Charles S. Cockell - Astrobiology

The term “natural kind” applies to a substance such as gold, whose characteristics can be exactly defined in terms of its physical properties (Figure 2.13). We can state the molecular mass, melting point, and a range of other definitive physical properties of gold that allow for an exact definition of what it is. The reason that it is a natural kind is that the word gold is used to denote a very specific type of atom. It is limited by something that can be described in very specific physical terms.

Figure 2.13 Gold can be uniquely defined using physical characteristics such as its melting temperature and atomic mass number. Is life in the same category of object?

A good example of a “non-natural kind” is a “chair” (Figure 2.14). If we define it as “something we can sit on,” then does that make my coffee table a chair? You might reply that a coffee table is not a chair because it has no vertical back on it. I might then show you a stool and claim that it too can therefore not be a chair and is more like a table. Thus, we launch into an endless circular discussion about what a chair is. The conversation is rather pointless because ultimately a chair is simply what we define it to be. If that includes coffee tables then so be it.

Figure 2.14 Perhaps life is more like a chair? Something whose definition depends more on human categorizations, rather than any fundamental physically derivable difference with other materials.

Similarly, maybe life is just a definition that encompasses an interesting segment of all organic chemistry that happens to do certain things, such as reproducing and growing. If we want to include viruses, then so be it; if not, then so be it. Perhaps the crucial point is that we all attempt to agree on a definition that we are going to use that includes all chemistry we are going to call “life.”

The notion that life might be a human definition and not rooted in physically separable characteristics might make some intuitive sense. Organic chemistry runs along a continuum from simple gases, such as methane, through to more complex organic molecules, through molecules that can replicate, such as certain ribonucleic acids, past molecules wrapped in protein that can replicate if given the correct environment (viruses) through to cellular materials and on to single-celled organisms and multicellular organisms. Perhaps life is a division we place somewhere along this continuum that will inevitably have certain materials whose inclusion on either side of the line will always be a matter of debate.

There is no doubt that studying the matter that we broadly refer to as “life” is enormously interesting. That is not just because we belong to this subset of matter (although that is certainly one motivation), but because life does do things and exhibits emergent properties that make it interesting as a form of material behavior. In that sense, attempting to refine our understanding of what constitutes and underpins many of these behaviors is an important task in advancing our understanding of this type of matter. However, it may not be possible, or even necessary, to attempt to encapsulate life in definitive single sentences that have no exceptions. Instead, one approach is to formulate operational definitions that broadly capture the type of material we are interested in studying under the term “life.” Joyce's definition, described earlier, is one such approach.

Operational definitions can also circumscribe the type of material we might look for elsewhere in the Universe. For example, to state that one objective of astrobiology is to discover if there are “self-sustaining chemical systems capable of undergoing Darwinian evolution” elsewhere seems like a sound starting point and a reasonable way to agree about the sorts of matter that interest most astrobiologists. Indeed, this definition is a satisfactory basis for the type of material that the rest of this book considers as “life.”

The concept of using working definitions of life may help us resolve some existing problems. For example, the argument about whether viruses are alive is a pervasive one. However, clearly if astrobiologists found virus-like entities on another planet, it would be of enormous interest. Although these entities cannot replicate on their own, and provided they could be shown not to be terrestrial contaminants inadvertently transported to the planet, then their presence would suggest the existence of other perhaps cellular entities, within which they can replicate. From an operational standpoint, the discovery of viruses on another planet would be highly significant. Thus, although we might argue about their place within a definition of life, in terms of the search for life elsewhere, they clearly would fall into the purview of living matter of interest to an astrobiologist. Consequently, from an operational point of view, it seems sensible to include them, and similar sorts of entities, in our quest for “life.”

A working definition of life should not stop us continuing the debate about defining what sort of matter should fall under our category of “life.” Quite apart from refining our understanding of biology, it would be a travesty to destroy some sort of entity elsewhere (if we ever find it) simply because it failed to fit within a narrow definition of life that we have constructed.

In this chapter, we discussed ideas about “life.” The definition of astrobiology's subject matter – life – has proven to be extremely intractable, but has been a quest for over 2000 years. Ultimately, “life” may be a human-defined term used to encompass a set of organic material of special interest. It may not be possible to reduce it to some simple physically identifiable and definitive sets of characteristics. However, in the absence of a definitive statement on what life is, we can produce working definitions of life that identify the types of matter that interest us and fit within the broad concept of “life.” This allows us to advance our efforts to understand the origin, evolution, and distribution of “life” in the Universe.

1 Write a short essay explaining why providing a single agreed definition of life is difficult.

2 Describe at least two features of things that are generally agreed to be alive. Discuss whether similar or identical features can be found in any other matter that is generally agreed to be non-living.

3 Life does not violate the Second Law of Thermodynamics. Provide at least two lines of evidence for this statement.

4 Discuss and describe one experiment that has been used to disprove spontaneous generation. What are the controls in this experiment?

5 Explain why attempting to derive an agreed upon operational definition of life might be important in searching for life elsewhere.

6 Describe at least one idea held by the ancients about the nature of life and discuss whether you think this idea was completely wrong or whether it had elements of truth in our efforts to understand the nature of living things.

7 What is a “dissipative structure?” Explain the relevance of this concept for understanding life.

8 “A virus is not alive.” Discuss this statement.