Molecular fossils of biological lipids are preserved in 2700-million-year-old shales from the Pilbara Craton, Australia. Sequential extraction of adjacent samples shows that these hydrocarbon biomarkers are indigenous and syngenetic to the Archean shales, greatly extending the known geological range of such molecules. The presence of abundant 2α-methylhopanes, which are characteristic of cyanobacteria, indicates that oxygenic photosynthesis evolved well before the atmosphere became oxidizing. The presence of steranes, particularly cholestane and its 28- to 30-carbon analogs, provides persuasive evidence for the existence of eukaryotes 500 million to 1 billion years before the extant fossil record indicates that the lineage arose.

The disappearance of iron formations from the geological record approximately 1.8 billion years (Gyr) ago was the consequence of rising oxygen levels in the atmosphere starting 2.45-2.32 Gyr ago. It marks the end of a 2.5-Gyr period dominated by anoxic and iron-rich deep oceans. However, despite rising oxygen levels and a concomitant increase in marine sulphate concentration, related to enhanced sulphide oxidation during continental weathering, the chemistry of the oceans in the following mid-Proterozoic interval (approximately 1.8-0.8 Gyr ago) probably did not yet resemble our oxygen-rich modern oceans. Recent data indicate that marine oxygen and sulphate concentrations may have remained well below current levels during this period, with one model indicating that anoxic and sulphidic marine basins were widespread, and perhaps even globally distributed. Here we present hydrocarbon biomarkers (molecular fossils) from a 1.64-Gyr-old basin in northern Australia, revealing the ecological structure of mid-Proterozoic marine communities. The biomarkers signify a marine basin with anoxic, sulphidic, sulphate-poor and permanently stratified deep waters, hostile to eukaryotic algae. Phototrophic purple sulphur bacteria (Chromatiaceae) were detected in the geological record based on the new carotenoid biomarker okenane, and they seem to have co-existed with communities of green sulphur bacteria (Chlorobiaceae). Collectively, the biomarkers support mounting evidence for a long-lasting Proterozoic world in which oxygen levels remained well below modern levels.

The transition from dominant bacterial to eukaryotic marine primary productivity was one of the most profound ecological revolutions in the Earth's history, reorganizing the distribution of carbon and nutrients in the water column and increasing energy flow to higher trophic levels. But the causes and geological timing of this transition, as well as possible links with rising atmospheric oxygen levels and the evolution of animals, remain obscure. Here we present a molecular fossil record of eukaryotic steroids demonstrating that bacteria were the only notable primary producers in the oceans before the Cryogenian period (720-635 million years ago). Increasing steroid diversity and abundance marks the rapid rise of marine planktonic algae (Archaeplastida) in the narrow time interval between the Sturtian and Marinoan 'snowball Earth' glaciations, 659-645 million years ago. We propose that the incumbency of cyanobacteria was broken by a surge of nutrients supplied by the Sturtian deglaciation. The 'Rise of Algae' created food webs with more efficient nutrient and energy transfers, driving ecosystems towards larger and increasingly complex organisms. This effect is recorded by the concomitant appearance of biomarkers for sponges and predatory rhizarians, and the subsequent radiation of eumetazoans in the Ediacaran period.

This study describes reconstruction of two highly unusual archaeal genomes by de novo metagenomic assembly of multiple, deeply sequenced libraries from surface waters of Lake Tyrrell (LT), a hypersaline lake in NW Victoria, Australia. Lineage-specific probes were designed using the assembled genomes to visualize these novel archaea, which were highly abundant in the 0.1-0.8 lm size fraction of lake water samples. Gene content and inferred metabolic capabilities were highly dissimilar to all previously identified hypersaline microbial species. Distinctive characteristics included unique amino acid composition, absence of Gvp gas vesicle proteins, atypical archaeal metabolic pathways and unusually small cell size (approximately 0.6 lm diameter). Multi-locus phylogenetic analyses demonstrated that these organisms belong to a new major euryarchaeal lineage, distantly related to halophilic archaea of class Halobacteria. Consistent with these findings, we propose creation of a new archaeal class, provisionally named 'Nanohaloarchaea'. In addition to their high abundance in LT surface waters, we report the prevalence of Nanohaloarchaea in other hypersaline environments worldwide. The simultaneous discovery and genome sequencing of a novel yet ubiquitous lineage of uncultivated microorganisms demonstrates that even historically well-characterized environments can reveal unexpected diversity when analyzed by metagenomics, and advances our understanding of the ecology of hypersaline environments and the evolutionary history of the archaea.

We review recent observations and models concerning the dynamics of Cryogenian global glaciation and their biological consequences.

The evolution of oxygenic photosynthesis had a profound impact on the Earth's surface chemistry, leading to a sharp rise in atmospheric oxygen between 2.45 and 2.32 billion years (Gyr) ago and the onset of extreme ice ages. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from approximately 2.7-Gyr-old shales in the Pilbara Craton, Australia, which contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10-20 per thousand) of (13)C relative to bulk sedimentary organic matter. Here we present micrometre-scale, in situ (13)C/(12)C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that originally yielded biomarkers. Our results show that both kerogen and pyrobitumen are strongly depleted in (13)C, indicating that indigenous petroleum is 10-20 per thousand lighter than the extracted hydrocarbons. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism approximately 2.2 Gyr ago and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78-1.68 Gyr ago and approximately 2.15 Gyr ago, respectively. Our results eliminate the evidence for oxygenic photosynthesis approximately 2.7 Gyr ago and exclude previous biomarker evidence for a long delay (approximately 300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45-2.32 Gyr ago.

Sterol biosynthesis is viewed primarily as a eukaryotic process, and the frequency of its occurrence in bacteria has long been a subject of controversy. Two enzymes, squalene monooxygenase and oxidosqualene cyclase, are the minimum necessary for initial biosynthesis of sterols from squalene. In this work, 19 protein gene sequences for eukaryotic squalene monooxygenase and 12 protein gene sequences for eukaryotic oxidosqualene cyclase were compared with all available complete and partial prokaryotic genomes. The only unequivocal matches for a sterol biosynthetic pathway were in the proteobacterium, Methylococcus capsulatus, in which sterol biosynthesis is known, and in the planctomycete, Gemmata obscuriglobus. The latter species contains the most abbreviated sterol pathway yet identified in any organism. Analysis shows that the major sterols in Gemmata are lanosterol and its uncommon isomer, parkeol. There are no subsequent modifications of these products. In bacteria, the sterol biosynthesis genes occupy a contiguous coding region and possibly comprise a single operon. Phylogenetic trees constructed for both enzymes show that the sterol pathway in bacteria and eukaryotes has a common ancestry. It is likely that this contiguous reading frame was exchanged between bacteria and early eukaryotes via lateral gene transfer or endosymbiotic events. The primitive sterols produced by Gemmata suggest that this genus could retain the most ancient remnants of the sterol biosynthetic pathway. S terol biosynthesis is nearly ubiquitous among eukaryotes; conversely, it is almost completely absent in prokaryotes (1). As a result, the presence of diverse steranes in ancient rocks is used as evidence for eukaryotic evolution Ͼ2.7 billion years ago (2). However, the occasional presence of sterols in prokaryotes is poorly understood. Sterol production by bacteria previously has been demonstrated only in the Methylococcales (3, 4) and Myxobacteriales (5,6).Understanding the evolution of sterol biosynthesis is of significant interest to biochemistry, evolutionary biology, and the geosciences, because the only known biosynthetic pathway requires molecular oxygen. The first step in this pathway is the epoxidation of the hydrocarbon squalene, in which the addition of 1 ⁄2O 2 is catalyzed by the enzyme squalene monooxygenase (SQMO) (7). Unless there are other unknown enzymes or abiogenic reactions capable of producing squalene epoxide, this would require the prior evolution of oxygenic photosynthesis. For sterol biosynthesis to date to the last common ancestor, a biogenic or abiogenic peroxidation could be a potential mechanism, although this has not yet been demonstrated.Cyclization of squalene epoxide to form the initial sterol proceeds immediately through the action of a second enzyme, oxidosqualene cyclase (OSC). It is believed that OSC evolved from the hopanoid pathway predecessor, bacterial squalenehopene cyclase (SHC) (8, 9). In eukaryotes, the initial sterols lanosterol and cycloartenol are merely biosynthetic intermediates; i.e.,...

Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories.oxygenic photosynthesis | eukaryotes | cyanobacteria | Great Oxidation Event | Pilbara