It is equally clear, however, that the data now available do not indicate when O2-producing photosynthetic cyanobacteria evolved from their AR-13324 in vivo anoxygenic photosynthetic bacterial precursors. The presence throughout much of Earth history of microbially laminated stromatolites, cyanobacterial and cyanobacterium-like microfossils, and of carbon isotopic compositions of carbonate and kerogenous carbon that fit both the direction and magnitude of the isotopic fractionation produced by modern oxygenic photoautotrophy

are consistent with, but are insufficient to establish the time of origin of O2-producing photosynthesis. CBL0137 chemical structure Thus, the earliest, Archean, stromatolites might have been formed by phototaxic anoxygenic photosynthetic bacteria, rather than by the cyanobacteria that dominate the upper surfaces of such structures today. Similarly, and despite the prevalence of assured cyanobacterial microscopic fossils in relatively young, Proterozoic,

Precambrian sediments, the filamentous and coccoidal microfossils of Archean terrains might represent remains of non-O2-producing photosynthesizers. And though the chemistry of ancient, Archean, organic matter XAV-939 mouse shows it to be unquestionably biogenic, the carbon isotopic data available from such sediments, backed even by voluminous data from younger deposits, cannot discriminate between its possible oxygenic and anoxygenic photosynthetic sources. It is certain that O2-producing photosynthesis evolved earlier, and perhaps much earlier, than the rise

of atmospheric oxygen in the Great Oxidation Event of ~2,450 Ma ago (Farquhar et al. 2000, 2007; Holland 2002; Canfield 2005), but how much earlier has yet to be established. Acknowledgments I thank J. Shen-Miller, A.B. Kudryavtsev, and C. Shi for reviews of this manuscript. This study is supported by CSEOL, the IGPP Center for the Study of Evolution and the Origin of Life at UCLA. Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any PLEKHM2 noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. References Allwood AC, Walter MR, Kamber BS, Marshall CP, Burch IW (2006) Stromatolite reef from the Early Archaean era of Australia. Nature 441:714–718CrossRefPubMed Allwood AC, Grotzinger JP, Knoll AH, Burch IW, Anderson MS, Coleman ML, Kanik I (2009) Controls on development and diversity of Early Archean stromatolites. Proc Natl Acad Sci USA 106:9548–9555CrossRefPubMed Allwood AC, Kamber BS, Walter MR, Burch IW, Kanik I (2010) Trace elements record depositional history of an Early Archean stromatolitic carbonate platform. Chem Geol 270:148–163CrossRef Barghoorn ES, Schopf JW (1965) Microorganisms from the late Precambrian of central Australia.