Conceiving of development with reference to homology can help identify developmental continuity where surface form shows considerable variation across age. I argue that there is a homology of structure between the object-centred, or triadic, interactions that emerge in infancy and later language. The structure of triadic interaction in infancy is first described as involving joint attention and joint engagement about a shared topic, and then a case is made that this structure is maintained through three levels of complexity in language—single word utterances, multiword utterances, and finally complex constructions. A focus on the homological relation between these social interactive structures may be useful in revealing developmental continuities where these may be obscured by quite different surface forms. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 55: 59–66, 2013.