What was the theory of spontaneous generation?

Early Greek thinkers thought units of life called spores were transferred to different planets including earth. ‘Panspermia’ is still a favourite idea for some astronomers For a long time it was also believed that life came out of decaying and rotting matter like straw, mud, etc. This was the theory of spontaneous generation. Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life.He showed that in pre-sterilised flasks, life did not come from killed yeast while in another flask open to air, new living organisms arose from ‘killed yeast’.