Clearly, if we are going to talk about biology, and organisms and cells and such, we have to define exactly what we mean by life. This raises a problem peculiar to biology as a science. We cannot define life generically because we know of only one type of life. We do not know whether this type of life is the only type of life possible or whether radically different forms of life exist elsewhere in the universe or even on Earth, in as yet to be recognized forms.
While you might think that we know of many different types of life, from mushrooms to whales, from humans to the bacterial communities growing on the surfaces of our teeth (that is what dental plaque is, after all), we will discover that the closer we look the more these different “types of life” are in fact all versions of a common underlying motif, they represent versions of a single type of life. Based on their common chemistry, molecular composition, cellular structure, and the way that they encode hereditary information in the form of molecules of deoxyribonucleic acid (DNA), all topics we will consider in depth later on, there is no reasonable doubt that all organisms are related, they are descended from a common ancestor.
We cannot currently answer the question of whether the origin of life is a simple, likely, and predictable event given the conditions that existed on the Earth when life first arose, or whether it is an extremely rare and unlikely event. In the absence of empirical data, one can question whether scientists are acting scientifically or more as lobbyists for their own pet projects when they talk about doing astrobiology or speculating on when and where we will discover alien life forms. That said, asking seemingly silly questions, provided that empirically-based answers can be generated, has often been the critical driver of scientific progress. Consider, for example, current searches for life on Earth, almost all of which are based on what we already know about life. Specifically, most of the methods used rely on the fact that all known organisms use DNA to encode their genetic information; these methods would not be expected to recognize dramatically different types of life; they certainly would not detect organisms that used a non-DNA method to encode genetic information. If we could generate living systems de novo in the laboratory we would have a better understanding of what functions are necessary for life and how to look for possible “non-standard” organisms using better methods. It might even lead to the discovery of alternative forms of life right here on Earth, assuming they exist.28 That said, until someone manages to create or identify such non-standard forms of life, it seems quite reasonable to concentrate on the characteristics of life as we know them.
So, let us start again in trying to produce a good definition, or given the fact that we know only of one version of life, a useful description of what we mean by life. First, the core units of life are organisms, which are individual living objects. From a structural and thermodynamic perspective, each organism is a bounded, non-equilibrium system that persists over time and, from a practical point of view, can produce one or more copies of itself. Even though organisms are composed of one or more cells, it is the organism that is the basic unit of life. It is the organism that produces new organisms.29
Why the requirement for and emphasis on reproduction? This is basically a pragmatic criterion. Assume that a non-reproducing form of life was possible. A system that could not reproduce runs the risk of death (or perhaps better put, extinction) by accident. Over time, the probability of death for a single individual will approach one–that is, certainty.30 (→) In contrast, a system that can reproduce makes multiple copies of itself and so minimizes, although by no means eliminates, the chance of accidental extinction, the death of all of its descendants. We see the value of this strategy when we consider the history of life. Even though there have been a number of mass extinction events over the course of life’s history,31 organisms descended from a single common ancestor that appeared billions of years ago continue to survive and flourish.