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Microfossils: Squaring up over ancient life
Abstract
The textbooks say that oxygen-producing microorganisms evolved some 3.5 billion years ago. But as that claim and its author come under attack, the history of life on Earth may have to be rewritten. Rex Dalton investigates.
... With it came the inference of a relatively advanced level of prokaryote evolution at 3.5 Ga, plus an early origin of photosynthetic oxygen production on Earth. Little attempt was made to test these assumptions until the authenticity of the 'microfossils' themselves was challenged almost a decade later (Dalton, 2002). ...
... The filaments do not exhibit biological behaviour; instead they are solitary, irregular and randomly orientated. Questions were also raised about the selectivity of the data chosen for publication, with suggestions that more complex objects exhibiting branching, a trait that would not have occurred in such primitive organisms, were withheld from publication (Packer quoted in Dalton, 2002). Although reported as 'thin-sections' (Schopf and Packer, 1987;Schopf, 1993), the thicknesses of the rock slices containing the type specimens are not standard (30 μm) but range from 193 to 380 μm (Brasier et al., 2005). ...
... These 3dimensional visualisations make it clear that this structure is a branch and cannot be attributed to folding of the filament (cf. Schopf quoted in Dalton, 2002). This filament also has an offset close to its lowermost termination that is seen in optical images and confirmed by the 3D model ( Fig. 15a and c, red arrow). ...
Filamentous microstructures from the 3.46. billion. year (Ga)-old Apex chert of Western Australia have been interpreted as remnants of Earth's oldest cellular life, but their purported biological nature has been robustly questioned on numerous occasions. Despite recent claims to the contrary, the controversy surrounding these famous microstructures remains unresolved.Here we interrogate new material from the original 'microfossil site' using high spatial resolution electron microscopy to decode the detailed morphology and chemistry of the Apex filaments. Light microscopy shows that our newly discovered filaments are identical to the previously described 'microfossil' holotypes and paratypes. Scanning and transmission electron microscopy data show that the filaments comprise chains of potassium- and barium-rich phyllosilicates, interleaved with carbon, minor quartz and iron oxides. Morphological features previously cited as evidence for cell compartments and dividing cells are shown to be carbon-coated stacks of phyllosilicate crystals. Three-dimensional filament reconstructions reveal non-rounded cross sections and examples of branching incompatible with a filamentous prokaryotic origin for these structures.When examined at the nano-scale, the Apex filaments exhibit no biological morphology nor bear any resemblance to younger . bona fide carbonaceous microfossils. Instead, available evidence indicates that the microstructures formed during fluid-flow events that facilitated the hydration, heating and exfoliation of potassium mica flakes, plus the redistribution and adsorption of barium, iron and carbon within an active hydrothermal system.
... In these cases, the growth process could break the symmetry of the faces of each form. These elongated crystals are so unusual that their presence in recent and ancient sediments and in the Martian meteorite ALH84001 has led to them being referred to as magnetofossils and being used as evidence for the past presence of magnetotactic bacteria in aquatic habitats and sediments Stolz et al. 1986Stolz et al. , 1990) and life on ancient Mars (McKay et al. 1996;Thomas-Keprta et al. 2000, 2002 (this is discussed at length later in this chapter). ...
... Moreover, much of the material thought to be fossilized microbes is subject to alternate interpretations. For example, microbial fossils supposedly representing cyanobacterial species from some of the oldest rocks on Earth, 3.5 billion year old cherts from western Australia (Schopf and Packer 1987;Schopf 1993), have recently been under intense scrutiny (Dalton 2002). Years after the original microbial fossil interpretation of the structures in these rocks had become "textbook orthodoxy" (Dalton 2002), Brasier et al. (2002) reexamined the structures and offered an alternative explanation for their formation, i.e., the structures are secondary artifacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass, or as Dalton (2002) puts it more simply, they represent "carbonaceous blobs, probably formed by the action of scalding water on minerals." ...
... For example, microbial fossils supposedly representing cyanobacterial species from some of the oldest rocks on Earth, 3.5 billion year old cherts from western Australia (Schopf and Packer 1987;Schopf 1993), have recently been under intense scrutiny (Dalton 2002). Years after the original microbial fossil interpretation of the structures in these rocks had become "textbook orthodoxy" (Dalton 2002), Brasier et al. (2002) reexamined the structures and offered an alternative explanation for their formation, i.e., the structures are secondary artifacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass, or as Dalton (2002) puts it more simply, they represent "carbonaceous blobs, probably formed by the action of scalding water on minerals." The point here illustrates that there is a significant need for unequivocal evidence for the past presence of microbes. ...
As stated in an Chapter 4, prokaryotes of both Domains or Superkingdoms, the Bacteria and the Archaea, mediate the formation of a large number of diverse minerals. They are known to do this either through biologically induced mineralization (BIM) (Lowenstam 1981) (the subject of Chapter 4 in this volume; Frankel and Bazylinski 2003) or biologically controlled mineralization (BCM). The latter has also been referred to as organic matrix-mediated mineralization (Lowenstam 1981) and boundary-organized biomineralization (Mann 1986). There are several important differences between BIM and BCM that will be discussed in detail in this chapter, most notably those dealing with aspects of the mineral crystals and the biomineralization process (see also Weiner and Dove 2003). However, there is another significant difference between the two modes of biomineralization and this is the aspect of functionality. Generally, in BIM there is no function to the biomineralized particles except, perhaps, as a solid substrate for attachment in the case of bacteria or as a form of protection against certain environmental conditions or attack from predators. However, it is easier to recognize the primary function of bone or shells produced by molluscs; two well-characterized examples of BCM by higher organisms. Functionality should always be examined when dealing with examples of biomineralization particularly in situations where the mineral product displays some qualities of both BIM and BCM.
In BCM, the organism exerts a great degree of crystallochemical control over the nucleation and growth of the mineral particles. For the most part, the minerals are directly synthesized at a specific location within or on the cell and only under certain conditions. The mineral particles produced by bacteria in BCM are characterized as well-ordered crystals with narrow size distributions, and specific, consistent particle morphologies. Because of these features, BCM processes are likely to be under specific chemical/biochemical and …
... We therefore propose that 'Earth's oldest fossils' are artefacts (pseudofossils) formed when carbonaceous glass associated with one of Earth's oldest hot springs recrystallized into spherulitic silica (Brasier et al., 2002). These conclusions have been followed by a flurry of important papers, published during the 'annus mirabilis' of astrobiology (Dalton, 2002; Fedo and Whitehouse, 2002; van Zuilen et al., 2002; Shoenberg et al., 2002). Each of these raises serious questions about hidden assumptions embedded within the Early Eden paradigm, as we discuss below. ...
... The Automontage technique used by us to illustrate this branching (Brasier et al., 2002) is a sophisticated computer merging of digital images, upgrading the traditional (cut and paste of single focal plane monochrome images) manual montage technique used by Schopf (1992 Schopf ( , 1993) to display 3D structures in 2D format. We note that Bonnie Packer (of Schopf and Packer, 1987) concurs with our use of the term 'branching' and that Schopf may originally have used this term himself (Dalton, 2002). Schopf has stated, however, that some of the structures are not actually branched but 'folded' or 'overlapping' (Dalton, 2002). ...
... We note that Bonnie Packer (of Schopf and Packer, 1987) concurs with our use of the term 'branching' and that Schopf may originally have used this term himself (Dalton, 2002). Schopf has stated, however, that some of the structures are not actually branched but 'folded' or 'overlapping' (Dalton, 2002). If this is accepted, it compounds the problem – for what once had seemed to be the 'end' of a filament must be reinterpreted either as something disrupted or from the middle. ...
We question the biogenicity of putative bacterial and cyanobacterial `microfossils' from 3465 Ma Apex cherts of the Warrawoona Group in Western Australia. They are challenged on the basis of integrated multidisciplinary evidence obtained from field and fabric mapping plus new high-resolution research into their context, sedimentology, filament morphology, `septation' and arrangement. They cannot be distinguished from (and are reinterpreted as) secondary artefacts of amorphous carbon that formed during devitrification of successive generations of carbonaceous hydrothermal dyke vein quartz. Similar structures occur within associated carbonaceous volcanic glass. The null hypothesis of an abiotic or prebiotic origin for such ancient carbonaceous matter is sustained until mutually supporting contextural, morphological and geochemical evidence for a bacterial rather than abiotic origin is forthcoming.
... We therefore propose that 'Earth's oldest fossils' are artefacts (pseudofossils) formed when carbonaceous glass associated with one of Earth's oldest hot springs recrystallized into spherulitic silica (Brasier et al., 2002). These conclusions have been followed by a flurry of important papers, published during the 'annus mirabilis' of astrobiology (Dalton, 2002; Fedo and Whitehouse, 2002; van Zuilen et al., 2002; Shoenberg et al., 2002). Each of these raises serious questions about hidden assumptions embedded within the Early Eden paradigm, as we discuss below. ...
... The Automontage technique used by us to illustrate this branching (Brasier et al., 2002) is a sophisticated computer merging of digital images, upgrading the traditional (cut and paste of single focal plane monochrome images) manual montage technique used by Schopf (1992 Schopf ( , 1993) to display 3D structures in 2D format. We note that Bonnie Packer (of Schopf and Packer, 1987) concurs with our use of the term 'branching' and that Schopf may originally have used this term himself (Dalton, 2002). Schopf has stated, however, that some of the structures are not actually branched but 'folded' or 'overlapping' (Dalton, 2002). ...
... We note that Bonnie Packer (of Schopf and Packer, 1987) concurs with our use of the term 'branching' and that Schopf may originally have used this term himself (Dalton, 2002). Schopf has stated, however, that some of the structures are not actually branched but 'folded' or 'overlapping' (Dalton, 2002). If this is accepted, it compounds the problem – for what once had seemed to be the 'end' of a filament must be reinterpreted either as something disrupted or from the middle. ...
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.
... In these cases, the growth process could break the symmetry of the faces of each form. These elongated crystals are so unusual that their presence in recent and ancient sediments and in the Martian meteorite ALH84001 has led to them being referred to as magnetofossils () and being used as evidence for the past presence of magnetotactic bacteria in aquatic habitats and sediments ( Stolz et al. 1986 Stolz et al. , 1990) and life on ancient Mars (McKay et al. 1996; Thomas-Keprta et al. 2000, 2002) (this is discussed at length later in this chapter). ...
... Moreover, much of the material thought to be fossilized microbes is subject to alternate interpretations. For example, microbial fossils supposedly representing cyanobacterial species from some of the oldest rocks on Earth, 3.5 billion year old cherts from western Australia (Schopf and Packer 1987; Schopf 1993), have recently been under intense scrutiny (Dalton 2002). Years after the original microbial fossil interpretation of the structures in these rocks had become " textbook orthodoxy " (Dalton 2002), Brasier et al. (2002) reexamined the structures and offered an alternative explanation for their formation, i.e., the structures are secondary artifacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass, or as Dalton (2002) puts it more simply, they represent " carbonaceous blobs, probably formed by the action of scalding water on minerals. ...
... The magnetite crystals have been examined to a great degree using a number of different techniques. The intracellular magnetite crystals produced by strain MV-1 display six distinctive properties that allow them to be distinguished from any known population of inorganically-produced magnetites (Thomas-Keprta et al. 2000, 2002). These six properties are: (i) narrow size range (a non-log-normal size distribution with the mean centered in the single-magnetic-domain size range); (ii) restricted widthto-length ratios (iii) high chemical purity (crystals are essentially stoichiometric magnetite); (iv) few crystallographic defects (crystals are defect-free with the exception of occasional twinning perpendicular to the [111] axis of elongation; (v) crystal morphology with unusual truncated hexa-octahedral geometry consisting of a combination of the three crystallographic forms for the 4 2 3 m m point group, the cube {100}, the octahedron {111}, and the rhombic dodecahedron {110}, with only 6 of the 12 possible {110} faces being expressed, namely, those that satisfy the relationship {110} @BULLET [111] = 0, with elongation defined to be along the [111] [111] axis [i.e., ...
... We therefore propose that 'Earth's oldest fossils' are artefacts (pseudofossils) formed when carbonaceous glass associated with one of Earth's oldest hot springs recrystallized into spherulitic silica (Brasier et al., 2002). These conclusions have been followed by a flurry of important papers, published during the 'annus mirabilis' of astrobiology (Dalton, 2002; Fedo and Whitehouse, 2002; van Zuilen et al., 2002; Shoenberg et al., 2002). Each of these raises serious questions about hidden assumptions embedded within the Early Eden paradigm, as we discuss below. ...
... The Automontage technique used by us to illustrate this branching (Brasier et al., 2002) is a sophisticated computer merging of digital images, upgrading the traditional (cut and paste of single focal plane monochrome images) manual montage technique used by Schopf (1992, 1993) to display 3D structures in 2D format. We note that Bonnie Packer (of Schopf and Packer, 1987) concurs with our use of the term 'branching' and that Schopf may originally have used this term himself (Dalton, 2002). Schopf has stated, however, that some of the structures are not actually branched but 'folded' or 'overlapping' (Dalton, 2002). ...
... We note that Bonnie Packer (of Schopf and Packer, 1987) concurs with our use of the term 'branching' and that Schopf may originally have used this term himself (Dalton, 2002). Schopf has stated, however, that some of the structures are not actually branched but 'folded' or 'overlapping' (Dalton, 2002). If this is accepted, it compounds the problem – for what once had seemed to be the 'end' of a filament must be reinterpreted either as something disrupted or from the middle. ...
We question the biogenicity of putative bacterial and cyanobacterial 'microfossils' from 3465 Ma Apex cherts of the Warrawoona Group in Western Australia. They are challenged on the basis of integrated multidisciplinary evidence obtained from field and fabric mapping plus new high-resolution research into their context, sedimentology, filament morphology, 'septation' and arrangement. They cannot be distinguished from (and are reinterpreted as) secondary artefacts of amorphous carbon that formed during devitrification of successive generations of carbonaceous hydrothermal dyke vein quartz. Similar structures occur within associated carbonaceous volcanic glass. The null hypothesis of an abiotic or prebiotic origin for such ancient carbonaceous matter is sustained until mutually supporting contextural, morphological and geochemical evidence for a bacterial rather than abiotic origin is forthcoming.
... This fi erce debate was echoed several years later when the ALH84001 skeptic Schopf found himself on the defensive as Oxford's Martin Brasier raised doubts about the biogenicity and phylogeny of the 3.5-billion-year-old terrestrial Apex chert microfossils, long thought to be conclusive evidence of Archean life (Dalton, 2002 ) . These microscopic fossils, which were originally characterized tentatively as cyanobacteria existing in a shallow sea bottom, were criticized based on their visual appearance as well as their geological context (Brasier et al., 2002 ) . ...
... Although heated at times (Dalton, 2002 ) , the debate served the purpose of clarifying the provenance of some of the oldest microfossils on Earth. Publications by Schopf and colleagues offer evidence to support the biogenicity of these fossils through more in-depth examinations of both morphology as well as the organic constituents of the microfossils (Schopf et al., , 2007Schopf, 2006 ) . ...
The biological record suggests that life on Earth arose as soon as conditions were favorable, which indicates that life either originated quickly, or arrived from elsewhere to seed Earth. Experimental research under the theme of “astrobiology” has produced data that some view as strong evidence for the second possibility, known as the panspermia hypothesis. While it is not unreasonable to consider the possibility that Earth’s life originated elsewhere and potentially much earlier, we conclude that the current literature offers no definitive evidence to support this hypothesis.
Chladni’s view, that they fall from the skies, pronounced in 1795, was ridiculed by the learned men of the times. (Rachel, 1881)
Evidence of life on Mars, even if only in the distant past, would finally answer the age-old question of whether living beings on Earth are alone in the universe. The magnitude of such a discovery is illustrated by President Bill Clinton’s appearance at a 1996 press conference to announce that proof had been found at last. A meteorite chipped from the surface of the Red Planet some 15 million years ago appeared to contain the fossil remains of tiny life-forms that indicated life had once existed on Mars. (Young and Martel, 2010)
... [43] The problem of using crystal morphology as a criterion to recognize the biological origin of minerals is an old one and is not limited to magnetite. The authenticity of many ancient fossils of prokaryotes, once accepted by the scientific community as valid, is currently under great scrutiny [Dalton, 2002]. An illustration of this point concerns the putative microbial fossils supposedly representing cyanobacterial species from 3.5 billion year old cherts from the Precambrian Warrawoona formation in Western Australia [Schopf and Packer, 1987;Schopf, 1993]. ...
... An illustration of this point concerns the putative microbial fossils supposedly representing cyanobacterial species from 3.5 billion year old cherts from the Precambrian Warrawoona formation in Western Australia [Schopf and Packer, 1987;Schopf, 1993]. Brasier et al. [2002] reexamined the putative fossils and offered an alternative explanation for their formation, i.e., the structures are secondary artifacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass while Dalton [2002] described these putative fossils as "carbonaceous blobs, probably formed by the action of scalding water on minerals." García-Ruiz et al. [2003,2009] were able to synthesize inorganic micron-sized filaments of silica-coated nanometer-sized carbonate crystals, arranged with strong orientational order that exhibit noncrystallographic morphologies (curved, helical) reminiscent of biological forms. ...
Over the years, nanometer-sized magnetite (Fe3O4) crystals have been recovered from many modern and ancient environments including sediments and soils and even meteorites. In some cases these crystals have been used as ``magnetofossils'' for evidence of the past presence of specific microbes. Magnetite nanocrystals can be formed by a number of different biological and inorganic mechanisms resulting in crystals with different physical and magnetic characteristics. Prokaryotes (bacteria) biomineralize magnetite through two methods that differ mechanistically, including: biologically induced mineralization (BIM) and biologically controlled mineralization (BCM). Magnetite nanocrystals produced by BIM are known to be synthesized by the dissimilatory iron-reducing bacteria, are deposited external to the cell, and generally are physically indistinguishable from magnetite particles formed inorganically. BCM magnetites, in contrast, are synthesized by the magnetotactic bacteria and some higher organisms and are precipitated intracellularly as membrane-bounded structures called magnetosomes. These magnetites appear to have unique crystal morphologies and a narrow size range leading to their original use as magnetofossils. Because of the discovery of nanometer-sized crystals of magnetite in the Martian meteorite ALH84001, the use of these criteria for the determination of whether magnetite crystals could constitute a prokaryotic biomarker was questioned. Thus, there is currently great debate over what criteria to use in the determination of whether specific magnetite crystals are biogenic or not. In the last decade, additional criteria have been established (e.g., the Magnetite Assay for Biogenicity), and new tools and technologies have been developed to determine the origin of specific types of magnetite crystals.
... These samples came from an outcrop (now often called the ' Awramik locality ') close to Panorama Homestead that was first found by the Archaean field geologist Roger Buick, at that time a member of the Bureau of Mineral Resources in Canberra. In 1980, a paper on inferred microfossils from this ' Awramik' site was accepted for publication in the journal Science, but Buick strongly disagreed with its conclusions and withdrew his name, and Science later declined to publish the article (Dalton 2002). When an expanded version appeared in the journal Precambrian Research (Awramik et al. 1983), Roger Buick wrote a strong rejoinder which set out a rigorous agenda for astrobiology, putting forward a shopping list of the criteria needed to determine biogenicity (Buick 1984). ...
... This conclusion was reinforced by her later observations in the laboratory, that many of the purported cyanobacteria-like structures had side branches unlike anything known from the Archaean to Middle Proterozoic fossil record. Unhappy about the way things were going, and allegedly including the selective nature of photographic evidence presented in Schopf & Packer (1987), Packer withdrew from the research project (Dalton 2002). ...
The importance of the search for life within the universe underpins many of the objectives of ESA and NASA. Fundamental to this objective is the recognition and understanding of the early fossil record on planet Earth. The history of study of ‘microfossils’ from the 3465 Ma Apex cherts of the Warrawoona Group in Western Australia is reviewed, as are the criteria put forward to test for for biogenicity of early microfossils. We propose that the null hypothesis of an abiotic or prebiotic origin of ancient microscopical artefacts is sustained until mutually supporting, contextural morphological and geochemical evidence for a biological origin is presented and an abiotic origin is falsified.
... [1,21,22] . [23] . ...
... The origin of Blue green algae was probably 2.7 billion years ago (Buick, 1992;Dalton 2002). They are known for their capacity of fixing atmospheric nitrogen. ...
The lentic ecosystems include pools, ponds, puddles, lakes, wetlands, reservoirs and so on. Algae play a significant role because of their dynamic eco-strategies and their amplitude in terms of diversity in morphological and physiological attributes. The present study was carried out in Sikkarayapuram stone quarries measuring 120 acres of land with twenty two deep rock pools located near Chennai city, surrounded by mid-range hills of Eastern Ghats of India. This nature’s treasure provides suitable environment for algal growth which resulted in 68 species belongs to the Class Cyanophyceae, Chlorophyceae, Bacillariophyceae and Euglenophyceae. Maximum number of 38 species was reported from Chlorophyceae followed by 16 species of Bacillariophyceae, 11 species in Cyanophyceae and 3 species of Euglenophyceae.
Keywords: Algae, lentic ecosystems, stone quarries, Sikkarayapuram
... The formal Brasier-Schopf debate at the 2002 Astrobiology Science Conference at the NASA Ames Research Center (Moffett Field, CA) has become the stuff of legend; good accounts of it are available elsewhere (Dalton 2002;Hazen 2005). The biogenicity of the Apex Chert microstructures remains controversial partly because of the complex geological history of their host rock, the presence of non-indigenous microfossil-like structures in the chert (Pinti et al. 2009) and the possibility that the Raman spectra of the carbon indicate multiple origins (e.g. ...
How did life on Earth begin? What does the search for life in the distant past tell us about the search for life on distant planets? How should the most ancient and ambiguous putative biosignatures be critically evaluated? How did the Earth–life system evolve through the dramatic upheavals of the Precambrian–Cambrian boundary? When and why did eukaryotes begin to produce mineralized skeletons? These are among the astrobiological questions to which palaeobiologist Martin Brasier made profound contributions in a career spanning nearly half a century and tragically cut short late last year. Here, we summarize and celebrate Martin's contributions to astrobiology.
... Microorganisms of this period were minute, poorly preserved, and constituted of simple morphologies that could have been produced even by non-biologic processes like mineralic microstructures that make it diffi cult to differentiate between true fossils and pseudofossils. Hence, Early Archaean palaeontological records are always viewed with scepticism (Brasier et al. 2002; Dalton 2002; Van Zuilen et al. 2002). Even those recorded from the Warrawoona Group of Western Australia have been challenged for their antiquity and biogenicity (Awramik et al. 1983; Buick 1984; Kaźmierczak and Barbara 2002). ...
... Studies confirmed that until our days their structure remains unchanged and, no matter how primitive they are, they still represent rather complicated and expertly organised forms of life [1]. Nevertheless, other reports estimated that the actual time of evolution of cyanobacteria is thought to be closer to 2.7 billion years ago [2,3]. Hence, evolutionary biologists estimate that algae could be the ancestors of plants. ...
... However, detecting ancient life on Earth has also been a challenge. This is because of the difficulties encountered in distinguishing ancient microfossils from pseudo-fossils [3][4][5] . Also, what has been interpreted as chemical (e.g. ...
The earliest evidence for amino acids on Earth is in Precambrian sedimentary rocks with varied metamorphic histories. Igneous rocks rarely contain such compounds, exceptions being those introduced via the migration of fluids into fractures subsequent to crystallization. Martian meteorites are excellent examples of ancient igneous rocks that apparently contain amino acids associated with minerals precipitated in rock fractures. The challenge has been to determine whether the organic compounds present in ancient terrestrial and extraterrestrial materials are indigenous and, if so, are representative of past life or pre-biotic synthesis. A summary of what is known to date about amino acids in ancient terrestrial and extraterrestrial materials is presented. Alternative approaches for distinguishing their origin(s) are discussed.
... The biogenicity of structures claimed to represent fossilized microorganisms, of which filamentous features in the 3460 Ma Apex chert of the Pilbara Craton are a prime example (Schopf, 1993;Schopf et al., 2002), have been questioned (Brasier et al., 2002(Brasier et al., , 2005 and heav-ily debated (see Dalton, 2002). Recent experiments have also generated filamentous microfossil-like structures, strikingly similar to those of the Apex chert, by abiotic mechanisms (Garcia-Ruiz et al., 2003). ...
Bioalteration of basaltic glass in pillow lava rims and glassy volcanic breccias (hyaloclastites) produces several distinctive traces including conspicuous petrographic textures. These biologically generated textures include granular and tubular morphologies that form during glass dissolution by microbes and subsequent precipitation of amorphous material. Such bioalteration textures have been described from upper, in situ oceanic crust spanning the youngest to the oldest oceanic basins (0–170 Ma). The granular type consists of individual and/or coalescing spherical bodies with diameters typically around 0.4 μm. These are by far the most abundant, having been traced up to ∼550 m depths in the oceanic crust. The tubular type is defined by distinct, straight to irregular tubes with diameters most commonly around 1–2 μm and lengths exceeding 100 μm. The tubes are most abundant between ∼50 m and 250 m into the volcanic basement. We advance a model for the production of these bioalteration textures and propose criteria for testing the biogenicity and antiquity of ancient examples.Similar bioalteration textures have also been found in hyaloclastites and well-preserved pillow lava margins of Phanerozoic to Proterozoic ophiolites and Archean greenstone belts. The latter include pillow lavas and hyaloclastites from the Mesoarchean Barberton Greenstone Belt of South Africa and the East Pilbara Terrane of the Pilbara Craton, Western Australia, where conspicuous titanite-mineralized tubes, have been found. Petrographic relationships and age data confirm that these structures developed in the Archean. Thus, these biologically generated textures may provide an important tool for mapping the deep oceanic biosphere and for tracing some of the earliest biological processes on Earth and perhaps other planetary surfaces.
... In summary, our re-imaging of the holotypes reveals that many of the structures are more sinuous than previously illustrated, and many have side branching features not previously described. Furthermore, reinterpretation of some filaments as 'folded' (Schopf in Dalton, 2002 ) would call into question the use of 'terminal cell shape' as a diagnostic criterion (Schopf, 1992bSchopf, , 1993) for each of the 11 'microfossil' taxa. ...
Structures resembling cyanobacterial microfossils from the ca. 3465 Ma old Apex chert of the Warrawoona Group in Western Australia have until recently been accepted as providing the oldest morphological evidence for life on Earth, and have been taken to support an early beginning for oxygen-releasing photosynthesis. Eleven species of filamentous prokaryote, principally distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils. They were contrasted with other microfossils that were dismissed as either unreliable or irreproducible. The aim of this paper is to provide a detailed account of research recently reported by us on the type and recollected material, involving optical and electron microscopy, digital image analysis and other techniques. All previously figured holotype materials are illustrated here, and the context for all the published materials is re-evaluated.The Apex chert ‘microfossils’ occur near the top of a 1.5-km long chert dyke complex associated with major synsedimentary growth faults. Highly localised, glassy felsic tuffs erupted explosively from this and other fissures during the early stages of volcanism, and were followed by the deposition of essentially hydrothermal black and white BaSO4 rich cherts that infiltrated the feeder dykes, underplating and dilating adjacent stratiform cherts before the start of the next volcanic cycle. The Apex chert ‘microfossils’ occur within multiple generations of these metalliferous hydrothermal vein cherts some 100 m down the dyke system. Comparable structures occur in associated volcanic vent glass and in hydrothermal cherts at least 1 km deep. We find no supporting evidence for a primary biological origin. We reinterpret the purported microfossil-like structures as pseudofossils that formed from the reorganization of carbonaceous matter, mainly during recrystallization from amorphous to spherulitic silica.
... Much of the material previously thought or believed by some to be fossilized microbes is subject to alternate interpretations. For example, microbiallike remains that supposedly represent cyanobacterial species from some of the oldest rocks on Earth, 3.5 billion year old cherts from western Australia (Schopf and Packer 1987; Schopf 1993), have recently been the subject of intense debate (Brasier et al. 2002; Dalton 2002). However, if mineral grains produced though BIM or BCM are unique or unusual enough that they can be easily recognized and accepted as not being abiologically formed by geological or chemical processes alone, and persist over long periods of geo logic time, they might prove be excellent fossil evidence for the past presence of certain microbes. ...
Biomineralization processes have traditionally been grouped into two distinct modes; biologically induced mineralization (BIM) and biologically controlled mineralization (BCM). In BIM, microbes cause mineral formation by sorbing solutes onto their cell surfaces or extruded organic polymers and/or releasing reactive metabolites which alter the saturation state of the solution proximal to the cell or polymer surface. Such mineral products appear to have no specific recognized functions. On the other hand, in BCM microbes exert a great degree of chemical and genetic control over the nucleation and growth of mineral particles, presumably because the biominerals produced serve some physiological function. Interestingly, there are examples where the same biomineral is produced by both modes in the same sedimentary environment. For example, the magnetic mineral magnetite (Fe3O4) is generated extracellularly in the bulk pore waters of sediments by various Fe(III)-reducing bacteria under anaerobic conditions, while some other anaerobic and microaerophilic bacteria and possibly protists form magnetite intracellularly within preformed vesicles. Differences in precipitation mechanisms might be caused by enzymatic activity at specific sites on the surface of the cell. Whereas one type of microbe might facilitate the transport of dissolved Fe(III) into the cell, another type will express its reductive enzymes and cause the reduction of Fe(III) external to the cell. Still other microbes might induce magnetite formation indirectly through the oxidation of Fe(II), followed by the reaction of dissolved Fe(II) with hydrolyzed Fe(III). The biomineralization of magnetite has significant effect on environmental iron cycling, the magnetization of sediments and thus the geologic record, and on the use of biomarkers as microbial fossils.
... Microorganisms of this period were minute, poorly preserved, and constituted of simple morphologies that could have been produced even by non-biologic processes like mineralic microstructures that make it diffi cult to differentiate between true fossils and pseudofossils. Hence, Early Archaean palaeontological records are always viewed with scepticism (Brasier et al. 2002;Dalton 2002;Van Zuilen et al. 2002). Even those recorded from the Warrawoona Group of Western Australia have been challenged for their antiquity and biogenicity (Awramik et al. 1983;Buick 1984;Kaźmierczak and Barbara 2002). ...
The discovery of Precambrian microfossils in 1954 opened a new vista of investigations in the field of evolution of life. Although the Precambrian encompasses 87% of the earth's history, the pace of organismal evolution was quite slow. The life forms as categorised today in the three principal domains viz. the Bacteria, the Archaea and the Eucarya evolved during this period. In this paper, we review the advancements made in the Precambrian palaeontology and its contribution in understanding the evolution of life forms on earth. These studies have enriched the data base on the Precambrian life. Most of the direct evidence includes fossil prokaryotes, protists, advanced algal fossils, acritarchs, and the indirect evidence is represented by the stromatolites, trace fossils and geochemical fossils signatures. The Precambrian fossils are preserved in the form of compressions, impressions, and permineralized and biomineralized remains.
... Conclusive evidence (a ''smoking gun'') is much more desirable than evidence that can be subjected to multiple interpretations. We have seen a number of disputes in the past that have lingered on without a clear resolution: the authenticity of 3.5-billion-year-old microfossils from Western Australia (Dalton, 2002), the interpretation of the Mars Viking results (Levin, 2007), and the existence of signs of martian life in the meteorite ALH84001 (Knoll, 1998). To avoid further ambiguities, we propose that the discovery of extant alien life should receive a much higher priority than a search for microfossils or other markers that simply suggest the past presence of life. ...
With the assumption that future attempts to explore our Solar System for life will be limited by economic constraints, we have formulated a series of principles to guide future searches: (1) the discovery of life that has originated independently of our own would have greater significance than evidence for panspermia; (2) an unambiguous identification of living beings (or the fully preserved, intact remains of such beings) is more desirable than the discovery of markers or fossils that would inform us of the presence of life but not its composition; (3) we should initially seek carbon-based life that employs a set of monomers and polymers substantially different than our own, which would effectively balance the need for ease of detection with that of establishing a separate origin; (4) a "follow-the-carbon" strategy appears optimal for locating such alternative carbon-based life. In following this agenda, we judge that an intensive investigation of a small number of bodies in our Solar System is more likely to succeed than a broad-based survey of a great number of worlds. Our priority for investigation is (1) Titan, (2) Mars, (3) Europa. Titan displays a rich organic chemistry and offers several alternative possibilities for the discovery of extant life or the early stages that lead to life. Mars has already been subjected to considerable study through landers and orbiters. Although only small amounts of methane testify to the inventory of reduced carbon on the planet, a number of other indicators suggest that the presence of microbial life is a possibility. Care will be needed, of course, to distinguish indigenous life from that which may have spread by panspermia. Europa appears to contain a subsurface ocean with the possibility of hydrothermal vents as an energy source. Its inventory of organic carbon is not yet known.
... Recent studies, however, have questioned the validity of some of the Earth's oldest fossils from the 3.5 to 3.4 Ga rocks of South Africa and Australia, implying that the structures mimicking bacterial fossils may be contaminants, fluid inclusions [1,2], or carbon threads formed abiologically in hydrothermal environments [6]. The controversy and the public debate on these findings have attracted much attention in the scientific community [9,10,15]. Any report of the record of early life, therefore, must hold up to the most rigorous inspection before the findings can be accepted as genuine and authentic. The earliest assured cyanobacterial microfossils were reported from Neoarchean rocks (2.7–2.5 Ga). ...
The oldest fossils found thus far on Earth are c. 3.49- and 3.46-billion-year-old filamentous and coccoidal microbial remains in rocks of the Pilbara craton, Western Australia, and c. 3.4-billion-year-old rocks from the Barberton region, South Africa. Their biogenicity was recently questioned and they were reinterpreted as contaminants, mineral artefacts or inorganic carbon aggregates. Morphological, geochemical and isotopic data imply, however, that life was relatively widespread and advanced in the Archean, between 3.5 and 2.5 billion years ago, with metabolic pathways analogous to those of recent prokaryotic organisms, including cyanobacteria, and probably even eukaryotes at the terminal Archean.
Since time immemorial, one of the fundamental questions that preoccupies the human mind is, how did all this life around us come into existence? It is not surprising that there is no quick and easy answer to this question, as we do not even know the size of the puzzle that lays in front of us. Therefore, the approach that needs to be adopted is a multidisciplinary understanding of biology, chemistry, physics, geology and their subsidiary scientific subjects (e.g., genetics, microbial biochemistry and geobiology) as well as reporting on ancient microorganisms preserved in sediments and volcanic lava deposits. Moreover, we should also ask what part did extremophiles play in the emergence of life. In this chapter, certain front runner hypotheses, ideas and the latest methodologies are brought into focus. In addition, the reader is guided through a possible scenario posed for chemical evolution, as elucidated by element (e.g., ¹³C/¹²C ratio) fractionation techniques, and introduced to a construct of the last universal common ancestor (LUCA), which gave rise to the three domains of life; namely, Archaea, Bacteria and Eukarya. In doing so, a possible role played by viruses is explored–in particular that of RNA viruses. Also brought to the fore, are the latest techniques (e.g., the use of 16S rRNA genes) which are assisting scientists to elucidate the routes to the emergence of life.
The current results of science from two research fields approaching the origin of life are reviewed. We will see how close we are to identifying the origin of life by following the biological evolutionary phylogenetic tree provided by evidence from the fossil record and the research results that suggest an “ultimate ancestor” by the analysis of biomolecules, such as genes and proteins.
On the basis of summarizing the basic conditions of breaking out red tide and the limit of iron on oceanic primary productivity, the relationship between iron in atmospheric dust and harmful algal bloom as well as iron transported by monsoon are discussed. It is significant to understand the way offering nutrient matter for stimulating red tide and to provide a new idea for forecasting harmful algal bloom.
Life has been built on the evolution and innovation of microbial metabolisms. Even with our scant understanding of the full diversity of microbial life, it is clear that microbes have become integral components of the biogeochemical cycles that drive our planet. The antiquity of life further suggests that various microbial metabolisms have been core and essential to global elemental cycling for a majority of Earth's history.
Cylindrospermopsis raciborskii is a toxic cyanobacterium of tropical origin. Recently, its
distribution has widened to various areas of the globe. Due to its toxicity, this expansion
has resulted in severe consequences for the environment and human health. In this work,
the geographical distribution of this species was reviewed, as well the hypotheses that
explain its origin and expansion routes. It is argued that this expansion is a result of several
factors. Besides adaptation to climate change, other characteristics of C. raciborskii, such
its resilience to different environmental parameters, and its nutrient uptake capability are
also relevant. Furthermore, the allelopathic activity of this species should also be influential
for its invasive potential. C. raciborskii strain LEGE 99043 was shown previously to inhibit
the growth of the microalgae A. falcatus. In this work, the allelopathic potential of this
strain was studied, as a response to the alteration of different environmental parameters,
and conditions of growth. Inhibitory activity was found in both low and high cellular
densities, and at distinct periods of growth. High light intensity, high temperature, and
absence of phosphate in the growth medium of C. raciborskii resulted in a further
enhancement of allelopathy. Allelopathic response of C. raciborskii to environmental
parameters, particularly absence of phosphate, has been not yet reported. Growth of C.
raciborskii in those parameters was optimum or even promoted. The observed activity
patterns suggest the action of several still unknown allelopathic compounds. In the future,
the characterization of these allelochemicals should reveal more about their
ecophysiological role.
The possibility of pseudofossils is a well-known obstacle to the identification of fossilized microorganisms (Cloud, 1973; Westall, 1999) and distinguishing abiotic from biotic origins is still a hotly debated topic (Dalton 2002; Schopf et al., 2002; Brasier et al., 2002). Here we report silica-carbonate aggregates, so-called ‘biomorphs’ that mimic—both morphologically and chemically-primitive microfossils.
Based on the in-depth investigation of the temporal and spatial relationship between aerosols and red tide events in the East China Sea (ECS), a model of the potential correlation between the paths of aerosol input and red tides in the ECS has been suggested. This study shows that red tides are closely related to aerosol events which come from northwest of the sea (winter monsoon direction) and descend to the ECS surface. Two principal paths of aerosol input are established that are relevant to the red tide events in the ECS. The first path contains aerosols from the northwest that are usually related to the red tide events covering an area of more than 1,000 km2. A second path is characterized by aerosols that first move from the northwest to east and then to south, finally settling in the red tide areas in the ECS. These aerosol paths are usually related to the red tide events that cover a smaller area, except in cases where stopover and follow-up re-supply of these aerosols may result in larger red tide events. The aerosols from southeast and southwest are not related with red tide events in the ECS. Downward vertical air currents play a key role in the relationship between ECS red tides and aerosol events. Without downdraughts, aerosols will have nothing to do with the red tide events. The study provides new information for discovering the occurrence mechanism of red tides in the ECS and essential parameters for red tide prediction and early warning.
The criticism by Pasteris and Wopenka of our use of laser-Raman imagery
to investigate the carbonaceous make-up of extremely ancient fossils
focuses only on their Raman signature; however, our interpretation that
the carbonaceous matter that makes up these specimens is biogenic is
based on several lines of evidence, of which Raman spectroscopy is only
one.
The origin of life on Earth continues to be one of the greatest mysteries of modern science. The fact that geochemical evidence for life may extend back to the earliest portion of the terrestrial rock record (approx. 4.0 Ga) makes it impossible to state with any certainty whether life originated on Earth or if it was introduced to Earth. Similarly, since a living system has never been synthesized in the laboratory, the requisite conditions and subsequent pathways for that origin remain unknown. It is this lack of definitive information that continues to inspire us to search outside the Earth for clues to life's origins. However, our ability to determine if life as we know it presently exists or existed elsewhere in our solar system or beyond depends on the establishment of criteria that can be used to determine its existence and to distinguish it from present-day life on Earth. The latter is critical, as it is extremely difficult to completely avoid contamination during sample collection and analysis. This is especially true with respect to materials returned to Earth, as is planned for future Mars' missions. Given their ubiquity and unique stable isotope compositions resulting from fractionations associated with their respective abiotic and biotic syntheses, amino acids are one of the classes of compounds best suited for establishing the existence of life as we know it. Criteria will be presented for detecting life and its possible precursors elsewhere in the solar system based on the stable isotope compositions of this important class of compounds.
The Precambrian Earth: Tempos and Events Edited by EG. Eriksson, W. Altermann, D.R. Nelson, W.U. Mueller and O. Catuneanu Developments in Precambrian Geology, Vol. 12 (K.C. Condie, Series Editor) 9 2004 Elsevier B.V. All rights reserved
Chapter 9
TOWARDS A SYNTHESIS
RG. ERIKSSON, O. CATUNEANU, D.R. NELSON, W.U. MUELLER AND W. ALTERMANN
The principal theme of this book is change through time, or tempos and events in the Precambrian (Preface). Each chapter portrays a different part of the Earth's history but there is a unifying theme: Earth's evolution. Chapter 1 explains the celestial origin of our planet and the early development of the Earth into core, mantle, crust and primitive atmosphere. Chapter 2 discusses the generation of continental crust, with the emphasis on granite-greenstone terranes. Chapter 3 builds further upon its predecessor, empha- sising the interaction between tectonism and mantle plumes through Precambrian time. Chapter 4 examines the volcanic attributes of the Archaean Earth and how they may have changed, as exemplified by plume-generated komatiites, the constant interaction between arc-plume volcanism and subaqueous caldera formation. Chapter 5 deals with the evolution of Earth's atmosphere and hydrosphere, and chapter 6 with related concepts of the evolu- tion of Precambrian life and bio-geology. Chapter 7 details sedimentation regimes through Precambrian time, while chapter 8 discusses the application of sequence stratigraphy to the Precambrian rock record.
9.1. EVOLUTION OF THE SOLAR SYSTEM AND THE EARLY EARTH
Investigation of pre-4 Ga Earth history relies largely upon study of the most ancient rocks thus far identified, and upon modelling of the differentiation of Earth's chemical reservoirs (Nelson, section 1.1). As the known preserved rock record dates from 4030 Ma (Stern and Bleeker, 1998; Bowring and Williams, 1999), more than 500 My of Earth's ear- liest evolution remains essentially speculative. It was only with the identification within meteorites of daughter products from radiogenic decay of long-extinct nuclides (firstly by Reynolds, 1960), that the timing of accretion and differentiation of the early Earth could be investigated (summarised by Nelson, in section 1.2). The short-lived parent nu- clides were synthesised during supernova explosions shortly before formation of our solar system; their short half-lives enable precise determination of the chronology of the earli- est history of the solar system (section 1.2). Collision and amalgamation of smaller, rocky planetesimals within a protoplanetary disk formed the terrestrial planets, including Earth. As proto-Earth and its Moon grew by these violent accretion processes, earlier differenti- ation products were largely obliterated; with the growth of embryonic planets the impact rate decreased and concomitantly, the likelihood of preservation of fragments of the early
Earth increased.
Current evidence (section 1.2) suggests that short-lived nuclides with atomic masses
< 140, together with a part of the heavy elements in our solar system, were synthesised
during a core-collapse supernova event at c. 4571 Ma (Lugmair and Shukolyukov, 2001;
Gilmour and Saxton, 2001 ). Formation of the Sun and solar system may have been initiated
by shock waves from this supernova explosion (probably one of a number of successive
such events); injection of short-lived nuclides into a nearby interstellar gas and dust cloud
may have triggered its collapse, forming a proto-Sun of radius c. five times its present
value, over a time period of < 105 years (Cameron, 1995; Nelson, section 1.2). Progressive
collapse from inner to outer parts of the cloud, together with conservation of angular
momentum, caused it to spin faster; colliding gas and dust particles orbiting the proto-
Sun in the same direction lost their energy, causing flattening of the cloud, especially near
its centre.
Gravitational energy was converted to heat during collapse of the nebula. At some time
during collapse, the density and temperature became high enough for hydrogen burning
to commence, and the proto-Sun began its violent T-Tauri phase (Cameron, 1995;
Nelson, section 1.2). More abundant Fe, Ni and silicate-rich components condensed within
lower temperature parts of the nebula in its medial to central parts, while volatile elements
(e.g., water, ammonia, methane ice) condensed in cold, outer parts of the accretionary disk.
Volatiles were possibly carried by the solar wind from inner to outer reaches of the emerging
solar system (Shu et al., 1994).
Spectroscopy and simulation modelling suggest only a few million years from star formation
and large scale accretion of disks into the young solar-type T-Tauri stars. Larger
planetesimals may have formed within ~< 2 My of solar system formation (Hutchison et al.,
2001). Coagulation consequent upon icy particle collisions within an ice sublimation belt
in the cold outer parts of the nebula being more efficient than that between metal or silicate
particles, large gas-rich proto-planets (Jupiter and Saturn precursors) formed before
the nebula gas dissipated (Cameron, 1995). Collision and amalgamation of chemically refractory
dust particles within the inner part of the disk occurred more slowly. Collisions of
smaller planetesimals with larger bodies continued to rework early planetary-sized bodies
for at least a further 100 My, and triggered large scale melting and magmatic differentiation
of silicate components of the larger planetesimals. Dating of meteoritic remnants from
these early differentiated planetary bodies indicates that planetesimals of at least 10s-100s
of kilometres in diameter underwent internal magmatic differentiation within < 10 My
after the supernova event. 187Re-187Os isotopic data from pallasites and iron meteorites
(Morgan et al., 1995; Shen et al., 1998; Horan et al., 1998) suggest formation of metallic
cores within c. ~< 50 My of formation of the solar system. There is intriguing evidence for
hydrothermal alteration processes involving aqueous fluids within planetesimals ~< 2 My
after solar system formation.
Planetary embryos had thus existed within <~ 5 My of the supernova event that triggered
formation of the solar system. Accretion of these embryos within a c. 0.5-2.5 AU range
of the Sun (Wetherill, 1994) was largely responsible for formation of the terrestrial plan
This has been the Year of the Baryon. Some low temperature ones were
seen at high redshift, some high temperature ones were seen at low
redshift, and some cooling ones were (probably) reheated. Astronomers
saw the back of the Sun (which is also made of baryons), a possible
solution to the problem of ejection of material by Type II supernovae
(in which neutrinos push out baryons), the production of R Coronae
Borealis stars (previously-owned baryons), and perhaps found the missing
satellite galaxies (whose failing is that they have no baryons). A few
questions were left unanswered for next year, and an attempt is made to
discuss these as well.
The synthesis of physical information on early Earth (or Mars) with recent knowledge arising from microbial genomic, proteomic and phylogenetic studies, strongly indicates that there was insufficient time (∼600 000 years) for life to arise and evolve to reach the biochemical complexity evident within the Last Common Community (LCC). If recent strong evidence of fossil cyanobacteria in carbonaceous meteorites is accepted, then the LCC would have existed prior to the origin of life on Earth and the planet would then have been seeded with representatives of the three domains once it became habitable. The existence of intermittently active cyanobacteria in comets opens the possibility for the evolution of microaerobic bacterial metabolism, elements of which appear at a deep level of the microbial phylogeny, at or below the depth of the LCC. It is also notable from a panspermia perspective that recent phylogenetic evidence indicates that the Gram-positive lineage (representatives of which are endowed with long-lived radiation-resistant spores) lies at the deepest level of domain Bacteria, with Archaea and Eukarya evolving from this lineage probably before 3.6 Gigayears ago (Gya).
Combining numerical diagnosis from atmospheric science with biogeochemical methods, a model of the potential correlation of
monsoons with red tide emergence in the East China Sea is constructed. The model is designed based on an in-depth investigation
of the time-space relationship of aerosol and red tide events in the East China Sea from 2005 to 2006, and a continuous monitoring
of atmospheric particulates at two stations. The study shows that every red tide event investigated has a close relationship
with aerosols coming from the northwest (wind direction in winter) along with subsidence flow. The elemental abundance of
total suspended particulate in Hangzhou and Tiantai is different from that of soil background levels, indicating atmospheric
particulates there are brought in by winter winds. There is a significant correlation between the content of iron and phosphorous
in atmospheric particulates, which mainly exist in binding materials between particulates. In addition, the confined absorption
of iron and phosphorous by red tides is related to the intensity of sunlight. These results provide new information regarding
the mechanism for the high frequency of red tides in the East China Sea. The results also provide a scientific basis for establishing
new pathways for pre-warning and forecasting of red tide disasters.
Keywordswinter monsoon–atmospheric particulate matter–red tide in East China Sea–iron nutrient element limitation
The segregated, linear and digital character of the genome has allowed us to apply information theory and other mathematical theorems about sequences or strings of symbols to make a quantitative rather than an anecdotal and ad hoc discussion of significant problems in molecular biology. This procedure has led us to avoid a number of illusions common in the literature. The application of these mathematical procedures will play a role in molecular biology analogous to that of thermodynamics in chemistry.
Microstructures in the ~3.5Gyr Apex Chert Formation were initially described as the oldest bacterial fossils on Earth over 20years ago. However, the identification of the structures (which resemble cyanobacteria) as biological in origin remains controversial. Here we determine the petrology and geochemistry of similar structures from the original Apex Chert locality using thin sections and Raman spectroscopy. Based on the microscopic examination of thin sections, we identify features similar to those previously identified as microfossils as a series of quartz and haematite-filled fractures. Raman spectroscopy of the fractures shows that carbonaceous material is not, as previously reported, associated with the structures, but is instead disseminated in the surrounding quartz matrix. We suggest that although the microstuctures analysed are not microfossils, the presence of carbonaceous material in the surrounding matrix is consistent with the existence of microbial life at this time, and with evidence of early Archaean life found at other sites. Furthermore, we caution against identifying microstructures as biological in origin without a full morphological and geochemical assessment.
The practice of physics and chemistrys - the making and breaking of molecular bonds arising from the interactions of matter and energys - is coincidental with the earliest matter/energy of the Cosmos. The scenario of an interconnected and tightly integrated global geochemical sulfur cycle extending from the atmosphere to the mantle on Archean Earth emerges from several recent studies. Like the environment in which it was generated and reared, primitive Earth, cobbled together from debris of the Solar System, was also a very likely and prolific laboratory for prebiotic syntheses. While isotopic and other geological evidence offer useful dating clues, the results may be equivocal. Biomarkers are potentially useful because each Domain, Archaea, Bacteria, and Eukarya, has signature membrane lipids with relatively stable complex carbon skeletons.
Microscopic carbonaceous structures found in ancient rocks could provide clues to early life on Earth if they turn out to be genuine fossil microorganisms. Here we show that thermal alteration of microbial remains embedded in a mineral matrix may significantly change their original morphology and produce structures that resemble those of what are claimed to be the Earth's oldest fossils. These observations may shed light on the controversy that surrounds these microfossils from the 3,465-Myr-old Apex Chert of the early Archaean Warrawoona Group in northwestern Australia.
In his article “Championing a 17th century underdog” (News of the Week, 11 July, p. [152][1]), Richard Stone reports on the current interest in the scientific achievements of Robert Hooke (1635–1703), including his remarkable insights in several areas of biology. One aspect of his many
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.
An elementary introduction to modern cosmology for general readers is
presented. The topics addressed include: the geometry of space and time,
special and general relativity, the big bang, the evolution of the
universe, stellar evolution, gravitational collapse to black holes,
energy extraction from black holes, the astrophysics of black holes, and
quantum particle creation near black holes.
The Proterozoic Biosphere is the first major study of the paleobiology of the Proterozoic Earth. It is a multidisciplinary work dealing with the evolution of the Earth, the environment, and life during the forty percent of Earth's history that extends from the middle of the Precambrian Eon (2500 Ma) to the beginning of the Paleozoic Era (550 Ma.). The Proterozoic Biosphere includes a vast amount of new data on Proterozoic organisms and their modern analogs. Prepared by the Precambrian Paleobiology Research Group, a multidisciplinary consortium of forty-one scientists from eight countries, this monograph will serve as a benchmark in the development of the science of the biochemistry and the organic chemistry of Proterozoic sediments. The three main goals of this study are: (1) to amass, evaluate, and synthesize the large body of paleobiologic data available from previous studies, eliminating mistakes so that future investigations will not be encumbered by them; (2) to generate new data and new analyses based on the reexamination of previous studies and on new investigations within an interdisciplinary framework; (3) to build toward the future by placing special emphasis on new or relatively neglected aspects of paleobiologic study and by highlighting major unsolved problems in the field.
A sample of chert from North Pole in the Archaean Pilbara block of Western Australia contains carbonaceous filaments that resemble microfossils. These occur in alternating light and dark laminae that look stromatolitic. However, the filaments are too simple in form for their origin to be determined, so they should be regarded as dubiofossils, perhaps biogenic, perhaps inorganic. Their host laminae were inorganically precipitated in a concordant fissure and thus cannot be stromatolitic. This fissure is younger than the surrounding silicified sediments of the ca. 3500 Ma old Warrawoona Group and possibly formed towards the end of the uplift and associated fracturing of the North Pole Dome, perhaps ca. 2750 Ma ago. The filaments are therefore contaminants in secondary chert.The filament-bearing rock was collected less than a metre from one of the localities (B) from which Awramik et al. reported early Archaean microfossils and possible microfossils. Their filaments from this locality were almost identical to those described here and were found in similar laminae. This suggests that their locality B filaments may also be contaminants in secondary chert. Other filaments found by Awramik et al. at North Pole come from an imprecisely located sample site (locality A) where the rock relationships are unknown. Since the host laminae of these filaments are not demonstrably primary and as cryptic concordant fissures filled with secondary minerals are common in locality A rocks, the filaments from this sample site could be contaminants too. Those that were assigned to Archaeotrichion should be treated as dubiofossils. Thus, the filaments described by Awramik et al. may not be fossil bacteria in ca. 3500 Ma old stromatolites, as they proposed, and are not necessarily the oldest known fossil organisms, as has been claimed.
Four morphotypes of structurally preserved, filamentous fossil bacteria have been discovered in petrographic thin sections of laminated, carbonaceous cherts from the ∼3500 Ma-old Warrawoona Group of northwestern Australia. These tubular and septate microfossils are interpreted here as being syngenetic with Warrawoona sedimentation; as such, they are apparently the oldest such fossils now known in the geological record. The diversity of this assemblage, and the evident complexity of its individual components, suggest that the beginnings of life on Earth may have appreciably pre-dated the deposition of the Warrawoona sediments.
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.
Geochemists are having a hard time working out why the atmosphere of the early Earth appears to have lacked oxygen for so long. Jon Copley considers the competing theories.
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.
Cellularly preserved filamentous and colonial fossil microorganisms have been discovered in bedded carbonaceous cherts from
the Early Archean Apex Basalt and Towers Formation of northwestern Western Australia. The cell types detected suggest that
cyanobacteria, and therefore oxygen-producing photosynthesis, may have been extant as early as 3.3 billion to 3.5 billion
years ago. These fossils are among the oldest now known from the geologic record; their discovery substantiates previous
reports of Early Archean microfossils in Warrawoona Group strata.
There is widespread textural evidence for microbial activity in the cherts of the Early Archean Onverwacht Group. Layers with fine carbonaceous laminations resembling fossil microbial mats are abundant in the cherty metasediments of the predominantly basaltic Hooggenoeg and Kromberg Formations. In rare cases, filamentous microfossils are associated with the laminae. The morphologies of the fossils, as well as the texture of the encompassing laminae suggest an affinity to modern mat-dwelling cyanobacteria or bacteria. A variety of spheroidal and ellipsoidal structures present in cherts of the Hooggenoeg and Kromberg Formations resemble modern coccoidal bacteria and bacterial structures, including spores. The development of spores may have enabled early microorganisms to survive the relatively harsh surficial conditions, including the effects of very large meteorite impacts on the young Earth.
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.
We can hope that the term 'palaeophosphatometry' won't catch on. The approach it describes has nonetheless delivered a plausible — if partial — answer to one of the main questions about Earth's history.
A newly discovered population of organic walled microstructures from the Swaziland System, South Africa, is considered to be biological on the following grounds: (i) the structures are carbonaceous and occasionally have internal organic contents; (ii) the population has a narrow unimodal size frequency distribution (average diameter, 2.5 micrometers; range, 1 to 4 micrometers); (iii) the structures are not strictly spherical, but are commonly flattened and folded like younger microfossils; (iv) the sedimentary context is consistent with biogenic origins; and (v) various stages of binary division are clearly preserved.
- R Buick
Buick, R. Precambr. Res. 39, 311-317 (1988).
- M D Brasier
Brasier, M. D. et al. Nature 416, 76–81 (2002).
- R Buick
Buick, R. Precambr. Res. 24, 157-172 (1984).
- A H Knoll
- E S Barghoorn
Knoll, A. H. & Barghoorn, E. S. Science 198, 396-398 (1977).
- M M Walsh
Walsh, M. M. Precambr. Res. 54, 271-293 (1992).
- J W Schopf
- B M Packer
Schopf, J. W. & Packer, B. M. Science 237, 70-73 (1987).
- J W Schopf
Schopf, J. W. Science 260, 640-646 (1993).
- A B Kudryavtsev
- J W Schopf
- D G Agresti
- T Wdowiak
Kudryavtsev, A. B., Schopf, J. W., Agresti, D. G. & Wdowiak, T. J.
Proc. Natl Acad. Sci. USA 98, 823-826 (2001).
- S M Awramik
- J W Schopf
- M R Walter
Awramik, S. M., Schopf, J. W. & Walter, M. R. Precambr. Res. 20,
357-374 (1983).
- S M Awramik
- J W Schopf
- M R Walter
Awramik, S. M., Schopf, J. W. & Walter, M. R. Precambr. Res. 39,
303-309 (1988).