One of the most promising technologies for discovering and studying nearby (< 30pc) Terrestrial exoplanets is nulling interferometry in the mid-infrared. This has been proposed by a number of researchers, dating back to the 1980;ties, and have now been studied technologically and scientifically since several years by the European Space Agency in the context of the Darwin mission study. Significant technology developments have been initiated in a number of critical techniques. Nevertheless, several problems remain to be overcome, relating mainly to the strict requirements on path compensation, thermal aspects due to the requirement that one need to operate in the mid-infrared in order to detect characteristic signatures of terrestrial type planets, and the significant level of complexity required by the mission as a whole. In this paper we discuss the current results in relation to the set of scientific goals that have been formulated in Europe for the topic of understanding the formation and evolution of planets like our own. Being one of the most ambitious undertakings of any space agency ever, one has to ask if there exist the possibility of dividing up the mission in steps, allowing significant simplifications and lowering of cost for each such step, while retaining for each one a compelling scientific case. Here we describe current investigations into this problem and outline the current level of understanding in this context and present possible scientific objectives for such a stepwise approach.