In The Gestural Origin of Language, Wilcox and Armstrong use evidence from and about sign languages to explore the origins of language as we know it today. According to their model, it is sign, not spoken languages, that is the original mode of human communication. The authors demonstrate that modern language is derived from practical actions and gestures that were increasingly recognised as having the potential to represent and hence to communicate. In other words, the fundamental ability that allows us to use language is our ability to use pictures of icons, rather than linguistic symbols. Evidence from the human fossil record supports the authors' claim by showing that we were anatomically able to produce gestures and signs before we were able to speak fluently. Although speech evolved later as a secondary linguistic communication device that eventually replaced sign language as the primary mode of communication, speech has never entirely replaced signs and gestures.