Apicomplexa are an ancient group of single-celled pathogens of humans and animals that include the etiological agents of such devastating plagues as malaria, toxoplasmosis, and coccidiosis. The defining feature of the Apicomplexa is the apical complex, the invasion machinery used to gain access to host cells. Evidence gathered from apicomplexans and their closest relatives argues that the apical complex is an extreme example of flagellum adaptability. The value of non-apicomplexan models, such as Chromera velia, is considered in an effort to understand the modern apical complex. The origin of the apical complex is unknown, but recent evidence points to a remarkable contribution from the flagellum to its evolution.