Earth's oldest (approximately 3.5 Ga) fossils and the 'Early Eden hypothesis': questioning the evidence

Hot Spring Microbial Community Elemental Composition: Hot Spring and Soil Inputs, and the Transition from Biocumulus to Siliceous Sinter

Hydrothermal systems host microbial communities that include some of the most deeply branching members of the tree of life, and recent work has suggested that terrestrial hot springs may have provided ideal conditions for the origin of life. Hydrothermal microbial communities are a potential source for biosignatures, and the presence of terrestrial hot spring deposits in 3.48 Ga rocks as well as on the surface of Mars lends weight to a need to better understand the preservation of biosignatures in these systems. Although there are general patterns of elemental enrichment in hydrothermal water dependent on physical and geochemical conditions, the elemental composition of bulk hydrothermal microbial communities (here termed biocumulus, including cellular biomass and accumulated non-cellular material) is largely unexplored. However, recent work has suggested both bulk and spatial trace element enrichment as a potential biosignature in hot spring deposits. To elucidate the elemental composition of hot spring biocumulus samples and explore the sources of those elements, we analyzed a suite of 16 elements in hot spring water samples and corresponding biocumulus from 60 hot springs sinter samples, and rock samples from 8 hydrothermal areas across Yellowstone National Park. We combined these data with values reported in literature to assess the patterns of elemental uptake into biocumulus and retention in associated siliceous sinter. Hot spring biocumuli are of biological origin, but organic carbon comprises a minor percentage of the total mass of both thermophilic chemotrophic and phototrophic biocumulus. Instead, the majority of hot spring biocumulus is inorganic material-largely silica-and the distribution of major and trace elements mimics that of surrounding rock and soil rather than the hot spring fluids. Analyses indicate a systematic loss of biologically associated elements during diagenetic transformation of biocumulus to siliceous sinter, suggesting a potential for silica sinter to preserve a trace element biosignature.

Dubiofossils from a Mars-analogue subsurface palaeoenvironment: The limits of biogenicity criteria

The search for a fossil record of Earth's deep biosphere, partly motivated by potential analogies with subsurface habitats on Mars, has uncovered numerous assemblages of inorganic microfilaments and tubules inside ancient pores and fractures. Although these enigmatic objects are morphologically similar to mineralized microorganisms (and some contain organic carbon), they also resemble some abiotic structures. Palaeobiologists have responded to this ambiguity by evaluating problematic filaments against checklists of "biogenicity criteria". Here, we describe material that tests the limits of this approach. We sampled Jurassic calcite veins formed through subseafloor serpentinization, a water-rock reaction that can fuel the deep biosphere and is known to have occurred widely on Mars. At two localities ~4 km apart, veins contained curving, branched microfilaments composed of Mg-silicate and Fe-oxide minerals. Using a wide range of analytical techniques including synchrotron X-ray microtomography and scanning transmission electron microscopy, we show that these features meet many published criteria for biogenicity and are comparable to fossilized cryptoendolithic fungi or bacteria. However, we argue that abiotic processes driven by serpentinization could account for the same set of lifelike features, and report a chemical garden experiment that supports this view. These filaments are, therefore, most objectively described as dubiofossils, a designation we here defend from criticism and recommend over alternative approaches, but which nevertheless signifies an impasse. Similar impasses can be anticipated in the future exploration of subsurface palaeo-habitats on Earth and Mars. To avoid them, further studies are required in biomimetic geochemical self-organization, microbial taphonomy and micro-analytical techniques, with a focus on subsurface habitats.

In Situ Fe and S isotope analyses in pyrite from the 3.2 Ga Mendon Formation (Barberton Greenstone Belt, South Africa): Evidence for early microbial iron reduction

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7-2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28- to 3.26-Gyr-old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ⁵⁶ Fe values of -1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass-independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ³³ S values > +3‰, while the other groups display more variable and closer to 0‰ Δ³³ S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ³³ S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass-independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro-organisms closely related to the last common ancestor had the ability to reduce Fe(III).

Organo-mineral associations in chert of the 3.5 Ga Mount Ada Basalt raise questions about the origin of organic matter in Paleoarchean hydrothermally influenced sediments

Hydrothermal and metamorphic processes could have abiotically produced organo-mineral associations displaying morphological and isotopic characteristics similar to those of fossilized microorganisms in ancient rocks, thereby leaving false-positive evidence for early life in the geological record. Recent studies revealed that geologically-induced alteration processes do not always completely obliterate all molecular information about the original organic precursors of ancient microfossils. Here, we report the molecular, geochemical, and mineralogical composition of organo-mineral associations in a chert sample from the ca. 3.47 billion-year-old (Ga) Mount Ada Basalt, in the Pilbara Craton, Western Australia. Our observations indicate that the molecular characteristics of carbonaceous matter are consistent with hydrothermally altered biological organics, although significantly distinct from that of organic microfossils discovered in a chert sample from the ca. 3.43 Ga Strelley Pool Formation in the same area. Alternatively, the presence of native metal alloys in the chert, previously believed to be unstable in such hydrothermally influenced environments, indicates strongly reducing conditions that were favorable for the abiotic formation of organic matter. Drawing definitive conclusions about the origin of most Paleoarchean organo-mineral associations therefore requires further characterization of a range of natural samples together with experimental simulations to constrain the molecular composition and geological fate of hydrothermally-generated condensed organics.

The Taphonomy of Proterozoic Microbial Mats and Implications for Early Diagenetic Silicification

The complex nature of growth and decomposition in microbial mats results in a broad range of microbial preservation. Such taphonomic variability complicates both the description of microbial elements preserved within geologic materials and the potential interpretation of microbial biomarkers. This study uses a taphonomic assessment to explore the preservation of different microbial components within silicified microbial mats of the late Mesoproterozoic (~1.0 Ga) Angmaat Formation, Bylot Supergroup, Baffin Island. The Angmaat Formation consists of unmetamorphosed and essentially undeformed strata that represent intertidal to supratidal deposition within an evaporative microbial flat. Early diagenetic silicification preserved microbial communities across a range of environments, from those episodically exposed to persistently submerged. Here, we present the development of a new methodology involving the use of high-resolution image mosaics to investigate the taphonomy of microfossils preserved in these mats. A taphonomic grade is assigned using a modified classification that accounts for both the taphonomic preservation state (good, fair, poor) of individual microfossils, as well as the degree of compaction of the overall mat. We show that although various taphonomic states occur within each of the silicified mats, the overall taphonomic assessment differentiates between well-preserved mats that are interpreted to have been silicified during active growth, to highly degraded and compacted mats that are interpreted to represent preservation during later stages of biological decomposition. These data indicate that even small changes in the timing of silicification may have substantial implications on our identification of microbial biomarkers and, therefore, our interpretation of early Earth ecosystems.

Manganese Oxides Resembling Microbial Fabrics and Their Implications for Recognizing Inorganically Preserved Microfossils

In the search for microfossils of early life on Earth, the demonstration of biogenicity is paramount. Traditionally, only syngenetic structures with cellular elaboration, hollow sheaths/cell walls, and indigenous kerogen have been considered bona fide fossils. Recent reports of inorganically preserved microfossils represent a shift from this practice. Such a shift, if accompanied by a robust set of biogenicity criteria, could have profound implications for the identification of biosignatures on early Earth and extraterrestrial bodies. Here, we reaffirm the conventional criteria by examining aggregates of inorganic filaments from the Pilbara region of Western Australia. These aggregates are preserved in bedded chert, and the filaments measure up to 1 μm in diameter and 100 μm in length. The aggregates superficially resemble kerogenous microbial fabrics and mycelial organisms. However, the filaments consist of manganese oxide, lack cellular elaboration, and show no evidence for hollow sheaths or cell walls. We conclude that the filaments are fibrous minerals of abiotic origin. The similarities between these pseudofossils and some filamentous fossils highlight the need for strict application of the conventional criteria for recognizing microfossils. In the absence of kerogen, morphologically simple structures should, at least, show evidence of cellular features to be considered bona fide fossils. Key Words: Fossil-Manganese oxide-Pilbara-Precambrian-Pseudofossil. Astrobiology 18, 249-258.

Raman Imaging Spectroscopy of a Putative Microfossil from the ∼3.46 Ga Apex Chert: Insights from Quartz Grain Orientation

The utility of nondestructive laser Raman for testing the biogenicity of microfossil-like structures in ancient rocks is promising, yet results from deposits like the ∼3.46 Ga Apex chert remain contentious. The essence of the debate is that associated microstructures, which are not purported to be microfossils, also contain reduced carbon that displays Raman D- and G-band peaks similar to those seen in the purported microfossils. This has led to the hypothesis that all features including reported microfossils are due to compression of nonfossil carbon during crystal growth around quartz spherulites or more angular crystals. In this scenario, the precursor to this macromolecular carbon may or may not have been of biogenic origin, while the arcuate and linear features described would be pseudofossils. To test this hypothesis, we have undertaken 2-D micro-Raman imaging of the Eoleptonema apex holotype and associated features using instrumentation with a high spatial and spectral resolution. In addition to this, we utilized the ratio of two Raman active quartz mode intensities (I129/I461) to assess quartz grain orientation and grain-splitting artifacts. These data lead us to conclude that the holotype of Eoleptonema apex is a sheet-shaped pseudofossil that appears to be a carbon infilled intragranular crack; therefore other holotypes should be carefully reexamined for syngenicity.

Deep questions about the nature of early-life signals: a commentary on Lister (1673) 'A description of certain stones figured like plants'

In 1673, Martin Lister explored the preservation of 'St Cuthbert's beads' plus other fossil crinoid remains from approximately 350 Ma Carboniferous limestone in northern England. He used taphonomic evidence (transport, disarticulation, burial and cementation) to infer an origin as petrified plant remains, in contrast with his views expressed elsewhere that fossil mollusc shells could have formed abiogenically, by 'plastic forces' within rock. Lister also observed pentagonal symmetry, now seen as characteristic of living echinoderm skeletons. A postscript from John Ray supports Lister's 'taphonomic' observations and accepts the biogenicity of these fossil 'vegetables'. Ray then concluded with a prophecy, predicting the discovery of comparable living fossils in remote ocean waters. These early discussions compare with current debates about the character of candidate microfossils from the early Earth and Mars. Interesting biomorphs are now tested against the abiogenic null hypotheses, making use of features such as those pioneered by Lister, including evidence for geological context, rules for growth and taphonomy. Advanced techniques now allow us to extend this list of criteria to include the nanoscale mapping of biology-like behaviour patterns plus metabolic pathways. Whereas the science of palaeobiology once began with tests for biogenicity, the same is now true for geobiology and astrobiology. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

Diverse capacity for 2-methylhopanoid production correlates with a specific ecological niche

Molecular fossils of 2-methylhopanoids are prominent biomarkers in modern and ancient sediments that have been used as proxies for cyanobacteria and their main metabolism, oxygenic photosynthesis. However, substantial culture and genomic-based evidence now indicates that organisms other than cyanobacteria can make 2-methylhopanoids. Because few data directly address which organisms produce 2-methylhopanoids in the environment, we used metagenomic and clone library methods to determine the environmental diversity of hpnP, the gene encoding the C-2 hopanoid methylase. Here we show that hpnP copies from alphaproteobacteria and as yet uncultured organisms are found in diverse modern environments, including some modern habitats representative of those preserved in the rock record. In contrast, cyanobacterial hpnP genes are rarer and tend to be localized to specific habitats. To move beyond understanding the taxonomic distribution of environmental 2-methylhopanoid producers, we asked whether hpnP presence might track with particular variables. We found hpnP to be significantly correlated with organisms, metabolisms and environments known to support plant-microbe interactions (P-value<10(-6)); in addition, we observed diverse hpnP types in closely packed microbial communities from other environments, including stromatolites, hot springs and hypersaline microbial mats. The common features of these niches indicate that 2-methylhopanoids are enriched in sessile microbial communities inhabiting environments low in oxygen and fixed nitrogen with high osmolarity. Our results support the earlier conclusion that 2-methylhopanoids are not reliable biomarkers for cyanobacteria or any other taxonomic group, and raise the new hypothesis that, instead, they are indicators of a specific environmental niche.

Raman imaging of metastable opal in carbonaceous microfossils of the 700-800 ma old Draken Formation

Opaline silica was detected, with Raman spectroscopy, in carbonaceous microfossils (especially Myxococcoides) in silicified filamentous microbial mats within dolomitized conglomerates of the Draken Formation (-800 to -700 Ma). High-resolution electron microscopy (HRTEM) and microprobe analyses were used to confirm the nature of this phase in the quartz matrix of the microbial mats. The silica likely precipitated in a microcrystalline form onto the organic macromolecules around, and within, the degrading microorganisms and preserved them by inhibiting the natural phase change to quartz. The Raman signal of opaline silica associated with carbonaceous matter and other biosignatures could be a potential indicator of biogenicity. This kind of association could be very useful during the future ExoMars mission (ESA/Roscosmos, 2018) that will search for traces of past life on Mars.

Are We from Outer Space?

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)

The origin of multicellularity in cyanobacteria

BACKGROUND

Cyanobacteria are one of the oldest and morphologically most diverse prokaryotic phyla on our planet. The early development of an oxygen-containing atmosphere approximately 2.45-2.22 billion years ago is attributed to the photosynthetic activity of cyanobacteria. Furthermore, they are one of the few prokaryotic phyla where multicellularity has evolved. Understanding when and how multicellularity evolved in these ancient organisms would provide fundamental information on the early history of life and further our knowledge of complex life forms.

RESULTS

We conducted and compared phylogenetic analyses of 16S rDNA sequences from a large sample of taxa representing the morphological and genetic diversity of cyanobacteria. We reconstructed ancestral character states on 10,000 phylogenetic trees. The results suggest that the majority of extant cyanobacteria descend from multicellular ancestors. Reversals to unicellularity occurred at least 5 times. Multicellularity was established again at least once within a single-celled clade. Comparison to the fossil record supports an early origin of multicellularity, possibly as early as the "Great Oxygenation Event" that occurred 2.45-2.22 billion years ago.

CONCLUSIONS

The results indicate that a multicellular morphotype evolved early in the cyanobacterial lineage and was regained at least once after a previous loss. Most of the morphological diversity exhibited in cyanobacteria today--including the majority of single-celled species--arose from ancient multicellular lineages. Multicellularity could have conferred a considerable advantage for exploring new niches and hence facilitated the diversification of new lineages.

Thermally altered Silurian cyanobacterial mats: a key to Earth's oldest fossils

Diagenetic changes in thermally altered cyanobacterial mats from early Silurian black radiolarian cherts of southwestern Poland (Bardzkie Montains, Sudetes) have been studied. These early diagenetically silicified mats are composed of variously degraded remains of benthic microbes that resemble some modern chroococcalean and pleurocapsalean cyanobacteria. Two modes of degradational processes have been recognized in the studied mats: (i) early postmortem biodegradation and (ii) late diagenetic thermal or thermobaric degradation. The latter led to partial transformation of the fossilized organic remnants of cyanobacterial sheaths and capsules, which resulted in the formation of objects morphologically distant from the original microbiota but preserved features that allow for their identification as bona fide biogenic structures. Some of these thermally generated Silurian fossils are highly similar to the controversial microfossil-like carbonaceous structures described from the Early Archean Apex Chert of Australia. This similarity opens a promising way for credible recognition of remnants of cyanobacteria and similar microbiota in other thermally metamorphosed Archean sedimentary rocks.

Was there a late Archean biospheric explosion?

There is a growing body of evidence which suggests that the evolution of the planet drives the evolution of the biosphere. There have been 2 significant stages in Earth history when atmospheric oxygen levels rose rapidly, and both appear to be associated with supercontinent cycles. The earlier biospheric event, which extends across the Archean-Proterozoic boundary (ca. 3.0-2.2 Ga), has received little attention and is the focus of this study. Recent work on the Pilbara Craton of Western Australia has shown that concretion formed by microbial activity during the diagenesis of these sediments are absent from early Archean sediments but abundant in late Archean and early Paleoproterozoic successions of the Hamersley Basin, appearing abruptly in sedimentary rocks younger than 2.7 Ga. This study suggests that their internal architecture may have been defined by the diffusion of humic acids and the formation of polymer gels during diagenesis. The data imply that the biosphere expanded suddenly shortly after 3.0 Ga and may have begun to raise the oxygen levels of the oceanic water column earlier than thought-possibly as much as 300 my earlier.

Question 2: Raman spectroscopic approach to analytical astrobiology: the detection of key biomolecular markers in the search for life

The recognition of extinct or extant life signatures in the terrestrial geological record is fundamentally dependent upon the understanding of both the structural morphology and chemical composition of relict biomaterials; the identification of cyanobacterial colonies that have adapted biogeologically their mineral matrices in early evolutionary processes is a fundamental step in the acquisition of analytical data from remote planetary probes designed for life-detection experiments, particularly on Mars and on the planetary satellite moons, Europa and Titan. A key factor in the assessment of early life signatures is the molecular presence of chemicals designed to protect the emerging organisms from the damaging effect of radiation exposure and of desiccation and temperature changes; in this respect the non-destructive capability of Raman spectroscopy to delineate the interfacial interactions between substrates and endolithic biology is now deemed an essential part of the ExoMars life-detection suite of instrumentation planned by the European Space Agency in the AURORA programme. A description of the scientific basis for the biogeological discrimination offered by Raman spectroscopy between organic and inorganic moieties in specimens from terrestrial Mars analogue sites is followed by selected examples of the type of analytical information provided, which will be complementary to the elementary and microscopic data obtained from other instrumentation on the same mission.

Nitrogen fixation in microbial mat and stromatolite communities from Cuatro Cienegas, Mexico

Nitrogen fixation (nitrogenase activity, NA) of a microbial mat and a living stromatolite from Cuatro Cienegas, Mexico, was examined over spring, summer, and winter of 2004. The goal of the study was to characterize the diazotrophic community through molecular analysis of the nifH gene and using inhibitors of sulfate reduction and oxygenic and anoxygenic photosynthesis. We also evaluated the role of ultraviolet radiation on the diazotrophic activity of the microbial communities. Both microbial communities showed patterns of NA with maximum rates during the day that decreased significantly with 3-3,4-dichlorophenyl-1',1'-dimethylurea, suggesting the potential importance of heterocystous cyanobacteria. There is also evidence of NA by sulfur-reducing bacteria in both microbial communities suggested by the negative effect exerted by the addition of sodium molybdate. Elimination of infrared and ultraviolet radiation had no effect on NA. Both microbial communities had nifH sequences that related to group I, including cyanobacteria and purple sulfur and nonsulfur bacteria, as well as group II nitrogenases, including sulfur reducing and green sulfur bacteria.

Chemotrophic filamentous microfossils from the Hollard Mound (Devonian, Morocco) as investigated by focused ion beam

The biologic origin of objects with microbe-like morphologies from the oldest preserved terrestrial sedimentary rocks remains a matter of controversy. Their biogenicity has been questioned, as well as the claim that they are convincing evidence of early life. Though minerals with microbe-like morphologies represent ambiguous evidence of life, they are, in a number of conditions, the only achievable information. In this study, the focused ion beam (FIB) electron microscopy technique was used for nano and micrometer-scale high-resolution imaging and in situ microsectioning of filamentous microfossils. The structural elements of these filaments, their spatial relationships with the host rock, and artifacts produced by alteration of the original morphology due to laboratory sample processing have been clearly defined. The in situ sectioning provided a means by which to investigate surface and subsurface microstructures and perform different analytical techniques on the same object, which minimizes sample destruction and avoids excessive manual handling and exposure of the specimen during analysis. Improvement in the morphological and compositional evaluation of the filaments has facilitated the development of a hypothesis regarding the metabolic pathway of the filamentous microfossils preserved in the Middle Devonian-aged Hollard Mound deposit, Anti-Atlas, Morocco. The results of this study demonstrate the potential of the FIB/SEM (scanning electron microscopy) system for detecting microbial-scale morphologies.

The carbon cycle on early Earth--and on Mars?

One of the goals of the present Martian exploration is to search for evidence of extinct (or even extant) life. This could be redefined as a search for carbon. The carbon cycle (or, more properly, cycles) on Earth is a complex interaction among three reservoirs: the atmosphere; the hydrosphere; and the lithosphere. Superimposed on this is the biosphere, and its presence influences the fixing and release of carbon in these reservoirs over different time-scales. The overall carbon balance is kept at equilibrium on the surface by a combination of tectonic processes (which bury carbon), volcanism (which releases it) and biology (which mediates it). In contrast to Earth, Mars presently has no active tectonic system; neither does it possess a significant biosphere. However, these observations might not necessarily have held in the past. By looking at how Earth's carbon cycles have changed with time, as both the Earth's tectonic structure and a more sophisticated biology have evolved, and also by constructing a carbon cycle for Mars based on the carbon chemistry of Martian meteorites, we investigate whether or not there is evidence for a Martian biosphere.

In situ infrared microspectroscopy of approximately 850 million-year-old prokaryotic fossils

In situ infrared (IR) and Raman microspectroscopy have been conducted on Neoproterozoic, organic-walled microfossils (prokaryotic fossils) in doubly polished, petrographic thin sections in order to detect their spectral signatures. The microfossils are very well preserved and occur in black chert from the approximately 850 million-year-old Bitter Springs Formation, Northern Territory, Australia. Raman microspectroscopy on two species of microfossils, one a filament and the other a coccoid, shows disordered peaks (D peak, 1340 cm(-1)) and graphite peaks (G peak, 1600 cm(-1)), indicating that they consist of disordered carbonaceous materials. IR micro-mapping results of the filament reveal that the distributions of peak heights at 2920 cm(-1) (aliphatic CH(2)), 1585 cm(-1) (aromatic C-C), and 1370 cm(-1) (aliphatic CH(3)) match the shape of the filamentous microfossil. These results suggest that IR microspectroscopy can be used for in situ characterization of organic polar signatures that morphologically indicate microfossils embedded in chert by using doubly-polished rock (petrographic) thin section samples. Further, these methods can be applied to controversial microfossil-like structures to test their biogenic nature.

Fossil evidence of Archaean life

Evidence for the existence of life during the Archaean segment of Earth history (more than 2500 Myr ago) is summarized. Data are presented for 48 Archaean deposits reported to contain biogenic stromatolites, for 14 such units reported to contain 40 morphotypes of putative microfossils, and for 13 especially ancient, 3200-3500 Myr old geologic units for which available organic geochemical data are also summarized. These compilations support the view that life's existence dates from more than or equal to 3500 Myr ago.

Photosynthesis in the Archean era

The earliest reductant for photosynthesis may have been H2. The carbon isotope composition measured in graphite from the 3.8-Ga Isua Supercrustal Belt in Greenland is attributed to H2-driven photosynthesis, rather than to oxygenic photosynthesis as there would have been no evolutionary pressure for oxygenic photosynthesis in the presence of H2. Anoxygenic photosynthesis may also be responsible for the filamentous mats found in the 3.4-Ga Buck Reef Chert in South Africa. Another early reductant was probably H2S. Eventually the supply of H2 in the atmosphere was likely to have been attenuated by the production of CH4 by methanogens, and the supply of H2S was likely to have been restricted to special environments near volcanos. Evaporites, possible stromatolites, and possible microfossils found in the 3.5-Ga Warrawoona Megasequence in Australia are attributed to sulfur-driven photosynthesis. Proteobacteria and protocyanobacteria are assumed to have evolved to use ferrous iron as reductant sometime around 3.0 Ga or earlier. This type of photosynthesis could have produced banded iron formations similar to those produced by oxygenic photosynthesis. Microfossils, stromatolites, and chemical biomarkers in Australia and South Africa show that cyanobacteria containing chlorophyll a and carrying out oxygenic photosynthesis appeared by 2.8 Ga, but the oxygen level in the atmosphere did not begin to increase until about 2.3 Ga.

The search for a topographic signature of life

Landscapes are shaped by the uplift, deformation and breakdown of bedrock and the erosion, transport and deposition of sediment. Life is important in all of these processes. Over short timescales, the impact of life is quite apparent: rock weathering, soil formation and erosion, slope stability and river dynamics are directly influenced by biotic processes that mediate chemical reactions, dilate soil, disrupt the ground surface and add strength with a weave of roots. Over geologic time, biotic effects are less obvious but equally important: biota affect climate, and climatic conditions dictate the mechanisms and rates of erosion that control topographic evolution. Apart from the obvious influence of humans, does the resulting landscape bear an unmistakable stamp of life? The influence of life on topography is a topic that has remained largely unexplored. Erosion laws that explicitly include biotic effects are needed to explore how intrinsically small-scale biotic processes can influence the form of entire landscapes, and to determine whether these processes create a distinctive topography.

Three-dimensional confocal optical imagery of precambrian microscopic organisms

A major difficulty that has long hindered studies of organic-walled Precambrian microbes in petrographic thin sections is the accurate documentation of their three-dimensional morphology. To address this need, we here demonstrate the use of confocal laser scanning microscopy. This technique, both non-intrusive and non-destructive, can provide data by which to objectively characterize, in situ and at submicron-scale resolution, the cellular and organismal morphology of permineralized (petrified) microorganisms. Application of this technique can provide information in three dimensions about the morphology, taphonomy, and fidelity of preservation of such fossils at a spatial resolution unavailable by any other means.

Raman imagery: a new approach to assess the geochemical maturity and biogenicity of permineralized precambrian fossils

Laser-Raman imagery is a non-intrusive, non-destructive analytical technique, recently introduced to Precambrian paleobiology, that can be used to demonstrate a one-to-one spatial correlation between the optically discernible morphology and kerogenous composition of permineralized fossil microorganisms. Made possible by the submicron-scale resolution of the technique and its high sensitivity to the Raman signal of carbonaceous matter, such analyses can be used to determine the chemical-structural characteristics of organic-walled microfossils and associated sapropelic carbonaceous matter in acid-resistant residues and petrographic thin sections. Here we use this technique to analyze kerogenous microscopic fossils and associated carbonaceous sapropel permineralized in 22 unmetamorphosed or little-metamorphosed fine-grained chert units ranging from approximately 400 to approximately 2,100 Ma old. The lineshapes of the Raman spectra acquired vary systematically with five indices of organic geochemical maturation: (1) the mineral-based metamorphic grade of the fossil-bearing units; (2) the fidelity of preservation of the fossils studied; (3) the color of the organic matter analyzed; and both the (4) H/C and (5) N/C ratios measured in particulate kerogens isolated from bulk samples of the fossil-bearing cherts. Deconvolution of relevant spectra shows that those of relatively well-preserved permineralized kerogens analyzed in situ exhibit a distinctive set of Raman bands that are identifiable also in hydrated organic-walled microfossils and particulate carbonaceous matter freed from the cherts by acid maceration. These distinctive Raman bands, however, become indeterminate upon dehydration of such specimens. To compare quantitatively the variations observed among the spectra measured, we introduce the Raman Index of Preservation, an approximate measure of the geochemical maturity of the kerogens studied that is consistent both with the five indices of organic geochemical alteration and with spectra acquired from fossils experimentally heated under controlled laboratory conditions. The results reported provide new insight into the chemical-structural characteristics of ancient carbonaceous matter, the physicochemical changes that accompany organic geochemical maturation, and a new criterion to be added to the suite of evidence by which to evaluate the origin of minute fossil-like objects of possible but uncertain biogenicity.

A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land

Background

The timescale of prokaryote evolution has been difficult to reconstruct because of a limited fossil record and complexities associated with molecular clocks and deep divergences. However, the relatively large number of genome sequences currently available has provided a better opportunity to control for potential biases such as horizontal gene transfer and rate differences among lineages. We assembled a data set of sequences from 32 proteins (~7600 amino acids) common to 72 species and estimated phylogenetic relationships and divergence times with a local clock method.

Results

Our phylogenetic results support most of the currently recognized higher-level groupings of prokaryotes. Of particular interest is a well-supported group of three major lineages of eubacteria (Actinobacteria, Deinococcus, and Cyanobacteria) that we call Terrabacteria and associate with an early colonization of land. Divergence time estimates for the major groups of eubacteria are between 2.5–3.2 billion years ago (Ga) while those for archaebacteria are mostly between 3.1–4.1 Ga. The time estimates suggest a Hadean origin of life (prior to 4.1 Ga), an early origin of methanogenesis (3.8–4.1 Ga), an origin of anaerobic methanotrophy after 3.1 Ga, an origin of phototrophy prior to 3.2 Ga, an early colonization of land 2.8–3.1 Ga, and an origin of aerobic methanotrophy 2.5–2.8 Ga.

Conclusions

Our early time estimates for methanogenesis support the consideration of methane, in addition to carbon dioxide, as a greenhouse gas responsible for the early warming of the Earths' surface. Our divergence times for the origin of anaerobic methanotrophy are compatible with highly depleted carbon isotopic values found in rocks dated 2.8–2.6 Ga. An early origin of phototrophy is consistent with the earliest bacterial mats and structures identified as stromatolites, but a 2.6 Ga origin of cyanobacteria suggests that those Archean structures, if biologically produced, were made by anoxygenic photosynthesizers. The resistance to desiccation of Terrabacteria and their elaboration of photoprotective compounds suggests that the common ancestor of this group inhabited land. If true, then oxygenic photosynthesis may owe its origin to terrestrial adaptations.