The first traces of life on Earth date back to the early Archean age. Microfossils of cyanobacteria-like prokaryotes within fossil stromatoliths demonstrate that life already existed 3.5 billion years ago (Awramik et al., 1983; Schopf and Packer, 1987; Schopf, 1993). Life had already originated much earlier, possibly by the end of the major period of meteorite impacts about 3.9 billion years ago (Schopf et al., 1983; Mojzsis et al., 1996). At that time, the Earth is generally assumed to have been much hotter than today (Ernst, 1983). Questions arise about possible physiological properties, modes of energy acquisition, and kinds of carbon sources of the earliest organisms which may have made their living in a world of fire and water.
Today most life forms known are mesophiles adapted to ambient temperatures within a range from 15 to 45 °C. Among bacteria, thermophiles (heat-lovers) have been recognized for some time, which grow optimally (fastest) between 45 and 70 °C. They thrive within Sun-heated soils, self-heated waste dumps, and thermal waters, and are closely related to mesophiles. Since Louis Pasteur's time it has generally been assumed that vegetative (growing) cells of bacteria (including most thermophiles) are quickly killed by temperatures of above 80 °C. In contrast, during recent years, hyperthermophilic bacteria and archaea (formerly the archaebacteria) with unprecedented properties have been isolated mostly from areas of volcanic activity (Stetter, 1986, 1992; Stetter et al., 1990).