Bacteria represent the oldest known group of organisms and two of the three domains of living things. They are responsible for much of earth's atmospheric oxygen, in prehistoric time and yet today. They have altered soils through nitrogen fixation and mineral utilization. They have been a source of organelles and photosynthetic pigments through endosymbiosis. Bacteria make a high cellulose diet feasible for cattle, reduce the world’s litter through decomposition and manufacture methane fuel. They are used in the manufacture of cheeses and antibiotics. Through genetic engineering, bacteria are increasingly becoming mankind's microscopic manufacturers of greatly needed chemicals. Alas, they also are responsible for much suffering and death of a wide range of eukaryotes including ourselves. These simple spheres, rods and spirals lack nuclei, endomembranes, and many organelles common to eukaryotes. They reproduce by binary fission, but achieve genetic diversity through mutation at a rate frustrating to medical practitioners. They are simple but versatile organisms.