Despite their importance little is known about how innovation ecosystems come into existence. We address this gap through an historical case study of Herceptin, a revolutionary drug developed for the treatment of ovarian and breast cancer, and the innovation ecosystem that emerged around this drug between 1978 and 1998. Through qualitative content analysis of a broad scope of archival documents (2474 in total), we define a cast of roles and determine their timing of entry onto the stage of ecosystem emergence, and in turn describe the interaction of these roles that govern emergence. We find that the locus of ecosystem emergence shifts gradually from discovery, resource provision and commitment, to the formation of connections and trust, and finally to complementarity and value creation. These activities are facilitated by specific roles that gain significance at various points in time. We additionally witness shifts in interaction dynamics, from individual level interactions early on, to interactions across levels, and finally to interactions at the organisational level. We synthesize these findings to propose a framework of a processual understanding of how innovation ecosystems come into existence.