Single-celled prokaryotic organisms (prokaryotic means: DNA not enclosed in a cell nucleus), many microscopic. Early in Earth's history, bacteria gradually altered the environment to support more complex forms of life (producing oxygen, for instance, in the atmosphere) even while moving into cells as organelles and decomposing organic matter into soil nutrients. Hydrothermophile bacteria discovered in samples drilled from deep in the earth give new support to Thomas Gold's idea that life originated in hot, high-pressure crevices underground.