Faced with the question ‘what is life?’ many scientists, and some philosophers, advance definitions of life. Defining life is especially popular among astrobiologists, many of whom are convinced that one cannot successfully search for truly novel forms of microbial life without a definition of life: How else will one recognize it if one encounters it? The extensive discussion of definitions of life in Part 2 (“Definition and nature of life”) of the CRC Handbook of Astrobiology (Kolb 2018) provides a salient illustration of this attitude. Along the same lines, a recent version of the NASA Astrobiology Strategy (Hays 2015) contains a large section devoted to “Key Research Questions for Defining Life” (p. 145).1 This chapter and the next explain why the scientific project of defining life is mistaken. Life is not the sort of thing that can be successfully defined. In truth, a definition of life is more likely to hinder than facilitate the discovery of novel forms of life.

There are universal theories in physics and chemistry but no universal theories in biology. The failure of biologists to come up with such a theory is not due to a lack of effort. Philosophers and scientists have struggled to formulate universal principles of life since at least the time of Newton. This chapter traces the history of these efforts back to their roots in the work of the ancient Greek philosopher Aristotle. Aristotle’s influence can be seen today in the view, which dominates contemporary biological thought about the nature and origin(s) of life, that the following abstract functional characteristics are basic to life: (1) the capacity to self-organize and maintain self-organization for an extended period of time against both external and internal perturbations and (2) the capacity to reproduce and (in light of Darwin’s theory of evolution) transmit to progeny adaptive characteristics. For the sake of simplicity, I refer to the former as “O” and to the latter as “R” throughout this chapter. As Section 1.2 discusses, the conceptual parallels between O and R and Aristotle’s ideas about life are remarkably close. He identified “nutrition” and “reproduction” as the basic functions of life and debated (as do so many contemporary researchers) which is more basic. Aristotle also bequeathed to biology the thorny problem of teleology – the notion that the allegedly basic functions of life (in their contemporary guise, metabolism and genetic-based reproduction) require a strange (to the modern scientific mind) form of causation that is intrinsically directed at achieving a future goal. As Aristotle argued, living things are not just fed, they feed themselves, and they are not just copied, they reproduce themselves. Characteristic O reflects this view in explicitly referring to the idea of self-organization. Similarly, characteristic R implicitly assumes that organisms contain an internal principle for generating organisms resembling themselves; external processes do not (like a 3D printer) duplicate them.

The most significant challenge facing the pursuit of a universal theory of life is the infamous “N = 1 problem.” In the late twentieth century biologists made an astonishing discovery. Life as we know it on Earth today descends from a last universal common ancestor (LUCA), and hence represents a single example of life. Logically speaking, one cannot safely generalize to all of life, wherever and whenever it may be found, on the basis of a single example. As Section 5.2 explains, the N = 1 problem of biology is not just a pernickety logical point. There are compelling scientific reasons for worrying that our sample of one may be unrepresentative of life. Biochemists and molecular biologists have established that life could differ from familiar Earth life in significant ways at the molecular and biochemical levels. In addition, astrobiologists have explored how the basic functions of familiar life (metabolism and genetic-based reproduction) might be realized by molecular compounds based on elements other than carbon under chemical and physical conditions differing from those thought to have been present on early Earth.

This book focuses on the search for a universal theory of life. It is concerned with the history of attempts to develop such a theory, diagnosing why these efforts have thus far been unsuccessful, and determining what is required to forge ahead and successfully pursue such a theory. It is of course possible that the diverse phenomena of life lack an objective natural unity, and hence that no such theory will ever be forthcoming. Indeed, this view has become popular among some biologists and many philosophers of biology. One of the central themes of the book is that skepticism about the prospects of universal biology is not only very premature but also potentially self-fulfilling: One does not want to short-circuit the potentially successful pursuit of universal biology by rejecting it out of hand.

This chapter explores the possibility of a shadow biosphere, that is, a form of microbial Earth life descended from an alternative abiogenesis.1 It is widely assumed that all life on Earth shares a common origin. Yet there is surprisingly little theoretical or empirical support for this belief, although it is true that all known life is so related. As Section 9.2 explains, the possibility that more than one form of life arose on Earth is consistent with (i) prevailing models of the origin of life (the RNA and SM (Small Molecule) Worlds, discussed in Section 5.4) and (ii) our current understanding of molecular biology and geochemical conditions on the early Earth. While the possibility that our planet hosted more than one abiogenesis is often conceded, many scientists nonetheless insist that any descendants would have been eliminated long ago by our microbial ancestors in a Darwinian competition for vital resources. As we shall see, this theoretical argument is undermined by what has been learned in recent years about the structure and dynamics of microbial communities.