Primordial organic chemistry and the origin of life
Primordial organic chemistry and origin of life
EVOLUTION↗
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The stability of the RNA bases: implications for the origin of life
High-temperature origin-of-life theories require that the components of the first genetic material are stable. We therefore have measured the half-lives for the decomposition of the nucleobases. They have been found to be short on the geologic time scale. At 100 degreesC, the growth temperatures of the hyperthermophiles, the half-lives are too short to allow for the adequate accumulation of these compounds (t1/2 for A and G approximately 1 yr; U = 12 yr; C = 19 days). Therefore, unless the origin of life took place extremely rapidly (<100 yr), we conclude that a high-temperature origin of life may be possible, but it cannot involve adenine, uracil, guanine, or cytosine. The rates of hydrolysis at 100 degreesC also suggest that an ocean-boiling asteroid impact would reset the prebiotic clock, requiring prebiotic synthetic processes to begin again. At 0 degreesC, A, U, G, and T appear to be sufficiently stable (t1/2 >/= 10(6) yr) to be involved in a low-temperature origin of life. However, the lack of stability of cytosine at 0 degreesC (t1/2 = 17, 000 yr) raises the possibility that the GC base pair may not have been used in the first genetic material unless life arose quickly (<10(6) yr) after a sterilization event. A two-letter code or an alternative base pair may have been used instead.
Non-NASA Center↗
Urey Prize Lecture - Planetary evolution and the origin of life
One of the principal questions concerning planetary evolution and life's origins relates to the early-earth organic material's origination in situ, outer solar system importation, or simple irrelevance to the emergence of organisms. Additional considerations encompass the character of interstellar organic material and its relationship to outer solar system organic compounds, and the possibility of life's emergence in the early Mars. Attention is given to the essentiality of liquid water for life-forms, in the role not only of a reaction medium among molecules but that of a basis for hydrophylic and hydrophobic groups' bonding.
Mckay, Christopher P.↗
The principle of cooperation and life's origin and evolution
In simple terms a living entity is a negentropic system that replicates, mutates and evoluves. A number of suggestions have been made, such as directed panspermia, atmospheric photosynthesis, genetic overtaking from inorganic processes, etc., as alternative models to the accepted Oparin-Haldane-Urey model of the origin of life on Earth. This has probably occurred because in spite of tremendous advances in the prebiotic synthesis of biochemical compounds, the fundamental problem of the appearance of the first life--a primordial replicating cell-ancestral to all other forms of extant life, has remained elusive. This is indeed a reflection on the different fundamental nature of the problem involved. Regardless of which were the fundamental processes which occurred on the primitive Earth, it has to end up with the fundamental characteristics of an ancestral protocell. The problem of the emergence of the first ancestral cell was one of synergistic macromolecular cooperation, as it has been discussed by authors recently (COSPAR XXV Plenary Meeting). An analogous situation must have occurred at the time of the appearance of the first eucaryotic organism. Procaryotic life appeared probably during the first 600 million years of Earth history when the Earth was sufficiently cool and continually bombarded (in the late accretion period) by comets and minor bodies of the solar system, when the sea had not yet acquired its present form.
Oro, J.↗
Evidence from Meteorites for Multiple Possible Amino Acid Alphabets for the Origins of Life
A key question for the origins of life is understanding which amino acids made up the first proteins synthesized during the origins of life. The canonical set of 20 - 22 amino acids used in proteins are all alpha-amino, alpha-hydrogen isomers that, nevertheless, show considerable variability in properties including size, hydrophobicity, and ionizability. Abiotic amino acid synthesis experiments such as Miller-Urey spark discharge reactions produce a set of up to 23 amino acids, depending on starting materials and reaction conditions, with significant abundances of both alpha- and non-alpha-amino acid isomers. These two sets of amino acids do not completely overlap; of the 23 spark discharge amino acids, only 11 are used in modern proteins. Furthermore, because our understanding of conditions on the early Earth are limited, it is unclear which set(s) of conditions employed in spark discharge or hydrothermal reactions are correct, leaving us with significant uncertainty about the amino acid alphabet available for the origins of life on Earth. Meteorites, the surviving remnants of asteroids and comets that fall to the Earth, offer the potential to study authentic samples of naturally-occurring abiotic chemistry, and thus can provide an alternative approach to constraining the amino acid library during the origins of life.
Burton, A. S.↗
Origins of life: a comparison of theories and application to Mars
The field of study that deals with the origins of life does not have a consensus for a theory of life's origin. An analysis of the range of theories offered shows that they share some common features that may be reliable predictors when considering the possible origins of life on another planet. The fundamental datum dealing with the origins of life is that life appeared early in the history of the Earth, probably before 3.5 Ga and possibly before 3.8 Ga. What might be called the standard theory (the Oparin-Haldane theory) posits the production of organic molecules on the early Earth followed by chemical reactions that produced increased organic complexity leading eventually to organic life capable of reproduction, mutation, and selection using organic material as nutrients. A distinct class of other theories (panspermia theories) suggests that life was carried to Earth from elsewhere--these theories receive some support from recent work on planetary impact processes. Other alternatives to the standard model suggest that life arose as an inorganic (clay) form and/or that the initial energy source was not organic material but chemical energy or sunlight. We find that the entire range of current theories suggests that liquid water is the quintessential environmental criterion for both the origin and sustenance of life. It is therefore of interest that during the time that life appeared on Earth we have evidence for liquid water present on the surface of Mars.
Review, Academic↗
RNA catalysis and the origins of life
The role of RNA catalysis in the origins of life is considered in connection with the discovery of riboszymes, which are RNA molecules that catalyze sequence-specific hydrolysis and transesterification reactions of RNA substrates. Due to this discovery, theories positing protein-free replication as preceding the appearance of the genetic code are more plausible. The scope of RNA catalysis in biology and chemistry is discussed, and it is noted that the development of methods to select (or predict) RNA sequences with preassigned catalytic functions would be a major contribution to the study of life's origins.
Orgel, Leslie E.↗
Life Sciences and Space Research 24 (4): Planetary Biology and Origins of Life; Topical Meeting of the COSPAR Interdisciplinary Scientific Commission F (Meeting F3) of the COSPAR Plenary Meeting, 29th, Washington, DC, Aug. 28-Sep. 5, 1992
The proceedings include sessions on extraterrestrial organic chemistry and the origins of life; life on Mars: past, present and future; planetary protection of Mars missions; chemical evolution on Titan; origins and early evolution of biological (a) energy transduction and membranes (b) information and catalysis; and carbon chemistry and isotopic fractionations in astrophysical environments.
Greenberg, J. M.↗
Extraterrestrial Organic Chemistry: From the Interstellar Medium to the Origins of Life
Extraterrestrially delivered organics in the origin of cellular life. Various processes leading to the emergence of cellular life from organics delivered from space to earth or other planetary bodies in the solar system will be reviewed. The focus will be on: (1) self-assembly of amphiphilic material to vesicles and other structures, such as micelles and multilayers, and its role in creating environments suitable for chemical catalysis, (2) a possible role of extraterrestrial delivery of organics in the formation of the simplest bioenergetics (3) mechanisms leading from amino acids or their precursors to simple peptides and, subsequently, to the evolution of metabolism. These issues will be discussed from two opposite points of view: (1) Which molecules could have been particularly useful in the protobiological evolution; this may provide focus for searching for these molecules in interstellar media. (2) Assuming that a considerable part of the inventory of organic matter on the early earth was delivered extraterrestrially, what does relative abundance of different organics in space tell us about the scenario leading to the origin of life.
Pohorille, Andrew↗
Impact of solar system exploration on theories of chemical evolution and the origin of life
The impact of solar system exploration on theories regarding chemical evolution and the origin of life is examined in detail. Major findings from missions to Mercury, Venus, the moon, Mars, Jupiter, Saturn, and Titan are reviewed and implications for prebiotic chemistry are discussed. Among the major conclusions are: prebiotic chemistry is widespread throughout the solar system and universe; chemical evolution and the origin of life are intimately associated with the origin and evolution of the solar system; the rate, direction, and extent of prebiotic chemistry is highly dependent upon planetary characteristics; and continued exploration will increase understanding of how life originated on earth and allow better estimates of the likelihood of similar processes occurring elsewhere.
Devincenzi, D. L.↗
The Martian Subsurface as a Potential Window into the Origin of Life
Few traces of Earth's geologic record are preserved from the time of life's emergence, over 3,800 million years ago. Consequently, what little we understand about abiogenesis - the origin of life on Earth - is based primarily on laboratory experiments and theory. The best geological lens for understanding early Earth might actually come from Mars, a planet with a crust that's overall far more ancient than our own. On Earth, surface sedimentary environments are thought to best preserve evidence of ancient life, but this is mostly because our planet has been dominated by high photosynthetic biomass production at the surface for the last approximately 2,500 million years or more. By the time oxygenic photosynthesis evolved on Earth, Mars had been a hyperarid, frozen desert with a surface bombarded by high-energy solar and cosmic radiation for more than a billion years, and as a result, photosynthetic surface life may never have occurred on Mars. Therefore, one must question whether searching for evidence of life in Martian surface sediments is the best strategy. This Perspective explores the possibility that the abundant hydrothermal environments on Mars might provide more valuable insights into life's origins.
Martian subsurface↗
The Origin of Life--Did It Occur at High Temperatures
A high-temperature origin of life has been proposed, largely for the reason that the hyperthermophiles are claimed to be the last common ancestor of modern organisms. Even if they are the oldest extant organisms, which is in dispute, their existence can say nothing about the temperatures of the origin of life, the RNA world, and organisms preceding the hyperthermophiles. There is no geological evidence for the physical setting of the origin of life because there are no unmetamorphosed rocks from that period. Prebiotic chemistry points to a low-temperature origin because most biochemicals decompose rather rapidly at temperatures of 100 C (e.g., half-lives are 73 min for ribose, 21 days for cytosine, and 204 days for adenine). Hyperthermophiles may appear at the base of some phylogenetic trees because they outcompeted the mesophiles when they adapted to lower temperatures, possibly due to enhanced production of heat-shock proteins.
Miller, Stanley L.↗
Oxidation state of the Earth's upper mantle during the last 3800 million years: Implications for the origin of life
A popular, as well as scientifically rigorous, scenario for the origin of life on Earth involves the production of organic molecules by interaction of lightning (or other forms of energy) with a chemically reducing atmosphere in the early history of Earth. Experiments since the 1950's have convincingly demonstrated that the yield of organic molecules is high when the atmosphere contains molecular hydrogen, methane, ammonia, and water vapor. Additional work has also shown that such a highly reducing atmosphere might not, however, have been sufficiently long-lived in the presence of intense solar ultraviolet radiation for life to have formed from it. One way of maintaining such an atmosphere would be to have a continual replenishment of the reduced gases by prolonged volcanic outgassing from a reducing of Earth's interior. The length of time that this replenishment might need to continue is in part constrained by the flux of asteroids onto the Earth's surface containing sufficient energy to destroy most, if not all, life that had developed up to that point in time. If a reducing atmosphere is a key ingredient for the origin of life on Earth, the time of the last environmental sterilization due to large impacts would be an important constraint. In a deep marine setting (e.g., hydrothermal vent), the last global sterilization might have occurred at 4200-4000 Ma. On the Earth's surface, the last global sterilization event might have occurred at 4000-3700 Ma. If these are meaningful constraints, how likely is it that a reducing atmosphere could have survived on the Earth until about 3800 Ma ago? Due to the importance of replenishing this atmosphere with reducing components by volcanic outgassing from the mantle, geochemical information on the history of the mantle's oxidation state would be useful for addressing this question. Geochemical and experimental data discussed in this abstract suggest that extrusive mafic volcanics derived from the upper mantle have had oxidation states near the fayalite-magnetite quartz buffer throughout the last 3800 Ma. At magmatic temperatures, the gases released from volcanoes having this oxidation state would have been, as they are today, composed dominantly of carbon dioxide and water vapor, and would not contain the ingredients for maintaining a reducing atmosphere. Consequently, geochemical data do not favor the survival of a reducing atmosphere until about 3800 Ma. Alternative venues and pathways for the origin of life need to be investigated (e.g., hydrothermal vents along oceanic ridges).
Delano, J. W.↗
Are comets connected to the origin of life
Possible connections between comets and the origin of life on earth are discussed. The orbital evolution of comets and their origin are considered within a framework for the origin of the solar system, with particular attention given to the origin of the biosphere, and the origin of the Oort cloud. Evidence suggesting that cometary nuclei are undifferentiated throughout is considered, and a model of the average composition of a mean new comet is obtained from observational data which is similar to that of an interstellar frost. The chemistry of the model composition giving rise to the species observed in cometary spectra is considered, as well as the relations of cometary to cosmic abundances of oxygen, carbon and sulfur. The characteristics of possible sites for prebiotic chemistry, including interstellar clouds, the protosolar nebula, comets in the Oort cloud, periodic comets and the primitive earth, are examined, and a possible role of comets in bringing the interstellar prebiotic chemistry to earth is suggested.
Delsemme, A. H.↗
Oparin and the origin of life - Cosmological considerations
The question of the origin of life is examined on a cosmological scale from early observations made by Oparin. Even by a comparison of the amount of data presently available from telescopic and spacecraft observations Oparin's conclusions have been confirmed. The concept of panspermia is rejected and details of the role of carbon on earth are presented. Formation of C2, CN and CH is explained and various aspects of the atmospheres of planets are considered. Finally, the origin of amides, amines and other nitrogenous derivatives from hydrocarbons is discussed.
Young, R. S.↗