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Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK
The antiquity of the carbon cycle, which began prior to 3.5 Ga ago, implies that a complex and diverse biosphere has existed for most of the Earth’s history. The earliest living community may have consisted of a variety of chemotrophic bacteria, including both archaea and eubacteria, that existed around hydrothermal systems. It is possible that photosynthesis began in thermophilic bacteria that originally developed infra-red thermotaxis for detecting hot vents, maximizing the chance of survival in the close vicinity of hydrothermal systems. The post-photosynthetic biosphere may have inhabited a set of distinct habitats, including marine littoral, open ocean, terrestrial and hydrothermal biomes. Sequestration of oxidation power, by partitioning into zones of oxidation and reduction, may have allowed an inflationary biosphere to develop, limited ultimately by the availability of crucial nutrients such as phosphorus. Biomes develop and collapse, and, in consequence, selective pressures over time would have created a cooperative, Gaian biosphere.
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