In this book, he puts these interests together with a solid and comprehensive survey of other background material relevant to the origins of language. The book also pushes Corballis' own specific hypothesis, that human languages were implemented mainly in manual gestures until about 50,000 years ago, at which point largely vocal language took over as an invented cultural innovation. This is an argument about the medium in which linguistic messages were expressed. Corballis believes that the human capacity for generative syntactic language may possibly be as old as one million years. The argument is much less about when true linguistic generativity arose than about the hypothesized relatively recent switch to the vocal medium.