Abstract
Life on Earth has evolved as a series of evolutionary transitions, during which lower-level units merged to form a new and more complex higher-level entity. Besides few canonical examples, many life forms exist for which it remains unclear whether or not they are about to complete the transition. This paucity of mechanistic understanding is likely due to an overemphasis on few model systems and a lack of criteria to compare disparate biological units. Here, we aim at filling this gap by proposing a new framework to classify different forms of biological organization, which considers two fundamental aspects: (i) the physiological component and (ii) the evolutionary component. Categorizing different biological units according to whether and how these aspects are represented yields six types of structural organization. Our framework allows to compare different organizational forms, and, in this way, provide insight into the evolutionary processes giving rise to these arrangements.