2. Definition of Life

Abstract

The definition of life is a long-standing debate with no general scientific consensus to be expected any time soon. The underlying problem is that living systems use compounds that are abundant in the surrounding environment and processes that are not intrinsically different from reactions that occur abiologically. There does not appear to exist a single characteristic property that is both intrinsic and unique to life. Rather we have to argue that life meets certain standards, or that it qualifies by the collective presence of a certain set of characteristics. The threshold for meeting this standard sounds arbitrary, and may well be arbitrary in the sense that life presumably arose through a long sequence of “emergent events”, each at a greater level of molecular complexity and order (Hazen 2002). If that notion is correct, any rigid distinction between life and non-life is a matter of subjective judgment. While our everyday experience with life on Earth makes the distinction between the living and non-living for the most part unambiguous, a consideration of life on other worlds, where conditions may be different, and/or where life may have evolved from its inorganic precedents to a lesser degree, requires us to formulate a more formal and objective definition for life. Before doing so, we will first address the limitations of commonplace assumptions about what constitutes life.