Schwartzman and Caldeira followed up the proposal of Charles Dismukes and coworkers that now extinct bacteria were performing oxygen-based photosynthesis before cyanobacteria came onto the scene. In a CO2 dominated world, these early oxygenic photosynthesizers split bicarbonate instead of water as the source of oxygen. They apparently boosted organic productivity and caused greater methane production by methanogens living in the ocean.
"It takes far less methane to maintain climatic temperatures than it does carbon dioxide," says Schwartzman.
As methane became dominant, CO2 levels dropped dramatically. Cyanobacteria then emerged and began oxygenic photosynthesis by splitting water as the source of oxygen. According to Schwartzman, only when atmospheric oxygen levels began to rise some 2.2 billion years ago did a CO2-concentrating mechanism emerge, an adaptation to declining CO2/02 ratios in the external environment.
Thus, global constraints on evolution appeared to have included carbon dioxide as well as oxygen levels in the atmosphere along with surface temperature. All the former have been strongly influenced by biological evolution in a complex set of feedbacks, an essential aspect of biospheric evolution. "The classical paradigm of evolution, that changes in the local environment lead to natural selection, should be rethought to include these feedbacks on a global scale. We hope that our hypothesis will be tested by looking more closely at the extant geologic record of the proposed transition as well as the insights from the study of photosynthesis and molecular biology of modern organisms," says Schwartzman.