"Journey to Diverse Microbial Worlds: Adaptation to Exotic Environments" , ed. Joseph Seckbach; a volume which is part of the book series on Cellular Origin and Life in Extreme Habitats. Kluwer Academic Publishers, Dordrecht, The Netherlands. Chapter 25, pp. 367-375.
INTRODUCTION TO ASTROBIOLOGY:
Origin, Evolution, Distribution and Destiny of Life in the Universe
FRANCES WESTALL2 AND JULIAN CHELA-FLORES3
1Hebrew University of Jerusalem, 91904, and 2Lunar and Planetary Institute, 3600, Bay Area Boulevard, Houston, TX 77058, and 3The Abdus Salam International Centre for Theoretical Physics (ICTP), Office 276, P.O.Box 586; Strada Costiera 11; 34136 Trieste, Italy, and Instituto de Estudios Avanzados, Apartado 17606, Parque Central, Caracas 1015A, Venezuela.
1. Are There Other Inhabited Worlds?
The only life that we know about in the universe
is life on our own planet Earth. We have no idea of how representative
it might be of life on other planets, although in the chapter
by one of us (JCF) it is conjectured that, given suitable environmental
conditions, if life started somewhere else it would be constrained
to take an evolutionary pathway to eukaryogenesis and multicellularity
(Chela-Flores 1998; Seckbach et al., 1998).
The question whether nature in an extraterrestrial context steers a predictable course is clearly an open question, but some hints from the basic laws of terrestrial biology (natural selection and a common ancestor for all the Earth biota) can be interpreted as evidence that to a large extent evolution is predictable and not contingent. Indeed, support from independent teams suggests that natural selection overrides the randomness of genetic drift; in other words, natural selection seems to be powerful enough to shape terrestrial organisms to similar ends, independent of historical contingency (Pennisi 2000, and references therein). On the other hand, some arguments militate in favor of a human-level of intelligence being reached by the conjectured universal constrained evolutionary pathway towards eukaryogenesis and multicellularity. Certainly, in an extraterrestrial environment the evolutionary steps that led to human beings would probably never repeat themselves; but that is hardly the relevant point: the role of contingency in evolution has little bearing on the emergence of a particular biological property (Conway-Morris 1998).
The inevitability of the emergence of particular biological properties is a phenomenon that has been recognized by students of evolution for a long time (evolutionary convergence). There is strong selective advantage for multicellularity of eukaryotic cells that have already become neurons; such an event occurred very early during multicellular evolution on Earth (Villegas et al., 2000). This argument strongly advocates in favor of the existence of other human-level of intelligence elsewhere in the cosmos. However, in spite of these persuasive arguments from the life sciences, some astronomers and paleontologists, independent of the evidence from biology, still defend the opposite point of view (Brownlee and Ward 2000).
2. Origin of Life on Earth
On the other hand, in trying to unravel the
mysteries of the origin(s) of life on Earth, we have unfortunately
little material upon which to work. The very fact that the Earth
is a living, dynamic entity (à la Gaia of James Lovelock)
means that the oldest rocks which hold the key to life's origins
have been destroyed by the inexorable process of plate tectonics.
The oldest rocks on Earth are found in the greenstone terrain
of Isua, S.W. Greenland, which is older than 3.8 billion years
(b.y.). Already these ancient rocks contain an isotopic signature
indicating that bacteria inhabited the environment in which the
rocks were formed (Schidlowski 1988; Mojzsis et al., 1996).
If full-fledged bacteria inhabited the Earth by 3.8 b.y. before the present (b.p.), then life must have started much earlier. Theoretically, it could have initiated at any time after water condensed on the Earth's surface: comets would have brought in the prebiotic molecules (Chyba and Sagan 1992) which, in as yet not-understood ways, self organized into primitive cellular structures developing, in turn, into the last common ancestor, or cenancestor (LCA). The LCA then gave rise to bacteria; but it should be underlined that what is emerging from the extensive analysis of a large number of phylogenetic trees constructed from a variety of macromolecules of life is that three primary cellular lines of evolutionary descent are established, between which extensive horizontal transfer events have taken place (Doolittle 1999, Becerra et al, 2000).
This critical development took place in conditions, which we would now consider inhospitable but which were normal for earliest life. The atmosphere was mildly reducing, consisting mostly of CO2 (Pollack et al., 1987), with subsequent consequences for a lower NO level (Kasting 1990). There is much discussion concerning the amount of oxygen in early Earth's atmosphere since there is evidence for at least localized pockets of subaerial oxidation (Ohmoto et al., 1999). The incident sunlight would have been about 30% less than at present because the Sun's nuclear furnace had not yet got into full swing (Sagan and Mullen 1972). It is assumed that the average temperatures were warm enough to keep water liquid on Earth owing to the greenhouse effect of the CO2 (with perhaps some CH4; cf., Kasting 1993). Temperatures may have been higher than at present due to the heavy meteorite/cometary bombardment, which characterized the Hadean and earliest Archaean epochs until about 3.8 b.y.b.p. (Maher and Stevenson 1988). There was no ozone layer to mitigate the deleterious effects of UV radiation. In addition, the moon was much closer to the Earth (Hartmann and Davis 1975; Cameron and Ward 1976) resulting in significant tidal influences on whatever surficial environments existed at the surface. Lastly, the aforementioned period of heavy bombardment could have sterilized the Earth a number of times (Sleep et al., 1989). Despite all of this, life started, developed, it flourished and remained.
3. Evolution of Life on Earth
Thus, one of the key characteristics of life,
its tenacity, developed early. The earliest bacteria may have
been thermophilic organisms of the domain Archaea (Woese 1987).
Already Darwin stated that life evolved in a warm little pond,
and most probably these thermophiles were the first organisms
on Earth (Copland 1936, Seckbach 1994, 1995, 2000). The species
of thermophilic Archaea, like many of the methanogens, lie near
the root in the tree of life (Valentine and Boone, in Seckbach
2000; Madigan and Marrs 1997). Recently it has been well established
that all life forms that cluster around the base of evolutionary
and phylogenetic trees are thermophiles (Pace 1997, Stetter 1998).
There is, however, a certain rebuttal to this theory that has
challenged the warm/hot origin of life, proposing that the first
cells were cryophilic (Galtiers et al., 1999).
Alternatively, the thermophilic signal may be an artifact of bacteria having been subjected to a thermophilic "bottle-neck" in the sense that during the period of heavy bombardment, the only bacteria to survive were those which either occupied the hydrothermal niche or which had taken refuge there (Baross and Hoffman 1985; Nisbet and Fowler 1996).
Until the rise of O2 in Earth's atmosphere and the development of ozone, most of the early microbes may have resided in sheltered, deep subterranean niches (Onstott et al, 1999). Thus, life could have started below the terrestrial surface, since the exposed land was inhospitable during the early history of our planet (Davies 1999).
One of the most important events in evolution was the advent of eukaryogenesis and multicellularity (Chela-Flores 2000a). The early Earth atmosphere was anoxic with significant rises in oxygen (>15% present atmospheric levels [PAL], Holland and Beukes 1990), occurring only at about 2.1 b.y.b.p. However, precursors with eukaryotic characteristics, as well as clear biochemical evidence for the existence of oxygenic cyanobacteria, have been identified in 2.7 b.y.-old shales from the Hammersley Basin, Australia (Brocks et al., 1999; Summons et al., 1999), before the significant 15% PAL was reached (Holland and Beukes 1990). Between 1.5 and 1.0 b.y.b.p. photosynthetic life became abundant enough to elevate atmospheric oxygen to nearly current level.
4. Distribution of Life, Here, There and Everywhere?
4.1. DISTRIBUTION OF LIFE IN THE UNIVERSE
Since the same laws of physics, chemical thermodynamics and carbon chemistry apply everywhere, there should be a high probability that other stars and satellites may harbor life, provided liquid water is available (de Duve, 1995; cf., "cosmic imperative", as discussed in Chela-Flores' chapter in this volume). Thus, the search for life (or even prebiotic conditions) on other solid bodies within our own solar system is of vital importance; equally important is the search for life in other solar systems by means for instance of the European Space Agency "Project Darwin" and the NASA initiative with "The Terrestrial Planet Finder". Both chapters within this section address these philosophical concepts, proposing practical considerations for the survivability of life under adverse conditions (McKay, in this volume) and also for the search for life, especially on Europa (Chela-Flores, in this volume).
4.2. DISTRIBUTION OF LIFE IN OUR SOLAR SYSTEM
Titan, a satellite of Saturn, with its thick,
CH4-containing atmosphere has been described as a "natural
exobiology laboratory (Jakosky, 1998, p.192). In fact, the Cassini-Huygens
spacecraft is due to rendezvous with Titan in 2004 to investigate
this natural chemical laboratory.
The planets that occur within the "habitable zone" of our solar system are Earth, Mars and Venus. They have had similar early histories and life may have developed on all three of these planets (McKay, 1997; Jakosky, 1998).
However, whereas Earth is now warm, wet and equable (its atmosphere is low in CO2, and high in N2), Mars is a cold and dry desert (with an atmosphere relatively high in CO2 and low in N2) the general atmospheric pressure is a fraction of that of the Earth), and Venus is an inferno, as far as life is concerned. As discussed in McKay's chapter (in this volume), the upper temperature limit for life is an uncomfortable 113º C (Blockl et al., 1997). This is maximum temperature limit of Pyrolobus fumarii (Stetter, 1998; see chapter by Seckbach and Oren, in this volume). The 400ºC at Venus' surface is therefore simply too high for liquid water and the molecular bonds of any biogenic organics would be broken down.
Likewise, McKay addresses the lower temperature limits of life, noting that there is abundant viable life in the cold dry deserts of Antarctica and the Arctic (Gilichinsky et al., 1992; McKay et al., 1994), as well as the fact that life can apparently survive for some millions of years in the Siberian tundra (Vorobyova et al., 1997). Chela-Flores (1998) underlines the fact that even eukaryotic organisms can survive in these environments. The deep ice samples at the Vostok Station, in which microorganisms were recently detected (Priscu et al. 1999, Karl et al. 1999), may be considered as an analogue of the environment present on Europa, the frozen Jovian moon which appears to have a subsurface ocean (Carr et al., 1998). Radar mapping of Lake Vostok revealed that liquid water exists below the icy crust. This water is possibly warmed up by pressure of the ice above and by geothermal sources below. Thus, these recent terrestrial observations may hold clues for the existence of life on other worlds.
The lack of water and the effects of UV radiation preclude the possibility of life at the surface of Mars (the high concentration of carbon dioxide in the Martian atmosphere screens off the UV radiation of wavelengths below 190 nm, but harmful UV-B radiation between 200 and 300 nm reaches the surface at full intensity). However, McKay (this volume) discusses the possibility of subsurface hydrothermal "islands" in the frozen aquifer of Mars, where life could potentially survive, but he also points out that it would have to contend with lethal doses of radiation from the surrounding rocks for significant periods of time.
The recent discovery of a possible biogenic signature within 3.9 b.y.-old carbonate globules in fractures of the Martian meteorite ALH84001 (McKay et al., 1996) has raised excitement among astrobiologists (Seckbach, 1997) and spawned fertile interest in this relatively new field (viz. NASA's Virtual Astrobiology Institute and related projects). Although the data from the Allan Hills meteorite, and also descriptions of structures having bacteriomorph shapes from the younger Martian meteorites, Nakhla and Shergotty (McKay et al., 1999) are tantalizing, there are as yet no firm conclusions concerning Martian biotics.
The problems are two-fold and need to be resolved before samples are returned from other astral bodies (Mars, cometary or asteroidal). (1) Recent work has underlined the problem of widespread, modern contamination of extraterrestrial materials (Steele et al., 2000), thus complicating interpretations. Furthermore, (2) we do not yet have a well-established database of truly biogenic structures, as opposed to biomorphic structures, although this is presently being addressed (McKay et al., 2000, Westall 1999).
One other aspect of the initial research by McKay et al. (1996), their description of possible "nanobacterial fossils" has led to interest in the size limits of life, with the surprising results that viable life may be very much smaller than originally believed (Kajander and Ciftcioglu 1998; Uwins et al., 1998; Gillet et al., 2000). The new developments in microbiology and micropalaeontology suggest that the diversity of microbial life in the Universe may turn out to be much larger than that presently encountered on Earth, thus presenting new challenges to the astrobiologist.
4.3. DESTINY OF LIFE IN THE UNIVERSE
The impact of extraterrestrial life on philosophy and theology goes beyond the scope of this book, which has been confined within the limits of science. But to complete this general overview of Astrobiology, we would like to comment on its last aspect, namely the destiny of life in the universe. This topic requires going beyond the frontiers of science. Other aspects of our culture have to be brought into a comprehensive dialogue, rather than within an intercultural debate (Chela-Flores, 2000b).
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