2PennState Astrobiology Research Center, University Park, PA 16802 University Park, PA 16802, USA
3University of Wisconsin Astrobiology Research Consortium, Madison, WI 53706, USA
Received September 9, 2013
Fossil evidence of photosynthesis, documented in the geological record by microbially laminated stromatolites, microscopic fossils, and carbon isotopic data consistent with the presence of Rubisco-mediated CO2-fixation, extends to ~3500 million years ago. Such evidence, however, does not resolve the time of origin of oxygenic photosynthesis from its anoxygenic photosynthetic evolutionary precursor. Though it is evident that cyanobacteria, the earliest-evolved O2-producing photoautotrophs, existed before ~2450 million years ago — the onset of the “Great Oxidation Event” (GOE) that forever altered Earth’s environment — O2-producing photosynthesis seems certain to have originated hundreds of millions of years earlier. How did Earth’s biota respond to the GOE? Four lines of evidence are here suggested to reflect this major environmental transition: (1) rRNA phylogeny-correlated metabolic and biosynthetic pathways document evolution from an anaerobic (pre-GOE) to a dominantly oxygen-requiring (post-GOE) biosphere; (2) consistent with the rRNA phylogeny of cyanobacteria, their fossil record evidences the immediately post-GOE presence of cyanobacterial nostocaceans characterized by specialized cells that protect their oxygen-labile nitrogenase enzyme system; (3) the earliest known fossil eukaryotes, obligately aerobic phytoplankton and putative algae, closely post-date the GOE; and (4) microbial sulfuretums are earliest known from rocks deposited during and immediately after the GOE, their apparent proliferation evidently spurred by an increase of environmental oxygen and a resulting upsurge of metabolically useable sulfate and nitrate. Though the biotic response to the GOE is a question new to paleobiology that is yet largely unexplored, additional evidence of its impact seems certain to be uncovered.
KEY WORDS: carbon isotopic fraction, cyanobacterial evolution, microfossils, oldest eucaryotes, oxygen-requiring biosynthesis, Precambrian, stromatolites