For hundreds of years, people believed that life forms could just appear. That is because they could not see microscopic organisms. In 1859, Louis Pasteur performed an important experiment that proved there were small, unseen bacteria in the air that caused milk to sour and food to rot. He later linked that to unseen organisms that also cause disease. To which part of the Cell Theory did Pasteur make the biggest contribution? Q: A: A) Cells are the basic units of life. B) All organisms are composed of cells. C) All cells come from pre-existing cells. Eliminate D) Cells contain DNA which is found specifically in the chromosome and RNA found in the cell nucleus and cytoplasm.