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.
2.9 Conclusions
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.
Questions for Review and Reflection
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.
Bibliography
Books
1 Bedau, M.A. and Cleland, C.E. (2018). The Nature of Life. Cambridge: Cambridge University Press.
2 Crick, F. (1981). Life Itself: Its Origin and Nature. New York: Simon & Shuster.
3 Maynard-Smith, J. (1986). The Problems of Biology. Oxford: Oxford University Press.
4 Schrödinger, E. (2012). What is Life? Cambridge: Cambridge University Press (originally published in 1944).
Papers
1 Bains, W. (2014). What do we think life is? A simple illustration and its consequences. International Journal of Astrobiology 13: 101–111.
2 Bedau, M.A. (1998). Four puzzles about life. Artificial Intelligence 4: 125–140.
3 Benner, S.A. (2010). Defining life. Astrobiology 10: 1021–1030.
4 Chao, L. (2000). The meaning of life. BioScience 50: 245–250.
5 Cleland, C.E. and Chyba, C.F. (2002). Defining “life.”. Origins of Life and Evolution of the Biosphere 32: 387–393.
6 Conrad, P.G. and Nealson, K.H. (2001). A non-earthcentric approach to life detection. Astrobiology 1: 15–24.
7 Fleischaker, G.R. (1990). Origins of life: an operational definition. Origins of Life and Evolution of the Biosphere 20: 127–137.
8 Koonin, E.V. (2012). Defining life: an exercise in semantics or a route to biological insights? Journal of Biomolecular Structure and Dynamics 29: 603–605.
9 Lange, M. (1996). Life, “artificial life,” and scientific explanation. Philosophy of Science 63: 225–244.
10 Machery, E. (2012). Why I stopped worrying about the definition of life … and why you should as well. Synthese 185: 145–164.
11 Mariscal, C. and Doolittle, W.F. (2018). Life and life only: a radical alternative to life definitionism. Synthese. https://doi.org/10.1007/s11229-018-1852-2.
12 Mix, L.J. (2015). Defending definitions of life. Astrobiology 15: 15–19.
13 Moreira, D. and López-García, P. (2009). Ten reasons to exclude viruses from the tree of life. Nature Reviews Microbiology 7: 306–311.
14 Nealson, K.H., Tsapin, A., and Storrie-Lombardi, M. (2002). Search for life in the universe: unconventional methods for an unconventional problem. International Microbiology 5: 223–230.
15 Ruiz-Mirazo, K., Peretó, J., and Moreno, A. (2004). A universal definition of life: autonomy and open-ended evolution. Origins of Life and Evolution of the Biosphere 34: 323–346.
16 Tsokolov, S.A. (2009). Why is the definition of life so elusive? Epistemological considerations. Astrobiology 9: 401–412.
17 Varela, F., Maturana, H., and Uribe, R. (1974). Autopoiesis: the organization of living systems. Biosystems 5: 187–196.
3 Matter and Life
Learning Outcomes
Understand the concept of atoms, ions, and molecules and their basic structure.
Be able to describe and explain different bonding types: ionic, covalent, metallic, van der Waals interactions, and hydrogen bonding.
Understand and give examples of the role of these bonding types in biological systems.