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As told by Maynard Smith and Szathmáry (The Major Transitions in Evolution, 1998) life’s major transitions involve information and individuality. With equal justification, however, we can mark evolutionary milestones in terms of physiological innovation. A physiological complement to Maynard Smith and Szathmáry’s list might include three major innovations associated with primary production (photosynthesis, oxygenic photosynthesis, and nitrogen fixation) and five that changed the face of heterotrophy (respiration, aerobic respiration, phagocytosis, bulk oxygen transport, and technology). Such a physiological perspective highlights interrelationships between evolving life and a physically dynamic planet. Geochemical data suggest that for much of the Proterozoic Eon, oxygen minimum zones of Earth’s oceans tended toward euxinia. Under these conditions, nitrogen limitation would have favored primary producers capable of nitrogen fixation, as the geobiological record suggests. Despite the presence of oxygenic photoautotrophs, continuing anoxygenic photosynthesis likely played an important role in sustaining the redox structure of Proterozoic oceans. Late in the Neoproterozoic Eon, however, tectonic events appear to have nudged the biosphere toward a new state. Widespread rifting correlates with a switch from predominantly sulfidic to ferruginous waters in the OMZ; broadly coeval expansion of eukaryotes is consistent with the low sulfide tolerance exhibited by most eukaryotic clades.

Four independent geochemical proxies suggest further redox transition 580-550 Ma, a time when rates of sediment accumulation increased markedly. Higher oxygen tensions and a receding challenge of anoxia likely facilitated animal diversification, but it was the evolution of anatomical mechanisms for bulk transfer that freed animals from the constraints of diffusion — ushering in the age of bilaterians.