| Reference |
Nutman AP, Bennett VC, Friend CR, Van Kranendonk MJ, Chivas AR. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature. 2016 Sep 22 537(7621):535-538. doi: 10.1038/nature19355. abstract & p.535 left column bottom paragraph, right column 2nd paragraph & p.537 right column bottom paragraphPubMed ID27580034
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| Comments |
Abstract: "Biological 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 minerals from the Isua supracrustal belt (ISB), southwest Greenland. Here [investigators] 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 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)." P.537 right column bottom paragraph: "The recognition of ~3,700-Myr-old biogenic stromatolites within Isua dolomites indicates that near the start of the preserved sedimentary record, atmospheric CO2 was being sequestered by biological activity (ref 27). The complexity and setting of the Isua stromatolites points to sophistication in life systems at 3,700 Ma, similar to that displayed by 3,480–3,400-Myr-old Pilbara stromatolites (refs 4, 8, 12). This implies that by ~3,700 Ma life already had a considerable prehistory, and supports model organism chronology that life arose during the Hadean (>4,000 Ma) (ref 6). A shallow-water depositional environment is not necessary to conclude biogenicity, since deep-water microbialites and stromatolites are known. However, a shallow-water environment is supported by the associated sedimentary structures such as cross-lamination and tempestite breccias." |