Introductory Overview: the Expanding
Human life, like all life, consists of matter and energy structured and directed by information. All life is part of an ecosystem; all ecosystems together constitute the biosphere the total configuration of living things interacting with each other and with non-living things. Every form of life continuously affects, and is affected by, its ecosystem.
The origins of life remain a mystery, but it seems safe to assume that interactions between living and non-living matter are as old as life itself. According to current insights, life probably began around 3.8 billion years ago, deep beneath the earth's surface near volcanic vents, feeding on chemicals such as sulphur. These earliest forms of life consisted solely of bacteria–unicellular organisms, some of which gradually 'migrated' and reached the surface of the seas where they made contact with air and sunlight, and where they acquired the ability to absorb solar energy by means of photosynthesis.
Originally all microbes were anaerobic, that is, unable to digest oxygen. Any oxygen contained in the compounds they used as nutrients was rejected by their metabolism and released into the atmosphere. Eventually this made the atmosphere so rich in oxygen as to be lethal to the anaerobic bacteria. By that time, however, some varieties had evolved a metabolism capable of coping with such high levels of oxygen. While the older varieties could survive only in anaerobic niches, these new varieties were able to thrive and reproduce in an atmosphere that had been filled with free oxygen by anaerobic life itself.
The dynamics of the biosphere thus brought about a drastic transformation of the non-living planetary atmosphere. From their earliest beginnings, organisms did not