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. Tice, The mats were once thriving in shallow-water, photic zone paleoenvironments below fair-231 weather wave base and show three distinct mat morphotypes: (1) alpha-type, fine carbonaceous laminations 232 that incorporate and loosely drape detrital grains underlying detrital grains and form silica, F) and in places ripped up, plastically deformed rolled-up fragments (Fig. 5 A and B), 1999.

B. 4a, ) beta-type, fine meshworks of filament-like strands (<5µm in diameter), which drape 234 underlying detrital grains (Fig. 4C and D); and (3) gamma-type, evenly spaced flat laminations that tightly 235 drape underlying sediments (Fig. 4E and F

. Tice, The formation of different mat morphotypes is 236 thought to be primarily controlled by local variations in ambient light intensity and/or current energy, 2009.

, Bulk organic carbon isotope measurements of the carbonaceous laminations, with ? 13 C org values 444 of the sedimentological and paleoenvironmental context of these fossil mats revealed that they are laterally 445 traceable for ?15 km in a ?1000 m-thick succession in the lower part of the Saddleback Syncline and show 446 distinct morphological adaptations to different hydrodynamic settings: (1) planar-type in coastal floodplain, Consequently, the first two morphotypes (alpha and beta) probably formed in shallower water, vol.447, 2009.

A. , Such facies dependent changes in the prevalent mat morphotypes are to be expected in a dynamic, tidally-449 influenced coastal environment and serve as an additional biogenicity indicator, 2006.

. Brasier, Moreover, the widespread occurrence of these fossil mats is consistent with a primary, 451 microbially mediated, cohesive erosion-resistant relief on the paleosurface that was locally deformed by 452 migrating gases and fluids, and in places eroded and, 2006.

. Homann, Based on the restriction of the mats to shallow-water, photic zone environments and their apparent absence 454 in subtidal settings, it is very likely that they were formed by phototrophic microbial communities, 2006.

. Gerdes, , 2000.

U. Coast, . Bose, ;. Chafetz, and . Taj, Even tough, no geochemical data 462 supporting the local presence of free oxygen (and thus oxygenic photosynthesis) at 3.22 Ga have been found 463 so far in the Moodies Group, fossil evidence indicative for ancient gas production, accumulation, and 464 migration is plentiful in the near vicinity of the mats. Now chert-filled cavities in the interior of some tufts 465 likely represent silicified gas bubbles that were trapped within the mat fabric, 2009.

. Bosak, Other types of silicified cavities include domes and bedding activity, due to the decay of organic matter, or alternatively by tidal-driven hydraulic pumping 470 of the ambient air trapped in pore space (Figs, pp.12-14, 2010.

. Homann, Some of these cavities 471 where also inhabited by microbial communities (see below in 5, vol.2, 2016.

. Homann, where a large variety of 474 sedimentary structures indicates that this succession records a transition from alluvial-fluvial (terrestrial) to 475 tide-influenced marine sedimentation (Heubeck and Lowe, Besides the main mat occurrence in the tidal marine deposits of the Saddleback Syncline, fossil microbial 473 mats also have been identified in the Dycedale Syncline, vol.476, 1994.

, The most widespread, pervasive, and probably also oldest trace of ancient life in the BGB are the remnants 614 of shallow-water microbial mats and biofilms. Mat-like laminations occur in almost all black-and-white-615 banded, but only the examples reported from 616 the 3.472 Ga Middle Marker, 1992.

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(. Chert and . Westall, ) are reasonably well studied to support their biogenic origin, 2006.

, well-preserved and -documented record of the Paleoarchean 621 life. Besides the necessity to carefully reinvestigate more potentially microfossil-and mat-bearing 622 carbonaceous cherts in the BGB it is also crucial to further support the already existing claims for early life 623 from these units with more detailed geochemical analysis such as e.g. carbon, nitrogen, and sulphur isotope 624 data. The microbial mats preserved in the tidal and, Especially the mats and microfossils preserved in the up to 400-m-thick Buck Reef Chert represent a 620 particularly widespread (~50 km along strike)

, Moodies Group are unique and currently not known from equivalent deposits from Austalia (Fig, p.16

. Noffke, The quality of 627 preservation of the delicate carbonaceous mat laminae, distinct morphotypes, and mat-associated cavities 628 in these coarse-grained and gravely siliciclastic deposits is truly exceptional and implies a rapid fossilization 629 driven by early diagenetic silicification of the sediments, ) has demonstrated the potential of detailed shallow marine, 1986.

. Homann, Besides impact-generated tsunamis it has been proposed that some of these 649 catastrophic events might have been large enough to cause partial boiling and sterilization of the oceans 650 and possibly triggered mass extinctions of low-temperature microbes, including most photosynthetic 651 microorganisms, vol.648, pp.1010-1011, 1986.

. Rasmussen, widespread 656 intertidal tufted microbial mats, laterally traceable for 15 km in a ?1000 m-thick succession, as well as 657 erosion-resistant fluvial microbial mats. The latter represent the oldest known direct fossil evidence for 658 terrestrial life on the continental surface and are ~500 Ma older than ~2.7 Ga old fluvio-lacustrine 659 stromatolites and coelobionts preserved in the Fortescue Group (Tumbiana and Hardey Formation, 660 Australia, The Moodies Group ecosystem was particularly diverse, advanced, and well-adapted and includes large 655 spheroidal microfossils, Earth's earliest evidence of cavity-dwelling microbes (coelobionts), 1992.

, It was predominantly thriving in shallow marine 676 environments in the photic zone but started to spread out to colonize fluvial habitats in emerged continental 677 surface environments and also occupied the first ecological niches, such as subsurface cavities. Traces of 678 ancient life in the BGB occur scattered throughout the entire stratigraphy confined to carbonaceous cherts 679 of the Onverwacht Group and siliciclastic deposits of the Moodies Group. However, their identification is 680 sometimes solely based on morphological attributes and not always accompanied by detailed and systematic, The deposits of the Barberton Greenstone Belt host a large variety of convincing macro-and microscopic, 675 as well as geochemical evidence for early microbial life, pp.1065-1066, 1120.

, Based on its universal and outstanding geological and paleobiological value the Barberton-Makhonjwa 687 Mountains were inscribed in the UNESCO World Heritage Site register in 2018, which will ultimately help 688 to protect these exceptional outcrops for future studies of Earth's early evolution

, Wlady Altermann, and Andrew Knoll for their helpful 695 comments. I am also indebted to Stefan Lalonde and Claire Earlie for useful discussions, This work was greatly supported by LabexMER ANR-10-LABX-19 and Prestige COFUND-GA-2013-693 609102 to M.H

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, A-D) Lenticular, disk-shaped objects with hollow centers and carbonaceous walls preserved in cherts from the Upper Onverwacht Group. Note that the structures occur isolated or in chain-like clusters of several specimen. (E-H) Morphologically very similar lenticular microfossils from the Buck Reef Chert, Photomicrographs of lenticular structures interpreted as microfossils, 1966.

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, A and B) Transmission electron microscopy (TEM) images of cell-like objects with granular, in places detached walls (arrow) from the Hooggenoeg Formation. C and D) Large spheroids and ellipsoids from the Buck Reef Chert. E and F) Clusters of thin-walled spheroids from the Buck Reef Chert. G and H) Groups of spheroidal microstructures with rounded to angular walls from the Buck Reef Chert. I) Putative organic microspheres (arrows) from the Msauli Chert, which show features resembling cell division (small image in the upper right corner), 2008.

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, Image (I) courtesy of Andrew H