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Evidence of Earlier Life on Earth Disputed:


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The disputed article......https://phys.org/news/2016-09-oldest-fossils-life-young-earth.html

Oldest fossils point to thriving life on young Earth September 1, 2016 

Australian researchers have found the world's oldest fossils, revealing that diverse life forms thrived on Earth 3.7 billion years ago.

 

Co-lead investigator Associate Professor Vickie Bennett from The Australian National University (ANU) said the research on stromatolite fossils found in Greenland provided a greater understanding of early habitats of life on Earth and could have implications for searching for life on Mars.

"This discovery turns the study of planetary habitability on its head," said Dr Bennett from ANU Research School of Earth Sciences.

"For the first time we have rocks that we know record the conditions and environments that sustained early life. Our research will provide new insights into chemical cycles and rock-water-microbe interactions on a young planet."



Read more at: https://phys.org/news/2016-09-oldest-fossils-life-young-earth.html#jCp

the paper:

https://www.nature.com/articles/nature19355

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures:

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 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-Myr-old 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.

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The Disputed Findings:

https://phys.org/news/2018-10-evidence-earliest-life-earth-disputed.html

Evidence of earliest life on Earth disputed

October 17, 2018:

When Australian scientists presented evidence in 2016 of life on Earth 3.7 billon years ago—pushing the record back 220 million years—it was a big deal, influencing even the search for life on Mars.

But that discovery, based on an analysis of primordial rocks in Greenland, has now been challenged, with another team of researchers arguing in a study published Wednesday that the structures presented as proof of microbial activity were, in fact, geologically forged by underground heat and pressure.

The truth hinges on whether the cone-shaped formations in question are genuine stromatolites, layered structures left in the wake of water-dwelling microorganisms.



Read more at: https://phys.org/news/2018-10-evidence-earliest-life-earth-disputed.html#jCp

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the paper:

https://www.nature.com/articles/s41586-018-0610-4

Reassessing evidence of life in 3,700-million-year-old rocks of Greenland

Abstract

The Palaeoarchean supracrustal belts in Greenland contain Earth’s oldest rocks and are a prime target in the search for the earliest evidence of life on Earth. However, metamorphism has largely obliterated original rock textures and compositions, posing a challenge to the preservation of biological signatures. A recent study of 3,700-million-year-old rocks of the Isua supracrustal belt in Greenland described a rare zone in which low deformation and a closed metamorphic system allowed preservation of primary sedimentary features, including putative conical and domical stromatolites1 (laminated accretionary structures formed by microbially mediated sedimentation). The morphology, layering, mineralogy, chemistry and geological context of the structures were attributed to the formation of microbial mats in a shallow marine environment by 3,700 million years ago, at the start of Earth’s rock record. Here we report new research that shows a non-biological, post-depositional origin for the structures. Three-dimensional analysis of the morphology and orientation of the structures within the context of host rock fabrics, combined with texture-specific analyses of major and trace element chemistry, show that the ‘stromatolites’ are more plausibly interpreted as part of an assemblage of deformation structures formed in carbonate-altered metasediments long after burial. The investigation of the structures of the Isua supracrustal belt serves as a cautionary tale in the search for signs of past life on Mars, highlighting the importance of three-dimensional, integrated analysis of morphology, rock fabrics and geochemistry at appropriate scales.

 

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