The aim of this contribution is to underline some problems related to what is called the cosmological anthropic principle. There are several statements (weaker or stronger) of this ‘principle’, which was initially introduced by Brandon Carter (cf. Demaret and Barbier, 1981; Barrow and Tipler, 1986; Demaret, 1991; Demaret and Lambert, 1996). Today, there are a huge number of references defining and discussing these statements and we do not want to enter such a discussion here. In fact, for our purpose, we can simply say that the ‘weak’ version expresses simply the causality principle: if human life exists in the Universe, then there exist precise constraints that render the emergence of such life possible. This can also be presented as an observational constraint. If, as human beings, we are observing the Universe now, the latter cannot be arbitrary. It has to be such that human life is possible. These ‘weak principles’ are in fact a translation of the fact that each empirical event or each phenomenon can be characterized by a set of necessary conditions. And the weak versions of what one called the anthropic principle are then nothing more than a logical implication: human life (H) implies necessary conditions for human life to exist (NCH). Or, if we are considering the observational constraint approach: the existence of human observers implies necessary constraints on the Universe that render this existence possible.

Preamble: the vertical inheritance of genetic material

Living systems are ephemeral vehicles of their immortal germlines. In the selfish-gene (Dawkins, 1976) or disposable-soma (Kirkwood, 1977) perspectives, genes do have a greater interest in regularly ‘buying a new vehicle and throwing away the old one’. Rather than being forever stuck in the same organism genes can reassort themselves with random samples of the genetic-material pool through sexual reproduction. As human beings we are thus familiar with the widespread biparental reproduction procedure. Two individuals – the parents – spawn reproductive cells – the gametes – which merge to produce offspring – the descendants. In diploid populations, each descendant receives a paternal copy and a maternal copy of the genome (Figure 19.1). Because of the recombination occurring in the germline each parental copy is itself a reassortment of the grandmaternal and grandpaternal genomes. From generation to generation gene copies are replicated. If they are transmitted to at least two descendants (Figure 19.1: stars), this will result in branching nodes in what we call ‘gene trees’ (Maddison, 1997).

This transmission of the genetic material is called ‘vertical’: DNA of an organism is inherited from its forebears. Vertical gene transfer implies that there is a tree structure describing the history of descent of the genetic material. By contrast, there is horizontal gene transfer when genetic material is passed on from donor organisms to receptors belonging to different species (see Chapter 20).

Titan before Cassini–Huygens

With a diameter of 5150 km, Titan is the largest satellite of Saturn. It was discovered in 1655 by Christiaan Huygens. The period of rotation of Titan around the Sun is that of Saturn, 29.5 years. With its obliquity of 27°, Saturn has seasons, each of 7 years' duration, and Titan's seasonality is the same thanks to the close alignment of its pole with Saturn's. In addition, Titan turns around Saturn – with synchronous rotation – within 16 Earth-days, thus Titan's solid surface rotates slowly; however, its atmosphere presents a super-rotation due to strong zonal winds. Titan's mean distance from the Sun is that of Saturn's – about 9.5 astronomical units (AU). This corresponds to a received solar flux at the top of its atmosphere just slightly more than 1% of the flux at the Earth. Moreover, distant from Saturn by about 20 Saturnian radii, Titan is far enough from the giant planet to avoid interactions with the rings, but still close enough to allow its atmosphere to interact with the electrons of the magnetosphere of Saturn, which thus play a role in its chemical evolution, together with the solar photons.

Titan is the only satellite in the Solar System having a dense atmosphere. The presence of its atmosphere was suggested in 1907 by José Comas-Sola based on his observations of the centre-to-limb darkening of Titan's disk.

Introduction

Survival of microorganisms in outer space, such as resistant bacterial endospores, is affected by harsh environmental conditions including microgravity, space vacuum leading to desiccation, wide variations in temperature and a strong radiation component of both galactic and solar origins (Nicholson et al., 2000). Solar extraterrestrial UV radiation is mostly deleterious due to its UV component consisting of genotoxic UVC (200 < λ < 280 nm) and more energetic vacuum–UV photons (140 < λ < 200 nm) that are able to ionize biomolecules but exhibit very low penetrating features. The galactic cosmic radiation (CGR) is composed predominantly of high-energy protons (85%), electrons, α-particles and high-charge (Z) and energy (E) nuclei (HZE). In addition, solar particle radiation that mostly consists of protons with very small amounts of α-particles and HZE ions is emitted during solar wind and erratic solar flares (Nicholson et al., 2000; Cucinotta et al., 2008). UVC and UVB photons (280 < λ < 320 nm) are, in the absence of shielding, the main lethal components of space radiation. However, an efficient protection against molecular effects of UV radiations is likely to occur when spores are embedded in micrometeorites according to the scenario that has been proposed for allowing interplanetary or interstellar transfer of microorganisms (Mileikowsky et al., 2000; Nicholson et al., 2000). In contrast, under the latter conditions, protection of microorganisms against the damaging effects of CGR, and more precisely, of highly penetrating HZE particles, is at best very limited.

Introduction

Because Earth is the only place where we are certain that life exists, the characteristics of terrestrial life underpin our search for life elsewhere. In essence, the search for extraterrestrial life begins here on Earth. In the mid-twentieth century, early astrobiologists had recognized this reality and began studying life in remote and extreme environments that could be considered as analogues to places on Mars or elsewhere (e.g. Kooistra et al., 1958; Cameron, 1963; Briot et al., 2004). Early work by NASA and the Jet Propulsion Laboratory included studies of arid-soil microbiology in various locations, including the Atacama Desert and the Antarctic Dry Valleys (Cameron et al., 1966; Cameron, 1969; Horowitz et al., 1969; Cameron et al., 1970). Testing of NASA's earliest life-detection instruments also took place at these and other extreme environments (Levin et al., 1962; Levin and Heim, 1965). In parallel, microbiologists were also studying experimentally the survivability and adaptation of microorganisms isolated from desert soils and exposed to Lunar and Martian conditions in the context of forward contamination of the Moon and Mars, as well as towards the possibility of the existence of extraterrestrial life (e.g. Fulton, 1958; Kooistra et al., 1958; Davis and Fulton, 1959; Packer et al., 1963).

In recent years, Earth-based microbiological research, especially in harsh or extreme environments, has greatly expanded our understanding of the nature and limits of life (e.g. Rothschild and Mancinelli, 2001; Steven et al., 2006; Pikuta et al., 2007; Southam et al., 2007).

Introduction

The modern study of the ‘habitability’ of circumstellar environments started almost half a century ago (Huang, 1959). The concept of a circumstellar habitable zone (CHZ) is relatively well defined, being tightly related to the requirement of the presence of liquid water as a necessary condition for life-as-we-know-it; the corresponding temperature range is a function of the luminosity of the star and of the distance of the planet from it. An important amount of recent work, drawing on various disciplines (planetary dynamics, atmospheric physics, geology, biology etc.) has refined considerably our understanding of various factors that may affect the CHZ; despite that progress, we should still consider the subject to be in its infancy (e.g. Chyba and Hand, 2005; Gaidos and Selsis, 2007; and references therein).

Habitability on a larger scale was considered a few years ago by Gonzalez et al. (2001), who introduced the concept of the galactic habitable zone (GHZ). The underlying idea is that various physical processes, which may favour the development or the destruction of complex life, may depend strongly on the temporal and spatial position in the Milky Way (MW). For instance, the risk of a supernova (SN) explosion sufficiently close to represent a threat to life is, in general, larger in the inner Galaxy than in the outer one, and has been larger in the past than at present.

Introduction

Meteorites have so far provided our best analytical window into the cosmochemical evolution of the elements that make up Earth's complex chemical systems. They are for the most part fragments of asteroids, i.e. of the small-sized and odd-shaped planetesimals that orbit the Sun in great numbers between Mars and Jupiter. According to the regular spacing of inner planets from the Sun (the Titius–Bode law), this orbit should be occupied by a planet, but it appears that the smaller lumps of early Solar-System material on their way to form a planet in this region fell under the strong gravity of the gas-giant Jupiter, which slung many away and left the rest unable to fully coalesce. Asteroids, therefore, are the remnants of a planet that never was and, just like comets and other smaller bodies in the Solar System that avoided the geological reprocessing of planet formation, may offer a pristine record of early Solar-System material. Yet, they have the important distinction of being concentrated in a crowded orbit and, subjected still to the gravitational pull of planets nearby and many collisions, regularly send their fragments to the Earth as meteorites, whose direct laboratory analyses secure unequivocal data of their extraterrestrial material.

In the case of carbonaceous meteorites, the delivery has taken on an astrobiological significance because this subgroup of meteorites contains abundant and diverse organic material, including compounds having identical counterparts in the biosphere, such as amino acids (e.g. Pizzarello et al., 2006).

Introduction

The word ‘cell’ was first used by the scientist Robert Hooke in the seventeenth century in his book Micrographia (1665), where he described observations made with his own handmade microscope (Hooke, 1665). In particular, he noticed small cavities in a piece of cork delimitated by cell walls of cellulose and suber that he called cells. However, neither Hooke nor his contemporaries realized the importance of this discovery and it was only during the nineteenth century that the cell stood out as a central dogma in biology: the cellular theory. This theory is based on the work of Matthias Schleiden and his friend Theodor Schwann, who showed that all living organisms (plants, animals, moulds) are made of microscopic building units: cells (Schwann and Schleyden, 1847). The cellular theory is based on two central ideas. First, the cell is the unit of structure, physiology and organization in all living beings. Thus, the cell retains a dual existence – as a distinct entity and as a building block in the construction of organisms: unicellular organisms correspond to a single and autonomous cell, whereas multicellular organisms are made of two up to several billions of often highly differentiated cells. The second concept of the cellular theory is that ‘every cell stems from another cell’, and was formulated by Rudolf Virchow in 1855 (Virchow, 1855).

Conceptual and historical framework

Botanists of the late nineteenth century were already familiar with microbial symbioses. In fact, the term ‘symbiosis’, meaning literally ‘living together’ was introduced by Anton de Bary and Albert Bernard Frank discussing lichens and mycorrhizae, respectively, at the end of the 1870s. However, until recent times, the idea that microbial associations are central in evolution remained almost marginal. The historian Jan Sapp (1994) proposed several reasons for that situation, including the traditional accent on microorganisms as causative agents of diseases and the prominent concepts of conflict and competition as major evolutionary driving forces. Darwin's metaphoric use of the term ‘struggle for existence’ included the ‘dependence of one being on another’, and he actually recognized the existence of species taking advantage of another species, but also emphasized the difficulty for his natural-selection theory to explain the emergence of structures in one species to benefit another (Darwin, 1859, p. 200). Lynn Margulis' contributions since the late 1960s on a symbiotic theory for cell evolution bridged the previous intellectual gap, and proposals made by forgotten biologists that had been dismissed were reopened – especially those of the Russian botany school (Khakhina, 1992; Margulis, 1993). Today there is a wide consensus on the essential role played by symbiosis during the origin and evolution of eukaryotic cells, although very passionate and fundamental debates still persist (de Duve, 2007).

Introduction

Water is known to be ubiquitous in the Universe, from the dark spots of the Sun up to the most distant galaxies. It is also a major component of Solar-System objects, especially in the outer parts beyond heliocentric distances of a few astronomical units (one astronomical unit = AU = average Earth–Sun distance = 149.6 × 106 km). It should be mentioned, however, that the Earth is the only place in the Solar System where water can be present in its three states: solid, liquid and vapour. So far, outside the Earth, water has always been found in the form of vapour or ice, although there are some indications that liquid water might be – or have been – present elsewhere in the Solar System. Liquid water was probably present in the past on the surface of Mars and also possibly Venus; it might currently exist in the interiors of some satellites of giant planets.

The presence of large amounts of water, vapour or ice in the Universe is a natural outcome of the large cosmic abundances of the hydrogen and oxygen elements which form the water molecule. In addition, the large abundance of water in the outer Solar System is also a natural consequence of the formation scenario of the Solar System, which led to the accretion of two classes of planets, the terrestrial and the giant ones, separated by the ‘snow line’, which basically corresponds to the heliocentric distance of water condensation in the primordial solar nebula.

Introduction

The awareness that genes and genomes are extraordinarily rich historical documents from which a wealth of evolutionary information can be retrieved has widened the range of phylogenetic studies to previously unsuspected heights. The development of efficient sequencing techniques, which now allows the rapid sequencing of complete cellular genomes, combined with the simultaneous and independent blossoming of computer science, has led not only to an explosive growth of databases and new sophisticated tools for their exploitation, but also to the recognition that, in spite of many lateral gene-transfer (LGT) events, different macromolecules are uniquely suited as molecular chronometers in the construction of nearly universal phylogenies.

Cladistic analysis of rRNA sequences is acknowledged as a prime force in systematics and from its very inception had a major impact on our understanding of early cellular evolution. The comparison of small-subunit ribosomal-RNA (16/18S rRNA) sequences led to the construction of a trifurcated unrooted tree in which all known organisms can be grouped in one of three major monophyletic cell lineages, i.e. the domains Bacteria (Eubacteria), Archaea (Archaeabacteria) and Eukarya (Eukaryotes) (Woese et al., 1990), which are all derived from an ancestral form, known as the last common ancestor (LCA).

This book aims at exploring several crucial issues related to the origin(s) and evolution of life in the Universe, starting from the only example of life known so far: terrestrial life. It is clear, though, that many of the circumstances that surrounded the emergence of life on Earth may have occurred, are occurring or will occur in other regions of our Galaxy or in other galaxies of our Universe. Therefore, the critical exploration of those conditions and the elaboration of models explaining the transition from the organic chemistry of the Universe to the biochemistry of terrestrial living forms are relevant at a much more global scale.

Just as with this volume, the field of astrobiology is by nature multidisciplinary. Astrophysicists, geologists, chemists, biologists, computer scientists and philosophers, as well as scientists working at the different interfaces between those disciplines, can all contribute to a better understanding of the processes and conditions that led to the emergence of life. The points of view and approaches of those different disciplines should not only superimpose, but also converge towards a unified explanation of the phenomenon of life in our Universe.

This book is an attempt to contribute to such an ambitious objective. It summarizes a series of lectures presented by selected speakers during two successive summer courses sponsored by the French Research Council (CNRS, Centre National de la Recherche Scientifique): Exobio'05 and Exobio'07, Ecole d'exobiologie du CNRS, which were respectively held in September 2005 and September 2007 in Propriano, Corsica (http://www.u-bordeaux1.fr/exobio07/).

Astrobiology, also known as bioastronomy or exobiology, is the study of the origin, evolution and distribution of life in the Universe. These are subjects which have been of interest to mankind throughout recorded history. Although questions of origins have most frequently invoked divine beings, non-supernatural speculation on these fundamental issues dates back at least to the Ionian school of pre-Socratic Greek philosophers. Anaximander, the successor to Thales, is reported as saying that all living creatures arose from the moist element (water) through the action of the Sun (Freeman, 1966), a prescient insight given current ideas that life as we know it requires water, that radiation acting on inorganic matter can produce the molecular components of life (amino acids, nucleic acids, etc.) and that the Sun is the ultimate energy source for almost all life on Earth. In fact, Anaximander seems to have gone further and suggested that human beings arose from fish-like creatures (presumably a natural result of life having originated in water).

Speculation about life beyond the Earth has also had a long tradition. Although Pythagoras himself is not known to have recorded his teachings, his school (in particular, Philolaus, ca. 400 BCE) is said to have written that the Moon appears Earth-like because it is inhabited with animals and plants (Dreyer, 1953). At roughly the same time the atomist school of Leucippus and Democritus taught that the Universe is infinite and contains innumerable worlds.

The origin of life and its existence elsewhere than on planet Earth

Studies of the origins of life are closely interwoven with exobiology (Raulin-Cerceau et al., 1998). It is highly probable that the full range of conditions present on Earth since its formation are present elsewhere. On a virtual trip through the Universe, we would travel not only in space, but also back in time into the Earth's biological history. The search for past, dormant or currently existing extraterrestrial life is one of the most thought-provoking challenges for biology. It is based on the certainty that liquid water and other key chemical and physical environmental conditions for the development of living organisms, as we know them, were, or are, present elsewhere in the Universe than on our planet. Any evidence of extraterrestrial life, from Mars sample analysis for example, would be of major interest for all biology. It would contribute to an understanding not only of the definition and origin of life, but also of the evolution and adaptation of molecular mechanisms in living cells, or of how organisms adapt and develop within ecosystems.

Why study life in extreme environments?

Life on Earth is almost everywhere! And because it is almost everwhere around us, we can hope to define the extreme limits for its existence by studying it here on Earth.