Human languages provide a kind of mirror on human thought, and so we want to understand the forces that have shaped the structures we see there. Evolution provides a source of structure at two levels. First, the human organism has evolved, with linguistic abilities of certain kinds, by genetic transmission and natural selection. And second, each particular language is a cultural artifact, transmitted by learning and selected by various cultural and natural forces. In each case, we can ask: what aspects of language structure can be explained by evolutionary forces? And what other forces are shaping human languages?