ATMOSPHERE EVOLUTION 203 Figure The phylogenetic tree of life based on comparative 16s rRNA sequences (Reproduced from Pace NR (2001) The universal nature of biochemistry Proceedings of the National Academy of Sciences USA 98: 805 808.) donor to reduce carbon dioxide to biologically useful organic compounds such as carbohydrates Liquid water and carbon dioxide are available to life in practically unlimited quantities; sunlight drives the reaction, and cyanobacteria that use it, although relative late-comers in phylogenetic terms, are arguably the most successful organisms in the history of life on Earth It is photosystem II that releases free oxygen as a by-product in the reversible reaction: 6CO2 ỵ 6H2 O þ light energy $ C6 H12 O6 þ 6O2 The gradual build-up of oxygen in the atmosphere was accomplished by the slow toilsome efforts of cyanobacteria over many hundreds of millions of years The immense importance of this metabolic reaction to the history of all life beyond the microbial level cannot be overstated (see Earth System Science) Rise in atmospheric oxygen Initially, the vast amounts of reduced iron (Fe2ỵ) and other chemical species dissolved in the oceans, and perhaps some reduced gases in the atmosphere (such as methane), provided a sink for the oxygen produced by photosynthesis As these sinks were exhausted, local concentrations of oxygen would have appeared, creating a dilemma for the early anaerobic microbial biosphere High levels of oxygen produce toxic radicals in the environment, which cause extensive damage to cell components This would have been a strong driving force for natural selection leading to adaptation: microbes that did not immediately