Biology – Quantitative Biology – Biomolecules
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...20923701b&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #237.01; Bulletin of the American Astronomical Society,
Biology
Quantitative Biology
Biomolecules
Scientific paper
Life has probably been present on Earth since the time of the oldest sedimentary rock record 3.8 Gyr ago, as indicated by graphite with light carbon isotope ratios consistent with derivation from organic matter. But certain evidence for life appears only at 3.52 Gyr, in the form of kerogen (insoluble organic matter) in sedimentary carbonate showing a -25‰ carbon isotope fractionation identical to that imparted by biological carbon fixation. Soon after, at 3.48 Gyr, the first visible evidence for life appears as stromatolites (sediment mounds constructed by microbes), as well as the first evidence for a specific metabolism (large negative sulfur isotope fractionations indicating microbial sulfate reduction). By 2.7 Gyr ago, molecular biomarkers (hydrocarbons derived from biomolecules with distinctive carbon skeletons such as steroids) indicate that all 3 Domains of life: bacteria, eukaryotes (organisms with compartmentalized cells like us) and archaea (bacteria-like organisms with different biochemistry, often inhabiting extreme environments); had evolved. The first multicellular eukaryotes appeared by 1.84 Gyr in the form of fossilized filamentous algae, after the atmosphere changed from anoxic to moderately oxygenated at 2.4 Gyr and following a series of extreme “Snowball Earth” glaciations between 2.4-2.2 Gyr. Planktonic algae diversified thereafter and modern algal groups arose 1.2 Gyr ago, apparently at the end of a prolonged period of ocean anoxia when the deep sea was sulfidic and presumably toxic. Animal evolution was delayed until 0.65 Gyr ago when biomarkers for sponges first appear in the record, evidently after a further rise in atmospheric oxygen to modern levels but surprisingly pre-dating the last of another series of “Snowball Earth” glaciations. These sponges co-existed with an enigmatic extinct group of large flat marine organisms called “Ediacaran fossils” that may have been ancestral to modern animal groups but might also have been a failed attempt at complex multicellular evolution.
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