The Archaea, or archaebacteria, constitute the fifth kingdom of living organisms, as distinct from true bacteria as from fungi, animals, and plants. They were very likely the first life forms, from which all other living things evolved, because they are naturally adapted to thrive in the anoxic extreme environments that prevailed when life originated on the earth. Less than 20 years ago they were discovered inhabiting hot springs, salt pans, animal rumens, deep sea vents, soils, and deeply buried sediments in environments in which other life forms could not survive. Their discovery has given a unique vantage on the principles of life, because they present unique life histories and metabolisms, in effect a novel, previously unrecognized type of life. Many of the Archaea are of significant commercial importance; for instance, PCR, which has revolutionized molecular biology, is based on enzymes found in a member of the Archaea that was originally found in a hot geyser pool in Yellowstone National Park.