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CALTECH GE 133 - The Emergence and Maintenance of Life

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From Protoplanets to Protolife:The Emergence and Maintenance of LifeEric GaidosUniversity of HawaiiFranck SelsisCentre de Recherche Astronomique de LyonDespite great advances in our understanding of the formation of the Solar System, theevolution of the Earth, and the chemical basis for life, we are not much closer than the ancientGreeks to an answer of whether life has arisen and persisted on any other planet. The origin oflife as a planetary phenomenon will probably resist successful explanation as long as we lackan early record of its evolution and additional examples. Plausible but meagerly-investigatedscenarios for the origin of important prebiotic molecules and their polymers on the Earthinvolving atmospheric chemistry, meteorites, deep-sea hot springs, and tidal flat sedimentshave been developed. Our view of the diversity of extant life, from which properties of a lastuniversal common ancestor (LUCA) can be inferred, has also improved in scope and resolution.It is widely thought that the geologic record shows that life emerged quickly after the endof prolonged bombardment of the Earth. New data and simulations contradict that view andsuggest that more than half a billion years of unrecorded Earth history may have elapsedbetween the origin of life and LUCA. The impact-driven exchange of material between theinner planets may have allowed earliest life to be more cosmopolitan. Indeed, terrestrial lifemay not have originated on the Earth, or even on any planet. Smaller bodies, e.g., the parentbodies of primitive meteorites, in which carbon molecules and catalytic transition metals wereabundant, and in which hydrothermal circulation persisted for millions of years, offer alternativeenvironments for the origin of life in our Solar System. However, only planet-sized bodiesoffer the stable physiochemical conditions necessary for the persistence of life. The searchfor past or present life on Mars is an obvious path to greater enlightenment. The absence ofintense geologic activity on Mars, which contributes to its inhospitable state today, has alsopreserved its ancient history. If life did emerge on Mars or was transferred from Earth, the lackof sterilizing impacts (due to a low gravity and no oceans) means that a more diverse biotamay have thrived than is represented by extant life on Earth. On the other hand, a habitablebut still lifeless early Mars is strong evidence against efficient transfer of life between planets.The subsurface oceans of some icy satellites of the outer planets represent the best locales tosearch for an independent origin of life in the Solar System because of the high dynamicalbarriers for transfer, intense radiation at their surfaces, and thick ice crusts. These also presentequally formidable barriers to our technology. The “ultimate” answer to the abundance of lifein the Cosmos will remain the domain of speculation until we develop observatories capable ofdetecting habitable planets - and signs of life - around the nearest million or so stars.1. INTRODUCTIONThis contribution’s place as the last chapter in Proto-stars and Planets V may betray a subtle conceit in how weview our place in a cosmic order that runs from the inter-stellar medium to planetary bodies. (Read in reverse order,the chapters would suggest a more humble search for ourorigins among wisps of interstellar gas and dust.) Never-theless, this sequence makes sense, both in a temporal andalso a physical order: It describes a gradation in phenom-ena in which physical and chemical inevitability (the lawsof gravity, classical and quantum mechanics, and electro-magnetism) which govern the collapse of the interstellarmedium and the formation of stars, are replaced by morestochastic processes such as accretion during planet forma-tion and evolution. For example, it may be inevitable thata cooling molecular cloud collapses, a disk forms, and thatrunaway growth of planetesimals occurs in that disk, butthe final masses, orbits, and surface environmentsof planetsmay not be predictable in more than a statistical sense. Ulti-mately it is no longer sufficient to describe what could hap-pen, one must also describe what did happen. Whereas starscan be described by a relatively small number of variables(age, rotation, and metallicity, for example), planets, partic-ularly terrestrial planets, cannot. In that context, the originand survival of life might be the ultimate contingency.On the other hand, what little we know about the originof life seems to suggests some element of inevitability. The1primary constituents of life (C, H, N, and O) are four of thefive most abundant elements in the universe. Some of themonomeric molecules of life (amino acids, sugars, etc.) arefound everywhere. Laboratory experiments have suggestedpossible pathways along which those monomers might be-come polymers, make copies of themselves and interact incomplex ways on which Darwinian selection can act. Evi-dence for life appears in Earth’s rock record as soon as thereis any geologic record at all.The dichotomy between chemical inevitability and his-torical contingency infuses studies of the origin and propen-sity of life in the universe (not to mention the question ofwhat life is), and it has spawned numerous popular bookson the subject. We leave resolution of that problem toscientists-cum-philosophers. In this review we concentrateon those lines of inquiry that have experienced especiallyfruitful development since the review of this subject byChyba et al. (2000) for Protostars and Planets IV, includ-ing new age constraints on the appearance of clement en-vironments and life on the Earth, a re-assessment of pre-dictions for the chemistry of the prebiotic atmosphere andoceans, the formulation of a dynamical scenario for a “late”cataclysmic bombardment that may have profoundly influ-enced the emergence of life, and the development of newtheories for the origin of Earth’s water. Because scienceknows so little about the origin of life on Earth and the po-tential environments for its origin elsewhere, we feel it isimportant to be open-minded - and even provocative - inthe scenarios that we consider. Our review is structured asfollows: We consider the timing and environment of theorigin of terrestrial life (Section 2) and our understandingof the combination of factors that permit Earth-like life topersist on a planet for an astronomically interesting periodof time (Section 3). Finally, we address how the search forlife


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