The aim of this article is to present an evolutionarily grounded explanation of why we speak in sentences. This question is seldomly addressed, neither in the Chomskian tradition nor in cognitive linguistics. I base my explanation on an analysis of different levels of communication. I identify four levels: praxis, instruction, coordination of common ground and coordination of meaning. The analysis will be focused on the evolutionary benefits of communicating about events as a way of coordinating actions. A cognitively grounded model (...) of events will be outlined. My central thesis is that the communicative role of sentences is to express events. (shrink)