In their book “Life in the Universe”, Dirk Shulze-Makuch and Louis Irwin make a very interesting analysis of what we mean by the term “life”, how to detect it and where it might be and in what form on other planets and satellites in the Solar System, as well as elsewhere in the Universe. The authors push their interpretations as to what could constitute a life form  deliberately, leaving one with the impression of having been intrigued at the same time as wryly shaking one’s head.  From the point of view of a “hard earther”, who appreciates solid, factual evidence, I found the book a fascinating blend of serious scientific analysis of what life is and a philosophical evaluation of what it could be.  Although I do not necessarily concur with the authors’ vision of the Solar System planets and satellites as repositories of life, I did appreciate the relatively detailed evocation of the principals of life, including life as we do not know it.  The book is well written and readable for an educated non-specialist.

Shulze-Makuch and Irwin start their book with an overview chapter setting out the main themes and theses before jumping into the core of the matter with an examination of the criteria of life.  After discussing problems related to traditional definitions of life, they conclude that the boundary between non-living and living must be transitional and that life cannot be defined by one single characteristic.  They propose a definition of life that is adapted to different environmental situations on different planets, not just the Earth.  Thus, they define life as a structure (1) composed of a bounded microenvironment in thermodynamic disequilibrium with its external environment, (2) that is capable of transforming energy and the environment to maintain a low entropy state, and (3) that is capable of information transmission and coding.  However, one wonders where the capability of evolution fits into this scenario?  After discussing the first cells and their possible earliest stages of evolution, they end the chapter with the intriguing observation that the chemical composition of life on Earth resembles more closely the composition of the stars than the planets!

The following chapter is a review of the early geological record of the Earth and life from which the authors conclude that the most primitive life forms must be simple, that life arose quickly when the conditions were conducive and that it was changes in the physical environment that led to an increase in complexity.  This is one of the few chapters where I had some difficulty with the superficiality of the review based on inadequate sources, probably because it is the areas in which I have had more reading.  Shulze-Makuch and Irwin base themselves on a book by Margulis and Sagan (1995) regarding the early geological and environmental evolution of the Earth.  Although brilliant, Margulis and her son are not earth scientists and their understanding of this field is rather limited.  There are other reviews that would have given Shulze-Makuch and Irwin better insight, such as Nisbet and Sleep (2001) (and, more recently Nisbet and Fowler, 2004; Westall, 2004).  Another problem is the obsession with complete sterilisation of the Earth during the late heavy bombardment signifying that life could only have started after the bombardment.  There is no evidence that the Earth was sterilised between 4.0-3.85 Ga, the scientists who made this hypothesis (Sleep et al., 1989) estimate that the Earth may have been hit by between 0 and 6 planet sterilising bolides!  Other studies indicate that even if the Earth had been hit by very large impactor, only the top 400 m of the ocean would have boiled off (Ryder, 2003).  If life arose quickly, as soon as the conditions were favourable, then it would have arisen as soon as the temperature of the condensed water was below 80°C (relatively abundant water existed at the surface probably by 4.4 Ga, Wilde et al., 2001).  Moreover, Shulze-Makuch and Irwin themselves state elsewhere in the book that the most common forms of life (as we know it) are probably chemosynthetic microorganisms that live in the subsurface and would therefore be protected from large-scale external perturbations.  Life did not start only after 3.85 Ga – the diversity and level of evolution demonstrated by the earliest microfossils of about 3.5 Ga indicate that it must have had a long period of evolution (Westall and Southam, in review).  I have digressed somewhat here, simply because this is the part of the book that I feel best capable of critically appraising on a scientific level……

Chapter 4 deals with the problem of energy.  This is a large chapter and one of the most important ones in the book.  The basic hypothesis here is that life needs energy in order to organise materials, maintain a low state of entropy and to perform work.  The authors first look at the energy sources for life as we know it, oxidation/reduction chemistry and light, noting that both these forms of energy are equally competitive from a purely energetic point of view.  They then go on to list and analyse individually other potential forms of energy, such as heat, pressure, stress, magnetic waves, kinetic energy, osmotic/ion gradients.  Although chemical and light energy is the most efficient, Shulze-Makuch and Irwin conclude that earliest life on Earth could have used other forms of energy that were later out-competed and that other, promising forms of energy include thermal, ionic/osmotic gradients and kinetic energy in a fluid. 

The following chapter considers the building blocks of life and explains the advantages of carbon – its ability to form millions of complex, stable molecules with itself, O, N and H, the variety of structures thus formed, their chirality, the polymer backbones, the energetically favourable redox reactions, the fact that water is an ideal solvent, and the fact that carbon is abundant in the universe.  Other polymer-forming elements, such as B, N, Si, P and S are discussed.  Si-based life does not seem to be a probability since it does not form ubiquitous polymeric compounds as does C.  The conditions under which life based on other elements (not necessarily just one) would have to exist are very different to those on Earth and the authors cite Titan as one possible example.  The advantages of life in a solvent rather than in a gas or solid are listed in Chapter 6, with water being the best solvent on warmer planets and other solvents, such as methanol, ammonium or water/ammonium mixtures being better on colder planets (for instance an H2O/NH3 mixture in the subsurface of Titan).  Schulze-Makuch and Irwin note that HCN, HF and H2S would not be suitable solvents under any temperature conditions.  Obviously the nature of the solvent is important with respect to the elemental composition of the building bricks of the particular life form on a particular planet.

Having established the advantages of carbon-based life forms in a liquid water solvent, Schulze-Makuch and Irwin then address the subsurface, surface, atmosphere and space habitats in which such life forms could exist.   The subsurface provides the most stable, protected environment and is probably the most widespread environment in which life is found/could be found on other planets and satellites. The surface environment, on the other hand, is subjected to extreme conditions and environmental changes including variations in temperature and humidity, wind, radiation effects, meteorite impacts, supernovae explosions and so on.  Although potentially more hostile, the surface environment does offer the advantage of access to a powerful source of energy, sunlight, and with it (as well as through environmental stresses) the potential for evolution to more complex life forms.  The authors discuss the possibility of life associated with water vapour droplets in the atmosphere of, for instance, Venus, and note that life in space will be limited to resistant dormant forms entombed in a protecting coating of dust or rock that could survive for possibly even up to a million years in a boulder on an interplanetary trajectory.

Exotic forms of life are the subject of the penultimate chapter; they include spin configurations based on p-hydrogen and o-oxygen, unbounded space clouds (Fred Hoyle’s “black cloud” scenario), and life on planets around a neutron star or a brown dwarf.  Other ideas such as plasma life or pure energy are put into the science fiction category while the authors note “we believe that in this book we have already pushed the limits of what it means to be alive” (p. 146).

The book concludes with a chapter on searching for life on other planets, cataloguing in a series of tables the types of signatures of life: biosignatures that record the functional processes of aggregates of organisms that alter their environment; geosignatures that are the result of the alteration of the geological environment owing to biological processes, such as limestone and ironstone deposits; and geoindicators that are planetary-scale signatures, such as an atmosphere or ice cover, indicating that a planet is capable of hosting life but not necessarily that it does actually contain life.  Unfortunately the discussion of the morphological biosignatures is dismissed with a reference to the current debate concerning the uncertain biogenicity of carbonaceous filaments described by Schopf (1993) originally as fossil cyanobacteria and by Brasier et al., (2002) as abiogenic artefacts.  This does not do justice to the wealth of documentation available on early life.  On the other hand, the authors do suggest that a variety of biosignatures relating to biofilm formation (composition, biostructures – e.g; bioetching or pitting but not actual fossils – and biominerals) constitute a reasonable biosignature.  This chapter contains other interesting tables in which the presence of various bio/geosignatures and geoindicators are listed for different planets and satellites in the Solar System and the consequent plausibility of life (senso lato) is assessed.  Again, with this chapter, because I have more personal experience, I feel that the treatment of the subject is somewhat cavalier.

All in all, I enjoyed reading this book.  Based on the parts that I was able to critically evaluate, I recognise that specialists in other fields may have problems with the way the authors have treated specific topics.  However, the authors themselves admit that the literature cited could be more extensive and there are a number of general books that should perhaps have been listed, for instance, Brack (1998) and Horneck and Baumstark-Khan (2002).  The structure of the book greatly aids its readability – each chapter starts with a brief overview of a particular topic, then comes a more or less detailed analysis of specific points, ending with a concluding paragraph.  At each point, the relevance of a specific topic for the possibilities of life on another planet or satellite is evoked.  There are also useful tables summarising various characteristics described in the chapter and their relevance to life in the universe.  Relevant figures also provide useful visual aids.  I recommend this book for interested non-specialists with the proviso that the authors have deliberately pushed their hypothesis to the extreme (as they state themselves) and, if the reader wishes to delve deeper into a particular field he/she should not rely only on the literature cited and should obtain more recent, up to date documentation.

References

Brack, A. (ed.), 1998. The molecular Origins of Life: assembling pieces of the puzzle. Cambridge Univ. Press, Cambridge.

Brasier, M.D., Green, O.R., Jephcoat, A.P., Kleppe, A.K., van Kranendonk, M., Lindsay, J.F., Steele, A., Grassineau, N. (2002). Questioning the evidence for Earth's oldest fossils. Nature, 416, 76-81.

Horneck, G. and Baumstark-Khan, C. (eds.), 2002. Astrobiology, the quest for the conditions of life, Springer Verlag, Berlin.

Margulis, L., and Sagan, D. (1995).  What is life? Simon & Schuster, New York.²

Nisbet, E.G., and N.H. Sleep (2001), The habitat and nature of early life, Nature, 409, 1083-1091.

Nisbet, E.G., and C.M.R. Fowler (2004), The early history of life. In Biogeochemistry pp 1-39 Vol. 8 (ed. W.H. Schelsinger) Treatise on Geochemistry, (eds H.D. Holland and K.K. Turekian), Elsevier-Pergamon, Oxford.

Ryder, G. (2003). Bombardment of the Hadean Earth: wholesome or deleterious. Astrobiology, 3, 3-6.

Schopf, J.W. (1993). Microfossils of the Early Archean Apex Chert: new evidence of the antiquity of life. Science, 260, 640-646.

Westall, F. (2004), Early life on Earth: The ancient fossil record, in Ehrenfreund, P., et al., eds., Astrobiology: Future Perspectives: Kluwer, Dordrecht, pp. 287–316.

Westall, F. and Southam, G. The early record of life. In Wilde, S.A., Valley, J.W., Peck, W.H., Graham, C.M. (2001). Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature, 409, 175-178.