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J. Seckbach and M. Walsh (eds.), From Fossils to Astrobiology, 515-520. © Springer Science + Business Media B. V. 2009

The Stromatolites (Burns et al., 2008) are the oldest fossils found on Earth, and they were formed mostly by massive colonies of cyanobacteria and other prokaryotes, now found in Precambrian rocks. The layered communities were responsible of precipitation of minerals, now mainly preserved as limestone. The structures are characterized by thin, alternating light and dark layers. The fossil cyanobacteria cells from Western Australia, dated from 3.5 billion years ago, are probably the oldest known fossils. Even older are the marine sediments and pillow lavas discovered in Greenland in which evidence of life has been observed, and these were determined to be 3.85 billion years old.

A "Living Fossil" is an informal term for any living species of organism that appears to be identical to a species otherwise only known from fossils. Such species have survived major extinction events and seemingly have not changed during their very long evolutionary history. For example, an alga, plant or animal once thought to be extinct and was found living in modern times. Some living specimen were identical to fossils dating from 400 million year ago, and such an organism is therefore considered a "living fossil".

Astrobiology (see further) is a multidisciplinary science investigating the origin of life, evolution, life in extreme environments, search and distribution of life on Earth and beyond, paleontology, physiology of radiation resistance, and more.

An understanding of the factors that enabled Earth to become so successfully colonized is important when we wish to learn more about the possibility that similar (or even different) life forms may exist elsewhere in the Universe. We now have a reasonably good understanding of the development of Earth as an ecosystem during the last two-and-a-half to three billion years. However, the question when did life originate on planet Earth remains still open, and the processes that led to the formation of early life on Earth are still enigmatic. The early events that led to the colonization of our planet by a diversity of microorganisms during the first half a billion years since living cells first appeared, remain elusive.

Our views on life on Earth in the earliest times are mainly based on the following factors: geochemical signatures (especially on stable isotope studies), limited fossil evidence, study of microbial life as it exists today, and on theoretical considerations based on our understanding of the physical and chemical properties of our planet during the first billion years of its existence. Stable isotope studies of the oldest rocks has pushed the date of the earliest appearance of life back as far as 3.85 billion years ago (Mojzsis et al., 1996; Schidlowski, 1988). On the other hand, when until recently it was assumed that microfossils with the morphology of several types of present-day cyanobacteria could be safely dated to 3,460 million years ago, the age and nature of these fossils of "Precambrian microbial mats" have been challenged in recent years (Brasier et al., 2002, 2006; Wacey et al., 2008).

Theoretical considerations to reconstruct the nature of early life on Earth have always been based on our (ever changing) views about the chemical composition of early Earth: presence or absence of oxygen, possible abundance of organic building blocks according to the Oparin-Haldane 'prebiotic soup' hypothesis, etc., and on thermodynamic constraints. Engelbert Broda's 1975 book on "The Evolution of Biogeochemical Processes", presented a for the time convincing scenario based on the "prebiotic soup" model. Since that time our understanding of the conditions on early Earth has greatly increased. Especially we know much more about the diversity of hyperthermophilic and other extre-mophilic microorganisms that inhabit our present-day planet. A number of very different possible scenarios for the origin of life and the nature of the first cells have been proposed in the past decades (see e.g Horgan, 1991; Pereto, 2005). The possibility that some kind of hyperthermophilic chemolithotroph may have been the first type of living organism has become an attractive alternative in our thinking. This recent concept is rather more accepted by the scientific community than a mesophilic heterotrophic fermentative bacterium swimming in a rich organic broth.

The understanding of the true nature of the first organisms on Earth will of course have a profound impact on our views how subsequently planet Earth, and possibly other planets in the universe as well, were colonized by life and as result became modified by the activity of living organisms.

A more recent development is the approach to reconstruct the order in which different types of metabolism (aerobic, anaerobic, fermentation, respiration, different kinds of chemolithotrophy and phototrophy) appeared, is 'phylogenomic dating'. Here both 16S rRNA genes and other slowly evolving genes are used as 'molecular clocks' in an attempt to reconstruct the history of life on Earth. The chapter by Blank in this volume presents a state-of-the-art overview of this intriguing field. The pioneering work of Carl Woese has shown that modern biomolecules are historical documents that can be used to learn even about the most distant past (Woese, 1987, 1994). However, the 'phylogenomic approach' still cannot be used for absolute dating of events in the early history of life on Earth. Therefore, calibration by means of other, independent methods will remain necessary. As long as the implications of the results obtained by such other methods it remains to be contested. Also the sequence information present within modern biomolecules will not yield a coherent picture of what happened on our planet over 3.5 billion years ago.

We clearly have learned a great deal since Oparin and Haldane proposed their theories about the origin of life in the 1920s. New information from disciplines as diverse as geology, paleontology, isotope geochemistry, microbiology, biogeochemistry, molecular biology and bioinformatics has contributed greatly to the discussion, and much of the current knowledge can be found in the preceding chapters. But unfortunately, in spite of all this information, we are still far removed from a generally accepted scenario of what kinds of life were found on Earth in the first half a billion years since it first appeared, and what kind of life we may expect to find elsewhere in the Universe.

Now all the major space agencies are planning new missions to the Moon, to the planets of our Solar System and to its outer boundaries, we have thought it timely to devote this volume to different aspects illustrating the above statements under the title "From Fossils to Astrobiology". The main conclusion that becomes obvious while reading the chapters in this book is that multiple disciplines, including geophysics, study of solar activity, space climate and astro-biology should be brought within a unified framework that takes the fossil evidence from earlier stages of life on Earth into account. This book was intended to provide a framework for disentangling the microbial and multicellular fossil record, interpreting its possible implications for the existence of life elsewhere.

Answering the question whether there is life in the Solar System beyond our own planet should be viewed as a complement to the astronomical approach for the search of evidence of the later stages of the evolutionary pathways towards intelligent behavior. This is possible with the ongoing SETI (Search for Extraterrestrial Intelligence) project using radio telescopes and other astronomical instruments.

Since we are able to overcome terrestrial gravity and to send spacecrafts to explore the solar system, the direct search for life elsewhere is becoming an inherent part of space exploration and one of the major approaches of astrobiology. Although only a few extraterrestrial planetary bodies have been explored in detail so far, from the data already collected one can distinguish different categories of targets of major interest for astrobiology in our solar system.

There are extraterrestrial environments where life, either extinct or extant, may be present. These places are characterized by conditions in the past that had been compatible with the development of complex prebiotic processes, and had existed for a period long enough to allow the emergence of life (or compatible with the importation of living system from other places), followed by habitable conditions. One of the main parameters driving habitability on a planetary body is the presence of liquid water. Mars, like Earth, very likely had large bodies of liquid water on its surface for long period of times - several hundreds of millions of years - in its early history. In spite of its evolution and drastic changes, Mars may have preserved traces of extraterrestrial bio-signatures, due to its lack of strong tectonic activity.

If life was - or still is - present on Mars, those traces may be recovered today from its subsurface. This makes the red planet the most attractive body in the solar system for searching for extraterrestrial life. But there are other places in the solar system where liquid water is probably present. This is the case for three out of the four Galilean satellites of Jupiter: Ganymede, Callisto and Europa. This is also the case for Titan, the largest satellite of Saturn, being the only satellite of the solar system having a dense atmosphere and the only planetary body having atmospheric condition close to that of the Earth. Evidence for the presence of an internal water ocean is dramatically supported by the recent observations of the shift of surface features on the satellite, observed by the Cassini-Huygens mission. This mission has also revealed the unexpected properties of another satellite of Saturn, Enceladus, ten times smaller than Titan. Surprising gigantic plumes of water ice particles and gas, together with organic molecules have been observed by the Cassini instruments coming from the south polar region of Enceladus.

This strongly suggests the presence of a large liquid water reservoir in the internal structure of this small satellite, together with an active organic chemistry. Although we have so far no direct evidence for the existence of these internal oceans, the most interesting cases are those of Europa and Enceladus, since if they exist, their internal liquid water reservoir may be in contact with rocky materials, facilitating redox reactions that provide chemical energy to sustain prebiotic processes as well as energy for living systems.

There are also extraterrestrial environments that show today similarities with the primitive Earth, before the emergence of life. The study of such environments is of tremendous importance since most of the conditions that were present on the early Earth have disappeared today, been erased by geological processes and by life itself. To understand the processes that allowed the origin of life on our planet, and verify our theories on chemical evolution leading to life, we need to check these concepts in a realistic environment. The availability of planetary bodies having analogies to the early Earth offers such an essential opportunity. This is, again, the case for Titan, which, thanks to the many observations from the Cassini-Huygens mission, looks now more and more like an evolving primitive Earth, with methane on Titan playing the role of water on Earth, and water ice that of silicates. And finally, there are extraterrestrial planetary bodies where a complex organic chemistry is occurring, the study of which offers a way to study and understand the general processes of complexification of matter in the universe, in a real planetary environment. This is the case for comets, but also for the two satellites of Saturn, again, Titan and Enceladus.

Thus, there are several locations of great interest in the solar system for looking at prebiotic processes or searching for bio-signatures, especially in the outer solar system: Europa, Titan and even Enceladus are among those important targets. Although the Cassini-Huygens mission is far form ended, and is officially extended until 2010, and likely to be extended-extended beyond, the scientific community is already working on a new mission to the Saturn system, able to explore Titan's surface with dedicated instruments from a balloon and surface probes, and Enceladus from surface probes.

However, from what we already know today about the habitable bodies, it seems likely that the Earth is the only place in the solar system where macroscopic life is present. To search for extraterrestrial macroscopic life, we probably have to look outside the solar system. The discovery of extrasolar planets opens new possibilities in that field. Within the last decade, almost 300 exoplanets have been detected. We have now to identify and develop efficient tools to search for clear signs of biological activity on these far planetary bodies. With our current technology it is out of question to search for fossils, but we should be able soon to search for atmospheric biomarkers. Searching for evolutionary biosignatures during the exploration of the Solar System is an important objective in our search for deeper insights into life on Earth. Finding traces of life in any of the candidate-sites that are known to space geophysicists, as we have pointed out above: Europa, Enceladus, Titan, or Mars, would add arguments towards the improved understanding of life on Earth, as we know it today. The study of fossil evidence of life on Earth can contribute considerably to the exploration of outer space, and that is one of the major messages of this book.

References

Blank, C.E. (2008). Phylogenomic dating and the relative ancestry of prokaryotic metabolism. This volume.

Brasier, M.D., Green, O.R., Jephcoat, A.P., Kleppe, A.K., van Kranendonk, M.J., Lindsay, J.F., Steele, A.

and Gassineau, N.V. (2002). Questioning the evidence for Earth's oldest fossils. Nature 416, 76-81. Brasier, M.D., McLoughlin, N., Green, O.R. and Wacey, D. (2006). A fresh look at the fossil evidence for early Archaean cellular life. Phil. Trans. R. Soc. B 361, 887-902. Broda, E. (1975). The Evolution of the Bioenergetic Processes. Pergamon, Oxford. Burns, B.P., Walter, M.R. and Neilan, B.A. (2008). Stromatolites. This volume. Horgan, J. (1991). In the beginning Sci. Am. 264(2), 116-125.

Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P. and Friend, C.R.L.

(1996). Evidence for life on Earth 3,800 million years ago. Nature 384, 55-59. Pereto, J. (2005). Controversies on the origin of life. Int. Microbiol. 8, 23-31.

Schidlowski, M. (1988). A 3,800-million-year isotopic record of life from carbon in sedimentary rocks. Nature 333, 313-318.

Wacey, D., McLoughlin, N. and Brasier, M.D. (2008). The search for windows into the earliest history of life on Earth and Mars. This volume. Woese, C.R. (1987). Bacterial evolution. Microbiol. Rev. 51, 221-271.

Woese, C.R. (1994). There must be a prokaryote somewhere: microbiology's search for itself. Microbiol. Res. 58, 1-9.

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