Definition of life

Abstract

The definition of life is a long-standing debate with no broadly accepted scientific consensus (Kolb 2007). The underlying problem in defining life is twofold. The first 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 second problem in defining life is linguistic. Life, grammatically, is a noun, which therefore calls for definition in terms of other nouns. Yet life is much more like a verb than a noun - more a process than an entity (Margulis and Sagan 1995). Defining life is analogous to defining wind. Wind is air in motion, a state of being. The molecules of wind are the same as those of air, but their dynamic state is their defining characteristic. Life can certainly be defined through a collection of properties, but the search for life wherever it occurs depends ultimately on an ability to recognize individual entities with the properties of being alive, as distinct from their non-living surroundings. And, while life is like the wind in its pervasive distribution, it differs from wind in having finite boundaries. The challenge of defining life in the context of a cosmic biology, therefore, is to specify the defining characteristics of a dynamic process that occurs within discrete boundaries that set the process apart from its environment. 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. © Springer-Verlag Berlin Heidelberg 2008.