Phosphorus in prebiotic chemistry

The prebiotic synthesis of phosphorus-containing compounds-such as nucleotides and polynucleotides-would require both a geologically plausible source of the element and pathways for its incorporation into chemical systems on the primitive Earth. The mineral apatite, which is the only significant source of phosphate on Earth, has long been thought to be problematical in this respect due to its low solubility and reactivity. However, in the last decade or so, at least two pathways have been demonstrated which would circumvent these perceived problems. In addition, recent results would seem to suggest an additional, extraterrestrial source of reactive phosphorus. It appears that the 'phosphorus problem' is no longer the stumbling block which it was once thought to be.     

The significance of the study about the biological effects of solar ultraviolet radiation using the Exposed Facility on the International Space Station.

It is believed that ultraviolet (UV) radiation from the sun participated in events related to the chemical evolution and birth of life on the primitive Earth. Although UV radiation would be also a driving force for the biological evolution of life on Earth, life space of the primitive living organisms would be limited in the UV-shielded place such as in the water at an early stage of the evolution of life. After the formation of stratospheric ozone layer through the production of oxygen by photoautotroph, living organisms were able to expand their domain from water to land. As a result, now, many kinds of living organisms containing human beings are flourishing on the ground. In the near future, increased transmission of harmful solar UV radiation may reach the Earth's surface due to stratospheric ozone layer depletion. In order to learn more about the biological effects of solar UV radiation with or without interruption by the ozone layer, the utilization of an Exposed Facility on the International Space Station is required. Experiments proposed for this facility would provide a tool for the scientific investigation of processes involved in the birth and evolution of life on Earth, and could also demonstrate the importance of protecting the Earth's future environment from future ozone layer depletion.     

Chirality and life

The crucial role of homochirality and chiral homogeneity in the self-replication of contemporary biopolymers is emphasized, and the experimentally demonstrated advantages of these chirality attributes in simpler polymeric systems are summarized. The implausibility of life without chirality and hence of a biogenic scenario for the origin of chiral molecules is stressed, and chance and determinate abiotic mechanisms for the origin of chirality are reviewed briefly in the context of their potential viability on the primitive Earth. It is concluded that all such mechanisms would be non-viable, and that the turbulent prebiotic environment would require an ongoing extraterrestrial source for the accumulation of chiral molecules on the primitive Earth. A scenario is described wherein the circularly polarized ultraviolet synchrotron radiation from the neutron star remnants of supernovae engenders asymmetric photolysis of the racemic constituents in the organic mantles on interstellar dust grains, whereupon these chiral constituents are transported repetitively to the primative Earth by direct accretion of the interstellar dust or through impacts of comets and asteroids.     

Multiple factors in the reptile extinctions of the Cretaceous period

The selective extinction of the dinosaurs and other giant reptiles has long been a topic of speculation and controversy. Everyone is familiar with the theory of the giant bollide colliding with Earth. But, would it not be more likely that that multiple factors acted over a relatively long period of time to produce this mass extinction?     

The ubiquity of iron

The importance of iron in living systems can be traced to the many complexes within which it is found, to its chemical mobility in undergoing oxidation-reduction reactions, and to the abundance of iron in Earth's crust. Iron is the most abundant element, by mass, in the Earth, constituting about 80% of the inner and outer cores of Earth. The molten outer core is about 8000 km in diameter, and the solid inner core is about 2400 km in diameter. Iron is the fourth most abundant element in Earth's crust. It is the chemically functional component of mononuclear iron complexes, dinuclear iron complexes, [2Fe-2S] and [4Fe-4S] clusters, [Fe-Ni-S] clusters, iron protophorphyrin IX, and many other complexes in protein biochemistry. Metals such as nickel, cobalt, copper, and manganese are present in the crust and could in principle function chemically in place of iron, but they are scarce in Earth's crust. Iron is plentiful because of its nuclear stability in stellar nuclear fusion reactions. It seems likely that other solid planets, formed by the same processes as Earth, would also foster the evolution of life and that iron would be similarly important to life on those planets as it is on Earth.     

Proline autocatalysis in the origin of biological enantioenriched chirality

Biological enantioenriched chirality is a phenomenon that in living organisms, amino acids and carbohydrates typically have the same absolute configuration. Perhaps one of the earliest attempts to delineate the origins of this phenomenon was a theory known as asymmetric autocatalysis, a reaction in which the structures of the chiral catalyst and the product are the same, and in which the chiral product acts as a chiral catalyst for its own production. In theory, this would mean that small asymmetries in the product will propagate rapidly. However, autocatalysis also relies on the cross‐inhibition of chiral states, something that would not likely be possible on primordial Earth. But recently, theories on asymmetric autocatalysis have begun to resurface as more recent findings indicate that other mechanisms exist to stabilize the homochiral states. In this study, I propose an autocatalytic cycle, and using density functional theory, prove that (1) it is plausible on primordial Earth, and (2) it propagates arbitrary asymmetries in proline. Thus, facilitating asymmetry in proline and allowing access to a wide variety of asymmetric proline‐catalyzed reactions, including those involved in the synthesis of amino acids and carbohydrates from achiral precursors.     

An automatically-returned Martian sample by 1985?

The success of the lunar sample analysis programs underscores the desirability of a returned Martian sample. A Mission which would bring back about 1 kg of soil is outlined. The vehicle would have a mass of about 15 tonnes on departure from Earth and would make extensive use of Viking and Mariner technology. Russian experience in the field of automatic soil sampling and automatic rendezvous would be invaluable and the Shuttle would make possible a tidier launch. Sterilisation or quarantine will be necessary to preclude back-contamination of Earth by hypothetical Martian micro-organisms. A prime quarantine facility designed to detect biogenic organic compounds and life processes could be set up at a Lunar base or in a Sky-lab. A single soil sample could be informative as to the general surface composition of Mars. Life detection would be a major task, followed closely by the chemistry of carbon and other life-related elements. However, knowledge of the detailed physics, chemistry and mineralogy of the Martian sample would be of inestimable value to planetary studies.     

Leg vein hemodynamics during bedrests simulating lunar trip

When contemplating future trips to the Moon whose gravity is one sixth of Earth gravity, the question is to know what the adaptive changes in the lower limb venous system would be. In fact, one can suppose that the presence of a partial gravity on the Moon would be able to attenuate venous hemodynamics adaptative changes observed in microgravity. In the present experiment changes in the venous hemodynamics of lower limbs have been studied with mercury strain gauge plethysmography during a simulated Moon mission including a 4 day trip to the Moon (-6 degrees bedrest), a stay of 6 days on the Moon (+11 degrees bedrest), and a 4-day trip back to Earth (-6 degrees bedrest). It was previously demonstrated that +11 degrees bedrest was a good model to simulate the effects of lunar gravity on the cardiovascular system (Vernikos-Danellis J 1986, personnal communication).     

Prebiotic photosynthetic reactions

Historically, numerous attempts have been made to mimic — by means of inorganic model reactions — the photosynthetic fixation of CO₂ by green plants. The literature in this field is strewn with claims and counter-claims. Two factors have led us to reexamine this subject: firstly; doubts concerning the highly reducing model for the atmosphere of the primitive Earth and secondly; recent results which demonstrate that photoreductive fixation is feasable on a suitable catalytic surface, for both CO₂ and N₂. The latter observation is of particular interest due to the well-known susceptability of NH₃ to photolytic destruction. Our review of the literature leads us to suggest that similar processes would have been plausible for the primitive Earth and could have been prebiotic precursors to an early development of CO₂-fixing autotrophs.     

Building a 2010 biodiversity conservation data baseline: contributions of the Group on Earth Observations

Characterizing, modeling, and monitoring the distribution and condition of the Earth’s biodiversity and ecosystems has a long history of being aided by Earth Observation data and the models and derivative products that they support. At the global scale, there need to be more coordinated efforts to acquire, process, analyze, and make available these observation data across all disciplines, particularly since many applications use the same primary and derived data. Towards that end, the Group on Earth Observations (GEO) is implementing the Global Earth Observing System of Systems. The purpose of this article is to provide information on GEO’s goals and activities relevant to biodiversity and ecosystems. The article also describes the data that would best serve as a year 2010 baseline in order to support the objectives of the Convention on Biological Diversity (CBD) as well as many other international agreements, programs, and activities. Indeed, an assessment of existing biodiversity observations with a concomitant recognition of the remaining gaps will promote the establishment of an integrated framework for biodiversity observations in support of the CBD’s efforts.     

Prebiotic materials from on and off the early Earth

One of the greatest puzzles of all time is how did life arise? It has been universally presumed that life arose in a soup rich in carbon compounds, but from where did these organic molecules come? In this article, I will review proposed terrestrial sources of prebiotic organic molecules, such as Miller-Urey synthesis (including how they would depend on the oxidation state of the atmosphere) and hydrothermal vents and also input from space. While the former is perhaps better known and more commonly taught in school, we now know that comet and asteroid dust deliver tons of organics to the Earth every day, therefore this flux of reduced carbon from space probably also played a role in making the Earth habitable. We will compare and contrast the types and abundances of organics from on and off the Earth given standard assumptions. Perhaps each process provided specific compounds (amino acids, sugars, amphiphiles) that were directly related to the origin or early evolution of life. In any case, whether planetary, nebular or interstellar, we will consider how one might attempt to distinguish between abiotic organic molecules from actual signs of life as part of a robotic search for life in the Solar System.     

Epistemological issues in the study of microbial life: alternative terran biospheres?

The assumption that all life on Earth today shares the same basic molecular architecture and biochemistry is part of the paradigm of modern biology. This paper argues that there is little theoretical or empirical support for this widely held assumption. Scientists know that life could have been at least modestly different at the molecular level and it is clear that alternative molecular building blocks for life were available on the early Earth. If the emergence of life is, like other natural phenomena, highly probable given the right chemical and physical conditions then it seems likely that the early Earth hosted multiple origins of life, some of which produced chemical variations on life as we know it. While these points are often conceded, it is nevertheless maintained that any primitive alternatives to familiar life would have been eliminated long ago, either amalgamated into a single form of life through lateral gene transfer (LGT) or alternatively out-competed by our putatively more evolutionarily robust form of life. Besides, the argument continues, if such life forms still existed, we surely would have encountered telling signs of them by now. These arguments do not hold up well under close scrutiny. They reflect a host of assumptions that are grounded in our experience with large multicellular organisms and, most importantly, do not apply to microbial forms of life, which cannot be easily studied without the aid of sophisticated technologies. Significantly, the most powerful molecular biology techniques available-polymerase chain reaction (PCR) amplification of rRNA genes augmented by metagenomic analysis-could not detect such microbes if they existed. Given the profound philosophical and scientific importance that such a discovery would represent, a dedicated search for 'shadow microbes' (heretofore unrecognized 'alien' forms of terran microbial life) seems in order. The best place to start such a search is with puzzling (anomalous) phenomena, such as desert varnish, that resist classification as 'biological' or 'nonbiological'.     

Possible origin of a membrane in the subsurface of the Earth

A location for the origin of life on Earth could have been an oil/water interface in the warm, subsurface environment of the Earth. The physico-chemical conditions of the subsurface would include elevated, but eventually cooling temperatures, anaerobic conditions, and protection from intense surface radiation. This type of subsurface oil/water environment may have been ideal for the assembly of the first simple membrane(s), where no enzyme catalysis was needed. Once a stable, simple, continuous closed membrane was formed, one central component of the first cell(s) would have been present; a semi-permeable open system that allowed the passage of both matter and energy in and out of the cell. Such an open system could also acquire novel functions, whereas a closed system would be unable to evolve.     

Impact constraints on the environment for chemical evolution and the continuity of life

The Moon and the Earth were bombarded heavily by planetesimals and asteroids that were capable of interfering with chemical evolution and the origin of life. In this paper, we explore the frequency of giant terrestrial impacts able to stop prebiotic chemistry in the probable regions of chemical evolution. The limited time available between impacts disruptive to prebiotic chemistry at the time of the oldest evidence of life suggests the need for a rapid process for chemical evolution of life. The classical hypothesis for the origin of life through the slow accumulation of prebiotic reactants in the primordial soup in the entire ocean may not be consistent with constraints imposed by the impact history of Earth. On the other hand, rapid chemical evolution in cloud systems and lakes or other shallow evaporating water bodies would have been possible because reactants could have been concentrated and polymerized rapidly in this environment. Thus, life probably could have originated near the surface between frequent surface sterilizing impacts. There may not have been continuity of life depending on sunlight because there is evidence that life, existing as early as 3.8 Gyr ago, may have been destroyed by giant impacts. The first such organisms on Earth where probably not the ancestors of present life.     

On the periodicity in the fine structure of histograms during nuclear decay

A possible explanation for the occurrence of periodicity in the fine structure of histograms during nuclear decay is proposed. The hypothesis states that, even in the absence of polarization, when the spin distribution of decaying nuclei over directions is, on the average, isotropic, the fine structure of histograms should depend, due to fluctuations, on the angles at which the sample is observed. There fore, when the Earth after some period would make a full furn, the distribution of fluctuations would return to the initial one, and, correspondingly, the fine structure of the histograms would repeat.     

The origin and early evolution of plants

Plant (archaeplastid) evolution has transformed the biosphere, but we are only now beginning to learn how this took place through comparative genomics, phylogenetics, and the fossil record. This has illuminated the phylogeny of Archaeplastida, Viridiplantae, and Streptophyta, and has resolved the evolution of key characters, genes, and genomes – revealing that many key innovations evolved long before the clades with which they have been casually associated. Molecular clock analyses estimate that Streptophyta and Viridiplantae emerged in the late Mesoproterozoic to late Neoproterozoic, whereas Archaeplastida emerged in the late-mid Palaeoproterozoic. Together, these insights inform on the coevolution of plants and the Earth system that transformed ecology and global biogeochemical cycles, increased weathering, and precipitated snowball Earth events, during which they would have been key to oxygen production and net primary productivity (NPP).     

Early anaerobic metabolisms

Before the advent of oxygenic photosynthesis, the biosphere was driven by anaerobic metabolisms. We catalogue and quantify the source strengths of the most probable electron donors and electron acceptors that would have been available to fuel early-Earth ecosystems. The most active ecosystems were probably driven by the cycling of H2 and Fe2+ through primary production conducted by anoxygenic phototrophs. Interesting and dynamic ecosystems would have also been driven by the microbial cycling of sulphur and nitrogen species, but their activity levels were probably not so great. Despite the diversity of potential early ecosystems, rates of primary production in the early-Earth anaerobic biosphere were probably well below those rates observed in the marine environment. We shift our attention to the Earth environment at 3.8Gyr ago, where the earliest marine sediments are preserved. We calculate, consistent with the carbon isotope record and other considerations of the carbon cycle, that marine rates of primary production at this time were probably an order of magnitude (or more) less than today. We conclude that the flux of reduced species to the Earth surface at this time may have been sufficient to drive anaerobic ecosystems of sufficient activity to be consistent with the carbon isotope record. Conversely, an ecosystem based on oxygenic photosynthesis was also possible with complete removal of the oxygen by reaction with reduced species from the mantle.     

Production of Guanine from NH4CN Polymerizations

The synthesis of adenine from the polymerization of concentrated ammonium cyanide solutions is well known. We show here that guanine is also produced by this reaction but at yields ranging from 10 to 40 times less than that of adenine. This synthesis is effective at both +80 and −20°C. Since high concentrations of NH₄CN are obtainable only by freezing, this prebiotic synthesis would be applicable to frozen regions of the primitive Earth, the Jovian satellite Europa and other icy satellites, and the parent body of the Murchison meteorite. Received: 18 September 1998 / Accepted: 31 March 1999     

The similarity of life across the universe

Is the hypothesis correct that if life exists elsewhere in the universe, it would have forms and structures unlike anything we could imagine? From the subatomic level in cellular energy acquisition to the assembly and even behavior of organisms at the scale of populations, life on Earth exhibits characteristics that suggest it is a universal norm for life at all levels of hierarchy. These patterns emerge from physical and biochemical limitations. Their potentially universal nature is supported by recent data on the astrophysical abundance and availability of carbon compounds and water. Within these constraints, biochemical and biological variation is certainly possible, but it is limited. If life exists elsewhere, life on Earth, rather than being a contingent product of one specific experiment in biological evolution, is likely to reflect common patterns for the assembly of living matter.