Milestones in Early Evolution

Abstract

During much of its early history Earth was dominated by an oxygen-poor, CO2+CO+methane-rich atmosphere, with several thousand to tens of thousands ppm CO2, inducing high-temperature low-pH acid ocean waters, extending beyond submarine fumaroles. Compensation of the low early solar radiation by the high greenhouse gas levels and the low albedo due to low continent/ocean ratio allowed presence of liquid water at the surface. The high water temperature resulted in little sequestration of CO2 accumulated in the atmosphere from episodic volcanism, impact cratering, metamorphic release of CO2, dissociation of methane from sediments and microbial activity. The low-oxygen levels of the Archaean hydrosphere limited marine life to extremophile cyanobacteria and, locally, photosynthesizing stromatolites, with limited release of oxygen about 3.5–3.4 Ga. Temperatures declined with the development of continental cratons and recycling of crustal material through the mantle in the Proterozoic and the Phanerozoic, including lowering of oceanic salinity due to sequestering of evaporite deposits in continental settings. Microbial methanogenesis involves reactions of CO2 with H2 or acetate (CH3CO 2 ─ ) produced from fermentation of photosynthetically produced organic matter. An overall increase with time in δ18O, shown by terrestrial sediments, reflects a long term recycling of cold crustal materials through the mantle. Long-term cooling of the atmosphere and hydrosphere was related to an overall intermittent temporal decline in atmospheric CO2, as shown by plant leaf pores. An abrupt disappearance of positive sulphur (MIF-S) anomalies at ~2.45 Ga suggests atmospheric enrichment in oxygen and development of an ozone layer related to progressive photosynthesis by algal activity. The origin of banded iron formations is interpreted in terms of microbial oxidation of ferrous (Fe+2) to ferric (Fe+3) iron under oxygen-poor atmospheric and hydrospheric conditions on the early Earth and direct chemo-lithotropic or photo-ferrotropic oxidation of ferrous to ferric iron. A biological significance of dolomite is corroborated by experimental studies that indicate precipitation of low-temperature dolomite in sedimentary systems and interstices of pillow lava under unoxidizing conditions and microbial mediation.