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Urzellen und Zellmodelle
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The Isua supracrustals of western Greenland are the oldest terrestrial rocks known, dated at > 3,750 m.y. Metamorphosed to lower amphibolite facies, they still can provide indications of their original environment of deposition. Graphite is widely dispersed throughout the meta-sediments, particularly within the ironstones. Most of the graphite is very well ordered. However, several samples show a marked disorder of crystallinity. These samples, when pyrolyzed at high temperatures, liberate organic fragments of low molecular weight (m/e < 200). The fragments suggest that at least some of the Isua graphite is derived from condensation of kerogen. Carbon isotope data has been interpreted as indication of photosynthetic fractionation. Whether the original organics, which are now seen as graphite, were biologically produced has not yet been unequivocally established. The pyrolyzed organics detected within the metasediments may well be the oldest molecular fossils yet found on the Earth.
Turbiditic and pelagic sedimentary rocks from the Isua supracrustal belt in west Greenland [more than 3700 million years ago
(Ma)] contain reduced carbon that is likely biogenic. The carbon is present as 2- to 5-micrometer graphite globules and has
an isotopic composition of δ13C that is about –19 per mil (Pee Dee belemnite standard). These data and the mode of occurrence indicate that the reduced
carbon represents biogenic detritus, which was perhaps derived from planktonic organisms.
We question the biogenicity of putative bacterial andcyanobacterial `microfossils' from3465 Ma Apex cherts of the Warrawoona Group in WesternAustralia. They arechallenged on the basis of integrated multidisciplinary evidenceobtained from field andfabric mapping plus new high-resolution research into theircontext, sedimentology,filament morphology, `septation' and arrangement. They cannotbe distinguished from(and are reinterpreted as) secondary artefacts of amorphouscarbon that formed duringdevitrification of successive generations of carbonaceoushydrothermal dyke vein quartz.Similar structures occur within associated carbonaceous volcanicglass. The nullhypothesis of an abiotic or prebiotic origin for such ancientcarbonaceous matter issustained until mutually supporting contextural, morphologicaland geochemicalevidence for a bacterial rather than abiotic origin is forthcoming.
Active transport across the vacuolar components of the eukaryotic endomembrane system is energized by a specific vacuolar H+-ATPase. The amino acid sequences of the 70- and 60-kDa subunits of the vacuolar H+-ATPase are approximately equal to 25% identical to the beta and alpha subunits, respectively, of the eubacterial-type F0F1-ATPases. We now report that the same vacuolar H+-ATPase subunits are approximately equal to 50% identical to the alpha and beta subunits, respectively, of the sulfur-metabolizing Sulfolobus acidocaldarius, an archaebacterium (Archaeobacterium). Moreover, the homologue of an 88-amino acid stretch near the amino-terminal end of the 70-kDa subunit is absent from the F0F1-ATPase beta subunit but is present in the alpha subunit of Sulfolobus. Since the two types of subunits (alpha and beta subunits; 60- and 70-kDa subunits) are homologous to each other, they must have arisen by a gene duplication that occurred prior to the last common ancestor of the eubacteria, eukaryotes, and Sulfolobus. Thus, the phylogenetic tree of the subunits can be rooted at the site where the gene duplication occurred. The inferred evolutionary tree contains two main branches: a eubacterial branch and an eocyte branch that gave rise to Sulfolobus and the eukaryotic host cell. The implication is that the vacuolar H+-ATPase of eukaryotes arose by the internalization of the plasma membrane H+-ATPase of an archaebacterial-like ancestral cell.
It is unknown when life first appeared on Earth. The earliest known microfossils (approximately 3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued. But the known examples of crustal rocks older than 3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences--a approximately 3,800-Myr-old banded iron formation from the Isua supracrustal belt, West Greenland, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr. The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present.
Advances in directed evolution and membrane biophysics make the
synthesis of simple living cells, if not yet foreseeable reality, an
imaginable goal. Overcoming the many scientific challenges along the way
will deepen our understanding of the essence of cellular life and its
origin on Earth.
Over the past decade, several liposome-based models for protocells have been developed. For example, liposome systems composed of polymerase enzymes encapsulated with their substrates have demonstrated that complex compartmentalized reactions can be carried out under conditions in which polymeric products are protected from degradation by hydrolytic enzymes present in the external medium. However, such systems do not have nutrient uptake mechanisms, which would be essential for primitive cells lacking the highly evolved nutrient transport processes present in all contemporary cells. In this report, we explore passive diffusion of solutes across lipid bilayers as one possible uptake mechanism. We have established conditions under which ionic substrates as large as ATP can permeate bilayers at rates capable of supplying an encapsulated template-dependent RNA polymerase. Furthermore, while allowing the permeation of monomer substrates such as ATP, bilayer vesicles selectively retained polymerization products as small as dimers and as large as a transfer RNA. These observations demonstrate that passive diffusion could be used by the earliest forms of cellular life for transport of important nutrients such as amino acids, phosphate, and phosphorylated organic solutes.
The continuity of abiotically formed bilayer membranes with similar structures in contemporary cellular life, and the requirement for microenvironments in which large and small molecules could be compartmentalized, support the idea that amphiphilic boundary structures contributed to the emergence of life. As an extension of this notion, we propose here a 'Lipid World' scenario as an early evolutionary step in the emergence of cellular life on Earth. This concept combines the potential chemical activities of lipids and other amphiphiles, with their capacity to undergo spontaneous self-organization into supramolecular structures such as micelles and bilayers. In particular, the documented chemical rate enhancements within lipid assemblies suggest that energy-dependent synthetic reactions could lead to the growth and increased abundance of certain amphiphilic assemblies. We further propose that selective processes might act on such assemblies, as suggested by our computer simulations of mutual catalysis among amphiphiles. As demonstrated also by other researchers, such mutual catalysis within random molecular assemblies could have led to a primordial homeostatic system displaying rudimentary life-like properties. Taken together, these concepts provide a theoretical framework, and suggest experimental tests for a Lipid World model for the origin of life.
The formation of lipid compounds during an aqueous Fischer-Tropsch-type reaction was studied with solutions of oxalic acid as the carbon and hydrogen source. The reactions were conducted in stainless steel vessels by heating the oxalic acid solution at discrete temperatures from 100 to 400 degrees C, at intervals of 50 degrees C for two days each. The maximum lipid yield, especially for oxygenated compounds, is in the window of 150-250 degrees C. At a temperature of 100 degrees C only a trace amount of lipids was detected. At temperatures above 150 degrees C the lipid components ranged from C12 to > C33 and included n-alkanols, n-alkanoic acids, n-alkyl formates, n-alkanals, n-alkanones, n-alkanes, and n-alkenes, all with essentially no carbon number preference. The n-alkanes increased in concentration over the oxygenated compounds at temperatures of 200 degrees C and above, with a slight reduction in their carbon number ranges due to cracking. It was also noted that the n-alkanoic acids increased while n-alkanols decreased with increasing temperature above 200 degrees C. At temperatures above 300 degrees C synthesis competes with cracking and reforming reactions. At 400 degrees C significant cracking was observed and polynuclear aromatic hydrocarbons and their alkylated homologs were detected. The results of this work suggest that the formation of lipid compounds by aqueous FTT reactions proceeds by insertion of a CO group at the terminal end of a carboxylic acid functionality to form n-oxoalkanoic acids, followed by reduction to n-alkanoic acids, to n-alkanals, then to n-alkanols. The n-alkenes are intermediate homologs for n-alkan-2-ones and n-alkanes. This proposed mechanism for aqueous FTT synthesis differs from the surface-catalyzed stepwise FT process (i.e., gaseous) of polymerization of methylene reported in the literature.
Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from about 3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer--Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth.
The isotopic composition of graphite is commonly used as a biomarker in the oldest (>3.5 Gyr ago) highly metamorphosed terrestrial rocks. Earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3.8-Gyr-old Isua supracrustal belt (ISB) in southern West Greenland have suggested the presence of a vast microbial ecosystem in the early Archean. This interpretation, however, has to be approached with extreme care. Here we show that graphite occurs abundantly in secondary carbonate veins in the ISB that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks (metasomatism). During these reactions, graphite forms from the disproportionation of Fe(II)-bearing carbonates at high temperature. These metasomatic rocks, which clearly lack biological relevance, were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life. The new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed Early Archaean rock record.
This paper deals with the difficulty of decoding the origins of natural structures through the study of their morphological features. We focus on the case of primitive life detection, where it is clear that the principles of comparative anatomy cannot be applied. A range of inorganic processes are described that result in morphologies emulating biological shapes, with particular emphasis on geochemically plausible processes. In particular, the formation of inorganic biomorphs in alkaline silica-rich environments are described in detail.
The clay montmorillonite is known to catalyze the polymerization of RNA from activated ribonucleotides. Here we report that montmorillonite accelerates the spontaneous conversion of fatty acid micelles into vesicles. Clay particles often become encapsulated in these vesicles, thus providing a pathway for the prebiotic encapsulation of catalytically active surfaces within membrane vesicles. In addition, RNA adsorbed to clay can be encapsulated within vesicles. Once formed, such vesicles can grow by incorporating fatty acid supplied as micelles and can divide without dilution of their contents by extrusion through small pores. These processes mediate vesicle replication through cycles of growth and division. The formation, growth, and division of the earliest cells may have occurred in response to similar interactions with mineral particles and inputs of material and energy.
We have synthesized inorganic micron-sized filaments, whose microstucture consists of silica-coated nanometer-sized carbonate
crystals, arranged with strong orientational order. They exhibit noncrystallographic, curved, helical morphologies, reminiscent
of biological forms. The filaments are similar to supposed cyanobacterial microfossils from the Precambrian Warrawoona chert
formation in Western Australia, reputed to be the oldest terrestrial microfossils. Simple organic hydrocarbons, whose sources
may also be abiotic and indeed inorganic, readily condense onto these filaments and subsequently polymerize under gentle heating
to yield kerogenous products. Our results demonstrate that abiotic and morphologically complex microstructures that are identical
to currently accepted biogenic materials can be synthesized inorganically.
The origin of life was an event probably unique in the Earth's history, and reconstructing this event is like assembling a puzzle made up of many pieces. These pieces are composed of information acquired from many different disciplines. The aim of this 1999 book is to integrate discoveries in astronomy, planetology, palaeontology, biology and chemistry, and use this knowledge to present plausible scenarios that give us a better understanding of the likely origin of life on Earth. Twenty-three top experts contribute chapters that discuss everything from the environment and atmosphere of the early Earth, through the appearance of organic molecules in the prebiotic environment, to primitive chiral chemical systems capable of self-replication and evolution by mutation. The book also discusses various clues to the origin of life that can be obtained by a study of the past and present microbial world, as well as from Saturn's moon Titan and the planet Mars. Chemists, biologists, earth scientists, and astronomers will find this book a thought-provoking summary of our knowledge of this extraordinary event.
The origin of life was an event probably unique in the Earth's history, and reconstructing this event is like assembling a puzzle made up of many pieces. These pieces are composed of information acquired from many different disciplines. The aim of this 1999 book is to integrate discoveries in astronomy, planetology, palaeontology, biology and chemistry, and use this knowledge to present plausible scenarios that give us a better understanding of the likely origin of life on Earth. Twenty-three top experts contribute chapters that discuss everything from the environment and atmosphere of the early Earth, through the appearance of organic molecules in the prebiotic environment, to primitive chiral chemical systems capable of self-replication and evolution by mutation. The book also discusses various clues to the origin of life that can be obtained by a study of the past and present microbial world, as well as from Saturn's moon Titan and the planet Mars. Chemists, biologists, earth scientists, and astronomers will find this book a thought-provoking summary of our knowledge of this extraordinary event.
Fossil remains of the most ancient, minute forms of life on Earth and
other planets are hard to recognize. Schopf et al. claim to have
identified the biological remnant material known as kerogen in
microscopic entities in rock by using Raman spectroscopic analysis. On
the basis of a substantial body of published evidence, however, we
contend that the Raman spectra of Schopf et al. indicate that these are
disordered carbonaceous materials of indeterminate origin. We maintain
that Raman spectroscopy cannot be used to identify microfossils
unambiguously, although it is a useful technique for pinpointing
promising microscopic entities for further investigation.
A microfossil named Ramsaysphaera has been identified in the ca. 3400-million-year-old Swartkoppie chert of South Africa. Ramsaysphaera closely resembles in size, shape and all preserved structural details, asporogenous yeasts which have been subjected to dehydration. A similar microfossil called Isuasphaera was recently detected in cherty layers of the ca. 3800-million-year-old Isua quartzite in. SW-Greenland. With the known data about the early earth, the question arises, when and where life originated and by what principles early evolution was ruled.
ISOTOPIC data for the Earth's oldest rocks1-7 imply that a considerable volume of continental crust existed during the early Archaean aeon (>3.0 Gyr ago), but it is not known when this crust first began to form emergent landmasses. Sedimentary geochemistry suggests8,9 that the area of exposed continent was negligible until late in the Archaean10, a contention supported by the fact that, until now, all greenstone supracrustal volcanic and sedimentary successions shown to have been deposited on eroded continental basement have yielded ages of
The enzymatic polymerization of ADP to poly(A), catalyzed by polynucleotide phosphorylase (PNPase) from Micrococcus luteus, has been studied in two supramolecular systems: (a) in reverse micelles formed by sodium bis-(2-ethylhexyl) sulfosuccinate in isooctane and (b) in oleic acid/oleate vesicles at pH 9. In the case of reverse micelles, the reaction proceeded with high yields and with a precipitation of poly(A) out of the micelles. In the case of vesicles, the poly(A) synthesis also proceeded and poly(A) remained entrapped inside the vesicles. The reaction has also in this case been studied under conditions of vesicle autopoietic self-reproduction, namely under conditions in which the vesicles are able to increase their concentrations due to an autocatalytic process which takes place within their boundaries. For this, PNPase was first entrapped inside the vesicles, followed by external addition of ADP and oleic anhydride. ADP permeated across the vesicle bilayer into the interior where PNPase catalyzed the formation of poly(A). In parallel to this endovesicular enzymatic poly(A) synthesis, oleic anhydride was hydrolyzed to oleic acid within the boundaries of the vesicles, which lead to an increase in size and number of vesicles. In this way, we realized a system in which self-reproduction was accompanied by a simultaneous growth of RNA inside the vesicles. This can be seen as a primitive model of a minimal cell.
An increased ratio of 12C to 13C, an indicator of the principal carbon-fixing reaction of photosynthesis, is found in sedimentary organic matter dating back to almost four thousand million years ago-a sign of prolific microbial life not long after the Earth's formation. Partial biological control of the terrestrial carbon cycle must have been established very early and was in full operation when the oldest sediments were formed.
Life has existed on earth for some 4 × 109 years. During most of this time, evolution took place at the level of cell evolution. The cells of presently existing organisms belong to two fundamentally different cell types, protocytes (of bacteria and archaea) and eucytes (of eukarya). Thanks to molecular phylogenetics, the path of evolution can now be traced back to its very beginnings, although the picture may be blurred by repeated horizontal gene transfer. A symbiogenetic origin of plastids and mitochondria is now very well documented, and it is being discussed also for some other constituents of eucytes, including even the cell nucleus. It could be demonstrated that not only did bacterial cells become incorporated into protoeucytes and transformed into organelles of their respective hosts, but also that endocytic eucytes have apparently been transformed to complex organelles by coevolution with host cells.
Das zelltrale Problem der Prim/irstruktur der EiweiBstoffe liegt heute in ihrer linearen Sequenz nnd deren Beziehung zlir Sekund~ir-, zur Terti~rstruktur und insbesondere zur physiologischen Aktivit~tt der Proteine. Diese Zusammenhfinge zu erfassen, ist am tiefsten bei den Hdmproteinen gellingen. Die rote Farbe dieser Pigmente hat schon immer das Interesse des Wissenschaftlers angeregt und es ist daher kein Wunder, dab hier eine Unzahl yon Ver6ffentlichungen seitens der Medizin, Genetik, Chemie und Physik vorliegen. Wir erleben heute den Beginn der Synthese dieser verschiedensten Arbeitsrichtlingen. Als Leitthema sei in diesem Beitrag die Konstanz und die Variabilit~tt der Proteinstruktur am Beispiel der H~tmoglobine und der Myoglobine demonstriert und dariiber hinaus die generelle Giiltigkeit dieser Beflinde an weiteren Beispielen gezeigt. Behandelt werden vor allem das H~imoglobin ulld das Myoglobin. Beide sind Proteine, also ]iiweil3stoffe, oder gellauer ausgedriickt Proteide. Sie enthMten noch ]iisell und einen niedermolekularen Porphyrin-Ring, beide auch Haem genannt, dessen Strliktur durch Synthese bewiesen wurde [1]. Das Haem wird auch die prosthetische Grlippe genannt lind ist bei allen H~imoglobinen nlld Myoglobinen identisch. Wir werden es daher bei lillseren Betrachtungen vernachl~issigen und nns nlir mit dem Protein-Anteil, der ca. 98 % des Gewichtes ausmacht, besch~iftigen. Myoglobin und H~moglobin haben eine sehr ~thnliche physiologische Funktion. Das Myoglobin dient zur Speicherling des Salierstoffes, der zur Mliskelarbeit ben6tigt wird. Es ist als Eiweil3stoff sehr einfach gebaut. Das Molekulargewicht betrfigt 16000. Alich als Protein ist Myoglobin relativ eillfach gebaut, es besteht aus einer einzigen Peptidkette, die bei S~ugern aus t 53 Aminos~uren aufgebaut ist [2]. Das H~moglobin ist wie wir sagen das respiratorische Protein des Blutes : es diellt zum Transport des Sauerstoffs. In der Lunge nimmt es Sauerstoff auf nnd transportiert es im Blutkreislauf an den Ort des Verbrauches, woes in der Lage ist, diesen wieder abzugebell. Die rote Farbe des Blutes riihrt vom H~imoglobin her, das im menschlichen K6rper jeweils in einer Menge yon ca. 600--700 g vorhanden ist und alle t 20 Tage vSltig erneuert bzw. abgebaut wird. Komplizierter als das Myoglobin, aber doch noch ~bersichtlich fiir den Biochemiker ist der Aufban des H~imoglobins. ]is besteht aus vier Peptid-Ketten [3, 4] und vier Haemen. Der Bau zeigt insofern eine gewisse RegelmiiBigkeit, als jeweils zwei Kettell identisch sind. Man nennt diese Peptid-Ketten c~- und/%Ketten und formuliert den Aufbau daher ganz generell ~2/52. Der Aufbau des Molekifls ist daher doppelt symmetrisch.
The determination of the sequence similarity of the ribosomal 16 S RNA of many bacteria and a few higher organisms has shown that the methanogenic, halophilic, and acido-thermophilic organisms are phylogenetically separated from the kingdoms of the Eubacteria and Eukaryotes thus representing a third kingdom called Archaebacteria. Many biochemical and molecular biological features support this conclusion.
Traditional schemes for the origin of cellular life on earth generally suppose that the chance assembly of polymer synthesis systems was the initial event, followed by incorporation into a membrane-enclosed volume to form the earliest cells. Here we discuss an alternative system consisting of replicating membrane vesicles, which we define as minimum protocells. These consist of vesicular bilayer membranes that self-assemble from relatively rare organic amphiphiles present in the prebiotic environment. If some of the amphiphiles are primitive pigment molecules asymmetrically oriented in the bilayer, light energy can be captured in the form of electrochemical ion gradients. This energy could then be used to convert relatively common precursor molecules into membrane amphiphiles, thereby providing an initial photosynthetic growth process, as well as an appropriate microenvironment for incorporation and evolution of polymer synthesis systems.
Hemoglobin and alkaline phosphatase were each encapsulated in phosphatidylcholine liposomes using a dehydration-rehydration cycle for liposome formation. In this method, liposomes prepared by sonication are mixed in aqueous solution with the solute desired to be encapsulated and the mixture is dried under nitrogen in a rotating flask. As the sample is dehydrated, the liposomes fuse to form a multilamellar film that effectively sandwiches the solute molecules. Upon rehydration, large liposomes are produced which have encapsulated a significant fraction of the solute. The optimal mass ratio of lipid to solute is approx. 1:2 to 1:3. This method has potential application in large-scale liposome production, since it depends only on a controlled drying and rehydration process, and does not require extensive use of organic solvents, detergents, or dialysis systems.
Amino acid sequence data from 57 different enzymes were used to determine the divergence times of the major biological groupings. Deuterostomes and protostomes split about 670 million years ago and plants, animals, and fungi last shared a common ancestor about a billion years ago. With regard to these protein sequences, plants are slightly more similar to animals than are the fungi. In contrast, phylogenetic analysis of the same sequences indicates that fungi and animals shared a common ancestor more recently than either did with plants, the greater difference resulting from the fungal lineage changing faster than the animal and plant lines over the last 965 million years. The major protist lineages have been changing at a somewhat faster rate than other eukaryotes and split off about 1230 million years ago. If the rate of change has been approximately constant, then prokaryotes and eukaryotes last shared a common ancestor about 2 billion years ago, archaebacterial sequences being measurably more similar to eukaryotic ones than are eubacterial ones.
The old question of a definition of minimal life is taken up again at the aim of providing a forum for an updated discussion. Briefly discussed are the reasons why such an attempt has previously encountered scepticism, and why such an attempt should be renewed at this stage of the inquiry on the origin of life. Then some of the definitions of life presently used are cited and briefly discussed, starting with the definition adopted by NASA as a general working definition. It is shown that this is too limited if one wishes to provide a broad encompassing definition, and some extensions of it are presented and discussed. Finally it is shown how the different definitions of life reflect the main schools of thought that presently dominate the field on the origin of life.
The universal phylogenetic tree not only spans all extant life, but its root and earliest branchings represent stages in the evolutionary process before modern cell types had come into being. The evolution of the cell is an interplay between vertically derived and horizontally acquired variation. Primitive cellular entities were necessarily simpler and more modular in design than are modern cells. Consequently, horizontal gene transfer early on was pervasive, dominating the evolutionary dynamic. The root of the universal phylogenetic tree represents the first stage in cellular evolution when the evolving cell became sufficiently integrated and stable to the erosive effects of horizontal gene transfer that true organismal lineages could exist.
Laser-Raman imagery is a sensitive, noninvasive, and nondestructive technique that can be used to correlate directly chemical composition with optically discernable morphology in ancient carbonaceous fossils. By affording means to investigate the molecular makeup of specimens ranging from megascopic to microscopic, it holds promise for providing insight into aspects of organic metamorphism and biochemical evolution, and for clarifying the nature of ancient minute fossil-like objects of putative but uncertain biogenicity.
NASA
A recent study by Mojzsis et al., (Nature 384, 55, 1996) found evidence of life in rocks in Greenland estimated by new isotopic data to be more than 3800 million years old. The author examines this study in relation to studies conducted on rocks between 3250 and 3800 million years old and presents reasons to agree and disagree with the interpretation of data.
Eleven taxa (including eight heretofore undescribed species) of cellularly preserved filamentous microbes, among the oldest fossils known, have been discovered in a bedded chert unit of the Early Archean Apex Basalt of northwestern Western Australia. This prokaryotic assemblage establishes that trichomic cyanobacterium-like microorganisms were extant and morphologically diverse at least as early as approximately 3465 million years ago and suggests that oxygen-producing photoautotrophy may have already evolved by this early stage in biotic history.
The literature of metabolism in proteinoids and proteinoid microspheres is reviewed and criticized from a biochemical and experimental point of view. Closely related literature is also reviewed in order to understand the function of proteinoids and proteinoid microspheres. Proteinoids or proteinoid microspheres have many activities. Esterolyis, decarboxylation, amination, deamination, and oxidoreduction are catabolic enzyme activities. The formation of ATP, peptides or oligonucleotides is synthetic enzyme activities. Additional activities are hormonal and inhibitory. Selective formation of peptides is an activity of nucleoproteinoid microspheres; these are a model for ribosomes. Mechanisms of peptide and oligonucleotide syntheses from amino acids and nucleotide triphosphate by proteinoid microspheres are tentatively proposed as an integrative consequence of reviewing the literature.
Unlike the familiar Phanerozoic history of life, evolution during the earlier and much longer Precambrian segment of geological time centred on prokaryotic microbes. Because such microorganisms are minute, are preserved incompletely in geological materials, and have simple morphologies that can be mimicked by nonbiological mineral microstructures, discriminating between true microbial fossils and microscopic pseudofossil 'lookalikes' can be difficult. Thus, valid identification of fossil microbes, which is essential to understanding the prokaryote-dominated, Precambrian 85% of life's history, can require more than traditional palaeontology that is focused on morphology. By combining optically discernible morphology with analyses of chemical composition, laser--Raman spectroscopic imagery of individual microscopic fossils provides a means by which to address this need. Here we apply this technique to exceptionally ancient fossil microbe-like objects, including the oldest such specimens reported from the geological record, and show that the results obtained substantiate the biological origin of the earliest cellular fossils known.
A theory for the evolution of cellular organization is presented. The model is based on the (data supported) conjecture that the dynamic of horizontal gene transfer (HGT) is primarily determined by the organization of the recipient cell. Aboriginal cell designs are taken to be simple and loosely organized enough that all cellular componentry can be altered and/or displaced through HGT, making HGT the principal driving force in early cellular evolution. Primitive cells did not carry a stable organismal genealogical trace. Primitive cellular evolution is basically communal. The high level of novelty required to evolve cell designs is a product of communal invention, of the universal HGT field, not intralineage variation. It is the community as a whole, the ecosystem, which evolves. The individual cell designs that evolved in this way are nevertheless fundamentally distinct, because the initial conditions in each case are somewhat different. As a cell design becomes more complex and interconnected a critical point is reached where a more integrated cellular organization emerges, and vertically generated novelty can and does assume greater importance. This critical point is called the "Darwinian Threshold" for the reasons given.
GEOCHEMISTRYOn page [1194][1], a group of researchers details how to cook up minerals in the laboratory that bear a striking resemblance to reported microfossils.
[1]: http://www.sciencemag.org/cgi/content/short/302/5648/1194
A minute, bacterium-like, rod-shaped organism, Eobacterium isolatum, has been found organically and structurally preserved in black chert from the Fig Tree Series (3.1 x 10(9) years old) of South Africa. Filamentous organic structures of probable biological origin, and complex alkanes, which apparently contain small amounts of the isoprenoid hydrocarbons pristane and phytane, are also indigenous to this Early Precambrian sediment. These organic remnants comprise the oldest known evidence of biological organization in the geologic record.
Molecular dynamics computer simulations of the structure and functions of a simple membrane are performed in order to examine whether membranes provide an environment capable of promoting protobiological evolution. Our model membrane is composed of glycerol 1-monooleate. It is found that the bilayer surface fluctuates in time and space, occasionally creating thinning defects in the membrane. These defects are essential for passive transport of simple ions across membranes because they reduce the Born barrier to this process by approximately 40%. Negative ions are transferred across the bilayer more readily than positive ions due to favorable interactions with the electric field at the membrane-water interface. Passive transport of neutral molecules is, in general, more complex than predicted by the solubility-diffusion model. In particular, molecules which exhibit sufficient hydrophilicity and lipophilicity concentrate near membrane surfaces and experience 'interfacial resistance' to transport. The membrane-water interface forms an environment suitable for heterogeneous catalysis. Several possible mechanisms leading to an increase of reaction rates at the interface are discussed. We conclude that vesicles have many properties that make them very good candidates for earliest protocells. Some potentially fruitful directions of experimental and theoretical research on this subject are proposed.
Entstehung des Lebens und frühe Evolution der Organismen
- O Kandler
- O. Kandler
Ein molekularbiologischer Kalender der Evolution? Strukturvergleiche analoger Proteine aus verschiedenen Organismen
- T Wieland
- G Pfleiderer
- T. Wieland
Spektrum der Wissenschaft
- S Simpson