From the beginning of time it was believed that living things could come from nonliving things. This process was known as spontaneous generation. However, in the middle of the 17th century and then through the next 100 years, this idea was disproved by three important experiments. We now know that a nonliving object or group of objects can not turn into a living organism. Spontaneous generation is impossible in the atmosphere that we have today.
In the early 1600's, people believed that living organisms could evolve from nonliving organisms. They proved this by saying that if a piece of meat was left out uncovered, that maggots would appear in a few days. These worms did not come from anything that they could see, so they assumed they came from the nonliving meat. In 1668, a man named Redi designed and completed an experiment that showed how this was not true. He took two pieces of raw meat, and left them out. He covered one so that nothing could get in, and left the other one open. The open one grew maggots, and the covered one did not, proving that the dead meat did not produce the worms as they had previously thought.
In the 1700's a man named Spallanzani proved Redi's idea to a further extent. He noticed microbial growth on boiled pond water after being exposed to the air. To prove that this growth came from something living in the air, and not from the nonliving water, he designed an experiment. He boiled pond water to kill all the microbial growths. He then poured that water into two separate test tubes. He sealed one so that no air could get in, and left one open to the air. The one that was left open slowly became more and more cloudy with microbial growths. The sealed tube stayed as clear as it had been when it was boiled. This experiment proved that the growths could not come from nonliving organisms, but had to have been transported there through the air. When Spallanzani presented his results to the public, he was criticized.