Startup process is a series of experiments before reaching a successful product. However, it is often difficult for startup founders to learn from their own experiences as well as from other startups. In order to capture the evolution of product and customer, we developed a theoretical model for early stage software startups. The model was adopted to describe five European software startups. Data was collected by interviews and documents. The model revealed the co-evolution patterns of product and customer knowledge. In addition, it discovers a common startup patterns among our cases, such as prototype-centric development and scale-up Agile. Last but not least, the model highlights signification pivots and possible relationship among them.