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In the article Oldest rocks, earliest life, heaviest impacts, and the Hadean–Archaean transition (Moorbath, 2005), the author states (based on previous referenced research) that the $\ce{^13C}$ depletion previously stated as being a biotracer of the presence of early life in the metamorphosed sedimentary Archaean aged (~3.8 billion year old) rocks found in Isua, Greenland is disputed, by what the author states as being a result of

serious metamorphic and tectonic complications for a biogenic interpretation.

Hence, the presence of the traces of very early life at that location (and age) are not confirmed.

Thus, what is the age of the earliest traces of life on Earth?

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    $\begingroup$ Depends on your definition of confirmed. For some people, the carbon isotope signature is enough of a confirmation. $\endgroup$ – Gimelist Dec 13 '14 at 16:21
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To complement @MarkRovetta answer, McLoughlin & Grosch reported this year during EGU findings of carbonaceous fragments that they think are biogenic in the 3.4 Ga Buck Reef Chert. Chemical tests are still ongoing however.

Schopf (2006) in his review of archean life reported a dozen of fossils in the 3 to 3.5Ga range. They are all "putative" fossil, however the biogenicity of the fossils he reports from the 2 to 3Ga range are frankly unquestionable.

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Fossils are our strongest, and to most people most accessible, evidence of the great age of life on earth. The fossils in the Burgess Shale are clearly the imprints of critters, but are a mere 505 million years old. The oldest cyanobacteria-like fossils known are nearly 3.5 billion years old, among the oldest fossils currently known. There is Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks, but requires knowledge of isotopic chemistry to understand (or dispute.) Life on earth originated before 3.5 billion years ago.

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  • $\begingroup$ yes, I mentioned the Isua Peninsula rocks in the question. Good answer though (+1) $\endgroup$ – user889 Dec 15 '14 at 1:15
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The oldest (fairly) definitive fossils date from about 3.48 billion years ago (Ga) and consist of sedimentary structures associated with microbial mats living in coastal environments.[1]

Beyond this there are is no known direct fossil evidence so instead we have to rely on geochemical evidence. As the OP mentioned, the oldest known sedimentary rock, the Isua greenstones, are dated to 3.8 Ga and contain isotopically light graphite which may indicate a biogenic origin, although this is hotly disputed.

Recently another paper has pushed the date for potentially biogenic carbon back even earlier to 4.1 Ga.[2] The authors analysed 10,000 zircons from the Jack Hills metasediments from Western Australia and managed to extract one which was uncracked and contained what they believe to be primary, isotopically closed graphite inculsions. The zircon has been dated by U-Pb dating to 4.10 ± 0.01 Ga and so the graphite inclusions must be at least as old as this. Isotopic analysis was done on the carbon and the δ13C value comes out at −24 ± 5‰. This is consistent with carbon which has undergone isotopic fractionation by enzymatic carbon fixation, which has an average value of δ13C = -25‰ over the past 3.5 Ga. Therefore the 4.1 Ga graphite may be evidence of biological activity at this very early stage in Earth's history.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3870916/
[2] http://www.pnas.org/content/112/47/14518.full.pdf

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