Early Evidence for photosynthesis

Résumé:
Though anoxygenic photosynthesis oxidizing H2S to sulfate (instead of
H2O to O2 in oxygenic photosynthesis) is hypothesized from biochemical
data to have evolved first, the geological evidence for it only goes
back to hydrocarbon biomarkers derived from photo-pigments of green
and purple anoxygenic photosynthetic bacteria in 1.65 Ga
rocks. Paradoxically, evidence for oxygenic photosynthesis is much
more ancient. There are 3 hypotheses regarding the timing of its
evolution: 1) extremely ancient such that the atmosphere has always
been oxygenated; 2) immediately before the atmosphere became
permanently and highly oxygenated during the "Great Oxidation Event"
~2.35 Ga ago; and 3) hundreds of millions of years before atmospheric
oxygenation but taking aeons for molecular oxygen to accumulate in the
atmosphere. There is abundant evidence for low or no environmental
oxygen in early Archean rocks, invalidating the first hypothesis, and
consistent but controversial evidence from hydrocarbon biomarkers,
redox-sensitive metals and minerals, and lacustrine stromatolites for
Archean oxygen production, apparently invalidating the second
hypothesis. In particular, biomarkers from contamination-proof fluid
inclusions from before the Great Oxidation Event support the third
hypothesis, implying that it indeed took aeons to oxidize the
reservoirs and fluxes of reduced volcanic gases, hydrothermal fluids
and volcanic crust. Thus, oxygenic photosynthetic cyanobacteria
apparently evolved well before their excreted oxygen permanently
polluted the atmosphere.