Cryogenian evolution of stigmasteroid biosynthesis

Cryogenian Origins of Multicellularity in Archaeplastida

Earth was impacted by global glaciations during the Cryogenian (720 to 635 million years ago; Ma), events invoked to explain both the origins of multicellularity in Archaeplastida and radiation of the first land plants. However, the temporal relationship between these environmental and biological events is poorly established, due to a paucity of molecular and fossil data, precluding resolution of the phylogeny and timescale of archaeplastid evolution. We infer a time-calibrated phylogeny of early archaeplastid evolution based on a revised molecular dataset and reappraisal of the fossil record. Phylogenetic topology testing resolves deep archaeplastid relationships, identifying two clades of Viridiplantae and placing Bryopsidales as sister to the Chlorophyceae. Our molecular clock analysis infers an origin of Archaeplastida in the late-Paleoproterozoic to early-Mesoproterozoic (1712 to 1387 Ma). Ancestral state reconstruction of cytomorphological traits on this time-calibrated tree reveals many of the independent origins of multicellularity span the Cryogenian, consistent with the Cryogenian multicellularity hypothesis. Multicellular rhodophytes emerged 902 to 655 Ma while crown-Anydrophyta (Zygnematophyceae and Embryophyta) originated 796 to 671 Ma, broadly compatible with the Cryogenian plant terrestrialization hypothesis. Our analyses resolve the timetree of Archaeplastida with age estimates for ancestral multicellular archaeplastids coinciding with the Cryogenian, compatible with hypotheses that propose a role of Snowball Earth in plant evolution.

Lost world of complex life and the late rise of the eukaryotic crown

Eukaryotic life appears to have flourished surprisingly late in the history of our planet. This view is based on the low diversity of diagnostic eukaryotic fossils in marine sediments of mid-Proterozoic age (around 1,600 to 800 million years ago) and an absence of steranes, the molecular fossils of eukaryotic membrane sterols^1,2. This scarcity of eukaryotic remains is difficult to reconcile with molecular clocks that suggest that the last eukaryotic common ancestor (LECA) had already emerged between around 1,200 and more than 1,800 million years ago. LECA, in turn, must have been preceded by stem-group eukaryotic forms by several hundred million years³. Here we report the discovery of abundant protosteroids in sedimentary rocks of mid-Proterozoic age. These primordial compounds had previously remained unnoticed because their structures represent early intermediates of the modern sterol biosynthetic pathway, as predicted by Konrad Bloch⁴. The protosteroids reveal an ecologically prominent 'protosterol biota' that was widespread and abundant in aquatic environments from at least 1,640 to around 800 million years ago and that probably comprised ancient protosterol-producing bacteria and deep-branching stem-group eukaryotes. Modern eukaryotes started to appear in the Tonian period (1,000 to 720 million years ago), fuelled by the proliferation of red algae (rhodophytes) by around 800 million years ago. This 'Tonian transformation' emerges as one of the most profound ecological turning points in the Earth's history.

Eukaryogenesis and oxygen in Earth history

The endosymbiotic origin of mitochondria during eukaryogenesis has long been viewed as an adaptive response to the oxygenation of Earth's surface environment, presuming a fundamentally aerobic lifestyle for the free-living bacterial ancestors of mitochondria. This oxygen-centric view has been robustly challenged by recent advances in the Earth and life sciences. While the permanent oxygenation of the atmosphere above trace concentrations is now thought to have occurred 2.2 billion years ago, large parts of the deep ocean remained anoxic until less than 0.5 billion years ago. Neither fossils nor molecular clocks correlate the origin of mitochondria, or eukaryogenesis more broadly, to either of these planetary redox transitions. Instead, mitochondria-bearing eukaryotes are consistently dated to between these two oxygenation events, during an interval of pervasive deep-sea anoxia and variable surface-water oxygenation. The discovery and cultivation of the Asgard archaea has reinforced metabolic evidence that eukaryogenesis was initially mediated by syntrophic H₂ exchange between an archaeal host and an α-proteobacterial symbiont living under anoxia. Together, these results temporally, spatially and metabolically decouple the earliest stages of eukaryogenesis from the oxygen content of the surface ocean and atmosphere. Rather than reflecting the ancestral metabolic state, obligate aerobiosis in eukaryotes is most probably derived, having only become globally widespread over the past 1 billion years as atmospheric oxygen approached modern levels.

Laboratory exploration of mineral precipitates from Europa's subsurface ocean

The precipitation of hydrated phases from a chondrite-like Na-Mg-Ca-SO4-Cl solution is studied using in situ synchrotron X-ray powder diffraction, under rapid- (360 K h-1, T = 250-80 K, t = 3 h) and ultra-slow-freezing (0.3 K day-1, T = 273-245 K, t = 242 days) conditions. The precipitation sequence under slow cooling initially follows the predictions of equilibrium thermodynamics models. However, after ∼50 days at 245 K, the formation of the highly hydrated sulfate phase Na2Mg(SO4)2·16H2O, a relatively recent discovery in the Na2Mg(SO4)2-H2O system, was observed. Rapid freezing, on the other hand, produced an assemblage of multiple phases which formed within a very short timescale (≤4 min, ΔT = 2 K) and, although remaining present throughout, varied in their relative proportions with decreasing temperature. Mirabilite and meridianiite were the major phases, with pentahydrite, epsomite, hydrohalite, gypsum, blödite, konyaite and loweite also observed. Na2Mg(SO4)2·16H2O was again found to be present and increased in proportion relative to other phases as the temperature decreased. The results are discussed in relation to possible implications for life on Europa and application to other icy ocean worlds.

The Archean origin of oxygenic photosynthesis and extant cyanobacterial lineages

The record of the coevolution of oxygenic phototrophs and the environment is preserved in three forms: genomes of modern organisms, diverse geochemical signals of surface oxidation and diagnostic Proterozoic microfossils. When calibrated by fossils, genomic data form the basis of molecular clock analyses. However, different interpretations of the geochemical record, fossil calibrations and evolutionary models produce a wide range of age estimates that are often conflicting. Here, we show that multiple interpretations of the cyanobacterial fossil record are consistent with an Archean origin of crown-group Cyanobacteria. We further show that incorporating relative dating information from horizontal gene transfers greatly improves the precision of these age estimates, by both providing a novel empirical criterion for selecting evolutionary models, and increasing the stringency of sampling of posterior age estimates. Independent of any geochemical evidence or hypotheses, these results support oxygenic photosynthesis evolving at least several hundred million years before the Great Oxygenation Event (GOE), a rapid diversification of major cyanobacterial lineages around the time of the GOE, and a post-Cryogenian origin of extant marine picocyanobacterial diversity.

Evolution of bacterial steroid biosynthesis and its impact on eukaryogenesis

Steroids are one of three major lipid components of the eukaryotic cellular membrane, along with glycerophospolipids and sphingolipids. Steroids have critical roles in eukaryotic endocytosis and thus may have been structural prerequisites for the endocytic acquisition of mitochondria during eukaryogenesis. The evolutionary history of the eukaryotic cellular membrane is poorly understood and, as such, has limited our understanding of eukaryogenesis. We address the evolution of steroid biosynthesis by combining ancestral sequence reconstruction and phylogenetic analyses of steroid biosynthesis genes. Our results indicate that steroid biosynthesis evolved within bacteria in response to the rise of oxygen and was later horizontally transferred to eukaryotes. Membrane properties of early eukaryotes are inferred to have been different than that of modern eukaryotes.

Steroids are components of the eukaryotic cellular membrane and have indispensable roles in the process of eukaryotic endocytosis by regulating membrane fluidity and permeability. In particular, steroids may have been a structural prerequisite for the acquisition of mitochondria via endocytosis during eukaryogenesis. While eukaryotes are inferred to have evolved from an archaeal lineage, there is little similarity between the eukaryotic and archaeal cellular membranes. As such, the evolution of eukaryotic cellular membranes has limited our understanding of eukaryogenesis. Despite evolving from archaea, the eukaryotic cellular membrane is essentially a fatty acid bacterial-type membrane, which implies a substantial bacterial contribution to the evolution of the eukaryotic cellular membrane. Here, we address the evolution of steroid biosynthesis in eukaryotes by combining ancestral sequence reconstruction and comprehensive phylogenetic analyses of steroid biosynthesis genes. Contrary to the traditional assumption that eukaryotic steroid biosynthesis evolved within eukaryotes, most steroid biosynthesis genes are inferred to be derived from bacteria. In particular, aerobic deltaproteobacteria (myxobacteria) seem to have mediated the transfer of key genes for steroid biosynthesis to eukaryotes. Analyses of resurrected steroid biosynthesis enzymes suggest that the steroid biosynthesis pathway in early eukaryotes may have been similar to the pathway seen in modern plants and algae. These resurrected proteins also experimentally demonstrate that molecular oxygen was required to establish the modern eukaryotic cellular membrane during eukaryogenesis. Our study provides unique insight into relationships between early eukaryotes and other bacteria in addition to the well-known endosymbiosis with alphaproteobacteria.

Cryogenian Glacial Habitats as a Plant Terrestrialisation Cradle - The Origin of the Anydrophytes and Zygnematophyceae Split

For tens of millions of years (Ma), the terrestrial habitats of Snowball Earth during the Cryogenian period (between 720 and 635 Ma before present-Neoproterozoic Era) were possibly dominated by global snow and ice cover up to the equatorial sublimative desert. The most recent time-calibrated phylogenies calibrated not only on plants but on a comprehensive set of eukaryotes indicate that within the Streptophyta, multicellular charophytes (Phragmoplastophyta) evolved in the Mesoproterozoic to the early Neoproterozoic. At the same time, Cryogenian is the time of the likely origin of the common ancestor of Zygnematophyceae and Embryophyta and later, also of the Zygnematophyceae-Embryophyta split. This common ancestor is proposed to be called Anydrophyta; here, we use anydrophytes. Based on the combination of published phylogenomic studies and estimated diversification time comparisons, we deem it highly likely that anydrophytes evolved in response to Cryogenian cooling. Also, later in the Cryogenian, secondary simplification of multicellular anydrophytes and loss of flagella resulted in Zygnematophyceae diversification as an adaptation to the extended cold glacial environment. We propose that the Marinoan geochemically documented expansion of first terrestrial flora has been represented not only by Chlorophyta but also by Streptophyta, including the anydrophytes, and later by Zygnematophyceae, thriving on glacial surfaces until today. It is possible that multicellular early Embryophyta survived in less abundant (possibly relatively warmer) refugia, relying more on mineral substrates, allowing the retention of flagella-based sexuality. The loss of flagella and sexual reproduction by conjugation evolved in Zygnematophyceae and zygomycetous fungi during the Cryogenian in a remarkably convergent way. Thus, we support the concept that the important basal cellular adaptations to terrestrial environments were exapted in streptophyte algae for terrestrialization and propose that this was stimulated by the adaptation to glacial habitats dominating the Cryogenian Snowball Earth. Including the glacial lifestyle when considering the rise of land plants increases the parsimony of connecting different ecological, phylogenetic, and physiological puzzles of the journey from aquatic algae to terrestrial floras.

Geological alteration of Precambrian steroids mimics early animal signatures

The absence of unambiguous animal body fossils in rocks older than the late Ediacaran has rendered fossil lipids the most promising tracers of early organismic complexity. Yet much debate surrounds the various potential biological sources of putative metazoan steroids found in Precambrian rocks. Here we show that 26-methylated steranes-hydrocarbon structures currently attributed to the earliest animals-can form via geological alteration of common algal sterols, which carries important implications for palaeo-ecological interpretations and inhibits the use of such unconventional 'sponge' steranes for reconstructing early animal evolution.

Algal origin of sponge sterane biomarkers negates the oldest evidence for animals in the rock record

The earliest fossils of animal-like organisms occur in Ediacaran rocks that are approximately 571 million years old. Yet 24-isopropylcholestanes and other C₃₀ fossil sterol molecules have been suggested to reflect an important ecological role of demosponges as the first abundant animals by the end of the Cryogenian period (>635 million years ago). Here, we demonstrate that C₃₀ 24-isopropylcholestane is not diagnostic for sponges and probably formed in Neoproterozoic sediments through the geological methylation of C₂₉ sterols of chlorophyte algae, the dominant eukaryotes at that time. These findings reconcile biomarker evidence with the geological record and revert the oldest evidence for animals back into the latest Ediacaran.

Discovery of the oldest known biomarkers provides evidence for phototrophic bacteria in the 1.73 Ga Wollogorang Formation, Australia

The discovery of mid-Proterozoic (1.8-0.8 billion years ago, Ga) indigenous biomarkers is a challenge, since biologically informative molecules of such antiquity are commonly destroyed by metamorphism or overprinted by drilling fluids and other anthropogenic petroleum products. Previously, the oldest clearly indigenous biomarkers were reported from the 1.64 Ga Barney Creek Formation in the northern Australian McArthur Basin. In this study, we present the discovery of biomarker molecules from carbonaceous shales of the 1.73 Ga Wollogorang Formation in the southern McArthur Basin, extending the biomarker record back in time by ~90 million years. The extracted hydrocarbons illustrate typical mid-Proterozoic signatures with a large unresolved complex mixture, high methyl alkane/n-alkane ratios and the absence of eukaryotic steranes. Acyclic isoprenoids, saturated carotenoid derivatives, bacterial hopanes and aromatic hopanoids and steroids also were below detection limits. However, continuous homologous series of low molecular weight C₁₄ -C₁₉ 2,3,4- and 2,3,6-trimethyl aryl isoprenoids (AI) were identified, and C₂₀ -C₂₂ AI homologues were tentatively identified. Based on elevated abundances relative to abiogenic isomers, we interpret the 2,3,6-AI isomer series as biogenic molecules and the 2,3,4-AI series as possibly biogenic. The biological sources for the 2,3,6-AI series include carotenoids of cyanobacteria and/or green sulphur bacteria (Chlorobiaceae). The lower concentrated 2,3,4-AI series may be derived from purple sulphur bacteria (Chromatiaceae). These degradation products of carotenoids are the oldest known clearly indigenous molecules of likely biogenic origin.

Niche expansion for phototrophic sulfur bacteria at the Proterozoic-Phanerozoic transition

Carotenoid pigments afford valuable clues about the chemistry and biology of both modern and ancient aquatic environments. This study reveals that fossil aromatic carotenoids—long considered biomarkers for anoxygenic, phototrophic sulfur bacteria and their physiological requirement for hydrogen sulfide and illumination—can also be biosynthesized by oxygen-producing cyanobacteria. Cyanobacterial aromatic carotenoids, which are distinct in their chemical structures and occurrence patterns, are the most commonly encountered compounds in Proterozoic marine settings as well as in lakes from more recent eras. In contrast, carotenoids diagnostic for green sulfur bacteria of the family Chlorobiaceae became both prevalent and abundant in marine paleoenvironments beginning in the Phanerozoic Eon. This expansion occurs as marine sulfate inventories increased toward the end of the Proterozoic Eon.

Fossilized carotenoid hydrocarbons provide a window into the physiology and biochemistry of ancient microbial phototrophic communities for which only a sparse and incomplete fossil record exists. However, accurate interpretation of carotenoid-derived biomarkers requires detailed knowledge of the carotenoid inventories of contemporary phototrophs and their physiologies. Here we report two distinct patterns of fossilized C₄₀ diaromatic carotenoids. Phanerozoic marine settings show distributions of diaromatic hydrocarbons dominated by isorenieratane, a biomarker derived from low-light-adapted phototrophic green sulfur bacteria. In contrast, isorenieratane is only a minor constituent within Neoproterozoic marine sediments and Phanerozoic lacustrine paleoenvironments, for which the major compounds detected are renierapurpurane and renieratane, together with some novel C₃₉ and C₃₈ carotenoid degradation products. This latter pattern can be traced to cyanobacteria as shown by analyses of cultured taxa and laboratory simulations of sedimentary diagenesis. The cyanobacterial carotenoid synechoxanthin, and its immediate biosynthetic precursors, contain thermally labile, aromatic carboxylic-acid functional groups, which upon hydrogenation and mild heating yield mixtures of products that closely resemble those found in the Proterozoic fossil record. The Neoproterozoic–Phanerozoic transition in fossil carotenoid patterns likely reflects a step change in the surface sulfur inventory that afforded opportunities for the expansion of phototropic sulfur bacteria in marine ecosystems. Furthermore, this expansion might have also coincided with a major change in physiology. One possibility is that the green sulfur bacteria developed the capacity to oxidize sulfide fully to sulfate, an innovation which would have significantly increased their capacity for photosynthetic carbon fixation.

Ediacaran reorganization of the marine phosphorus cycle

The evolution of macroscopic animals in the latest Proterozoic Eon is associated with many changes in the geochemical environment, but the sequence of cause and effect remains a topic of intense research and debate. In this study, we use two apparently paradoxical observations—that massively phosphorus-rich rocks first appear at this time, and that the median P content of rocks does not change—to argue for a change in internal marine P cycling associated with rising sulfate levels. We argue that this change was self-sustaining, setting in motion a cascade of biogeochemical transformations that led to conditions favorable for major ecological and evolutionary change.

The Ediacaran Period (635 to 541 Ma) marks the global transition to a more productive biosphere, evidenced by increased availability of food and oxidants, the appearance of macroscopic animals, significant populations of eukaryotic phytoplankton, and the onset of massive phosphorite deposition. We propose this entire suite of changes results from an increase in the size of the deep-water marine phosphorus reservoir, associated with rising sulfate concentrations and increased remineralization of organic P by sulfate-reducing bacteria. Simple mass balance calculations, constrained by modern anoxic basins, suggest that deep-water phosphate concentrations may have increased by an order of magnitude without any increase in the rate of P input from the continents. Strikingly, despite a major shift in phosphorite deposition, a new compilation of the phosphorus content of Neoproterozoic and early Paleozoic shows little secular change in median values, supporting the view that changes in remineralization and not erosional P fluxes were the principal drivers of observed shifts in phosphorite accumulation. The trigger for these changes may have been transient Neoproterozoic weathering events whose biogeochemical consequences were sustained by a set of positive feedbacks, mediated by the oxygen and sulfur cycles, that led to permanent state change in biogeochemical cycling, primary production, and biological diversity by the end of the Ediacaran Period.

Free and kerogen-bound biomarkers from late Tonian sedimentary rocks record abundant eukaryotes in mid-Neoproterozoic marine communities

Lipid biomarker assemblages preserved within the bitumen and kerogen phases of sedimentary rocks from the ca. 780-729 Ma Chuar and Visingsö Groups facilitate paleoenvironmental reconstructions and reveal fundamental aspects of emerging mid-Neoproterozoic marine communities. The Chuar and Visingsö Groups were deposited offshore of two distinct paleocontinents (Laurentia and Baltica, respectively) during the Tonian Period, and the rock samples used had not undergone excessive metamorphism. The major polycyclic alkane biomarkers detected in the rock bitumens and kerogen hydropyrolysates consist of tricyclic terpanes, hopanes, methylhopanes, and steranes. Major features of the biomarker assemblages include detectable and significant contribution from eukaryotes, encompassing the first robust occurrences of kerogen-bound regular steranes from Tonian rocks, including 21-norcholestane, 27-norcholestane, cholestane, ergostane, and cryostane, along with a novel unidentified C₃₀ sterane series from our least thermally mature Chuar Group samples. Appreciable values for the sterane/hopane (S/H) ratio are found for both the free and kerogen-bound biomarker pools for both the Chuar Group rocks (S/H between 0.09 and 1.26) and the Visingsö Group samples (S/H between 0.03 and 0.37). The more organic-rich rock samples generally yield higher S/H ratios than for organic-lean substrates, which suggests a marine nutrient control on eukaryotic abundance relative to bacteria. A C₂₇ sterane (cholestane) predominance among total C₂₆ -C₃₀ steranes is a common feature found for all samples investigated, with lower amounts of C₂₈ steranes (ergostane and crysotane) also present. No traces of known ancient C₃₀ sterane compounds; including 24-isopropylcholestanes, 24-n-propylcholestanes, or 26-methylstigmastanes, are detectable in any of these pre-Sturtian rocks. These biomarker characteristics support the view that the Tonian Period was a key interval in the history of life on our planet since it marked the transition from a bacterially dominated marine biosphere to an ocean system which became progressively enriched with eukaryotes. The eukaryotic source organisms likely encompassed photosynthetic primary producers, marking a rise in red algae, and consumers in a revamped trophic structure predating the Sturtian glaciation.

Food sources for the Ediacara biota communities

The Ediacara biota represents the first complex macroscopic organisms in the geological record, foreshadowing the radiation of eumetazoan animals in the Cambrian explosion. However, little is known about the contingencies that lead to their emergence, including the possible roles of nutrient availability and the quality of food sources. Here we present information on primary producers in the Ediacaran based on biomarker molecules that were extracted from sediments hosting Ediacaran macrofossils. High relative abundances of algal steranes over bacterial hopanes suggest that the Ediacara biota inhabited nutrient replete environments with an abundance of algal food sources comparable to Phanerozoic ecosystems. Thus, organisms of the Ediacara biota inhabited nutrient-rich environments akin to those that later fuelled the Cambrian explosion.

A one-billion-year-old multicellular chlorophyte

Chlorophytes (representing a clade within the Viridiplantae and a sister group of the Streptophyta) probably dominated marine export bioproductivity and played a key role in facilitating ecosystem complexity before the Mesozoic diversification of phototrophic eukaryotes such as diatoms, coccolithophorans and dinoflagellates. Molecular clock and biomarker data indicate that chlorophytes diverged in the Mesoproterozoic or early Neoproterozoic, followed by their subsequent phylogenetic diversification, multicellular evolution and ecological expansion in the late Neoproterozoic and Palaeozoic. This model, however, has not been rigorously tested with palaeontological data because of the scarcity of Proterozoic chlorophyte fossils. Here we report abundant millimetre-sized, multicellular and morphologically differentiated macrofossils from rocks approximately 1,000 million years ago. These fossils are described as Proterocladus antiquus new species and are interpreted as benthic siphonocladalean chlorophytes, suggesting that chlorophytes acquired macroscopic size, multicellularity and cellular differentiation nearly a billion years ago, much earlier than previously thought.

Neoproterozoic origin and multiple transitions to macroscopic growth in green seaweeds

The Neoproterozoic Era records the transition from a largely bacterial to a predominantly eukaryotic phototrophic world, creating the foundation for the complex benthic ecosystems that have sustained Metazoa from the Ediacaran Period onward. This study focuses on the evolutionary origins of green seaweeds, which play an important ecological role in the benthos of modern sunlit oceans and likely played a crucial part in the evolution of early animals by structuring benthic habitats and providing novel niches. By applying a phylogenomic approach, we resolve deep relationships of the core Chlorophyta (Ulvophyceae or green seaweeds, and freshwater or terrestrial Chlorophyceae and Trebouxiophyceae) and unveil a rapid radiation of Chlorophyceae and the principal lineages of the Ulvophyceae late in the Neoproterozoic Era. Our time-calibrated tree points to an origin and early diversification of green seaweeds in the late Tonian and Cryogenian periods, an interval marked by two global glaciations with strong consequent changes in the amount of available marine benthic habitat. We hypothesize that unicellular and simple multicellular ancestors of green seaweeds survived these extreme climate events in isolated refugia, and diversified in benthic environments that became increasingly available as ice retreated. An increased supply of nutrients and biotic interactions, such as grazing pressure, likely triggered the independent evolution of macroscopic growth via different strategies, including true multicellularity, and multiple types of giant-celled forms.

A nematode sterol C4α-methyltransferase catalyzes a new methylation reaction responsible for sterol diversity

Primitive sterol evolution plays an important role in fossil record interpretation and offers potential therapeutic avenues for human disease resulting from nematode infections. Recognizing that C4-methyl stenol products [8(14)-lophenol] can be synthesized in bacteria while C4-methyl stanol products (dinosterol) can be synthesized in dinoflagellates and preserved as sterane biomarkers in ancient sedimentary rock is key to eukaryotic sterol evolution. In this regard, nematodes have been proposed to convert dietary cholesterol to 8(14)-lophenol by a secondary metabolism pathway that could involve sterol C4 methylation analogous to the C2 methylation of hopanoids (radicle-type mechanism) or C24 methylation of sterols (carbocation-type mechanism). Here, we characterized dichotomous cholesterol metabolic pathways in Caenorhabditis elegans that generate 3-oxo sterol intermediates in separate paths to lophanol (4-methyl stanol) and 8(14)-lophenol (4-methyl stenol). We uncovered alternate C3-sterol oxidation and Δ⁷ desaturation steps that regulate sterol flux from which branching metabolite networks arise, while lophanol/8(14)-lophenol formation is shown to be dependent on a sterol C4α-methyltransferse (4-SMT) that requires 3-oxo sterol substrates and catalyzes a newly discovered 3-keto-enol tautomerism mechanism linked to S-adenosyl-l-methionine-dependent methylation. Alignment-specific substrate-binding domains similarly conserved in 4-SMT and 24-SMT enzymes, despite minimal amino acid sequence identity, suggests divergence from a common, primordial ancestor in the evolution of methyl sterols. The combination of these results provides evolutionary leads to sterol diversity and points to cryptic C4-methyl steroidogenic pathways of targeted convergence that mediate lineage-specific adaptations.­­

Microbial assemblage and palaeoenvironmental reconstruction of the 1.38 Ga Velkerri Formation, McArthur Basin, northern Australia

The ca. 1.38 billion years (Ga) old Roper Group of the McArthur Basin, northern Australia, is one of the most extensive Proterozoic hydrocarbon-bearing units. Organic-rich black siltstones from the Velkerri Formation were deposited in a deep-water sequence and were analysed to determine their organic geochemical (biomarker) signatures, which were used to interpret the microbial diversity and palaeoenvironment of the Roper Seaway. The indigenous hydrocarbon biomarker assemblages describe a water column dominated by bacteria with large-scale heterotrophic reworking of the organic matter in the water column or bottom sediment. Possible evidence for microbial reworking includes a large unresolved complex mixture (UCM), high ratios of mid-chained and terminally branched monomethyl alkanes relative to n-alkanes-features characteristic of indigenous Proterozoic bitumen. Steranes, biomarkers for single-celled and multicellular eukaryotes, were below detection limits in all extracts analysed, despite eukaryotic microfossils having been previously identified in the Roper Group, albeit largely in organically lean shallower water facies. These data suggest that eukaryotes, while present in the Roper Seaway, were ecologically restricted and contributed little to export production. The 2,3,4- and 2,3,6-trimethyl aryl isoprenoids (TMAI) were absent or in very low concentration in the Velkerri Formation. The low abundance is primary and not caused by thermal destruction. The combination of increased dibenzothiophene in the Amungee Member of the Velkerri Formation and trace metal redox geochemistry suggests that degradation of carotenoids occurred during intermittent oxygen exposure at the sediment-water interface and/or the water column was rarely euxinic in the photic zone and likely only transiently euxinic at depth. A comparison of this work with recently published biomarker and trace elemental studies from other mid-Proterozoic basins demonstrates that microbial environments, water column geochemistry and basin redox were heterogeneous.

Bisnorgammacerane traces predatory pressure and the persistent rise of algal ecosystems after Snowball Earth

Eukaryotic algae rose to ecological relevance after the Neoproterozoic Snowball Earth glaciations, but the causes for this consequential evolutionary transition remain enigmatic. Cap carbonates were globally deposited directly after these glaciations, but they are usually organic barren or thermally overprinted. Here we show that uniquely-preserved cap dolostones of the Araras Group contain exceptional abundances of a newly identified biomarker: 25,28-bisnorgammacerane. Its secular occurrence, carbon isotope systematics and co-occurrence with other demethylated terpenoids suggest a mechanistic connection to extensive microbial degradation of ciliate-derived biomass in bacterially dominated ecosystems. Declining 25,28-bisnorgammacerane concentrations, and a parallel rise of steranes over hopanes, indicate the transition from a bacterial to eukaryotic dominated ecosystem after the Marinoan deglaciation. Nutrient levels already increased during the Cryogenian and were a prerequisite, but not the ultimate driver for the algal rise. Intense predatory pressure by bacterivorous protists may have irrevocably cleared self-sustaining cyanobacterial ecosystems, thereby creating the ecological opportunity that allowed for the persistent rise of eukaryotic algae to global importance.

Hydrocarbon biomarkers preserved in carbonate veins of potentially Paleoproterozoic age, and implications for the early biosphere

Research on the early rise of oxygenic photosynthesis and eukaryotes has recently encountered a major pitfall, as some hopane and sterane biomarkers reported in Archaean rocks are the results of contamination. Following an extensive petrological framework in the Pilbara Craton, Western Australia, oil-bearing fluid inclusions and solid bitumens were identified in replacement and hydrothermal carbonate veins cross-cutting Archaean metasedimentary rocks. The 2.55-2.63 billion years old metasedimentary rocks were found to be depleted of indigenous biomarkers. Here we show novel biomarker results from the solvent extraction of the carbonate veins. Volcanic rock blanks, outside rinses, and instrumental blanks showed no biomarkers, and the surrounding rocks were metamorphosed to a sufficiently high extent to not yield any biomarkers, but the biomarkers found in the veins are most likely indigenous. Biomarkers detected include C_21-22 ααα- and αββ-steranes (pregnanes), C_27-29 αββ-steranes, C_19-26 tricyclic terpanes, C_29-30,34 αβ-hopanes, C₃₀ 17α-diahopane, and trisnorhopanes, which are in the range 2-180 pg/g. The extracted organic matter is highly mature, based on the biomarker configurations and calculated vitrinite reflectance that ranges from 2.4-3.0 (methylphenanthrene index), 1.4-1.9 (methyladamantane index), and 1.4-2.3 (methyldiamantane index). As the biomarkers are highly mature and the biomarker assemblages have a distinctive pattern to each vein type the likelihood of sample contamination by recent, less mature, biomarkers from a different assemblage is unlikely. The detection of steranes suggests that molecular oxygen was available when the veins were formed, possibly between 2.2 and 1.8 billion years ago, but no evidence for oxygenic photosynthesis in the form of cyanobacterial biomarkers has been found. Carbonate minerals that seem to better preserve biomarkers, such as concretions or veins, show the growing importance of new and exciting opportunities to seek biomarkers in the early Earth rock record, and potentially on other planets. Our results demonstrate for that first time that biomarkers can be found in veins cutting through highly metamorphosed Archaean rocks, and gives an insight into ancient environments.

The slow rise of complex life as revealed through biomarker genetics

Organic molecules preserved in ancient rocks can function as 'biomarkers', providing a unique window into the evolution of life. While biomarkers demonstrate intriguing patterns through the Neoproterozoic, it can be difficult to constrain particular biomarkers to specific organisms. The goal of the present paper is to demonstrate the utility of biomarkers when we focus less on which organisms produce them, and more on how their underlying genetic pathways evolved. Using this approach, it becomes clear that there are discrepancies between the biomarker, fossil, and molecular records. However, these discrepancies probably represent long time periods between the diversification of eukaryotic groups through the Neoproterozoic and their eventual rise to ecological significance. This 'long fuse' hypothesis contrasts with the adaptive radiations often associated with the development of complex life.

1.1-billion-year-old porphyrins establish a marine ecosystem dominated by bacterial primary producers

The average cell size of marine phytoplankton is critical for the flow of energy and nutrients from the base of the food web to higher trophic levels. Thus, the evolutionary succession of primary producers through Earth's history is important for our understanding of the radiation of modern protists ∼800 million years ago and the emergence of eumetazoan animals ∼200 million years later. Currently, it is difficult to establish connections between primary production and the proliferation of large and complex organisms because the mid-Proterozoic (∼1,800-800 million years ago) rock record is nearly devoid of recognizable phytoplankton fossils. We report the discovery of intact porphyrins, the molecular fossils of chlorophylls, from 1,100-million-year-old marine black shales of the Taoudeni Basin (Mauritania), 600 million years older than previous findings. The porphyrin nitrogen isotopes (δ¹⁵N_por = 5.6-10.2‰) are heavier than in younger sedimentary sequences, and the isotopic offset between sedimentary bulk nitrogen and porphyrins (ε_por = -5.1 to -0.5‰) points to cyanobacteria as dominant primary producers. Based on fossil carotenoids, anoxygenic green (Chlorobiacea) and purple sulfur bacteria (Chromatiaceae) also contributed to photosynthate. The low ε_por values, in combination with a lack of diagnostic eukaryotic steranes in the time interval of 1,600-1,000 million years ago, demonstrate that algae played an insignificant role in mid-Proterozoic oceans. The paucity of algae and the small cell size of bacterial phytoplankton may have curtailed the flow of energy to higher trophic levels, potentially contributing to a diminished evolutionary pace toward complex eukaryotic ecosystems and large and active organisms.

Tracking the rise of eukaryotes to ecological dominance with zinc isotopes

The biogeochemical cycling of zinc (Zn) is intimately coupled with organic carbon in the ocean. Based on an extensive new sedimentary Zn isotope record across Earth's history, we provide evidence for a fundamental shift in the marine Zn cycle ~800 million years ago. We discuss a wide range of potential drivers for this transition and propose that, within available constraints, a restructuring of marine ecosystems is the most parsimonious explanation for this shift. Using a global isotope mass balance approach, we show that a change in the organic Zn/C ratio is required to account for observed Zn isotope trends through time. Given the higher affinity of eukaryotes for Zn relative to prokaryotes, we suggest that a shift toward a more eukaryote-rich ecosystem could have provided a means of more efficiently sequestering organic-derived Zn. Despite the much earlier appearance of eukaryotes in the microfossil record (~1700 to 1600 million years ago), our data suggest a delayed rise to ecological prominence during the Neoproterozoic, consistent with the currently accepted organic biomarker records.

The transition from a cyanobacterial to algal world and the emergence of animals

The Neoproterozoic, 1000-541 million years (Myr) ago, saw the transition from a largely bacterial world to the emergence of multicellular grazers, suspension feeders and predators. This article explores the hypothesis that the first appearance of large, multicellular heterotrophs was fueled by an elevated supply of nutrients and carbon from the bottom of the food chain to higher trophic levels. A refined record of molecular fossils of algal sterols reveals that the transition from dominantly bacterial to eukaryotic primary production in open marine habitat occurred between 659 and 645 Myr ago, in the hot interlude between two Snowball Earth glaciations. This bacterial-eukaryotic transition reveals three characteristics: it was rapid on geological timescales, it followed an extreme environmental catastrophe and it was permanent - hallmarks of an ecological hysteresis that shifted Earth's oceans between two self-stabilizing steady states. More than 50 million years of Snowball glaciations and their hot aftermath may have purged old-world bacterial phytoplankton, providing empty but nutrient-rich ecospace for recolonization by larger algae and transforming the base of the food web. Elevated average and maximum particle sizes at the base of the food chain may have provided more efficient energy and nutrient transfer to higher trophic levels, fueling an arms race toward larger grazers, predators and prey, and the development of increasingly complex feeding and defense strategies.