Experts investigate communicative flexibility (in both form and usage of signals) as the foundation of the evolution of complex communication systems, including human language. The evolutionary roots of human communication are difficult to trace, but recent comparative research suggests that the first key step in that evolutionary history may have been the establishment of basic communicative flexibility--the ability to vocalize freely combined with the capability to coordinate vocalization with communicative intent. The contributors to this volume investigate how some species (particularly ancient hominids) broke free of the constraints of "fixed signals," actions that were evolved to communicate but lack the flexibility of language--a newborn infant's cry, for example, always signals distress and has a stereotypical form not modifiable by the crying baby. Fundamentally, the contributors ask what communicative flexibility is and what evolutionary conditions can produce it. The accounts offered in these chapters are notable for taking the question of language origins farther back in evolutionary time than in much previous work. Many contributors address the very earliest communicative break of the hominid line from the primate background; others examine the evolutionary origins of flexibility in, for example, birds and marine mammals. The volume's interdisciplinary theoretical perspectives illuminate issues that are on the cutting edge of recent research on this topic. Contributors Stéphanie Barbu, Curt Burgess, Josep Call, Laurance Doyle, Julia Fischer, Michael Goldstein, Ulrike Griebel, Kurt Hammerschmidt, Sean Hanser, Martine Hausberger, Laurence Henry, Allison Kaufman, Stan Kuczaj, Robert F. Lachlan, Brian MacWhinney, Radhika Makecha, Brenda McCowan, D. Kimbrough Oller, Michael Owren, Ron Schusterman, Charles T. Snowdon, Kim Sterelny, Benoît Testé, Gert Westermann