. (2016). Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, 537 (7621), 535-538. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures AbstractBiological activity is a major factor in Earth's chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth's evolution is when did life arise and impact hydrosphere-atmosphere-lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800-3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals1,2 from the Isua supracrustal belt (ISB), southwest Greenland3. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myrold Dresser Formation of the Pilbara Craton, Australia4,5. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth's sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life's origin in the Hadean eon (>4,000 Ma)6. Keywords emergence, life, shown, discovery, rapid, 3, structures, 700, million, year, old, microbial Disciplines Medicine and Health Sciences | Social and Behavioral Sciences Publication DetailsNutman, A. P., Bennett, V. C., Friend, C. R. L., Van Kranendonk, M. J. & Chivas, A. R. (2016). Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, 537 (7621), 535-538. 127Low deformation lacuna site 'C' preserves a 30 cm thick breccia lens in dolomite-rich 128 metasedimentary rocks is associated with dolostone (Fig. 2d) 139The barium-rich character of some layers of the dolomitic succession (up to 1 wt% BaO) now 140represented by halogen-rich barian phlogopite ± barian muscovite ± hyalophane (Extended and La anomalies and low total REE contents (Fig. 3, Extended Data Table 2 and Fig. 4). 149This REE+Y signature is preserved in even the most Ti-Al rich 'marly' layer (analysis A-2 Palaeoarchaean stromatolites (Fig. 1). 179(ii) The presence of originally low temperature dolomite, which requires microbial 180 activity for precipitation 24,25 .

High-precision Fe isotopic data for 104 samples, including modern and ancient (≥ 3.7 Ga) subduction-related magmas and mantle peridotites, are presented. These data demonstrate that mid-ocean ridge and oceanicisland basalts (MORBs and OIBs) have on average small, but distinctly (~+ 0.06‰) higher 56 Fe/ 54 Fe ratios than both modern and Eoarchean boninites and many island arc basalts (IABs) that are interpreted to form by large degrees of flux melting of depleted mantle sources. Additionally boninites and many IABs have iron isotopic compositions similar to chondrites, fertile mantle peridotites, Eoarchean mantle peridotites, and basalts from Mars and Vesta. The observed variations are best explained by the bulk silicate Earth having a near-chondritic iron isotopic composition, with~+ 0.3‰ equilibrium isotope fractionation between Fe 3+ and Fe 2+ and preferential extraction of isotopically heavier, incompatible Fe 3+ during mantle melting to form oceanic crust (as represented by MORBs and OIBs). A quantitative model that relates the iron isotopic composition of basaltic magmas to the degree of partial melting, Fe 3+ /Fe 2+ ratio, and buffering capacity of the mantle is presented. The concept that redox conditions may influence iron isotopic fractionation during melting provides a new approach for understanding the redox conditions of magma genesis on early Earth and Mars. Experimental and theoretical work is required to establish iron isotopic fractionation as an oxybarometer of mantle melting.

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