Recent studies related to microbial cells preserved in material from Mars indicates that microbes had been in the universe for a long time. Microbiologists estimate that living organisms have been on earth for 3.5 billion years. Whether you believe the initial living cell was specially created or happened from the random interaction of molecules is not important in this course. During the discussion of the origin of life in the mid-1880s, scientists including Pasteur, Needham, and Schwann discussed the Theory of Spontaneous Generation and the Theory of Biogenesis. As early as 40 B.C. scientists tried to explain the origin of life. However, Francesco Redi covered meat in a jar with gauze and maggots developed outside not inside the meat. Later it was discovered that heating reduced microbes; but it was much later when resistant endospores were determined to be responsible for growth in liquid food. As cotton plugs and long swan-necked tubes were used to prevent air contamination, scientists became convinced that "life came only from previously living organisms."