The lungs of the Earth: Review of the carbon cycle and mass extinction of species

The ability of carbon to combine with oxygen or/and hydrogen, leading to the formation of complex molecules such as amino acids, carbohydrates, lipids, proteins and nucleic acids, in the presence of water, forms the basis of the chemistry of advanced life. The carbon, oxygen, nitrogen and sulphur cycles, mediated by the atmosphere-ocean-land system, constitute the "lungs of the biosphere", allowing the exchange of essential components of biological molecules. The capture of atmospheric carbon dioxide through photosynthesis, release of oxygen, respiration and burial of carbon produce the balance on which the biosphere depends. The atmospheric concentration of carbon-dominated greenhouse gases plays a key role regulating terrestrial temperatures. The mean global temperature of ~14.9°C allows the existence on the Earth surface of aqueous media where metabolic microbiological processes are performed, among other by chemo-bacteria, microbes and algae. The geological record displays a close correspondence between paleo-CO ₂ levels and paleo-temperature trends, allowing the identification of environmental factors that underlie the evolution and extinction of species. Unoxidizing atmospheric and low-pH hydrosphere conditions on the early Earth, dominated by methane, CO ₂ and CO, constrained the appearance of oxygenating organisms, with the exception of minor oxygen release by stromatolites. An increase in photosynthetic oxygen about 2.45 Ga was associated with proliferation of phytoplankton. Glaciation followed by the "Cambrian Explosion" of life at 543 Ma is considered responsible for development of complex proteins and abundant marine life. The anthropogenic extraction and transfer from the Earth's crust to the atmosphere of carbon, including coal, oil, tar sand, shale oil, methane gas, coal seam gas and other forms of hydrocarbon, constitutes the most significant shift in composition of the atmosphere since the PETM hyperthermal event (~56 Ma) and the K-T boundary extinction (~66 Ma), with worrying consequences for the planetary habitat.