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Proof of life: Reevaluating oldest known Archean trace fossil for indications of early biology

In the hunt for early life, geobiologists seek evidence of ancient microbes in the form of trace fossils – geological records of biological activity – embedded in lavas beneath the ocean floor. Filamentous titanite (a calcium titanium silicate mineral) microtextures found in 3.45 billion-year-old volcanic pillow lavas of the Barberton greenstone belt of South Africa, have been argued previously2 to be Earth's oldest trace fossil, representing the mineralized remains of microbial tunnels in seafloor volcanic glass. However, scientists at the University of Bergen, Norway have reported new data based on in situ U-Pb (uranium-lead) dating, metamorphic temperature mapping constraints and morphological observations that bring the biological origin of these fossils into serious question. The new age determined for the titanite microtextures is much younger than the eruptive and seafloor hydrothermal age of the previously proposed bioalteration model. As a result, the researchers have analyzed these fossils' syngenicity (age as estimated by a textural, chemical, mineral, or biological feature formed at the same time as its encapsulating material) and biogenicity (any chemical and/or morphological signature preserved over a range of spatial scales in rocks, minerals, ice, or dust particles that are uniquely produced by past or present organisms). The scientists conclude that the oldest bona fide biogenic trace fossil now reverts to roughly 1.7 Ga microborings in silicified stromatolites found in China, and that the search for subsurface life – both on the early Earth as well as in extraterrestrial mafic–ultramafic rocks1, such as Martian basalts – be based not only on new biosignatures, but on new detection techniques as well.

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