What Is Life?
An early question that needs to be confronted, indeed a question that in the last analysis requires definition, is: What is life? Most biologists would agree that self-replication, genetic continuity, is a fundamental trait of the life process. Systems that generally would be deemed nonbiological can exhibit a sort of self-replication, however (2). Examples would be the growth of a crystal lattice or a propagating clay structure. Crystals and clays propagate, unquestionably, but life they are not. There is no locus of genetic continuity, no organism. Such systems do not evolve, do not change in genetic ways to meet new challenges. Consequently, the definition of life should include the capacity for evolution as well as self-replication. Indeed, the mechanism of evolutionnatural selectionis a consequence of the necessarily competing drives for self-replication that are manifest in all organisms. The definition based on those processes, then, would be that life is any self-replicating, evolving system.
The processes of self-replication and evolution are not reliably detectable, even in the terrestrial setting. Consequently, in the practical search for life elsewhere we need to incorporate information on the nature of the chemistries that can provide the basis for self-replication and evolution. Considering the properties of molecules likely to be needed to replicate and evolve, it is predictable that life that we encounter anywhere in the universe will be composed of organic chemicals that follow the same general principles as our own organic-based terrestrial life. The operational definition of life then becomes: Life is a self-replicating, evolving system expected to be based on organic chemistry.