For much of the course of life on Earth, Diatoms are microscopic, and are still, extremely important. More than 10,000 living diatom species are known, with about the same number of named fossil forms.Over ninety percent of the biosphere is plant life, of which diatoms make up about a quarter by weight. They are enormously abundant in the upper layers of the world's oceans, providing high-grade nutrition to creatures as diverse as protozoans and baleen whales, and supplying the atmosphere with around a quarter of its oxygen. Most diatoms are much less than half a millimetre in size, but their oil-rich, silica-shelled bodies, sinking to the ocean floor in vast numbers over long periods of time, have been transformed into the petroleum deposits of the world, and their skeletons have formed thick strata of diatomaceous earth which has found application in human products as varied as dynamite and (in earlier times) toothpaste.