Chapter 2

What is Life?

Synopsis

What is life? although most people have an intuitive understanding of whether something is alive, we still lack of a concrete definition of life.This simple question is surprisingly difficult to answer; yet, it is fundamental to understanding life as a universal phenomenon and to search for it elsewhere. Imagine the difficulty of identifying life on other worlds without a clear understanding of what similarities it may or may not share with life on Earth. Or, consider the limitations of interpreting possible origin-of-life scenarios on Earth without distinguishing between primitive life-forms and the non-living entities from which they must have emerged. So far, we must admit that defining life, despite significant effort invested on it, has been elusive. This may be due to limitations in our understanding of the fundamental components of life, or in the way we frame it to be empirically and iteratively tested. However, the very exercise of studying life’s most essential characteristics has revealed many exciting insights and may uncover its true nature in the future.

We begin this chapter by providing a historical framework of the concept. We then discuss key features of scientifically useful definitions, and outline why they matter. Next, we further discuss a diversity of definitions before outlining the central properties (biochemical, genetical and physical) of life as we know it and we give examples across the life spectrum. Finally, we give examples of how definitions of life are guiding astrobiology research on the origins of life, the synthesis of artificial life, and the hunt for life on other worlds. We close by providing thoughts on future research directions that may help better probe and constrain the concept of life as an integral part of the wider universe.