Figure 1.

Variations in atmospheric oxygen concentrations over geological time. Green arrows and texts represent major evolutionary and geological events that are believed to be in direct connection with atmospheric O₂ concentration (Graham et al., 1995, 1997; Dudley, 1998; Bishopric, 2005; Raymond and Segre, 2006; Fisher and Burggren, 2007; Koch and Britton, 2007; Harrison et al., 2010; Taylor and McElwain, 2010). Similar comparisons have been discussed previously (Koch and Britton, 2007). Arrows are positioned in time based on approximations derived from geological evidence. Atmospheric O₂ levels were estimated and plotted based on available geochemical studies (Berner, 2006; Olson, 2006; Raymond and Segre, 2006; Berner et al., 2007; Taylor and McElwain, 2010). (A) Atmospheric O₂ concentration across geological time. This representation includes the periods over which life first appeared on Earth and the evolution of oxygenic photosynthesis, which was responsible for the subsequent rise in atmospheric O₂. (B) A zoom in on the region highlighted in red on (A). This expanded view includes the time span ranging from the Ediacaran period of the late Pre-Cambrian to the present Quaternary period (not named in graph) of the Cenozoic Era. During these periods, atmospheric O₂ concentration varied greatly; these changes influenced the evolutionary trends of animal life, including the development of the cardiovascular and respiratory systems in vertebrates. Symbols on the bottom axis represent the geological Periods according to the 2009 Geologic Time Scale (Walker and Geissman, 2009). Pre-Є, Pre-Cambrian; Є, Cambrian; O, Ordovician; S, Silurian; D, Devonian; C, Carboniferous; P, Permian; Tr, Triassic; J, Jurassic; K, Cretaceous; T, Tertiary.