Estimates of the maximum time required to originate life

Fossils of the oldest microorganisms exist in 3.5 billion year old rocks and there is indirect evidence that life may have existed 3.8 billion years ago (3.8 Ga). Impacts able to destroy life or interrupt prebiotic chemistry may have occurred after 3.5 Ga. If large impactors vaporized the oceans, sterilized the planets, and interfered with the origination of life, life must have originated in the time interval between these impacts which increased with geologic time. Therefore, the maximum time required for the origination of life is the time that occurred between sterilizing impacts just before 3.8 Ga or 3.5 Ga, depending upon when life first appeared on Earth. If life first originated 3.5 Ga, and impacts with kinetic energies between 2×10³⁴ and 2×10³⁵ were able to vaporize the oceans, using the most probable impact flux, we find that the maximum time required to originate life would have been 67 to 133 million years (My). If life first originated 3.8 Ga, the maximum time to originate life was 2.5 to 11 My. Using a more conservative estimate for the flux of impacting objects before 3.8 Ga, we find a maximum time of 25 My for the same range of impactor kinetic energies. The impact model suggests that it is possible that life may have originated more than once.     

Martian paleolakes and waterways: Exobiological implications

The problems of how warm and wet Mars once was and when climate transitions may have occurred are not well understood. Mars may have had an early environment similar to Earth's that was conductive to the ermergence of life. In addition, increasing geologic evidence indicates that water, upon which terrestrial life depends, has been present on Mars throughout its history. This evidence suggests that life could have developed not only on early Mars but also over longer periods of time in longer lasting, more clement local environments. Indications of past or present life most likely would be found in areas where liquid water existed in sufficient quantities to provide for the needs of biological systems. We suggest that paleolakes may have provided such environments. Unlike the case on Earth, this record of the origin and evolution of life has probably not been erased by extensive deformation of the Martian surface. Our work has identified eleven prospective areas where large lacustrine basins may once have existed. These areas are important for future biological, geological, and climatological investigations.Presented at the International Symposium on The Biological Exploration of Mars, October 26–27, 1990, Tallahassee, FL, U.S.A.     

When did oxygenic photosynthesis evolve?

The atmosphere has apparently been oxygenated since the 'Great Oxidation Event' ca 2.4 Ga ago, but when the photosynthetic oxygen production began is debatable. However, geological and geochemical evidence from older sedimentary rocks indicates that oxygenic photosynthesis evolved well before this oxygenation event. Fluid-inclusion oils in ca 2.45 Ga sandstones contain hydrocarbon biomarkers evidently sourced from similarly ancient kerogen, preserved without subsequent contamination, and derived from organisms producing and requiring molecular oxygen. Mo and Re abundances and sulphur isotope systematics of slightly older (2.5 Ga) kerogenous shales record a transient pulse of atmospheric oxygen. As early as ca 2.7 Ga, stromatolites and biomarkers from evaporative lake sediments deficient in exogenous reducing power strongly imply that oxygen-producing cyanobacteria had already evolved. Even at ca 3.2 Ga, thick and widespread kerogenous shales are consistent with aerobic photoautrophic marine plankton, and U-Pb data from ca 3.8 Ga metasediments suggest that this metabolism could have arisen by the start of the geological record. Hence, the hypothesis that oxygenic photosynthesis evolved well before the atmosphere became permanently oxygenated seems well supported.     

Electrons, life and the evolution of Earth's oxygen cycle

The biogeochemical cycles of H, C, N, O and S are coupled via biologically catalysed electron transfer (redox) reactions. The metabolic processes responsible for maintaining these cycles evolved over the first ca 2.3 Ga of Earth's history in prokaryotes and, through a sequence of events, led to the production of oxygen via the photobiologically catalysed oxidation of water. However, geochemical evidence suggests that there was a delay of several hundred million years before oxygen accumulated in Earth's atmosphere related to changes in the burial efficiency of organic matter and fundamental alterations in the nitrogen cycle. In the latter case, the presence of free molecular oxygen allowed ammonium to be oxidized to nitrate and subsequently denitrified. The interaction between the oxygen and nitrogen cycles in particular led to a negative feedback, in which increased production of oxygen led to decreased fixed inorganic nitrogen in the oceans. This feedback, which is supported by isotopic analyses of fixed nitrogen in sedimentary rocks from the Late Archaean, continues to the present. However, once sufficient oxygen accumulated in Earth's atmosphere to allow nitrification to out-compete denitrification, a new stable electron 'market' emerged in which oxygenic photosynthesis and aerobic respiration ultimately spread via endosymbiotic events and massive lateral gene transfer to eukaryotic host cells, allowing the evolution of complex (i.e. animal) life forms. The resulting network of electron transfers led a gas composition of Earth's atmosphere that is far from thermodynamic equilibrium (i.e. it is an emergent property), yet is relatively stable on geological time scales. The early coevolution of the C, N and O cycles, and the resulting non-equilibrium gaseous by-products can be used as a guide to search for the presence of life on terrestrial planets outside of our Solar System.     

Palaeoclimates: the first two billion years

Earth's climate during the Archaean remains highly uncertain, as the relevant geologic evidence is sparse and occasionally contradictory. Oxygen isotopes in cherts suggest that between 3.5 and 3.2 Gyr ago (Ga) the Archaean climate was hot (55-85 degrees C); however, the fact that these cherts have experienced only a modest amount of weathering suggests that the climate was temperate, as today. The presence of diamictites in the Pongola Supergroup and the Witwatersrand Basin of South Africa suggests that by 2.9 Ga the climate was glacial. The Late Archaean was relatively warm; then glaciation (possibly of global extent) reappeared in the Early Palaeoproterozoic, around 2.3-2.4 Ga. Fitting these climatic constraints with a model requires high concentrations of atmospheric CO2 or CH4, or both. Solar luminosity was 20-25% lower than today, so elevated greenhouse gas concentrations were needed just to keep the mean surface temperature above freezing. A rise in O2 at approximately 2.4 Ga, and a concomitant decrease in CH4, provides a natural explanation for the Palaeoproterozoic glaciations. The Mid-Archaean glaciations may have been caused by a drawdown in H2 and CH4 caused by the origin of bacterial sulphate reduction. More work is needed to test this latter hypothesis.     

Atmospheric composition and climate on the early Earth

Oxygen isotope data from ancient sedimentary rocks appear to suggest that the early Earth was significantly warmer than today, with estimates of surface temperatures between 45 and 85 degrees C. We argue, following others, that this interpretation is incorrect-the same data can be explained via a change in isotopic composition of seawater with time. These changes in the isotopic composition could result from an increase in mean depth of the mid-ocean ridges caused by a decrease in geothermal heat flow with time. All this implies that the early Earth was warm, not hot.A more temperate early Earth is also easier to reconcile with the long-term glacial record. However, what triggered these early glaciations is still under debate. The Paleoproterozoic glaciations at approximately 2.4Ga were probably caused by the rise of atmospheric O2 and a concomitant decrease in greenhouse warming by CH4. Glaciation might have occurred in the Mid-Archaean as well, at approximately 2.9Ga, perhaps as a consequence of anti-greenhouse cooling by hydrocarbon haze. Both glaciations are linked to decreases in the magnitude of mass-independent sulphur isotope fractionation in ancient rocks. Studying both the oxygen and sulphur isotopic records has thus proved useful in probing the composition of the early atmosphere.     

Bolide impacts and the oxidation state of carbon in the Earth's early atmosphere

A one-dimensional photochemical model was used to examine the effect of bolide impacts on the oxidation state of Earth's primitive atmosphere. The impact rate should have been high prior to 3.8 Ga before present, based on evidence derived from the Moon. Impacts of comets or carbonaceous asteroids should have enhanced the atmospheric CO/CO₂ ratio by bringing in CO ice and/or organic carbon that can be oxidized to CO in the impact plume. Ordinary chondritic impactors would contain elemental iron that could have reacted with ambient CO₂ to give CO. Nitric oxide (NO) should also have been produced by reaction between ambient CO₂ and N₂ in the hot impact plumes. High NO concentrations increase the atmospheric CO/CO₂ ratio by increasing the rainout rate of oxidized gases. According to the model, atmospheric CO/CO₂ ratios of unity or greater are possible during the first several hundred million years of Earth's history, provided that dissolved CO was not rapidly oxidized to bicarbonate in the ocean. Specifically, high atmospheric CO/CO₂ ratios are possible if either: (1) the climate was cool (like today's climate), so that hydration of dissolved CO to formate was slow, or (2) the formate formed from CO was efficiently converted into volatile, reduced carbon compounds, such as methane. A high atmospheric CO/CO₂ ratio may have helped to facilitate prebiotic synthesis by enhancing the production rates of hydrogen cyanide and formaldehyde. Formaldehyde may have been produced even more efficiently by photochemical reduction of bicarbonate and formate in Fe⁺⁺-rich surface waters.     

Carbon isotopes support the presence of extensive land floras pre-dating the origin of vascular plants

Multiple lines of evidence indicate that Earth's land masses became green some 2.7 Ga ago, about 1 billion years after the advent of life. About 2.2 billion years later, land plants abruptly appear in the fossil record and diversify marking the onset of ecologically complex terrestrial communities that persist to the present day. Given this long history of land colonization, surprisingly few studies report direct fossil evidence of emergent vegetation prior to the continuous record of life on land that starts in the mid-Silurian (ca. 420–425 Ma ago). Here we compare stable carbon isotope signatures of fossils from seven Ordovician–Silurian (450–420 Ma old) Appalachian biotas with signatures of coeval marine organic matter and with stable carbon isotope values predicted for Ordovician and Silurian liverworts (BRYOCARB model). The comparisons support a terrestrial origin for fossils in six of the biotas analyzed, and indicate that some of the fossils represent bryophyte-grade plants. Our results demonstrate that extensive land floras pre-dated the advent of vascular plants by at least 25 Ma. The Appalachian fossils represent the oldest direct evidence of widespread colonization of continents. These findings provide a new search image for macrofossil assemblages that contain the earliest stages of land plant evolution. We anticipate they will fuel renewed efforts to search for direct fossil evidence to track the origin of land plants and eukaryotic life on continents further back in geologic time.     

Fermentative capacity of baker's yeast exposed to hyperbaric stress

Baker's yeast suspensions were incubated at different pressures (from 1 bar to 6 bar) and different gases [air, O(2) and a mixture of 8% (v/v) CO(2), 21% O(2) and N(2)]. Raising the air pressure from 1 bar to 6 bar stimulated cell growth but had no effect on leavening ability or viability of the cells. A 50% reduction of the CO(2) produced in dough occurred with 6 bar O(2) which also stopped growth. The fermentative capacity of the cells was stimulated by the cells exposure to increased CO(2) partial pressure up to 0.48 bar.     

When, where, and in what environment could the RNA world appear and evolve?

The environment necessary for the existence, amplification, and evolution of the RNA world, the difficulties of the abiogenous synthesis of RNA, and paradoxical situations with the stability of RNA, its functions, and the place of RNA in the geological history of the Earth are discussed. The chemical instability of the covalent structure of RNA in the aqueous medium is incompatible with the necessity of water for formation of its functionally active conformations (“water paradox”). The stable double-helical structure of RNA required for replication is incompatible with the stable compact conformations of single-stranded RNA molecules that are necessary for catalytic functions (conformational paradox). There was a very short time gap (or no gap at all) between the end of the massive meteorite bombardment of the Earth (3.9 Ga ago) and the appearance of the first evidence of cellular life (bacteria) in the Earth’s rocks (3.8–3.85 Ga ago or even earlier) (geological paradox). It is concluded that the RNA world could not appear, exist, or evolve into cellular forms of life on the Earth. This paper briefly discusses the possibility of an extraterrestrial origin of the RNA world and its extraterrestrial evolution with a subsequent distribution in space (mainly by comets) of the cellular form of life as more resistant to the environment as compared with free RNA.     

The terrestrial biota prior to the origin of land plants (embryophytes): a review of the evidence

It is often assumed that life originated and diversified in the oceans prior to colonizing the land. However, environmental constraints in chemical evolution models point towards critical steps leading to the origin of life as having occurred in subaerial settings. The earliest fossil record does not include finds from terrestrial deposits, so much of our understanding about the presence of a terrestrial microbial cover prior to the Proterozoic is based on inference and geochemical proxies that indicate biospheric carbon cycling during the Archaean. Our assessment is that by 2.7 Ga, microbial ecosystems in terrestrial settings were driven by oxygen‐generating, photosynthetic cyanobacteria. Studies of modern organisms indicate that both the origin and primary diversification of the eukaryotes could have occurred in terrestrial settings, shortly after 2.0 Ga, but there is no direct fossil evidence of terrestrial eukaryotes until about 1.1 Ga. At this time, it appears that the diversity of life in non‐marine habitats exceeded that found in marine settings where sulphidic seas may have impaired eukaryotic physiology and retarded evolution. Geochemical proxies indicate the establishment of an extensive soil‐forming microbial cover by 850 Ma, and it is possible that a rise in atmospheric oxygen at this time was due to the evolutionary expansion of green algae into terrestrial habitats. Direct fossil evidence of the earliest terrestrial biotas in the Phanerozoic consists of problematical palynomorphs from the Cambro‐Ordovician of Laurentia. These indicate that the evolution of the first land plants (embryophytes) during the Middle Ordovician took place within a landscape that included aeroterrestrial algae which were actively adapting to selection in subaerial settings.     

Seaweeds in cold seas: evolution and carbon acquisition

Much evidence suggests that life originated in hydrothermal habitats, and for much of the time since the origin of cyanobacteria (at least 2.5 Ga ago) and of eukaryotic algae (at least 2.1 Ga ago) the average sea surface and land surface temperatures were higher than they are today. However, there have been at least four significant glacial episodes prior to the Pleistocene glaciations. Two of these (approx. 2.1 and 0.7 Ga ago) may have involved a 'Snowball Earth' with a very great impact on the algae (sensu lato) of the time (cyanobacteria, Chlorophyta and Rhodophyta) and especially those that were adapted to warm habitats. By contrast, it is possible that heterokont, dinophyte and haptophyte phototrophs only evolved after the Carboniferous-Permian ice age (approx. 250 Ma ago) and so did not encounter low (相似文献   

Prebiotic Organic Microstructures

Micro- and sub-micrometer spheres, tubules and fiber-filament soft structures have been synthesized in our experiments conducted with 3?MeV proton irradiations of a mixture of simple inorganic constituents, CO, N(2) and H(2)O. We analysed the irradiation products, with scanning electron microscopy (SEM) and atomic force microscopy (AFM). These laboratory organic structures produced a wide variety of proteinaceous and non-proteinaceous amino acids after HCl hydrolysis. The enantiomer analysis for D,L-alanine confirmed that the amino acids were abiotically synthesized during the laboratory experiment. We discuss the presence of CO(2) and the production of H(2) during exothermic processes of serpentinization and consequently we discuss the production of hydrothermal CO in a ferromagnesian silicate mineral environment. We also discuss the low intensity of the Earth's magnetic field during the Paleoarchaean Era and consequently we conclude that excitation sources arising from cosmic radiation were much more abundant during this Era. We then show that our laboratory prebiotic microstructures might be synthesized during the Archaean Eon, as a product of the serpentinization process of the rocks and of their mineral contents.     

A molecular timeline for the origin of photosynthetic eukaryotes

The appearance of photosynthetic eukaryotes (algae and plants) dramatically altered the Earth's ecosystem, making possible all vertebrate life on land, including humans. Dating algal origin is, however, frustrated by a meager fossil record. We generated a plastid multi-gene phylogeny with Bayesian inference and then used maximum likelihood molecular clock methods to estimate algal divergence times. The plastid tree was used as a surrogate for algal host evolution because of recent phylogenetic evidence supporting the vertical ancestry of the plastid in the red, green, and glaucophyte algae. Nodes in the plastid tree were constrained with six reliable fossil dates and a maximum age of 3,500 MYA based on the earliest known eubacterial fossil. Our analyses support an ancient (late Paleoproterozoic) origin of photosynthetic eukaryotes with the primary endosymbiosis that gave rise to the first alga having occurred after the split of the Plantae (i.e., red, green, and glaucophyte algae plus land plants) from the opisthokonts sometime before 1,558 MYA. The split of the red and green algae is calculated to have occurred about 1,500 MYA, and the putative single red algal secondary endosymbiosis that gave rise to the plastid in the cryptophyte, haptophyte, and stramenopile algae (chromists) occurred about 1,300 MYA. These dates, which are consistent with fossil evidence for putative marine algae (i.e., acritarchs) from the early Mesoproterozoic (1,500 MYA) and with a major eukaryotic diversification in the very late Mesoproterozoic and Neoproterozoic, provide a molecular timeline for understanding algal evolution.     

Micro-scale sulphur isotope evidence for sulphur cycling in the late Archean shallow ocean

We report in situ secondary ion mass spectrometer sulphur isotope data for sedimentary pyrite from the 2.52 Ga Upper Campbellrand Subgroup, Transvaal, South Africa. The analysed sedimentary rocks represent a transition in depositional environment from very shallow to deeper water, with strong sedimentological, facies distribution and geochemical evidence for the presence of a shallow redox chemocline. Data were obtained directly in thin section in order to preserve petrographic context. They reveal a very large extent of isotopic fractionation both in mass‐independent (MIF) and in mass‐dependent fractionation (MDF) on unprecedentedly small scale. In the shallow‐water microbical carbonates, three types of pyrite were identified. The texturally oldest pyrite is found as small, isotopically little fractionated grains in the microbial mats. Large (several mm) spheroidal pyrite concretions, which postdate the mat pyrite, record strong evidence for an origin by bacterial sulphate reduction. Rare pyrite surrounding late fenestral calcite is inferred to have formed from recycled bacterial pyrite on account of the slope of its correlated MIF and MDF array. This latter type of pyrite was also found in an interbedded black shale and a carbonate laminite. In a deeper water chert, pyrite with very heavy sulphur indicates partial to almost complete sulphate reduction across a chemocline whose existence has been inferred independently. The combined picture from all the studied samples is that of a sulphate availability‐limited environment, in which sulphur was cycled between reservoirs according to changing redox conditions established across the chemocline. Cycling apparently reduced the extent of recorded sulphur isotope fractionation relative to what is expected from projection in the correlated MIF and MDF arrays. This is consistent with regionally relatively high free oxygen concentrations in the shallow water, permitting locally strong MDF. Our new observations add to the growing evidence for a complex, fluctuating evolution of free atmospheric oxygen between c. 2.7 Ga and 2.3 Ga.     

On the early evolutionary stage of the geosphere and biosphere and the problem of early glaciations

The early evolutionary stages of the geosphere and biosphere are determined by three interrelated factors: (1) continuous cooling of the surface and interior (mantle) of the Earth (the mean temperatures of the mantle and surface decreased by a factor of 1.5–2 and 3–4, respectively; the mean heat flow was reduced by approximately one order of magnitude, and viscosity, by three orders); (2) continuous stepwise oxidation of the surface, which was particularly well pronounced from 3.8 to 1.8 Ga; and (3) periodic and correlated fluctuations of conditions in the geosphere and biosphere of varying extent and nature. The major boundaries of this evolution were about 4 Ga (the origin of rather thick and heterogeneous earth’s crust, the origin of life); about 3 Ga (appearance of a strong magnetic field, an increase in photosynthetic activity); about 1.8–1.9 Ga (appearance of an oxidized atmosphere, the first supercontinent, possibly, the first superplumes from the nucleus); and about 0.75 Ga (acceleration of subduction, “watering” of the upper mantle, elevation of continents with vast land masses, shelves, large rivers, and the first great glaciations). The significance and correlations of the earliest events (before and about 4 Ga) and events about 750 Ma are widely debated. In the Late Archean and Early Proterozoic (before 1.8 Ga), the biosphere was dominated by cyanobacteria, the dynamics and developmental peaks of which are marked by the presence of widespread stromatolite buildups in carbonaceous rocks (initially, mostly dolomitic matter). About 700–750 Ma, intense and frequent glaciations developed, marking the cooling of the Earth. The greatest glaciation apparently occurred about 640 Ma, which gave rise to the discussion of the model of the Snowball Earth. The emergence and evolution of skeletons in animals is sometimes thought to be connected with glaciations. These events are correlated and accounted for by great endogenous changes. One of the major events in endogenous history is the onset about 750 Ma of periodic manifestation of mantle flows (superplumes), which explain further periodicity of the biosphere evolution. In conclusion, extrapolation of future evolution and successive collapse of biosphere segments in the course of transformation of the Sun into a red star and warming of the Earth surface are proposed.     

Ultraviolet Radiation and the Photobiology of Earth's Early Oceans

During the Archean era (3.9–2.5 Ga ago) the earth was dominatedby an oceanic lithosphere. Thus, understanding how life aroseand persisted in the Archean oceans constitutes a majorchallenge in understanding early life on earth. Using aradiative transfer model of the late Archean oceans, thephotobiological environment of the photic zone and the surfacemicrolayer is explored at the time before the formation of a significant ozonecolumn. DNA damage rates might have been approximately threeorders of magnitude higher in the surface layer of the Archeanoceans than on the present-day oceans, but at 30 m depth,damage may have been similar to the surface of the present-dayoceans. However at this depth the risk of being transported to surface waters in the mixed layer was high. The mixed layer mayhave been inhabited by a low diversity UV-resistant biota. Butit could have been numerically abundant. Repair capabilitiessimilar to Deinococcus radiodurans would be sufficient tosurvive in the mixed layer. Diversity may have beengreater in the region below the mixed layer and above the lightcompensation point corresponding to today's `deep chlorophyllmaximum'. During much of the Archean the air-water interface wasprobably an uninhabitable extreme environment for neuston. Thehabitability of some regions of the photic zone is consistentwith the evidence embodied in the geologic record, whichsuggests an oxygenated upper layer in the Archean oceans. Duringthe early Proterozoic, as ozone concentrations increased to acolumn abundance above 1 × 10¹⁷ cm^-2, UV stresswould have been reduced and possibly a greater diversity oforganisms could have inhabited the mixed layer. However,nutrient upwelling from newly emergent continental crusts mayhave been more significant in increasing total planktonic abundance inthe open oceans and coastal regions than photobiologicalfactors. The phohobiological environment of the Archean oceanshas implications for the potential cross-transfer of life betweenother water bodies of the early Solar System, possibly on earlyMars or the water bodies of a wet, early Venus.     

The effects of heavy meteorite bombardment on the early evolution — The emergence of the three Domains of life

A characteristic of many molecular phylogenies is that the three domains of life (Bacteria, Archaea, Eucarya) are clearly separated from each other. The analyses of ancient duplicated genes suggest that the last common ancestor of all presently known life forms already had been a sophisticated cellular prokaryote. These findings are in conflict with theories that have been proposed to explain the absence of deep branching lineages. In this paper we propose an alternative scenario, namely, a large meteorite impact that wiped out almost all life forms present on the early Earth. Following this nearly complete frustation of life on Earth, two surviving extreme thermophilic species gave rise to the now existing major groups of living organisms, the Bacteria and Archaea. [The latter also contributed the major portion to the nucleo-cytoplasmic component of the Eucarya]. An exact calibration of the molecular record with regard to time is not yet possible. The emergence of Eucarya in fossil and molecular records suggests that the proposed late impact should have occurred before 2100 million years before present (BP). If the 3500 million year old microfossils [Schopf, J. W. 1993: Science 260: 640–646] are interpreted as representatives of present day existing groups of bacteria (i.e., as cyanobacteria), then the impact is dated to around 3700 million years BP.The analysis of molecular sequences suggests that the separation between the Eucarya and the two prokaryotic domains is less deep then the separation between Bacteria and Archaea. The fundamental cell biological differences between Archaea and Eucarya were obtained over a comparatively short evolutionary distance (as measured in number of substitution events in biological macromolecules).Our interpretation of the molecular record suggests that life emerged early in Earth's history even before the time of the heavy bombardment was over. Early life forms already had colonized extreme habitats which allowed at least two prokaryotic species to survive a late nearly ocean boiling impact. The distribution of ecotypes on the rooted universal tree of life should not be interpreted as evidence that life originated in extremely hot environments.     

Ancient Fungal Life in North Pacific Eocene Oceanic Crust

We present evidence that eukaryotic life has existed in an extreme environment, inside the oceanic crust. Up to now only prokaryotes have been discovered within deep marine sediments and glass-rims of pillow basalts, no higher life forms are described as yet. This study demonstrates unique filamentous fossil structures observed within carbonate-filled vesicles of a massive lava flow unit from the upper oceanic crust in the North Pacific (ODP Site 1224). Based on morphological traits including branching, septa and central pores, the filaments are interpreted as fungi. The chemical composition of the fungal structures differs from the surrounding crystalline carbonate matrix in the deep basaltic rocks. Small open space between the fungi and the carbonate cement and undisturbed filamentous growth through different calcite crystals indicate endolithic fungal growth after the calcium carbonate filling. The presence of euhedral pyrite crystals within the carbonate cements points out anaerobic conditions in this habitat. Our results provide for the first time evidence for eukaryotic, fungal life in deep ocean basaltic rocks.     

Geological constraints on detecting the earliest life on Earth: a perspective from the Early Archaean (older than 3.7 Gyr) of southwest Greenland

At greater than 3.7 Gyr, Earth's oldest known supracrustal rocks, comprised dominantly of mafic igneous with less common sedimentary units including banded iron formation (BIF), are exposed in southwest Greenland. Regionally, they were intruded by younger tonalites, and then both were intensely dynamothermally metamorphosed to granulite facies (the highest pressures and temperatures generally encountered in the Earth's crust during metamorphism) in the Archaean and subsequently at lower grades until about 1500 Myr ago. Claims for the first preserved life on Earth have been based on the occurrence of greater than 3.8 Gyr isotopically light C occurring as graphite inclusions within apatite crystals from a 5 m thick purported BIF on the island of Akilia. Detailed geologic mapping and observations there indicate that the banding, first claimed to be depositional, is clearly deformational in origin. Furthermore, the mineralogy of the supposed BIF, being dominated by pyroxene, amphibole and quartz, is unlike well-known BIF from the Isua Greenstone Belt (IGB), but resembles enclosing mafic and ultramafic igneous rocks modified by metasomatism and repeated metamorphic recrystallization. This scenario parsimoniously links the geology, whole-rock geochemistry, 2.7 Gyr single crystal zircon ages in the unit, an approximately 1500 Myr age for apatites that lack any graphite, non-MIF sulphur isotopes in the unit and an inconclusive Fe isotope signature. Although both putative body fossils and carbon-12 enriched isotopes in graphite described at Isua are better explained by abiotic processes, more fruitful targets for examining the earliest stages in the emergence of life remain within greater than 3.7 Gyr IGB, which preserves BIF and other rocks that unambiguously formed at Earth's surface.