Life, despite the lack of any generally accepted definition of life, physiologists regard as living any system capable of eating, metabolizing, excreting, breathing, moving, growing, and reproducing, and able to respond to external stimuli. Metabolically, life is a property of any object surrounded by a definite boundary and capable of exchanging materials with its surroundings. Biochemically, life subsists in cellular systems containing both nucleic acids and proteins. For the geneticist, life belongs to systems able to perform complex transformations of organic molecules and to construct from raw materials copies of themselves capable of evolution by natural selection. As to the origin of life, many believe it was created by God. Scientists believe in the formation of organic substances in the atmosphere over 2 billion years ago; they joined water to form a “nutrient broth” that evolved into life.