“It seems plain and self-evident, yet it needs to be said: the isolated knowledge obtained by a group of specialists in a narrow field has in itself no value whatsoever, but only in its synthesis with all the rest of knowledge and only inasmuch as it really contributes in this synthesis toward answering the demand, ‘Who are we?’” Erwin Schro¨dinger M.Gargaudetal.(Ed.):LecturesinAtrobiology,Vol.I,pp.V–XXVI (cid:1)c Springer-VerlagBerlinHeidelberg2005 Foreword Astrobiology, also known as bioastronomy or exobiology, refers to a vast area of scientific research. The formation of the solar system, its accretion and the formationoftheplanets,theoriginofthemoleculesoutofwhichlivingbeingsare made, the traces of present and past life within the solar system and elsewhere, aswellasthe searchforextra-solarplanets,areallpartofastrobiology.Andthe above list is not exhaustive. For obvious reasons, astrobiology is a field without the traditional barriers between astronomers,chemists, physicists, geologists,and biologists or between experimentalistsandtheorists,observersandthosewhomodelthe observations. As such, a single researcher cannot possess all the knowledge necessary to be an “astrobiologist”. One can even go a step further and say that while astrobi- ology clearly exists as a field of scientific research, there are no astrobiologists. Astrobiology exists at a higher level of organizationwhere the knowledge is not thatofanindividualbutthatofaresearchcommunitywhosemembersallshare the same interest for the fundamental questions concerning the emergence of life, its evolution, and how life is distributed on Earth and throughout the uni- verse.Eachpersoncontributesinpiecing togetherthis vastpuzzlethroughtheir knowledge and their experimental and theoretical tools. As often, if not always, when treating questions dealing with the past or with a sort of “elsewhere” where one cannot go and that one can only study indirectly,wemustbesatisfiedwithplausiblescenariosratherthanclearproofor other certainties.Inthis way,the strategyofthe astrobiologistissimilar to that of an archeologist or a paleontologist. There exists, however, major differences betweenthepathofachemistinterestedintheoriginoflife,andthusinprebiotic chemicalevolution,thatof a biologistwantingto followtime back startingwith currentlife, andthat ofa paleontologistsearchingfor the tracesof primitive life and its evolution and extinctions. While paleontologists have some hard data at hand (fossils and other physi- cal traces), the situation is very different for chemists, who are obliged to build a plausiblescenarioforthe appearanceoflife basedonhypotheses developedby specialists in other fields (composition of the primitive terrestrial atmosphere, addition of extraterrestrialorganic material, etc.). For the most part, these hy- potheses areunverifiable.The biologist,onthe other hand,tries to use phyloge- netic tools to find and understand LUCA, the first common ancestor who must have been preceded by other micro-organismswith no descendants. VIII Foreword Similarly, there is an important difference between the strategies of a ge- ologist, expert in the transition between the Tertiary and Cretaceous periods, and the planetologist who would like to describe the Earth during the period of intense meteoritic bombardment. The former disposes of observations and measures (iridium content, sediment ashes, shocked quartz, etc.), which pro- vide a reasonable explanation for the great biological Cretaceous Tertiary crisis causedby a major meteorite impact. The latter only has access to indirectdata based on observations of lunar craters but also simulations, which are of course based on theoretical models. Since everyscientisthasa limited areaofexpertise,the scenariothat he/she proposes can only be validated by the constraints and parameters that he/she knowsandmasters.Suchanindividualstrategycanthusleadtoasmanyscenar- ios as there are researchers. A multidisciplinary approach has the advantage of subjecting each individual proposition to a much larger number of constraints. This naturally leads to the rejection of “weak” scenarios and to the emergence of more robust hypotheses. For example, it is pointless for a chemist to invoke the roleofaprebioticchemicalreactionifthe conditionsneededforthereaction to occurarecompletely incompatible withthe primitive Earthconditions deter- mined by the planetologists. This simple example illustrates the importance of interdisciplinary discussion for all those who consider themselves to be astrobi- ologists.The CNRSsummerschoolssuchasProprianoin1999and2003andLa Colle-sur-Loup in 2001 have contributed to strengthening the dialog within the French scientific community. The goalof the first summer school, Exobio’99 in Propriano,was to provide participants with an objective image of what we know today about the early Earth conditions – the oceans, the proto-continents, the atmosphere, and even the climate – but also of what we know about the solar system during the first billion years of its history.Some stages in the chemicalevolutionthat may have occurredontheyoungplanetEarth,withadifferentsolarradiation,lessintense in the visible part of the spectrum but much more intense in the RX region were also discussed during the first summer school. The discussion then moves towards the biological evolution, the early stages of which are still very poorly understood. The problems related to the exploration of Mars and Titan were then addressed. The second summer school, Exobio ’01 in La Colle-sur-Loup, was more oriented towards the chemistry, molecular biology, biochemistry, and biologi- cal evolution of early Earth. Its main theme was the study of organisms re- ferred to as extremophiles, which could provide information on the nature of the first unicellular organisms that populated the young oceans. Among the specific topics addressed were the autoformation of biological membranes, the possible origins of the homochirality of the constituents of living beings, the protometabolisms that may be inferred from the study of metabolisms, and the possible role of ribozymes before the emergence of catalysis by proteinic enzymes. Foreword IX The texts that follow represent the first volume of the series “Lectures in Astrobiology” and are the result of the first two schools. The chapters were written for readers already familiar with the general topic of the origin of life and life “elsewhere” but not to the extent to which each specialist is in his own discipline. As such, they are meant as much for students as for established scientists seekingto broadentheir horizonsinthe vastfield ofthe originsoflife. We hope these texts will initiate vocations and incite researchers and students specializedinoneoftheindividualfieldstojointhebroadforumofastrobiology. It is undeniable that the questions forming the basis of astrobiology are among the big questions that humanity has asked itself since its inception and which recent decades have attempted to answer; answers that seem more and more plausible although necessarily partial. Acknowledgements This book is the product of a multidisciplinary community, the members of which all question the knowledge from their disciplines at origin in order to build together the complex structure, which this area of research represents. Oneneedsaveryopenmindaswellastheabilitytoquestiononesideasthrough the recent discoveries in other fields. A group of international reviewers and ourselves have read the set of texts that follow. The final versions of these texts, after multiple rewritings and long discussions, sometimes required the opinions of five or six specialists. We would therefore like to thank all of the authors who have accepted these remarks, criticisms, and multiple discussions warmly, but also all of the “spe- cialist reviewers” who, through their expertise, have contributed to the general coherence of this work. Last but not least, the editors call upon the reader’s indulgence concerning some (or many!) misusages of the English language; English is not the mother tongue of the large majority of authors. Muriel Gargaud Bernard Barbier Herv´e Martin Jacques Reisse Fromlefttoright:Herv´eMartin(geochemist),MurielGargaud(astrophysicist),Jacques Reisse (chemist), Bernard Barbier (biochemist) Preface Thetwentiethcenturywillberememberedasthecenturyofscientificrevolutions. It started with the discoveries of physics, which revealed the fine structure of matterandthefundamentallawsofnature.Thencamecosmology,whichtraced thehistoryoftheUniverse,fromtheBigBangtothedizzyingrecessionofmyriad whirling galaxies. Finally, with the advances of biochemistry, cell biology, and molecular biology, life itself has disclosed its secrets. These revolutions, in turn, have spawned new technologies – nuclear power, space travel, informatics, bioengineering – that could not even be conceived one century earlier; they have also opened new fields of inquiry that had been relegatedbeforetotherealmoftheunknowable,objectsonlyofgratuitousspec- ulation or imaginative fiction. Among these new fields, the origin and evolution of life on Earth have become topics of intense theoretical and experimental re- search. The latestoffspring ofthis upheavalis exobiology,the science ofextraterres- triallife,alsoknownasastrobiologyorbioastronomy.Ofallbranchesofscience, itis the mostuniversalandall-encompassing,involvingvirtually everyscientific discipline. It is also the emptiest, being so far without known object. No sign of life beyond our planet has yet been uncovered. Whether or not its quest will one day be fulfilled, exobiology has already produced many valuable fruits and is bound to produce many more in the fu- ture. It has brought together and impelled physicists, chemists, cosmologists, astronomers, planetologists, geologists, paleontologists, biologists of all kinds, andotherspecialistswhohaduntilthenlaboredeachintheisolationoftheirin- dividualdisciplinestointeract.Ithasstimulatedmanyinvestigationsthatwould otherwise have been performed with considerably less vigor, perhaps not at all. Ithasrevealedanumberofsignificantfactsonthecosmicpropertiesandinterre- lationships outofwhichlife andmind emergedonourplanetandmay,perhaps, have done so elsewhere in our galaxy or in others. It has even alerted philoso- phers and theologians to a very real possibility that, only 400 years ago, was a heresy punishable by death. It has evoked new dreams in the collective imag- ination of humans who, ever since their distant ancestors started contemplating the skies, have asked the question: Is there life out there? Are there others like us elsewhere?