Fig. 1.

The levels of life. All life-forms follow a simultaneous trajectory within the three parallel state spaces or levels governed by material properties, constraint surfaces, and optimization principles. In L1, each phylogeny illustrates a possible evolutionary trajectory, each of which is associated with a different material origin. A history-centric approach to life equates life with a complete phylogenetic history. Extant-centric approaches seek commonality across the terminal branches of phylogenies. All points in L1 map many-to-one to points in L2. The set of points in L2 describes the space of physical constraints to include the limitations of physical laws. Evolved constraints are the sub-set of points in L2 that we describe as the physics of living systems. All points in L1 and L2 obey action or optimization principles that are defined by the set of points in L3. A small set of optimization principles such as the maximization of fitness and related concepts define the space of living action principles in L3. A principle-centric approach to life defines life in terms of the entry and restriction of a material trajectory within L1 that is constrained in L2 and only moving within the restricted space of living optimization principles in L3. Each material phylogeny in L1 is likely to be different across the universe, but can still map onto similar or identical sets of physical constraints in L2. For example, the blue and red phylogenies in L1 map onto the same set of constraints in L2. These in turn project into the space of the living in L3. In addition, living systems might produce non-living descendants. Here, we have shown in orange how a putative AI might originate from the terminal biotic branch of the green phylogeny and venture outside biology to be governed by the constraints of engineering in L2 and non-living optimization principles in L1. The reverse is also possible where abiotic materials give rise through biotechnology to new biotic life-forms. The non-unique trajectories through L1-L3 allow for the possibility of multiple life (Color figure available online; Image credit: Mesa Schumacher)