- Radu Popa: Portland State University, Portland, OR, USA. rpopa@pdx.edu
The criteria used for defining life are influenced by various philosophical visions about life, ranging from holism to reductionism and from mechanistic-reductionism to vitalism. Using different scenarios about the origin and evolution of life as well as properties of energy-dissipative systems, artificial life simulations and basic tenets of xenobiology, guidelines can be established for formulating a definition of life. A definition of life is proposed that is parametric, non-Earth-centric, quantitative and capable of discriminating 'living entities' from 'life'. Living entities are defined as self-maintained systems, capable of adaptive evolution individually, collectively or as a line of descend. Life is a broader concept indicating that the capacity to express these attributes is either virtual or actual. At least four major phase transitions can be recognized during the origin of life (reflexive activity; self-regulated homeostasis; the advent of informatons and the origin of adaptive evolution); these make the origin and evolution of early life an example of 'punctuated gradualism'. Such phase transitions can be used to identify a boundary in early evolution where life began. This contribution identifies the step in the evolution of a dynamic system when digital control of the system's state becomes dominant over analogical control, and genetic information is irreversibly used for adaptive evolution, as the boundary between non-living and living systems.