Earliest known life forms
The earliest known life forms on Earth may be as old as 4.1 billion years (or Ga) according to biologically fractionated graphite inside a single zircon grain in the Jack Hills range of Australia.
[2] The earliest evidence of life found in a stratigraphic unit, not just a single mineral grain, is the 3.7 Ga metasedimentary rocks containing graphite from the Isua Supracrustal Belt in Greenland.
[3] The earliest direct known life on Earth are stromatolite fossils which have been found in 3.480-billion-year-old geyserite uncovered in the Dresser Formation of the Pilbara Craton of Western Australia.
[21][22][23][24] In July 2020, marine biologists reported that aerobic microorganisms (mainly) in "quasi-suspended animation" were found in organically poor sediment 76.2 m (250 ft) below the seafloor in the South Pacific Gyre (SPG) ("the deadest spot in the ocean").
[25] Microbes have been found in the Atacama Desert in Chile, one of the driest places on Earth,[26] and in deep-sea hydrothermal vent environments which can reach temperatures over 400°C.
The oldest disputed geochemical evidence of life is isotopically light graphite inside a single zircon grain from the Jack Hills in Western Australia.
[42] Reassessment of the Akilia graphite show that metamorphism, Fischer-Tropsch mechanisms in hydrothermal environments, and volcanic processes may be responsible for enrichment lighter carbon isotopes.
[52][53][54] Further mineralogical analysis disagrees with the initial findings of internal convex-up laminae, a critical criterion for stromatolite identification, suggesting that the structures may be deformation features (i.e. boudins) caused by extensional tectonics in the Isua Supracrustal Belt.
[55][56] The earliest direct evidence of life are stromatolites found in 3.48 billion-year-old chert in the Dresser formation of the Pilbara Craton in Western Australia.
[58] However, while most scientists accept their biogenicity, abiotic explanations for these fossils cannot be fully discarded due to their hydrothermal depositional environment and debated geochemical evidence.
[63] Claims of the earliest life using fossilized microorganisms (microfossils) are from hydrothermal vent precipitates from an ancient sea-bed in the Nuvvuagittuq Belt of Quebec, Canada.
[64][65] These findings may be better explained by abiotic processes: for example, silica-rich waters,[66] "chemical gardens,"[67] circulating hydrothermal fluids,[68] and volcanic ejecta[69] can produce morphologies similar to those presented in Nuvvuagittuq.
The 3.48 Ga Dresser formation hosts microfossils of prokaryotic filaments in silica veins, the earliest fossil evidence of life on Earth,[70] but their origins may be volcanic.
[73][74] "Putative filamentous microfossils," possibly of methanogens and/or methanotrophs that lived about 3.42-billion-year-old in "a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt, have been identified in South Africa.
"[1] A diverse set of microfossil morphologies have been found in the 3.43 Ga Strelley Pool Formation including spheroid, lenticular, and film-like microstructures.
[76] The early lithification of these structures allowed important chemical tracers, such as the carbon-to-nitrogen ratio, to be retained at levels higher than is typical in older, metamorphosed rock units.
[36][37] Model Hadean-like geothermal microenvironments were demonstrated to have the potential to support the synthesis and replication of RNA and thus possibly the evolution of primitive life.
[92] Porous rock systems, comprising heated air-water interfaces, were shown to facilitate ribozyme catalyzed RNA replication of sense and antisense strands and then subsequent strand-dissociation.
