My contribution to this volume is that of a theologian interested in the relationship between science and religion. I will be asking whether what we are calling “fitness for life” and “biochemical fine-tuning” are consistent with, and perhaps even supportive of, the ageless religious convictions that the universe is here for a reason and that life is the intended consequence of divine love, wisdom, and creative power.

Today, it is particularly striking to many scientists that cosmic constants, physical laws, biochemical pathways, and terrestrial conditions are just right for the emergence and flourishing of life. It is not surprising, of course, that, as life exists, the cosmic and chemical conditions for it had to have been formatted for such an emergence. It would be remarkable, however, if the format could have been otherwise, and hence not right for life. During the universe's history, it now seems that only a very restricted set of physical conditions operative at several major junctures of emergence could have opened the gateways to life (Hogan, 2000). So, what principles lie behind the narrowing of the gateways that allowed only those conditions preparatory to life to flow through while excluding any cosmological principles, physical parameters, and chemical laws that would not have permitted such an outcome?

In the long, unfolding story of nature's development, any conceivable series of physical conditions or constants other than those that would lead to life have been tossed aside.

Crane Brinton, Harvard historian, friend of Lawrence J. Henderson, and fellow member of The Saturday Club, wrote the obituary for Henderson in the Club's third commemorative volume (Brinton, 1958, p. 207). Noting that Henderson was somewhat out of the ordinary – crossing the Charles River on several occasions to keep appointments at the Medical School (Boston) and the College (Cambridge) and then recrossing it to get to the Business School (Boston) – Brinton went on to note Henderson's other non-traditional characteristics: “Ticketed as a biological chemist, he later took the title physiologist and, although he would not have liked the name, at the end of his career he was a sociologist [emphasis added].”

Brinton went on: “A cross section of his publications may indeed be so drawn up as to seem an academic scandal.” Brinton ran through the publications, from the well-known The Fitness of the Environment (1913) and The Order of Nature (1917); the more esoteric On the Excretion of Acid from the Animal Organism (1910, 1911); the simple volume Blood: A Study in General Physiology (1928); the unexpected transcript of an interview on the experiments in the Liberty Bread Shop (Brinton, 1958, p. 208); in his later life, The Study of Man (1941); to Pareto's General Sociology: A Physiologist's Interpretation (1935). Brinton jocularly added that a piece by Henderson – a biographical memoir on the life of the poet Edwin Arlington Robinson (a close friend from his student days) written as a memoir for the American Academy of Arts and Sciences – is to be found in the Woodberry Poetry Room of Harvard's Lamont Library.

The chapters in this volume, written from a wide variety of perspectives, explore the possibility of extending the theme of “fine-tuning” beyond the domain of cosmology, where it first entered into serious discussion in the mid-1970s, to other sciences such as biochemistry and biology. As a prelude to this investigation, it seems worthwhile to explore the theme of fine-tuning itself in some detail, given the ambiguities that still surround it and the vigor of the continuing disagreement as to what its implications are. How did fine-tuning make its way into the cosmological discussion? What precisely did – and does – it amount to? What were – and still are – the responses to it? How is one to evaluate those responses? Achieving a measure of clarity on these issues should make it easier to appreciate the search for fine-tuning or its analogs elsewhere in the sciences.

The infinities of space and time in Newtonian mechanics were not propitious to the formulation of a cosmology, a theory of the cosmic whole, although the notion of gravity gave a hint, at least, as to how material complexity could form. The unification of space and time by Einstein's general theory in a non-Euclidean geometrical framework offered new possibilities, and Hubble's subsequent confirmation of galactic expansion pointed Lemaître to a universe model that would, from a “primeval atom,” expand into the universe we know.

This is a book about whether our universe is “biocentric.” The Oxford English Dictionary defines this term as “treating life as a central fact” [1]; thus a biocentric universe is one predisposed towards producing life (life's centrality is implicit if “the fitness of the environment [for life] far precedes the existence of the living organisms” [2]). To date, this unusual idea has been most thoroughly explored (and most widely publicized) under the umbrella term “Anthropic Principle” in physics [3]. In essence, this principle refers to a suite of fundamental physical parameters, dimensionless constants that interact to imbue our universe with such interrelated phenomena as a diverse periodic table of elements, a preponderance of carbon and water, stars that emit energy, and planets that orbit them [4]. It asserts that, without clear explanation at present, the constants responsible for this state of affairs appear finely tuned in our universe to values peculiarly sympathetic with life's emergence.

Even if we accept this view of physics at face value, we remain a long logical leap from establishing truly biocentric credentials for our universe. Understanding “what is” versus “what might have been” for physics must be met by an equivalent understanding in biology. Thus the interface of biochemistry, where physics becomes biology, deserves especially close scrutiny. In this context, the first and perhaps most important point of this chapter is to emphasize that in considering physics and biology, two fundamentally different sets of expectations collide.

Like others (see for instance Oparin, 1968; Monod, 1970), I too am convinced that the origin of life will be eventually understood in scientific, rational terms. However, I recognize, of course, that so far it remains an unsolved problem, and even an unfathomable one. We scientists are playing with partial explanatory hypotheses and have few hard facts at our disposal. Therefore, we must remain humble in attempting to describe each scenario and be careful to discuss each one's constraints, recognizing where the scenarios may be flexible to synthesis with other hypotheses. Thus, my present discussion explores some areas in which there may indeed be some flexibility in terms of what solutions could potentially lead to living systems – in other words, where some “coarse-tuning” might be tolerable.

Specifically, I address the following series of questions:

1. Is life necessarily based on carbon?

2. Must the “bricks of life” have originated by some process closely related to Miller's “spark tube” experiment, or do other possibilities exist?

3. Is it necessary that any kind of life be associated with water from the start?

4. Is it necessary for any kind of life to be cellular and for the cells to be bounded in water by membranes?

5. Are the n-acyl phospholipids of “classical” membranes, or the more recently discovered archaeal lipids, plausible constituents of primitive membranes?

6. Could early membrane-forming amphiphiles have simply been polyprenyl phosphates, following the “Strasbourg scenario” (Birault et al., 1996)?

7. Are there automatic consequences of the self-organization of amphiphiles (such as polyprenyl phosphates) in water into membranes, and does this lead to novel properties? Specifically, I consider the available evidence for the following features of membranes:

8. […]

Introduction

Mars is the world that has generated the most interest in life beyond the Earth. There are three reasons why Mars is the prime target for a search for signs of life. First, there is direct evidence that Mars had liquid water on its surface in the past, and there is the possibility that there is liquid water in the subsurface at the present time. Second, Mars has an atmosphere, albeit a thin one, that contains CO2 and N2. Third, conditions on Mars are cold and dry and thus are favourable for the preservation of evidence of organic remains of life that may have formed under more clement past conditions.

Mars may be cold and dry today but there is compelling evidence that earlier in its history Mars did have liquid water. This evidence comes primarily from the images taken from orbital spacecraft. Figure 12.1 from Malin and Carr (1999) shows an image of a canyon on Mars and represents probably the best evidence for extended and repeated, if not continuous, flows of liquid water on Mars.Water is the common ecological requirement for life on Earth. No organisms are known that can grow or reproduce without liquid water. Thus, the evidence that sometime in its early history Mars had liquid water is the primary motivation for the search for evidence of life (McKay, 1997).

Introduction

A decade has passed since the first discovery of an extrasolar planet by Mayor and Queloz (1995) and its confirmation by Marcy et al. (1997). Since this groundbreaking discovery, about 190 planets have been found around nearby stars, as of May 2006 (Schneider, 2006). Here we shall review the main methods astronomers use to detect extrasolar planets, and the data we can derive from those observations.

Aitken (1938) examined the observational problem of detecting extrasolar planets. He showed that their detection, either directly or indirectly, lay beyond the technical horizon of his era. The basic difficulty in directly detecting planets is the brightness ratio between a typical planet, which shines mainly by reflecting the light of its host star, and the star itself. In the case of Jupiter and the Sun, this ratio is 2.5 × 10–9. If we keep using Jupiter as a typical example, we expect a planet to orbit at a distance of the order of 5 astronomical units (AU, where 1 AU = 1.5 × 1011 m = distance from Earth to Sun) from its host star. At a relatively small distance of 5 parsecs from the Solar System, this translates into a mean angular separation of the sources on the sky of 1 arcsecond. Therefore, with present technology, it is extremely demanding to directly image any extrasolar planet inside the overpowering glare of its host star, particularly from the ground, where the Earth's atmosphere seriously affects the observations.

Introduction

The main themes of this chapter concern the phenomenon of emergence, the origin of life as a primary example of emergence, and how evolution begins with the inception of cellular life. The physical properties of certain molecular species are relevant to life's origins, because these properties lead to the emergence of more complex structures by self-assembly. One such property is the capacity of amphiphilic molecules such as soap to form membranous boundary structures, familiar examples being soap bubbles and cell membranes. A second example is the chemical bonding that allows biopolymers such as nucleic acids and proteins to assemble into functional sequences. Self-assembly processes can produce complex supramolecular structures with certain properties of the living state. Such structures are able to capture energy available in the environment and initiate primitive reactions associated with metabolism, growth, and replication. At some point approximately 4 billion years ago, cellular compartments appeared that contained macromolecular systems capable of catalysed growth and replication. Because each cellular structure would be slightly different from all others, Darwinian evolution by natural selection could begin, with the primary selective factor being competition for energy and nutrients.

Defining emergence

Researchers increasingly use the term emergence to describe processes by which more complex systems arise from seemingly simpler systems, typically in an unpredictable fashion. This usage is just the opposite of reductionism, the belief that any phenomenon can be explained by understanding the parts of that system.

Introduction: life beyond the habitable zone

As the horizons of the field of astrobiology have extended to the rapidly growing population of known extrasolar planetary systems, the concept of a habitable zone around each star has been of particular interest. With each discovery, the question is raised of whether the new planet is in the so-called habitable zone, or whether hypothetical orbits within that zone would be stable. In this context, the habitable zone is commonly defined as a range of distances from the star where temperatures would be in a range for life to be viable and for water to exist near the surface in a phase able to support living organisms (Kasting et al., 1993; Menou and Tabachnik, 2003; Raymond and Barnes, 2005). This restrictive definition can contribute to a relatively pessimistic view of the potential for extra-terrestrial life (e.g., Ward and Brownlee (2000)). It has even been used to advocate planetary exploration objectives, purportedly motivated by the search for life, that are restricted to the terrestrial planets in our Solar System (e.g., Hubbard (2005)).

Unless corrected, this widespread semantic usage of the term habitable zone would leave out one of the most likely places for exploration to reveal extraterrestrial life: Jupiter's moon Europa. The analyses cited above neglect tidal friction, which is known in the case of Jupiter's Galilean satellites to provide significant heating, comparable in fact to the solar heating in the habitable (and inhabited) terrestrial-planet zone.

Introduction

This chapter presents one of the very rare exobiological hypotheses. The main thesis is that there could be life in the dark dune spots (DDSs) of the southern polar region of Mars, at latitudes between –60° and –80°. The spots have a characteristic annual morphological cycle and it is suspected that liquid water forms in them every year. We propose that a consortium of simple organisms (similar to bacteria) comes to life each year, driven by sunlight absorbed by the photosynthetic members of the consortium. A crucial feature of the proposed habitat is that life processes take place only under the cover of water ice/frost/snow. By the time this frost disappears from the dunes, the putative microbes, named Mars surface organisms (MSOs) must revert to a dormant state. The hypothesis has been worked out in considerable detail, it has not been convincingly refuted so far, and it is certainly testable by available scientific methods. We survey some of the history of, the logic behind, the testable predictions of, and the main challenges to the DDS-MSO hypothesis.

History

The spots in question were observed on images made by the Mars orbiter camera (MOC) on board the Mars Global Surveyor (MGS) spacecraft between 1998 and 1999 (images are credited to NASA/JPL/Malin Space Science Systems). These features appear in the southern and northern polar regions of the planet in the spring, and range in diameter from a few dozen to a few hundred metres.

Introduction

While the notion of worlds beyond our Earth is ancient, the specific idea of planets orbiting distant stars is relatively new. Over two millennia ago Epicurus stated “there are infinite worlds both like and unlike this world of ours,” but he was not speaking of Earth-like planets orbiting Sun-like stars. Indeed, planets orbiting stars would have been a meaningless issue to the Greeks, as the Sun was not recognized as a star, nor the Earth as a planet (Chapter 1).

One of the earliest and most eloquent spokespersons for what is now called astrobiology, and among the first to grasp the implications of the Sun being a star and the Earth a planet, was the mystical Roman Catholic monk Giordano Bruno. In On the Infinite Universe and Worlds (1584) he wrote:

Bruno then concludes with the revolutionary slogan:

Bruno's reward for this prescience and for other heresies was condemnation by the Church, followed by immolation in a public square in Rome in 1600.

Introduction

Prokaryotic microorganisms were the only form of life for at least 80 percent of our evolutionary history (Schopf and Packer, 1987). Multicellular organisms including plants, animals and fungi evolved a mere 0.5–1.0 Ga from single-celled eukaryotic ancestors. Geologists and paleontologists debate the age of life on this planet and when the major microbial lineages first diverged (see Chapter 12 for details). Cyanobacterium-like fossils suggest that life emerged at least 3.45 Ga (Schopf et al., 2002; Schopf and Packer, 1987), but the biogenic origins of these structures are contested (Brasier et al., 2002; Section 12.2.1). The chemical record documents prokaryotic metabolisms that may have existed 3.47–3.85 Ga (Mojzsis et al., 1996) and eukaryotic biosignatures that may be as old as 2.7 Gyr (Brocks et al., 1999). Yet, these are still imprecise interpretations (some might be more recent microbial contamination) and do not set absolute limits on the possible origins of life on Earth. Early periods of heavy bombardment between 4.1 and 3.8 Ga might constrain when life first appeared on Earth, although microorganisms living off chemical energy at kilometer depths could have survived even the largest impact events.

By the standards of multicellular plants and animals, single-cell organisms look relatively simple (Patterson and Sogin, 1993), yet they transformed the atmosphere, the waters, the surface, and the subsurface of the Earth.

Introduction

Picture a future triumph in robotic space exploration: in a complex mission to Mars, a sample has been collected from the martian subsurface near a newly discovered hydrothermally active site at 30°N latitude. Ten years in the detailed planning and execution, the mission's Earth return capsule with its sample canister has landed in the Utah desert. The sample is now under extensive analysis and testing in an ultra-clean containment facility – and initial observations have shown positive indications that it contains life. Only after later testing is completed, checked, rechecked, and repeated is it shown unequivocally that the lifeform contained within the sample is a soil bacterium common to the dirt of an old Soviet launch facility in Baikonur, Kazakhstan, and which has apparently been alive on Mars since a spacecraft crash-landed there in 1972 …

Or picture, as did novelist Michael Crichton (1969) in the very year of the first lunar sample-return mission (Apollo 11), a spacecraft returning to Earth containing a dangerous extraterrestrial organism – The Andromeda Strain – not related in any way to Earth-life and operating by rules scarcely understood even after hundreds of humans have met their grisly demise …

Once you have those events in mind, you are developing a feel for what planetary protection might be, and what it is meant to prevent.

Introduction

How does life begin? Can life arise elsewhere than the Earth? These questions are among the most fundamental and challenging in all of biology. Charles Darwin once wrote to a friend, “It is mere rubbish, thinking at present of the origin of life; one might as well think of the origin of matter.” (Letter to J. D. Hooker, March 29, 1863.) Darwin made this comment when the knowledge required to think about the origin of life and matter simply did not exist. Now, 150 years later, we understand much more. We know that new elements are constantly being synthesized by nuclear fusion of hydrogen and helium in the interiors of stars, then expelled into interstellar space when stars reach the ends of their lives. This matter is the source of new stars and planetary systems, and it is literally true that planets like the Earth and the biogenic elements that give rise to life are composed of “star dust” (Chapter 3). We also know that liquid water once existed on Mars, and perhaps still does beneath the martian surface, suggesting that microbial life may exist elsewhere than on the Earth. Probably most important is that we understand living cells in unprecedented detail, even to the point of knowing the entire sequence of three billion nucleotide bases in the human genome, and we have begun to manipulate the genetic blueprint of life.

Introduction

Habitable planets are those bodies that provide environments, materials and processes that are advantageous for the formation and long-term evolution of life. Understanding the processes that lead to the formation of such planets is a central issue in astrobiology. We are of course handicapped in this quest since Earth is the only example of a planet with proven habitability – the only one known to have provided thermal, chemical, and other physical conditions that allowed life to form and survive for ~3.5 Gyr.

This chapter emphasizes the formation of Earth-like planets, those with environments capable of supporting complex life comparable to Earth's plants and animals. The focus on life comparable to Earth's multicellular organisms is partly due to the practical consideration that we better understand the environmental constraints of such life. Despite this restricted focus, note that most astrobiologists consider that the dominant form of life in the Universe, as it has been on Earth over most of its history, is probably far simpler and more rugged, analogous to Earth's bacteria and archaea (Section 3.2).

This interpretation of habitability is highly Earth-centric and assumes that life elsewhere is similar to terrestrial life and requires environments similar to those of terrestrial organisms. The actual cosmic limits of life are of course unknown, but the Earth-centric view is a reasonable, albeit conservative, place to start. Until there are detailed data on other inhabited planets, discussions of extraterrestrial life would be prudently biased by what is known from our Earth experience.