Based on the currently known paleontological and biogeochemical record of life in the oldest terrestrial sediments and moderate extrapolations thereof, it may be stated with fair confidence that microbial (prokaryotic and archaeoprokaryotic) ecosystems had been prolific on early Earth from at least 3.8 Ga ago. While the information encoded in the oldest record (>3.5 Ga) is commonly impaired by a metamorphic overprint, the evidence for the existence of life at times <3.5 Ga seems so firmly established as to be virtually unassailable. This holds for both the morphological (cellular) record and the biogeochemical data. Specifically, the 13C/12C signature of fossil organic carbon conveys a remarkably consistent signal of biologically mediated (enzymatic) carbon isotope fractionations over ∼ 4 Ga of recorded geological history, suggesting an extreme degree of evolutionary conservatism in the biochemistry of (photo) autotrophic carbon fixation.
Postulating a universality of biological principles in analogy to the proven universality of the laws of physics and chemistry, it may be reasonably expected that the principal properties of extraterrestrial life are similar to those that characterize Earth-bound biology. Hence, the record of life preserved in Earth‘s oldest sediments should provide a sound baseline for the interpretation of extraterrestrial analogues. Inter alia, enzymatic reactions in exobiological systems ought to be beset with isotopic fractionations resembling those in earthly biochemistry, with 13C/12C values eventually retrieved from Martian rocks likely to constrain current conjectures on the existence of former life on Mars.
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