The Galileo satellite during its recent passes close to the Earth recorded a planet with an unusual red-absorbing pigment, a poisonous atmosphere, simultaneously rich in oxygen and in methane, with strong, modulated, narrow-band, radio emissions in the MHz frequencies (Sagan et al. 1993). To an observer visiting the solar system, these features; the photo-oxidisable pigment chlorophyll, abundant atmospheric oxygen, the existence of reducing conditions and intelligent life might well appear self-contradictory. While intelligent life is a recent event, the presence of other forms of life based on photosynthesis and survival under both oxygen-rich atmospheres and reducing conditions go back to the earliest times (Table 1). Life on Earth has evolved over nearly 4 G years under atmospheric environments ranging from anoxia, to hypoxia, to hyperoxia (relative to the present day), and not always in that sequence.