364-6613/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1364-6613(03)00136-0