DOI: 10.1126/science.285.5430.1033 , 1033 (1999); 285Science et al.Jochen J. Brocks,EukaryotesArchean Molecular Fossils and the Early Rise of www.sciencemag.org (this information is current as of September 1, 2009 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/285/5430/1033version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/285/5430/1033#otherarticles, 6 of which can be accessed for free: cites 12 articlesThis article 367 article(s) on the ISI Web of Science. cited byThis article has been http://www.sciencemag.org/cgi/content/full/285/5430/1033#otherarticles 72 articles hosted by HighWire Press; see: cited byThis article has been http://www.sciencemag.org/cgi/collection/paleoPaleontology : subject collectionsThis article appears in the following http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. is aScience1999 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on September 1, 2009 www.sciencemag.orgDownloaded fromArchean Molecular Fossils andthe Early Rise of EukaryotesJochen J. Brocks,1,2* Graham A. Logan,2Roger Buick,1Roger E. Summons2Molecular fossils of biological lipids are preserved in 2700-million-year-oldshales from the Pilbara Craton, Australia. Sequential extraction of adjacentsamples shows that these hydrocarbon biomarkers are indigenous and synge-netic to the Archean shales, greatly extending the known geological range ofsuch molecules. The presence of abundant 2␣-methylhopanes, which are char-acteristic of cyanobacteria, indicates that oxygenic photosynthesis evolved wellbefore the atmosphere became oxidizing. The presence of steranes, particularlycholestane and its 28- to 30-carbon analogs, provides persuasive evidence forthe existence of eukaryotes 500 million to 1 billion years before the extant fossilrecord indicates that the lineage arose.Microfossils (1), stromatolites (2), and sedi-mentary carbon isotope ratios (3) all indicatethat microbial organisms inhabited the oceansin Archean times [⬎2500 million years ago(Ma)]. But these lines of evidence are not veryinformative about what these microbes were orhow they lived. Potentially, a better insight intoprimordial biological diversity can be obtainedfrom molecular fossils derived from cellularand membrane lipids (“biomarkers”). Althoughsuch soluble hydrocarbons were first extractedfrom Archean rocks more than 30 years ago,their significance was generally discounted af-ter amino acids of recent origin were found inthe same rocks (4). Prevailing models of ther-mal maturation dictated that complex hydrocar-bons should not survive the metamorphism ex-perienced by all Archean terrains. However,indications of greater hydrocarbon stability (5)and observations of oil in Archean fluid inclu-sions (6) suggest that these maturation modelsare unduly pessimistic and that biomarkerscould indeed be preserved in low-grade Arche-an metasedimentary rocks. Furthermore, sys-tematic sampling strategies, improved analyti-cal techniques, and greater geochemical knowl-edge (7) should make their recognition easierand their interpretation more rigorous. We nowreport molecular fossils in late Archean shalesthat have suffered only minimal metamor-phism. These molecular fossils reveal that theArchean biota was considerably more complexthan currently recognized and that the domainsEucarya and Bacteria were already extant.Samples came from depths of around700 m in diamond drill core WRL#1, collarednear Wittenoom in the Pilbara Craton of north-western Australia (8). They represent the⬃2600-Ma Marra Mamba Formation (9)(lowermost Hamersley Group), the underly-ing Roy Hill and Warrie Members of the⬃2690-Ma Jeerinah Formation (10), and the⬃2715-Ma Maddina Formation (FortescueGroup) (9), all of which have only beenmetamorphosed to prehnite-pumpellyite fa-cies in this area (11)(⬃200° to ⬃300°C).The closely spaced sampling of differentlithologies allowed comparison of rocks withvarying compositions, porosities, and kero-gen contents but with identical postdeposi-tional histories. Most analyses were of finelylaminated kerogenous shales from the RoyHill Shale and Marra Mamba Formation thatwere deposited in a marine continental-slopeenvironment below storm-wave base undersuboxic conditions (12). Those from theMarra Mamba Formation were interbeddedwith oxide-facies banded iron formation.Black chert from the Warrie Member, quartzsandstone and vein dolomite from the RoyHill Member, and terrestrial basalt from theMaddina Formation served as controls forlaboratory procedures.Although these rocks are well-preservedby Archean standards, they are nevertheless1School of Geosciences, University of Sydney, Sydney,NSW 2006, Australia.2Australian Geological SurveyOrganisation (AGSO), Canberra, ACT 2601, Australia.*To whom correspondence should be addressed. E-mail: [email protected] or [email protected]. 1. Drill core WRL#1 showingthe gas chromatograms of extract-able hydrocarbons at differentdepths (in meters). The kerogen-poor samples on the left (whitesquares on drill core) yielded lowquantities of extractable organicmatter, whereas the kerogen-richshales on the right (black squareson drill core) yielded relativelyhigh concentrations.R ESEARCH A RTICLEwww.sciencemag.org SCIENCE VOL 285 13 AUGUST 1999 1033 on September 1, 2009 www.sciencemag.orgDownloaded fromhighly mature and therefore contain onlysmall quantities of extractable hydrocarbons.Hence, they are especially prone to moderncontamination by petroleum products duringdrilling, storage, and analysis. Accordingly,each sample underwent three solvent rinses toensure that extracted bitumen was intimatelyassociated with the rock and not superficial orlining cracks. After each rinse was analyzedchromatographically, the rock was brokeninto smaller fragments to expose fresh bed-ding surfaces, fractures, and fissures. If thelast rinse proved clean, the sample was