This book puts forward a new approach to language change, the punctuated equilibrium model. This is based on the premise that during most of the 100,000 or more years that humans have had language, states of equilibrium have existed during which linguistic features diffused across the languages in a given area so that they gradually converged on a common prototype. From time to time, the state of equilibrium would be punctuated, with the expansion and split of peoples and of languages. Dixon argues that the commonly used family tree model of language change is appropriate during a period of punctuation, but not during the longer periods of equilibrium (and that the original languages of humankind cannot be reconstructed).