Software ecosystems is an emerging trend within the software industry, implying a shift from closed organizations and processes towards open structures, where actors external to the software development organization are becoming increasingly involved in development. This forms an ecosystem of organizations that are related through the shared interest in a software product, leading to new opportunities and new challenges to the industry and its organizational environment. To understand why and how this change occurs, we have followed the development of a software product line organization for a period of approximately five years. We have studied their change from a waterfall-like approach, via agile software product line engineering, towards an emerging software ecosystem. We discuss implications for practice, and propose a nascent theory on software ecosystems. We conclude that the observed change has led to an increase in collaboration across (previously closed) organizational borders, and to the development of a shared value consisting of two components: the technology (the product line, as an extensible platform), and the business domain it supports. Opening up both the technical interface of the product and the organizational interfaces are key enablers of such a change.