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Throughout this textbook, the word “life” is pervasive. You will come across discussions on “the origin of life,” “life in extremes,” “evidence of life in the fossil record,” “life on other planets,” “biosignatures of life,” and so forth. However, implicit in all these discussions is the idea that we all agree on what life is. This chapter will explore the idea of “life.”

Attempts to understand what constitutes life are important for many areas of astrobiology. If we seek to understand how and when the origin of life occurred, we must agree when the transition from mere molecules to “living” things occurred, and that requires that we have some agreement on what constitutes a living thing. If we are to seek life on other worlds, we must agree what it is we are looking for. We must agree what types of entities would be of interest as exemplars of living things and what types of materials would not constitute life.

Similar concerns exist in a field such as synthetic biology. Scientists must agree when they have artificially created a living thing in the laboratory. Another application of the concept of life is in the computational field of artificial life, where computer programming is used to make artificial digital entities that reproduce and evolve. To discern whether a digital entity created in the computer could be described as “artificial life,” we must agree on what a living thing is.

There are numerous books and papers that explore the characteristics of the material we call “life.” This textbook is not intended as an exhaustive scientific and philosophical review. The objective of this chapter is to introduce some of the basic ideas so that you can explore this topic for yourself. However, given the widespread use of the word life, it is an important topic to explore.

As with astrobiology itself, it is always useful to explore some historical perspective since it helps to provide an understanding of what people thought in the past and why certain paradigms might still persist.